The B AB AR detector

Emanuel Oliva

The B AB AR detector

2002, Nuclear Instruments & Methods in Physics Research Section A-accelerators Spectrometers Detectors and Associated Equipment

Abstract

BABAR, the detector for the SLAC PEP-II asymmetric e + e − B Factory operating at the Υ (4S) resonance, was designed to allow comprehensive studies of CP -violation in B-meson decays. Charged particle tracks are measured in a multi-layer silicon vertex tracker surrounded by a cylindrical wire drift chamber. Electromagentic showers from electrons and photons are detected in an array of CsI crystals located just inside the solenoidal coil of a superconducting magnet. Muons and neutral hadrons are identified by arrays of resistive plate chambers inserted into gaps in the steel flux return of the magnet. Charged hadrons are identified by dE/dx measurements in the tracking detectors and in a ring-imaging Cherenkov detector surrounding the drift chamber. The trigger, data acquisition and data-monitoring systems, VME-and network-based, are controlled by custom-designed online software. Details of the layout and performance of the detector components and their associated electronics and software are presented.

SLAC-PUB-8569 BABAR-PUB-01/08 hep-ex/0105044 April, 2001 The BABAR Detector The BABAR Collaboration Abstract BABAR, the detector for the SLAC PEP-II asymmetric e+ e− B Factory operating at the Υ (4S) resonance, was designed to allow comprehensive studies of CP -violation in B-meson decays. Charged particle tracks are measured in a multi-layer silicon vertex tracker surrounded by a cylindrical wire drift chamber. Electromagentic showers from electrons and photons are detected in an array of CsI crystals located just inside the solenoidal coil of a superconducting magnet. Muons and neutral hadrons are identified by arrays of resistive plate chambers inserted into gaps in the steel flux return of the magnet. Charged hadrons are identified by dE/dx measurements in the tracking detectors and in a ring-imaging Cherenkov detector surrounding the drift chamber. The trigger, data acquisition and data-monitoring systems, VME- and network-based, are controlled by custom-designed online software. Details of the layout and performance of the detector components and their associated electronics and software are presented. Submitted to Nuclear Instruments and Methods Stanford Linear Accelerator Center, Stanford University, Stanford, CA 94309 Work supported in part by Department of Energy contract DE-AC03-76SF00515. i The BABAR Collaboration B. Aubert, A. Bazan, A. Boucham, D. Boutigny, I. De Bonis, J. Favier, J.-M. Gaillard, A. Jeremie, Y. Karyotakis, T. Le Flour, J.P. Lees, S. Lieunard, P. Petitpas, P. Robbe, V. Tisserand, K. Zachariadou Laboratoire de Physique des Particules, F-74941 Annecy-le-Vieux, France A. Palano Universit` a di Bari, Dipartimento di Fisica and INFN, I-70126 Bari, Italy G.P. Chen, J.C. Chen, N.D. Qi, G. Rong, P. Wang, Y.S. Zhu Institute of High Energy Physics, Beijing 100039, China G. Eigen, P.L. Reinertsen, B. Stugu University of Bergen, N-5007 Bergen, Norway B. Abbott, G.S. Abrams, L. Amerman, A.W. Borgland, A.B. Breon, D.N. Brown, J. Button-Shafer, A.R. Clark, S. Dardin, C. Day, S.F. Dow, Q. Fan, I. Gaponenko, M.S. Gill, F.R. Goozen, S.J. Gowdy, A. Gritsan, Y. Groysman, C. Hernikl, R.G. Jacobsen, R.C. Jared, R.W. Kadel, J. Kadyk, A. Karcher, L.T. Kerth, I. Kipnis, S. Kluth, J.F. Kral, R. Lafever, C. LeClerc, M.E. Levi, S.A. Lewis, C. Lionberger, T. Liu, M. Long, L. Luo, G. Lynch, P. Luft, E. Mandelli, M. Marino, K. Marks, C. Matuk, A.B. Meyer, R. Minor, A. Mokhtarani, M. Momayezi, M. Nyman, P.J. Oddone, J. Ohnemus, D. Oshatz, S. Patton, M. Pedrali-Noy, A. Perazzo, C. Peters, W. Pope, M. Pripstein, D.R. Quarrie, J.E. Rasson, N.A. Roe, A. Romosan, M.T. Ronan, V.G. Shelkov, R. Stone, P.D. Strother,1 A.V. Telnov, H. von der Lippe, T.F. Weber, W.A. Wenzel, G. Zizka Lawrence Berkeley National Laboratory and University of California, Berkeley, CA 94720, USA P.G. Bright-Thomas, C.M. Hawkes, A. Kirk, D. J. Knowles, S.W. O’Neale, A.T. Watson, N.K. Watson University of Birmingham, Birmingham, B15 2TT, UK T. Deppermann, H. Koch, J. Krug, M. Kunze, B. Lewandowski, K. Peters, H. Schmuecker, M. Steinke Ruhr Universit¨ at Bochum, Inst. f. Experimentalphysik 1, D-44780 Bochum, Germany J.C. Andress, N.R. Barlow, W. Bhimji, N. Chevalier, P.J. Clark, W.N. Cottingham, N. De Groot, N. Dyce, B. Foster, A. Mass, J.D. McFall, D. Wallom, F.F. Wilson University of Bristol, Bristol BS8 1TL, UK K. Abe, C. Hearty, J.A. McKenna, D. Thiessen University of British Columbia, Vancouver, BC, Canada V6T 1Z1 B. Camanzi, T.J. Harrison,2 A.K. McKemey, J. Tinslay Brunel University, Uxbridge, Middlesex UB8 3PH, UK E.I. Antohin, V.E. Blinov, A.D. Bukin, D.A. Bukin, A.R. Buzykaev, M.S. Dubrovin, V.B. Golubev, V.N. Ivanchenko, G.M. Kolachev, A.A. Korol, E.A. Kravchenko, S.F. Mikhailov, A.P. Onuchin, A.A. Salnikov, S.I. Serednyakov, Yu.I. Skovpen, V.I. Telnov, A.N. Yushkov Budker Institute of Nuclear Physics, Novosibirsk 630090, Russia 1 Now at Queen Mary, University of London, London, E1 4NS, UK 2 Now at University of Birmingham, Birmingham B15 2TT, UK ii J. Booth, A.J. Lankford, M. Mandelkern, S. Pier, D.P. Stoker, G. Zioulas University of California at Irvine, Irvine, CA 92697, USA A. Ahsan, K. Arisaka, C. Buchanan, S. Chun University of California at Los Angeles, Los Angeles, CA 90024, USA R. Faccini,3 D.B. MacFarlane, S.A. Prell, Sh. Rahatlou, G. Raven, V. Sharma University of California at San Diego, La Jolla, CA 92093, USA S. Burke, D. Callahan, C. Campagnari, B. Dahmes, D. Hale, P.A. Hart, N. Kuznetsova, S. Kyre, S. L. Levy, O. Long, A. Lu, J. May, J.D. Richman, W. Verkerke, M. Witherell, S. Yellin University of California at Santa Barbara, Santa Barbara, CA 93106, USA J. Beringer, J. DeWitt, D.E. Dorfan, A.M. Eisner, A. Frey, A.A. Grillo, M. Grothe, C.A. Heusch, R.P. Johnson, W. Kroeger, W.S. Lockman, T. Pulliam, W. Rowe, H. Sadrozinski, T. Schalk, R.E. Schmitz, B.A. Schumm, A. Seiden, E.N. Spencer, M. Turri, W. Walkowiak, M. Wilder, D.C. Williams University of California at Santa Cruz, Santa Cruz, CA 95064, USA E. Chen, G.P. Dubois-Felsmann, A. Dvoretskii, J.E. Hanson, D.G. Hitlin, Yu.G. Kolomensky,4 S. Metzler, J. Oyang, F.C. Porter, A. Ryd, A. Samuel, M. Weaver, S. Yang, R.Y. Zhu California Institute of Technology, Pasadena, CA 91125, USA S. Devmal, T.L. Geld, S. Jayatilleke, S.M. Jayatilleke, G. Mancinelli, B.T. Meadows, M.D. Sokoloﬀ University of Cincinnati, Cincinnati, OH 45221, USA P. Bloom, B. Broomer, E. Erdos, S. Fahey, W.T. Ford, F. Gaede, W.C. van Hoek, D.R. Johnson, A.K. Michael, U. Nauenberg, A. Olivas, H. Park, P. Rankin, J. Roy, S. Sen, J.G. Smith, D.L. Wagner University of Colorado, Boulder, CO 80309, USA J. Blouw, J.L. Harton, M. Krishnamurthy, A. Soﬀer, W.H. Toki, D.W. Warner, R.J. Wilson, J. Zhang Colorado State University, Fort Collins, CO 80523, USA T. Brandt, J. Brose, G. Dahlinger, M. Dickopp, R.S. Dubitzky, P. Eckstein, H. Futterschneider, M.L. Kocian, R. Krause, R. M¨uller-Pfeﬀerkorn, K.R. Schubert, R. Schwierz, B. Spaan, L. Wilden Technische Universit¨ at Dresden, D-01062 Dresden, Germany L. Behr, D. Bernard, G.R. Bonneaud, F. Brochard, J. Cohen-Tanugi, S. Ferrag, G. Fouque, F. Gastaldi, P. Matricon, P. Mora de Freitas, C. Renard, E. Roussot, S. T’Jampens, C. Thiebaux, G. Vasileiadis, M. Verderi Ecole Polytechnique, F-91128 Palaiseau, France A. Anjomshoaa, R. Bernet, F. Di Lodovico, F. Muheim, S. Playfer, J.E. Swain University of Edinburgh, Edinburgh EH9 3JZ, UK 3 Jointly appointed with Universit` a di Roma La Sapienza, Dipartimento di Fisica and INFN, I-00185 Roma, Italy 4 Now at LBNL and University of California, Berkeley, CA 94720, USA iii M. Falbo Elon College, Elon College, NC 27244, USA C. Bozzi, S. Dittongo, M. Folegani, L. Piemontese, A..C. Ramusino Universit` a di Ferrara, Dipartimento di Fisica and INFN, I-44100 Ferrara, Italy E. Treadwell Florida A&M University, Tallahassee, FL 32307, USA F. Anulli,5 R. Baldini-Ferroli, A. Calcaterra, R. de Sangro, D. Falciai, G. Finocchiaro, P. Patteri, I.M. Peruzzi,5 M. Piccolo, Y. Xie, A. Zallo Laboratori Nazionali di Frascati dell’INFN, I-00044 Frascati, Italy S. Bagnasco, A. Buzzo, R. Contri, G. Crosetti, P. Fabbricatore, S. Farinon, M. Lo Vetere, M. Macri, S. Minutoli, M.R. Monge, R. Musenich, M. Pallavicini, R. Parodi, S. Passaggio, F.C. Pastore, C. Patrignani, M.G. Pia, C. Priano, E. Robutti, A. Santroni Universit` a di Genova, Dipartimento di Fisica and INFN, I-16146 Genova, Italy R. Bartoldus, T. Dignan, R. Hamilton, U. Mallik University of Iowa, Iowa City, IA 52242, USA J. Cochran, H.B. Crawley, P.A. Fischer, J. Lamsa, R. McKay, W.T. Meyer, E.I. Rosenberg Iowa State University, Ames, IA 50011-3160, USA J.N. Albert, C. Beigbeder, M. Benkebil, D. Breton, R. Cizeron, S. Du, G. Grosdidier, C. Hast, A. H¨ocker, H. M. Lacker, V. LePeltier, A.M. Lutz, S. Plaszczynski, M.H. Schune, S. Trincaz-Duvoid, K. Truong, A. Valassi, G. Wormser Laboratoire de l’Acc´el´erateur Lin´eaire, F-91898 Orsay, France O. Alford, D. Behne, R.M. Bionta, J. Bowman, V. Brigljevi´c, A. Brooks, V.A. Dacosta, O. Fackler, D. Fujino, M. Harper, D.J. Lange, M. Mugge, T.G. O’Connor, H. Olson, L. Ott, E. Parker, B. Pedrotti, M. Roeben, X. Shi, K. van Bibber, T.J. Wenaus, D.M. Wright, C.R. Wuest, B. Yamamoto Lawrence Livermore National Laboratory, Livermore, CA 94550, USA M. Carroll, P. Cooke, J.R. Fry, E. Gabathuler, R. Gamet, M. George, M. Kay, S. McMahon,6 A. Muir, D.J. Payne, R.J. Sloane, P. Sutcliﬀe, C. Touramanis University of Liverpool, Liverpool L69 3BX, UK M.L. Aspinwall, D.A. Bowerman, P.D. Dauncey, I. Eschrich, N.J.W. Gunawardane, R. Martin, J.A. Nash, D.R. Price, P.Sanders, D.Smith University of London, Imperial College, London, SW7 2BW, UK D.E. Azzopardi, J.J. Back, P. Dixon, P.F. Harrison, D. Newman-Coburn,7 R.J.L. Potter, H.W. Shorthouse, M.I. Williams, P.B. Vidal Queen Mary, University of London, London, E1 4NS, UK 5 Jointlyappointed with Univ. di Perugia, I-06100 Perugia, Italy 6 Now at University of California at Irvine, Irvine, CA 92697, USA 7 Deceased iv G. Cowan, S. George, M.G. Green, A. Kurup, C.E. Marker, P. McGrath, T.R. McMahon, F. Salvatore, I. Scott, G. Vaitsas University of London, Royal Holloway and Bedford New College, Egham, Surrey TW20 0EX, UK D. Brown, C. L. Davis, Y. Li, J. Pavlovich University of Louisville, Louisville, KY 40292, USA J. Allison, R.J. Barlow, J.T. Boyd, J. Fullwood, F. Jackson, A. Khan,8 G.D. Laﬀerty, N. Savvas, E.T. Simopoulos, R.J. Thompson, J.H. Weatherall University of Manchester, Manchester M13 9PL, UK R. Bard, C. Dallapiccola,9 A. Farbin, A. Jawahery, V. Lillard, J. Olsen, D.A. Roberts, J.R. Schieck University of Maryland, College Park, MD 20742, USA G. Blaylock, K.T. Flood, S.S. Hertzbach, R. Kofler, C.S. Lin, S. Willocq, J. Wittlin University of Massachusetts, Amherst, MA 01003, USA B. Brau, R. Cowan, F. Taylor, R.K. Yamamoto Massachusetts Institute of Technology, Cambridge, MA 02139, USA D.I. Britton, R. Fernholz,10 M. Houde, M. Milek, P.M. Patel, J. Trischuk McGill University, Montr´eal, QC, Canada H3A 2T8 F. Lanni, F. Palombo Universit` a di Milano, Dipartimento di Fisica and INFN, I-20133 Milano, Italy J.M. Bauer, M. Booke, L. Cremaldi, R. Kroeger, M. Reep, J. Reidy, D.A. Sanders, D.J. Summers University of Mississippi, University, MS 38677, USA J. F. Arguin, M. Beaulieu, J. P. Martin, J. Y. Nief, R. Seitz, P. Taras, A. Woch, V. Zacek Universit´e de Montr´eal, Lab. Ren´e J.A. L´evesque, Montr´eal, QC, Canada, H3C 3J7 H. Nicholson, C.S. Sutton Mount Holyoke College, South Hadley, MA 01075, USA C. Cartero, N. Cavallo,11 G. De Nardo, F. Fabozzi, C. Gatto, L. Lista, D. Piccolo, C. Sciacca Universit` a di Napoli Federico II, Dipartimento di Fisica and INFN, I-80126 Napoli, Italy N. M. Cason, J. M. LoSecco University of Notre Dame, Notre Dame, IN 46556, USA J.R. G. Alsmiller, T.A. Gabriel, T. Handler, J. Heck Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA 8 Now at University of Edinburgh, Edinburgh EH9 3JZ, UK 9 Now at University of Massachusetts, Amherst, MA 01003, USA 10 Now at Princeton University, Princeton, NJ 08544, USA 11 Also with Universit` a della Basilicata, I-85100 Portenza, Italy v M. Iwasaki, N.B. Sinev, University of Oregon, Eugene, OR 97403, USA R. Caracciolo, F. Colecchia, F. Dal Corso, F. Galeazzi, M. Marzolla, G. Michelon, M. Morandin, M. Posocco, M. Rotondo, S. Santi, F. Simonetto, R. Stroili, E. Torassa, C. Voci Universit` a di Padova, Dipartimento di Fisica and INFN, I-35131 Padova, Italy P. Bailly, M. Benayoun, H. Briand, J. Chauveau, P. David, C. De la Vaissi`ere, L. Del Buono, J.-F. Genat, O. Hamon, Ph. Leruste, F. Le Diberder, H. Lebbolo, J. Lory, L. Martin, F. Martinez-Vidal,12 L. Roos, J. Stark, S. Versill´e, B. Zhang Universit´e Paris VI et VII, F-75252 Paris, France P.F. Manfredi, L. Ratti, V. Re, V. Speziali Universit` a di Pavia, Dipartimento di Fisica and INFN, I-27100 Pavia, Italy E.D. Frank, L. Gladney, Q.H. Guo, J.H. Panetta University of Pennsylvania, Philadelphia, PA 19104, USA C. Angelini, G. Batignani, S. Bettarini, M. Bondioli, F. Bosi, M. Carpinelli, F. Forti, A. Gaddi, D. Gagliardi, M.A. Giorgi, A. Lusiani, P. Mammini, M. Morganti, F. Morsani, N. Neri, A. Profeti, E. Paoloni, F. Raﬀaelli, M. Rama, G. Rizzo, F. Sandrelli, G. Simi, G. Triggiani Universit` a di Pisa, Scuola Normale Superiore, and INFN, I-56010 Pisa, Italy M. Haire, D. Judd, K. Paick, L. Turnbull, D. E. Wagoner Prairie View A&M University, Prairie View, TX 77446, USA J. Albert, C. Bula, M.H. Kelsey, C. Lu, K.T. McDonald, V. Miftakov, B. Sands, S.F. Schaﬀner, A.J.S. Smith, A. Tumanov, E.W. Varnes Princeton University, Princeton, NJ 08544, USA F. Bronzini, A. Buccheri, C. Bulfon, G. Cavoto, D. del Re, F. Ferrarotto, F. Ferroni, K. Fratini, E. Lamanna, E. Leonardi, M.A. Mazzoni, S. Morganti, G. Piredda, F. Safai Tehrani, M. Serra, C. Voena Universit` a di Roma La Sapienza, Dipartimento di Fisica and INFN, I-00185 Roma, Italy R. Waldi Universit¨ at Rostock, D-18051 Rostock, Germany P.F. Jacques, M. Kalelkar, R.J. Plano Rutgers University, New Brunswick, NJ 08903, USA T. Adye, B. Claxton, J. Dowdell, U. Egede, B. Franek, S. Galagedera, N.I. Geddes, G.P. Gopal, J. Kay,13 J. Lidbury, S. Madani, S. Metcalfe,13,14 G. Markey,13 P. Olley, M. Watt, S.M. Xella Rutherford Appleton Laboratory, Didcot, Oxon., OX11 0QX, UK 12 Now at Universit` a di Pisa, I-56010 Pisa, Italy 13 AtCLRC Daresbury Laboratory, Daresbury, Warrington, Cheshire, WA4 4AD, UK 14 Now at Stanford Linear Accelerator Center, Stanford, CA 94309, USA vi R. Aleksan, P. Besson,7 P. Bourgeois, P. Convert, G. De Domenico, A. de Lesquen, S. Emery, A. Gaidot, S. F. Ganzhur, Z. Georgette, L. Gosset, P. Graﬃn, G. Hamel de Monchenault, S. Herv´e, M. Karolak, W. Kozanecki, M. Langer, G.W. London, V. Marques, B. Mayer, P. Micout, J.P. Mols, J.P. Mouly, Y. Penichot, J. Rolquin, B. Serfass, J.C. Toussaint, M. Usseglio, G. Vasseur, C. Yeche, M. Zito DAPNIA, Commissariat a ` l’Energie Atomique/Saclay, F-91191 Gif-sur-Yvette, France N. Copty, M.V. Purohit, F.X. Yumiceva University of South Carolina, Columbia, SC 29208, USA I. Adam, A. Adesanya, P.L. Anthony, D. Aston, J. Bartelt, J. Becla, R. Bell, E. Bloom, C.T. Boeheim, A.M. Boyarski, R.F. Boyce, D. Briggs, F. Bulos, W. Burgess, B. Byers, G. Calderini, R. Chestnut, R. Claus, M.R. Convery, R. Coombes, L. Cottrell, D.P. Coupal, D.H. Coward, W.W. Craddock, S. DeBarger, H. DeStaebler, J. Dorfan, M. Doser, W. Dunwoodie, J.E. Dusatko, S. Ecklund, T.H. Fieguth, D.R. Freytag, T. Glanzman, G.L. Godfrey, G. Haller, A. Hanushevsky, J. Harris, A. Hasan, C. Hee, T. Himel, M.E. Huﬀer, T. Hung, W.R. Innes, C.P. Jessop, H. Kawahara, L. Keller, M.E. King, L. Klaisner, H.J. Krebs, U. Langenegger, W. Langeveld, D.W.G.S. Leith, S.K. Louie, S. Luitz, V. Luth, H.L. Lynch, J. McDonald, G. Manzin, H. Marsiske, T. Mattison,15 M. McCulloch, M. McDougald, D. McShurley, S. Menke, R. Messner, S. Metcalfe, M. Morii,16 R. Mount, D. R. Muller, D. Nelson, M. Norby, C.P. O’Grady, L. Olavson, J. Olsen, F.G. O’Neill, G. Oxoby, P. Paolucci,17 T. Pavel, J. Perl, M. Pertsova, S. Petrak, G. Putallaz, P.E. Raines, B.N. Ratcliﬀ, R. Reif, S.H. Robertson, L.S. Rochester, A. Roodman, J.J. Russel, L. Sapozhnikov, O.H. Saxton, T. Schietinger, R.H. Schindler, J. Schwiening, G. Sciolla,18 J.T. Seeman, V.V. Serbo, S. Shapiro, K. Skarpass Sr., A. Snyder, E. Soderstrom, A. Soha, S.M. Spanier, A. Stahl, P. Stiles, D. Su, M.K. Sullivan, M. Talby, H.A. Tanaka, J. Va’vra, S.R. Wagner, R. Wang, T. Weber, A.J.R. Weinstein, J.L. White, U. Wienands, W.J. Wisniewski, C.C. Young, N. Yu Stanford Linear Accelerator Center, Stanford, CA 94309, USA P.R. Burchat, C.H. Cheng, D. Kirkby, T.I. Meyer, C. Roat Stanford University, Stanford, CA 94305-4060, USA R. Henderson, N. Khan TRIUMF, Vancouver, BC, Canada V6T 2A3 S. Berridge, W. Bugg, H. Cohn, E. Hart, A.W. Weidemann University of Tennessee, Knoxville, TN 37996, USA T. Benninger, J.M. Izen, I. Kitayama, X.C. Lou, M. Turcotte University of Texas at Dallas, Richardson, TX 75083, USA F. Bianchi, M. Bona, F. Daudo, B. Di Girolamo, D. Gamba, P. Grosso, A. Smol, P..P. Trapani, D. Zanin Universit` a di Torino, Dipartimento di Fisica and INFN, I-10125 Torino, Italy 15 Now at University of British Columbia, Vancouver, BC, Canada V6T 1Z1 16 Now at Harvard University, Cambridge, MA 02138 17 Now at Universit` a di Napoli Federico II, I-80126 Napoli, Italy 18 Now at Massachusetts Institute of Technology, Cambridge, MA 02139, USA vii L. Bosisio, G. Della Ricca, L. Lanceri, A. Pompili, P. Poropat, M. Prest, I. Rashevskaia, E. Vallazza, G. Vuagnin Universit` a di Trieste, Dipartimento di Fisica and INFN, I-34127 Trieste, Italy R.S. Panvini Vanderbilt University, Nashville, TN 37235, USA A. De Silva,19 R. Kowalewski, J.M. Roney University of Victoria, Victoria, BC, Canada V8W 3P6 H.R. Band, E. Charles, S. Dasu, P. Elmer, J.R. Johnson, J. Nielsen, W. Orejudos, Y. Pan, R. Prepost, I.J. Scott, J. Walsh,12 S.L. Wu, Z. Yu, H. Zobernig University of Wisconsin, Madison, WI 53706, USA T.B. Moore, H. Neal Yale University, New Haven, CT 06511, USA 19 Now at TRIUMF, Vancouver, BC, Canada V6T 2A3 1 The BABAR Detector The BABAR Collaboration BABAR, the detector for the SLAC PEP-II asymmetric e+ e− B Factory operating at the Υ (4S) resonance, was designed to allow comprehensive studies of CP -violation in B-meson decays. Charged particle tracks are measured in a multi-layer silicon vertex tracker surrounded by a cylindrical wire drift chamber. Electromagnetic showers from electrons and photons are detected in an array of CsI crystals located just inside the solenoidal coil of a superconducting magnet. Muons and neutral hadrons are identified by arrays of resistive plate chambers inserted into gaps in the steel flux return of the magnet. Charged hadrons are identified by dE/dx measurements in the tracking detectors and in a ring-imaging Cherenkov detector surrounding the drift chamber. The trigger, data acquisition and data-monitoring systems , VME- and network-based, are controlled by custom-designed online software. Details of the layout and performance of the detector components and their associated electronics and software are presented. 1. Introduction struct the decay vertices of the two B mesons, to determine their relative decay times, and thus to The primary physics goal of the BABAR ex- measure the time dependence of their decay rates. periment is the systematic study of CP -violating The crucial test of CP invariance is a comparison asymmetries in the decay of neutral B mesons of the time-dependent decay rates for B 0 and B 0 to CP eigenstates. Secondary goals are precision to a self-conjugate state. For the cleanest exper- measurements of decays of bottom and charm imental test, this requires events in which one B mesons and of τ leptons, and searches for rare meson decays to a CP eigenstate that is fully re- processes that become accessible with the high constructed and the other B meson is tagged as luminosity of the PEP-II B Factory [1]. The de- a B 0 or a B 0 by its decay products: a charged sign of the detector is optimized for CP violation lepton, a charged kaon, or other flavor sensitive studies, but it is also well suited for these other features such as a low momentum charged pion physics topics. The scientific goals of the BABAR from a D∗ decay. experiment were first presented in the Letter of The very small branching ratios of B mesons to Intent [2] and the Technical Design Report [3]; CP eigenstates, typically 10−4 , the need for full detailed physics studies have been documented reconstruction of final states with two or more in the BABAR Physics Book [4] and earlier work- charged particles and several π 0 s, plus the need shops [5]. to tag the second neutral B, place stringent re- The PEP-II B Factory is an asymmetric e+ e− quirements on the detector, which should have collider designed to operate at a luminosity of 3 × 1033 cm−2 s−1 and above, at a center-of- • a large and uniform acceptance down to mass energy of 10.58 GeV, the mass of the Υ (4S) small polar angles relative to the boost di- resonance. This resonance decays exclusively to rection; B 0 B 0 and B + B − pairs and thus provides an ideal laboratory for the study of B mesons. In PEP- • excellent reconstruction eﬃciency for II, the electron beam of 9.0 GeV collides head- charged particles down to 60 MeV/c and for on with the positron beam of 3.1 GeV resulting photons to 20 MeV; in a Lorentz boost to the Υ (4S) resonance of • very good momentum resolution to separate βγ = 0.56. This boost makes it possible to recon- small signals from background; 2 Detector CL Instrumented Flux Return (IFR)) 0 Scale 4m I.P. Barrel Superconducting BABAR Coordinate System Coil y 1015 1749 x Electromagnetic Cryogenic 1149 4050 1149 Calorimeter (EMC) Chimney z 370 Drift Chamber (DCH) Cherenkov Detector Silicon Vertex (DIRC) Tracker (SVT) IFR Magnetic Shield 1225 Endcap for DIRC Forward 3045 End Plug Bucking Coil 1375 Support Tube 810 e– e+ Q4 Q2 Q1 3500 B1 Floor 3-2001 8583A50 Figure 1. BABAR detector longitudinal section. • excellent energy and angular resolution for B flavor-tagging, and for the reconstruction the detection of photons from π 0 and η 0 de- of exclusive states; modes such as B 0 → cays, and from radiative decays in the range K ± π ∓ or B 0 → π + π − , as well as in charm from 20 MeV to 4 GeV; meson and τ decays; • very good vertex resolution, both transverse • a flexible, redundant, and selective trigger and parallel to the beam direction; system; • low-noise electronics and a reliable, high • eﬃcient electron and muon identification, bandwidth data-acquisition and control sys- with low misidentification probablities for tem; hadrons. This feature is crucial for tagging the B flavor, for the reconstruction of char- • detailed monitoring and automated calibra- monium states, and is also important for tion; the study of decays involving leptons; • an online computing and network system • eﬃcient and accurate identification of that can control, process, and store the ex- hadrons over a wide range of momenta for pected high volume of data; and 3 0 Scale 4m IFR Barrel BABAR Coordinate System y Cutaway Superconducting Section x Coil z DIRC EMC DCH SVT IFR Cylindrical RPCs Corner Plates Earthquake Tie-down Gap Filler Plates 3500 Earthquake Isolator Floor 3-2001 8583A51 Figure 2. BABAR detector end view. • detector components that can tolerate sig- and operation of the detector. Finally, a detailed nificant radiation doses and operate reliably presentation of the design, construction, and per- under high-background conditions. formance of all BABAR detector systems is pro- vided. To reach the desired sensitivity for the most in- teresting measurements, data sets of order 108 B 2. Detector Overview mesons will be needed. For the peak cross section at the Υ (4S) of about 1.1 nb, this will require an The BABAR detector was designed and built by integrated luminosity of order 100 fb−1 or three a large international team of scientists and en- years of reliable and highly eﬃcient operation of gineers. Details of its original design are docu- a detector with state-of-the art capabilities. mented in the Technical Design Report [3], issued In the following, a brief overview of the princi- in 1995. pal components of the detector, the trigger, the Figure 1 shows a longitudinal section through data-acquisition, and the online computing and the detector center, and Figure 2 shows an end control system is given. This overview is followed view with the principal dimensions. The detector by brief descriptions of the PEP-II interaction re- surrounds the PEP-II interaction region. To max- gion, the beam characteristics, and of the impact imize the geometric acceptance for the boosted of the beam generated background on the design Υ (4S) decays, the whole detector is oﬀset rela- 4 tive to the beam-beam interaction point (IP) by Material (X0) 0.37 m in the direction of the lower energy beam. The inner detector consists of a silicon ver- tex tracker, a drift chamber, a ring-imaging 1 Cherenkov detector, and a CsI calorimeter. These detector systems are surrounded by a supercon- EMC ducting solenoid that is designed for a field of 1.5 T. The steel flux return is instrumented for -1 10 DRC muon and neutral hadron detection. The polar angle coverage extends to 350 mrad in the forward DCH direction and 400 mrad in the backward direction, defined relative to the high energy beam. As in- -2 SVT dicated in the two drawings, the right handed co- 10 ordinate system is anchored on the main tracking 0 0.5 1 1.5 2 2.5 3 system, the drift chamber, with the z-axis coin- Polar Angle θ (rad) ciding with its principal axis. This axis is oﬀset relative to the beam axis by about 20 mrad in the Figure 3. Amount of material (in units of radia- horizontal plane. The positive y-axis points up- tion lengths) which a high energy particle, origi- ward and the positive x-axis points away from the nating from the center of the coordinate system center of the PEP-II storage rings. at a polar angle θ, traverses before it reaches the The detector is of compact design, its trans- first active element of a specific detector system. verse dimension being constrained by the 3.5 m el- evation of the beam above the floor. The solenoid taken to keep material in the active volume of radius was chosen by balancing the physics re- the detector to a minimum. Figure 3 shows the quirements and performance of the drift chamber distribution of material in the various detector and calorimeter against the total detector cost. systems in units of radiation lengths. Each curve As in many similar systems, the calorimeter was indicates the material that a high energy particle the most expensive single system and thus consid- traverses before it reaches the first active element erable eﬀort was made to minimize its total vol- of a specific detector system. ume without undue impact on the performance of either the tracking system or the calorimeter 2.1. Detector Components itself. The forward and backward acceptance of An overview of the coverage, the segmentation, the tracking system are constrained by compo- and performance of the BABAR detector systems nents of PEP-II, a pair of dipole magnets (B1) is presented in Table 1. followed by a pair of quadrupole magnets (Q1). The charged particle tracking system is made of The vertex detector and these magnets are placed two components, the silicon vertex tracker (SVT) inside a support tube (4.5 m long and 0.217 m in- and the drift chamber (DCH). ner diameter) that is cantilevered from beamline The SVT has been designed to measure angles supports. The central section of this tube is fab- and positions of charged particles just outside the ricated from a carbon-fiber composite. beam pipe. The SVT is composed of five layers Since the average momentum of charged par- of double-sided silicon strip detectors that are as- ticles produced in B-meson decay is less than sembled from modules with readout at each end, 1 GeV/c, the precision of the measured track thus reducing the inactive material in the accep- parameters is heavily influenced by multiple tance volume. The inner three layers primarily Coulomb scattering. Similarly, the detection eﬃ- provide position and angle information for the ciency and energy resolution of low energy pho- measurement of the vertex position. They are tons are severely impacted by material in front mounted as close to the water-cooled beryllium of the calorimeter. Thus, special care has been beam pipe as practical, thus minimizing the im- 5 pact of multiple scattering in the beam pipe on between the crystals is held to a minimum. The the extrapolation to the vertex. The outer two individual crystals are read out by pairs of silicon layers are at much larger radii, providing the co- PIN diodes. Low noise analog circuits and fre- ordinate and angle measurements needed for link- quent, precise calibration of the electronics and ing SVT and DCH tracks. energy response over the full dynamic range are The principal purpose of the DCH is the mo- crucial for maintaining the desired performance. mentum measurement for charged particles. It The instrumented flux return (IFR) is designed also supplies information for the charged particle to identify muons and to detect neutral hadrons. trigger and a measurement of dE/dx for particle For this purpose, the magnet flux return steel in identification. The DCH is of compact design, the barrel and the two end doors is segmented with 40 layers of small, approximately hexagonal into layers, increasing in thickness from 2 cm on cells. Longitudinal information is derived from the inside to 10 cm at the outside. Between these wires placed at small angles to the principal axis. steel absorbers, single gap resistive plate cham- By choosing low-mass wires, and a helium-based bers (RPCs) are inserted which detect stream- gas mixture the multiple scattering inside the ers from ionizing particles via external capacitive DCH is minimized. The readout electronics are readout strips. There are 19 layers of RPCs in mounted on the backward endplate of the cham- the barrel sectors and 18 layers in the end doors. ber, minimizing the amount of material in front Two additional cylindrical layers of RPCs with of the calorimeter endcap. four readout planes are placed at a radius just The DIRC, the detector of internally reflected inside the magnet cryostat to detect particles ex- Cherenkov light, is a novel device providing sep- iting the EMC. aration of pions and kaons from about 500 MeV/c to the kinematic limit of 4.5 GeV/c. Cherenkov 2.2. Electronics, Trigger, Data Acquisition light is produced in 4.9 m long bars of synthetic and Online Computing fused silica of rectangular cross section, 1.7 cm × The electronics, trigger, data acquisition, and 3.5 cm, and transported by total internal reflec- online computing represent a collection of tightly tion, preserving the angle of emission, to an ar- coupled hardware and software systems. These ray of photomultiplier tubes. This array forms systems were designed to maximize the physics the backward wall of a toroidal water tank that is data acceptance, maintainability, and reliabil- located beyond the backward end of the magnet. ity while managing complexity, and minimizing Images of the Cherenkov rings are reconstructed deadtime, and cost. from the position and time of arrival of the signals • Front-End Electronics (FEE) assemblies, in the photomultiplier tubes. located on the detector, consist of signal The electromagnetic calorimeter (EMC) is de- processing and digitization electronics along signed to detect electromagnetic showers with ex- with the data transfer via optical fiber to cellent energy and angular resolution over the en- the data acquisition system. ergy range from 20 MeV to 4 GeV. This coverage • A robust and flexible two-level trigger copes allows the detection of low energy π 0 s and η 0 s with the full beam-beam interaction rate. from B decays and higher energy photons and The first level, Level 1 (L1), is implemented electrons from electromagnetic, weak, and radia- in hardware, the other, Level 3 (L3), in soft- tive processes. The EMC is a finely segmented ware. Provision is made for an intermediate array of projective geometry made of thallium trigger (Level 2) should severe conditions doped cesium iodide (CsI(Tl)) crystals. The crys- require additional sophistication. tals are arranged in modules that are supported individually from an external support structure. • The Online Dataflow (ODF), handles digi- This structure is built in two sections, a barrel tized data from the FEE through the event and a forward endcap. To obtain the desired res- building. ODF includes the fast control and olution, the amount of material in front of and in- timing system (FCTS). 6 Table 1 Overview of the coverage, segmentation, and performance of the BABAR detector systems. The nota- tion (C), (F), and (B) refers to the central barrel, forward and backward components of the system, respectively. The detector coverage in the laboratory frame is specified in terms of the polar angles θ1 (forward) and θ2 (backward). The number of readout channels is listed. The dynamic range (resolution) of the FEE circuits is specified for pulse height (time) measurements by an ADC (TDC) in terms of the number of bits (nsec). Performance numbers are quoted for 1 GeV/c particles, except where noted. The performances for the SVT and DCH are quoted for a combined Kalman fit (for the definition of the track parameters, see Section 7.) θ1 No. ADC TDC No. System (θ2 ) Channels (bits) (ns) Layers Segmentation Performance SVT 20.1◦ 150K 4 - 5 50-100 µm r − φ σd0 = 55 µm (-29.8◦ ) 100-200 µm z σz0 = 65 µm DCH 17.2◦ 7,104 8 2 40 6-8 mm σφ = 1 mrad (-27.4◦ ) drift distance σtanλ = 0.001 σpt /pt = 0.47% σ(dE/dx) = 7.5% DIRC 25.5◦ 10,752 - 0.5 1 35 × 17 mm2 σθC = 2.5 mrad (-38.6◦ ) (r∆φ × ∆r) per track 144 bars EMC(C) 27.1◦ 2 × 5760 17–18 — 1 47 × 47 mm2 σE /E = 3.0% (-39.2◦ ) 5760 cystals σφ = 3.9 mrad EMC(F) 15.8◦ 2 × 820 1 820 crystals σθ = 3.9 mrad (27.1◦ ) IFR(C) 47◦ 22K+2K 1 0.5 19+2 20-38 mm 90% µ± eﬀ. (-57◦ ) 6-8% π ± mis-id IFR(F) 20◦ 14.5K 18 28-38 mm (loose selection, (47◦ ) 1.5–3.0 GeV/c) IFR(B) -57◦ 14.5K 18 28-38 mm (-26◦ ) • A farm of commercial Unix processors and stants generation in near realtime. Physics associated software, Online Event Process- event data are transferred to an object ing (OEP), provides the realtime environ- database [6] and are made available for fur- ment within which complete events are pro- ther analyses. cessed by the L3 trigger algorithms, partial event reconstruction is performed for mon- • An Online Run Control (ORC) system im- itoring, and event data are logged to an in- plements the logic for managing the state of termediate storage. the detector systems, starting and stopping runs, and performing calibrations as well as • Software running on a second farm of providing a user control interface. processors, Online Prompt Reconstruction (OPR), completely reconstructs all physics • A system to control and monitor the detec- events, and performs monitoring and con- tor and its support systems, Online Detec- 7 tor Control (ODC), is based upon the Ex- ability. Most components underwent comprehen- perimental Physics Industrial Control Sys- sive mean time between failure (MTBF) studies. tem (EPICS) toolbox [7]. This system in- All circuits underwent a burn-in procedure prior cludes communication links with PEP-II. to installation with the goal of minimizing initial failure rates. 2.2.1. Electronics All BABAR detector systems share a common 2.2.2. Trigger electronics architecture. Event data from the de- The trigger system operates as a sequence of tector flows through the FEE, while monitoring two independent stages, the second conditional and control signals are handled by a separate, upon the first. The L1 trigger is responsible for parallel system. All FEE systems are mounted interpreting incoming detector signals, recogniz- directly on the detector to optimize performance ing and removing beam-induced background to a and to minimize the cable plant, thereby avoiding level acceptable for the L3 software trigger which noise pickup and ground loops in long signal ca- runs on a farm of commercial processors. bles. All detector systems utilize standard BABAR L1 consists of pipelined hardware processors interfaces to the data acquisition electronics and designed to provide an output trigger rate of < 2 kHz. The L1 trigger selection is based on data ∼ software. Each FEE chain consists of an amplifier, a digi- from DCH, EMC, and IFR. The maximum L1 re- tizer, a trigger latency buﬀer for storing data dur- sponse latency for a given collision is 12 µs. Based ing the L1 trigger processing, and an event buﬀer on both the complete event and L1 trigger infor- for storing data between the L1 Accept and sub- mation, the L3 software algorithms select events sequent transfer to the data acquisition system of interest which are then stored for processing. (see Figure 4). Custom integrated circuits (ICs) The L3 output rate is administratively limited to have been developed to perform the signal pro- 120 Hz so as not to overload the downstream stor- cessing. The digital L1 latency buﬀers function age and processing capacity. as fixed length data pipelines managed by com- BABAR has no fast counters for triggering pur- mon protocol signals generated by the FCTS. All poses, and bunch crossings are nearly continuous de-randomizing event buﬀers function as FIFOs at a 4.2 ns spacing. Dedicated L1 trigger proces- (first-in-first-out) capable of storing a fixed num- sors receive data continuously clocked in from the ber of events. During normal operation, analog DCH, EMC, and IFR detector systems. These signal processing, digitization, and data readout processors produce clocked outputs to the fast occur continuously and simultaneously. control system at 30 MHz, the time granularity of resultant L1 Accept signals. The arrival of an L1 Accept signal by the data acquisition sys- L1 Latency Event AMP Digitizer Buffer Buffer tem causes a portion of each system’s L1 latency buﬀer to be read out, ranging from about 500 ns for the SVT to 4–16 µs for the EMC. Absolute to L1 trigger L1 trigger accept timing information for the event, i.e., associating an event with a particular beam crossing, is deter- Figure 4. Schematic diagram of the Front-End mined oﬄine, using DCH track segment timing, Electronics (FEE). Analog signals arrive from the waveforms from the EMC, and accelerator timing left, proceed conditionally through the indicated fiducials. steps and are injected into the remainder of the data acquisition system. 2.2.3. Data Acquisition and Online Computing Since many of the front-end circuits are inac- The data acquisition and computing systems, cessible or require significant downtime for ac- responsible for the transport of event data from cess, stringent requirements were placed on reli- the detector FEE to mass storage with a min- 8 imum of dead time are shown schematically in running the VxWorks [8] realtime operating sys- Figure 5. These systems also interface with the tem and Unix processors running the Solaris trigger to enable calibrations and testing. Other operating system. ODF provides configuration parts of these systems provide for the control and and readout of the FEE over fiber links to monitoring of the detector and supporting facili- the ROMs; data transport, buﬀering, and event ties. building from the ROMs to the Unix farm over a switched 100 Mbits/s Ethernet network; mask- Hardware ing and prescaling of L1 triggers; and logical par- titioning of DAQ hardware into multiple, inde- The data acquisition system hardware consists pendent data acquisition systems for parallel cal- of VME crates, specialized VME-based proces- ibrations and diagnostics. Additional feature ex- sors called readout modules (ROMs), the FCTS, a traction (FEX) code in the ROMs extracts phys- Unix processor farm, various server machines and ical signals from the raw data, performs gain and an Ethernet network. A ROM consists of a Mo- pedestal correction, data sparsification, and data torola MVME2306 PowerPC single board com- formatting. Data from electronics calibrations puter, event buﬀers, an interface to the FCTS, are accumulated in the ROMs, channel response and a custom Personality Card that connects functions are evaluated, results are compared to with the FEE circuits via 1.2 Gbits/s fiber optic reference data and subsequently applied in fea- cables. The ROM provides the standard interface ture extraction. Calibration data are stored in a between the detector specific FEE, the FCTS, conditions database. and the event builder. There are 157 ROMs in the system located in 19 physical VME crates Online Event Processing (OEP) divided into 24 logical crates by virtue of seg- mented backplanes. The FCTS system consists OEP receives and processes data from the ODF of a VME crate plus individual Fast Control Dis- event builder on each of the Unix processors. tribution Modules in each of the data acquisition OEP orchestrates the following tasks: L3 trigger VME crates. The Unix processor farm consists of algorithms; fast monitoring to assure data qual- 32 Sun workstations. ity; and logging the selected events to disk while The detector monitoring and control system merging the multiple data output streams to a consists of a standard set of components, includ- single file. ing Motorola MVME177 single-board computers, and other VME modules. With the exception of Online Prompt Reconstruction (OPR) the solenoid magnet, which has its own control OPR bridges the online and oﬄine systems [9]. and monitoring, all BABAR detector components This system reads raw data recorded to disk by use this system. OEP and, operating on a farm of 150 Unix pro- The online computing system relies on a com- cessors, selects physics events, performs complete plex of workstation consoles and servers with reconstruction, performs rolling calibrations, col- 0.8 Tbytes of attached storage, all interconnected lects extensive monitoring data for quality assur- with switched 100 Mbits/s and 1 Gbits/s Ethernet ance, and writes the result into an event store. networks. Multiple 1 Gbits/s Ethernet links con- A rolling calibration is the generation of recon- nect the experimental hall with the SLAC com- struction constants during normal event process- puting center. ing, which are then applied to the processing of subsequent data. Online Dataflow (ODF) The ODF software connects, controls and man- Online Detector Control (ODC) ages the flow of data in the acquisition hard- and Run Control (ORC) ware with little dead time. This code is di- The ODC system controls and extensively vided between embedded processors in the ROMs monitors the electronics, the environment, and 9 raw processed digital analog digital event signals signals data Event Bldg BABAR FrontEnd VME Dataflow Intermediate L3 Trigger detector Electronics Crates Event Store Monitoring trigger L1 Accept, clocks data and trigger data L1 Trigger 24 Fast Control Processor trigger and Timing lines Figure 5. Schematic diagram of the data acquisition. assures the safety of the detector. Its implementa- of the design and operational experience of PEP- tion is based on EPICS, providing detector-wide II can be found in references [10] and [11]. standardization for control and monitoring, diag- nostics and alarm handling. ODC also provides Table 2 communication with PEP-II and the magnet con- PEP-II beam parameters. Values are given both trol system. Monitoring data are archived in an for the design and for typical colliding beam oper- ambient database. ation in the first year. HER and LER refer to the The ORC system ties together the various com- high energy e− and low energy e+ ring, respec- ponents of the online system and provides the op- tively. σLx , σLy , and σLz refer to the horizontal, erator with a single graphical interface to control vertical, and longitudinal rms size of the luminous detector operation. Complex configurations are region. stored in a configuration database; keys to the configuration used for any run are stored along Parameters Design Typical with the data. The event store, conditions, ambi- ent, and configuration databases are implemented Energy HER/LER (GeV) 9.0/3.1 9.0/3.1 in an object database [6], while other data are Current HER/LER (A) 0.75/2.15 0.7/1.3 stored in a relational database. # of bunches 1658 553-829 Bunch spacing (ns) 4.2 6.3-10.5 σLx ( µm) 110 120 σLy ( µm) 3.3 5.6 3. The PEP II Storage Rings and Their σLz (mm) 9 9 Impact on the BABAR Detector Luminosity (1033 cm−2 s−1 ) 3 2.5 3.1. PEP-II Storage Rings Luminosity ( pb−1 /d) 135 120 PEP-II is an e+ e− storage ring system designed to operate at a center of mass (c.m.) energy PEP-II typically operates on a 40–50 minute of 10.58 GeV, corresponding to the mass of the fill cycle. At the end of each fill, it takes about Υ (4S) resonance. The parameters of these en- three minutes to replenish the beams. After a ergy asymmetric storage rings are presented in loss of the stored beams, the beams are refilled in Table 2. PEP-II has surpassed its design goals, approximately 10–15 minutes. BABAR divides the both in terms of the instantaneous and the inte- data into runs, defined as periods of three hour grated daily luminosity, with significantly fewer duration or less during which beam and detector bunches than anticipated. A detailed description 10 conditions are judged to be stable. While most the two beams, and the position, angles, and size of the data are recorded at the peak of the Υ (4S) of the luminous region. resonance, about 12% are taken at a c.m. energy 40 MeV lower to allow for studies of non-resonant 3.3.1. Luminosity background. While PEP-II measures radiative Bhabha scat- tering to provide a fast monitor of the relative lu- 3.2. Impact of PEP-II on BABAR Layout minosity for operations, BABAR derives the abso- The high beam currents and the large num- lute luminosity oﬄine from other QED processes, ber of closely-spaced bunches required to produce primarily e+ e− , and µ+ µ− pairs. The measured the high luminosity of PEP-II tightly couple the rates are consistent and stable as a function of issues of detector design, interaction region lay- time. For a data sample of 1 fb−1 , the statistical out, and remediation of machine-induced back- error is less than 1%. The systematic uncertainty ground. The bunches collide head-on and are on the relative changes of the luminosity is less separated magnetically in the horizontal plane than 0.5%, while the systematic error on the ab- by a pair of dipole magnets (B1), followed by solute value of the luminosity is estimated to be a series of oﬀset quadrupoles. The tapered B1 about 1.5%. This error is currently dominated by dipoles, located at ± 21 cm on either side of the uncertainties in the Monte Carlo generator and IP, and the Q1 quadrupoles are permanent mag- the simulation of the detector. It is expected that nets made of samarium-cobalt placed inside the with a better understanding of the eﬃciency, the field of the BABAR solenoid, while the Q2, Q4, and overall systematic error on the absolute value of Q5 quadrupoles, located outside or in the fringe the luminosity will be significantly reduced. field of the solenoid, are standard iron magnets. The collision axis is oﬀ-set from the z-axis of the 3.3.2. Beam Energies BABAR detector by about 20 mrad in the horizon- During operation, the mean energies of the tal plane [12] to minimize the perturbation of the two beams are calculated from the total mag- beams by the solenoidal field. netic bending strength (including the eﬀects of The interaction region is enclosed by a water- oﬀ-axis quadrupole fields, steering magnets, and cooled beam pipe of 27.9 mm outer radius, com- wigglers) and the average deviations of the ac- posed of two layers of beryllium (0.83 mm and celerating frequencies from their central values. 0.53 mm thick) with a 1.48 mm water channel be- While the systematic uncertainty in the PEP-II tween them. To attenuate synchrotron radiation, calculation of the absolute beam energies is es- the inner surface of the pipe is coated with a 4 µm timated to be 5–10 MeV, the relative energy set- thin layer of gold. In addition, the beam pipe ting for each beam is accurate and stable to about is wrapped with 150 µm of tantalum foil on ei- 1 MeV. The rms energy spreads of the LER and ther side of the IP, beyond z = +10.1 cm and HER beams are 2.3 MeV and 5.5 MeV, respec- z = −7.9 cm. The total thickness of the cen- tively. tral beam pipe section at normal incidence cor- To ensure that data are recorded close to the responds to 1.06% of a radiation length. peak of the Υ (4S) resonance, the observed ratio of The beam pipe, the permanent magnets, and BB enriched hadronic events to lepton pair pro- the SVT were assembled and aligned, and then duction is monitored online. Near the peak of the enclosed in a 4.5 m-long support tube which spans resonance, a 2.5% change in the BB production the IP. The central section of this tube was fabri- rate corresponds to a 2 MeV change in the c.m. cated from a carbon-fiber epoxy composite with energy, a value that is close to the tolerance to a thickness of 0.79% of a radiation length. which the energy of PEP-II can be held. How- ever, a drop in the BB rate does not distinguish 3.3. Monitoring of Beam Parameters between energy settings below or above the Υ (4S) The beam parameters most critical for BABAR peak. The sign of the energy change must be de- performance are the luminosity, the energies of termined from other indicators. The best mon- 11 itor and absolute calibration of the c.m. energy 2000 is derived from the measured c.m. momentum of fully reconstructed B mesons combined with Events/ 2MeV/c2 the known B-meson mass. An absolute error of 1.1 MeV is obtained for an integrated luminosity of 1 fb−1 . This error is presently limited by the 1000 uncertainty in the B-meson mass [13] and by the detector resolution. The beam energies are necessary input for the calculation of two kinematic variables that are 0 commonly used to separate signal from back- ground in the analysis of exclusive B-meson de- 5.24 5.26 5.28 5.30 2-2001 cays. These variables, which make optimum use 8583A36 Energy Substituted Mass (GeV/c2) of the measured quantities and are largely uncor- Figure 6. The energy-substituted mass for a sam- related, are Lorentz-invariants which can be eval- ple of 6,700 neutral B mesons reconstructed in uated both in the laboratory and c.m. frames. the final states D(∗)− π + , D(∗)− ρ+ , D(∗)− a+ 1 , and The first variable, ∆E, can be expressed in J/ψ K ∗0 . The background is extrapolated from Lorentz invariant form as events outside the signal region. √ ∆E = (2qB q0 − s)/2 s, (1) ∗ √ and the B-meson energy is substituted by Ebeam . ∗ where s = 2Ebeam is the total energy of the Figure 6 shows the mES distribution for a sam- + − e e system in the c.m. frame, and qB and q0 = ple of fully reconstructed B mesons. The resolu- (E0 , p 0 ) are the Lorentz vectors representing the ∗ tion in mES is dominated by the spread in Ebeam , momentum of the B candidate and of the e+ e− σEbeam ∗ = 2.6 MeV. system, q0 = qe+ + qe− . In the c.m. frame, ∆E takes the familiar form 3.3.3. Beam Direction ∗ ∗ The direction of the beams relative to BABAR ∆E = EB − Ebeam , (2) is measured iteratively run-by-run using e+ e− → here EB∗ is the reconstructed energy of the B me- e+ e− and e+ e− → µ+ µ− events. The resultant son. The ∆E distribution receives a sizable con- uncertainty in the direction of the boost from the laboratory to the c.m. frame, β, is about 1 mrad, tribution from the beam energy spread, but is generally dominated by detector resolution. dominated by alignment errors. This translates into an uncertainty of about 0.3 MeV in mES . β The second variable is the energy-substituted mass, mES , defined as mES 2 = qB 2 . In the lab- is consistent to within 1 mrad with the orienta- oratory frame, mES can be determined from the tion of the elongated beam spot (see below). It is measured three-momentum p B of the B candi- stable to better than 1 mrad from one run to the date without explicit knowledge of the masses of next. the decay products: 3.3.4. Beam Size and Position The size and position of the luminous region are mES = (s/2 + p B ·p0 )2 /E02 − p2B . (3) critical parameters for the decay-time-dependent In the c.m. frame ( p0 = 0), this variable takes the analyses and their values are monitored contin- familiar form uously online and oﬄine. The design values for the size of the luminous region are presented in mES = Ebeam ∗2 − p∗2 Table 2. The vertical size is too small to be mea- B, (4) sured directly. It is inferred from the measured where p∗B is the c.m. momentum of the B meson, luminosity, the horizontal size, and the beam cur- derived from the momenta of its decay products, rents; it varies typically by 1–2 µm. 12 The transverse position, size, and angles of the ter the BABAR acceptance. The remaining syn- luminous region relative to the BABAR coordinate chrotron radiation background is dominated by system are determined by analyzing the distribu- x-rays (scattered from tungsten tips of a mask) tion of the distance of closest approach to the generated by beam tails in the high-field region of z-axis of the tracks in well measured two-track the HER low-β quadrupoles. This residual back- events as a function of the azimuth φ. The lon- ground is relatively low and has not presented gitudinal parameters are derived from the lon- significant problems. gitudinal vertex distribution of the two tracks. A combined fit to nine parameters (three aver- 3.4.2. Beam-Gas Scattering age coordinates, three widths, and three small Beam-gas bremsstrahlung and Coulomb scat- angles) converges readily, even after significant tering oﬀ residual gas molecules cause beam par- changes in the beam position. The uncertain- ticles to escape the acceptance of the ring if their ties in the average beam position are of the order energy loss or scattering angle are suﬃciently of a few µm in the transverse plane and 100 µm large. The intrinsic rate of these processes is pro- along the collision axis. Run-by-run variations in portional to the product of the beam current and the beam position are comparable to these mea- the residual pressure (which itself increases with surement uncertainties, indicating that the beams current). Their relative importance, as well as the are stable over the period of a typical run. The resulting spatial distribution and absolute rate of fit parameters are stored run-by-run in the con- lost particles impinging the vacuum pipe in the ditions database. These measurements are also vicinity of the detector, depend on the beam op- checked oﬄine by measuring the primary vertices tical functions, the limiting apertures, and the in multi-hadron events. The measured horizontal entire residual-pressure profile around the rings. and longitudinal beam sizes, corrected for track- The separation dipoles bend the energy-degraded ing resolution, are consistent with those measured particles from the two beams in opposite direc- by PEP-II. tions and consequently most BABAR detector sys- tems exhibit occupancy peaks in the horizontal ◦ 3.4. Beam Background Sources plane, i.e., the LER background near φ = 0 and ◦ The primary sources of steady-state accelera- HER background near φ = 180 . tor backgrounds are, in order of increasing im- During the first few months of operation and portance: synchrotron radiation in the vicinity of during the first month after a local venting of the interaction region; interactions between the the machine, the higher pressures lead to signifi- beam particles and the residual gas in either ring; cantly enhanced background from beam-gas scat- and electromagnetic showers generated by beam- tering. The situation has improved significantly beam collisions [14–16]. In addition, there are with time due to scrubbing of the vacuum pipe other background sources that fluctuate widely by synchrotron radiation. Towards the end of the and can lead to very large instantaneous rates, first year of data-taking, the dynamic pressure in thereby disrupting stable operation. both rings had dropped below the design goal, and the corresponding background contributions 3.4.1. Synchrotron Radiation were much reduced. Nevertheless, beam-gas scat- Synchrotron radiation in nearby dipoles, the tering remains the primary source of radiation interaction-region quadrupole doublets and the damage in the SVT and the dominant source of B1 separation dipoles generates many kW of background in all detectors systems, except for power and is potentially a severe background. the DIRC. The beam orbits, vacuum-pipe apertures and synchrotron-radiation masks have been designed 3.4.3. Luminosity Background such that most of these photons are channeled Radiative Bhabha scattering results in energy- to a distant dump; the remainder are forced to degraded electrons or positrons hitting aperture undergo multiple scatters before they can en- limitations within a few meters of the IP and 13 spraying BABAR with electromagnetic shower de- 600 Dose Budget bris. This background is directly proportional HER+LER (non-horiz) to the instantaneous luminosity and thus is ex- LER (horiz) Dose Integral / kRad pected to contribute an increasing fraction of the HER (horiz) total background in the future. Already this is 400 the dominant background in the DIRC. 3.4.4. Background Fluctuations In addition to these steady-state background 200 sources, there are instantaneous sources of radia- tion that fluctuate on diverse time scales: • beam losses during injection, 0 -200 0 200 • intense bursts of radiation, varying in du- 3-2001 8583A52 Day of Year 2000 ration from a few ms to several minutes, currently attributed to very small dust par- Figure 7. The integrated radiation dose as mea- ticles, which become trapped in the beam, sured by PIN diodes located at three diﬀerent and positions, showing contributions from the HER • non-Gaussian tails from beam-beam inter- (φ = 180◦ ), and the LER (φ = 0◦ ) in the hori- actions (especially of the e+ beam) that are zontal plane, and from both beams combined else- highly sensitive to adjustments in collima- where. Also shown is the SVT radiation budget tor settings and ring tunes. for the first 500 days of operation. These eﬀects typically lead to short periods of diodes in the middle are exposed to about ten high background and have resulted in a large times more radiation than the others. These mid- number of BABAR-initiated beam aborts (see be- plane diodes are connected to the beam abort low). system, while the remaining eight diodes at the top and bottom are used to monitor the radia- 3.5. Radiation Protection tion dose delivered to the SVT. The accuracy of and Monitoring the measured average dose rate is better than 0.5 A system has been developed to monitor the in- mRad/s. The integrated dose, as measured by stantaneous and integrated radiation doses, and the SVT diodes, is presented in Figure 7. to either abort the beams or to halt or limit the The radiation level at the DCH and the EMC rate of injection, if conditions become critical. In is more than two orders of magnitude lower than addition, DCH and IFR currents, as well as DIRC at the SVT. To amplify the signal, the PIN diodes and IFR counting rates, are monitored; abnor- for the DCH and EMC are mounted on small mally high rates signal critical conditions. CsI(Tl) crystals (with a volume of about 10 cm3 ). Radiation monitoring and protection systems These silicon diodes are installed in sets of four. are installed for the SVT, the DCH electronics, Three sets are mounted on the front face of the and the EMC. The radiation doses are measured endcap calorimeter and one set on the backward with silicon photodiodes. For the SVT, 12 diodes endplate of the DCH, close to the readout elec- are arranged in three horizontal planes, at, above, tronics. The signals of the four diodes in each and below the beam level, with four diodes in each set are summed, amplified, and fed into the ra- plane, placed at z = +12.1 cm and z = −8.5 cm diation protection electronics. Only one of the and at a radial distance of 3 cm from the beam three diode sets of the EMC is used at any given line [17]. The diode leakage current, after cor- time. The DCH and the EMC use identical hard- rection for temperature and radiation damage ef- ware and decision algorithms. They limit injec- fects, is proportional to the dose rate. The four 14 tion rates whenever an instantaneous dose equiv- alent to about 1 Rad/day is exceeded. 80 The SVT employs a diﬀerent strategy and cir- cuitry to assess whether the measured radiation levels merit a beam abort or a reduction in single- 60 Trips / day beam injection rate. Every beam dump initi- ated by BABAR is followed by a 10–15 minute period of injection with significant radiation ex- 40 posure. Thus, to minimize the ratio of the in- tegrated radiation dose to the integrated lumi- 20 nosity, it has been beneficial to tolerate transient high-dose events as long as the integrated dose remains less than the typical dose accumulated 0 during injection. To diﬀerentiate between very 0 100 200 300 high instantaneous radiation and sustained high Day of Year 2000 dose rates, trip thresholds are enforced on two diﬀerent time scales: an instantaneous dose of Figure 8. Daily rates of beam aborts initiated 1 Rad accumulated over 1 ms, and an average by the SVT radiation protection diodes, summed of 50 mRad/s measured over a 5-minute period. over regular data-taking and PEP-II injection. During injection, higher thresholds are imposed, since an aborted injection will delay the return to ferent detector systems varies significantly. Ta- taking data. ble 3 lists the limits on the instantaneous and Figure 8 shows the daily rate of beam aborts integrated background levels in terms of the to- initiated by the SVT protection diodes during the tal dose and instantaneous observables. These year 2000. Initially, as many as 80 beam aborts limits are estimates derived from beam tests and were triggered per day, while the average for sta- experience of earlier experiments. For each de- ble operation was significantly below ten at the tector system, an annual radiation allowance has end of the run. The measures described above, been established taking into account the total es- combined with a significant reduction in large timated lifetime of the components and the ex- background fluctuations, have been very eﬀective pected annual operating conditions. The typical in protecting the detector against radiation dam- values accumulated for the first year of operation age, as well increasing the combined live time of are also presented in the table. the machine and detector to greater than 75%. Systematic studies of background rates were performed with stable stored beams. Measure- 3.6. Impact of Beam-Generated ments of the current-dependence of the back- Background on BABAR grounds were carried out for single beams, two Beam-generated backgrounds aﬀect the detec- beams not colliding, and colliding beams with the tor in multiple ways. They cause radiation dam- goal to identify the principal background sources, age to the detector components and the electron- to develop schemes of reducing these sources, and ics and thus may limit the lifetime of the ex- to extrapolate to operation at higher luminosity periment. They may also cause electrical break- [16]. These experimental studies were comple- down and damage or generate large numbers of mented by Monte Carlo simulations of beam-gas extraneous signals leading to problems with band- scattering and of the propagation of showers in width limitations of the data acquisition system the detector. The studies show that the rela- and with event reconstruction. Backgrounds can tive importance of the single-beam and luminos- degrade resolution and decrease eﬃciency. ity background contributions varies, as illustrated The impact of the beam-generated background in Figure 9. Data for the IFR are not shown be- on the lifetime and on the operation of the dif- cause this system is largely insensitive to beam- 15 Table 3 BABAR background tolerance. Operational limits are expressed either as lifetime limits (radiation-damage and aging-related quantities), or in terms of instantaneous observables (DCH current, DIRC and L1- trigger rates). Limiting factor Operational First-year Detector system and impact limit typical SVT sensors Integrated dose: 2 MRad 0.33 MRad and electronics radiation damage (hor.-plane modules) 0.06 MRad (other modules) SVT sensors Instantaneous dose: 1 Rad/ms N/A diode shorts DCH: electronics Integrated dose: 20 kRad ≤ 100 Rad radiation damage DCH: wire current Accumulated charge: 100 mC/cm 8 mC/cm wire aging DCH: total current HV system limitations 1000 µA 250 µA (steady-state) DIRC PMTs Counting rate: 200 kHz 110 kHz (steady-state, TDC deadtime well-shielded sector) EMC crystals Integrated dose: 10 kRad 0.25 kRad radiation damage (worst case) L1 trigger Counting rate: 2 kHz 0.7 kHz DAQ dead time (steady-state) generated backgrounds, except for the outer layer fidence that the SVT can sustain operation for of the forward endcap, due to insuﬃcient shield- several more years (see Figure 7). ing of the external beam line components. DCH: For the DCH, the currents on the wires The experience of the first year of operation are the main concern, both because of the lim- and the concern for future operation for each of ited capacity of the HV power supplies and the the detectors are summarized as follows. eﬀect of wire aging. The currents drawn are ap- SVT: The most significant concern for the proximately uniformly distributed among the 44 SVT with regard to machine background is the HV supplies, one for each quadrant of superlay- integrated radiation dose. The instantaneous and ers 2–10, and two per quadrant for superlayer 1. integrated dose rates in the radiation protection Consequently, the total current limit is close to diodes are representative, to within about a fac- the sum of the limits of the individual supplies. tor of two, of the radiation doses absorbed by During stable operation the total chamber cur- the SVT modules. The exposure in the horizon- rent is 200–300 µA. However, radiation spikes can tal planes is an order of magnitude larger than lead to currents that occasionally exceed the limit elsewhere, averaging 15–25 mRad/s during sta- of 1000 µA, causing HV supplies to trip. Other ble beam operation. The highest integrated dose background eﬀects are measured to be well be- is 450 kRad, roughly 1 kRad/day. This dose is low the estimated lifetime limits and thus are not about 30% below the allowance, giving some con- a serious issue at this time. The average wire 16 cap crystals. RadFETs [18] are realtime integrat- LER Only HER Only Collisions ing dosimeters based on solid-state Metal Oxide >1MeV >10MeV Background Fraction 1.00 Semiconductor (MOS) technology. The absorbed dose increases approximately linearly with the in- 0.75 tegrated luminosity. The highest dose to date was observed in the innermost ring of the end- 0.50 cap, close to 250 Rad, while the barrel crystals accumulated about 80 Rad. The observed reduc- 0.25 tion in light collection of 10–15% in the worst place, and 4–7% in the barrel, is consistent with 1-2001 0 8583A31 SVT DCH DIRC EMC EMC TRG expectation (see Section 9). The energy resolution is dependent on the sin- gle crystal readout threshold, currently set at Figure 9. Fractional steady-state background 1 MeV. During stable beam conditions the aver- contributions in BABAR detector systems, mea- age crystal occupancy for random triggers is 16%, sured for single beams and colliding beams under with 10% originating from electronics noise in the stable conditions (I + = 1.25 A, I − = 0.75 A, absence of any energy deposition. The spectrum L = 2.3 × 1033 cm−2 s−1 ) in July 2000. The con- of photons observed in the EMC from the LER tributions are derived from the measured doses in and HER is presented in Figure 10. The HER the horizontal plane for the SVT, the total cur- produces a somewhat harder spectrum. The av- rents in the DCH, the rates in the DIRC photo- erage occupancy for a threshold of 1 MeV and the multipliers, the occupancy and number of pho- average number of crystals with a deposited en- tons above 10 MeV in the EMC, and the L1 trig- ergy of more than 10 MeV are shown in Figure ger rates. 11 as a function of beam currents for both single and colliding beams. The occupancy increases occupancy has not exceeded 1–2% during stable significantly at smaller polar angles, in the for- operation, but the extrapolation to future opera- ward endcap and the backward barrel sections, tion at higher luminosity and currents remains a and in the horizontal plane. The rate increase is major concern. approximately linear with the single beam cur- DIRC: The DIRC radiators, made of syn- rents. Background rates recorded with separated thetic fused silica, were tested up to doses of beams are consistent with those produced by sin- 100 kRad without showing any measurable eﬀects gle beams. For colliding beams, there is an addi- and thus radiation damage is not a concern. The tional flux of photons originating from small angle present operational limit of the DIRC is set by radiative Bhabha scattering. This eﬀect is larger the TDC electronics which induce significant dead for low energy photons and thus it is expected time at rates above 250 kHz, well above the sta- that at higher luminosities the low energy back- ble beam rate of 110 kHz in well shielded areas. ground will raise the occupancy and thereby limit Roughly half of the present rate is luminosity- the EMC energy resolution. related and can be attributed to radiative Bhabha L1 Trigger: During stable beam operation, scattering. The counting rate is due to debris the typical L1 trigger rate is below 1 kHz, more from electromagnetic showers entering the water- than a factor of two below the data acquisition filled stand-oﬀ box. Eﬀorts are underway to im- bandwidth limit of about 2.0–2.5 kHz. Experi- prove the shielding of the beam pipe nearby. ence shows that background bursts and other rate EMC: The lifetime of the EMC is set by the spikes can raise the data volume by as much as a reduction in light collection in the CsI crystals factor of two and thus it is necessary to aim for due to radiation damage. The cumulative dose steady state rates significantly below the stated absorbed by the EMC is measured by a large set limit. For the L1 trigger, the dominant sources of RadFETs placed in front of the barrel and end- of DCH triggers are particles generated by inter- 17 Crystals / Event / 2.5 MeV 10 2 Beams in Collision 10 18 HER only Crystal Occupancy (%) Beams Separated LER only 1 -1 14 10 -2 10 10 -3 (a) 10 Number of Crystals with E>10 MeV 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 Beams in Collision Single Crystal Energy (GeV) 8 Beams Separated Figure 10. The energy spectrum of photons recorded in the EMC by random triggers with 6 single beams at typical operating currents, LER at 1.1A and HER at 0.7A. The electronic noise 4 has been subtracted. actions in vacuum flanges and the B1 magnets 2 (see Figure 86 in Section 11). This eﬀect is most (b) pronounced in the horizontal plane. At present, 0 the HER background is twice as high as that of 0 200 400 600 the LER, and the colliding beams contribute less HER Current (mA) 1-2001 8583A8 than half of the combined LER and HER single beam triggers. Figure 11. Average rates in the EMC for ran- dom triggers as a function of the HER current for a fixed LER current of 1.1A, both for separated 3.7. Summary and Outlook and colliding beams; a) the single crystal occu- Towards the end of the first year of data-taking, pancy for thresholds of 1 MeV and b) the number PEP-II routinely delivered beams close to de- of crystals with a deposited energy greater than sign luminosity. Due to the very close coop- 10 MeV. The solid curves represent a fit to the eration with the PEP-II operations team, the colliding beam data, the dashed curves indicate machine-induced backgrounds have not been a the sum of rates recorded for single beams. major problem once stable conditions were estab- lished. The background monitoring and protec- the sources and the impact of machine-related tion system has become a reliable and useful tool background on BABAR, among them upgrades to to safeguard the detector operation. the DCH power supply system and to the DIRC Currently planned upgrades are expected to TDC electronics, the addition of localized shield- raise the luminosity to 1.5 × 1034 cm−2 s−1 within ing against shower debris (especially for the DIRC a few years. The single beam backgrounds will stand-oﬀ box ), new vacuum chambers, adjustable increase with beam currents and the luminosity collimators, and additional pumping capacity in background is projected to exceed, or at best critical regions upbeam of the interaction point. remain comparable to, the beam-gas contribu- With the expected increase in LER current tion. Measures are being prepared to reduce 18 HER + LER in Collision port for the detector components. Figures 1 and 2 HER + LER show key components of the BABAR magnet sys- 600 HER tem and some of the nearby PEP-II magnets. The magnet coil cryostat is mounted inside the L1 Rate (Hz) hexagonal barrel flux return by four brackets on 400 each end. The flux return end doors are each split vertically and mounted on skids that rest on the floor. To permit access to the inner detec- 200 tor, the doors can be raised and moved on rollers. At the interface between the barrel and the end doors, approximately 60% of the area is occupied 0 0 400 by structural steel and filler plates; the remaining 1-2001 8583A7 HER Current (mA) space is reserved for cables and utilities from the inner detectors. A vertical, triangular chase cut Figure 12. The L1 trigger rate as a function of into the backward end doors contains the cryostat the HER current for single beam only, for both chimney. Table 4 lists the principal parameters of beams, separated and colliding (with a LER cur- the magnet system. The total weight of the flux rent of 1.1A). return is approximately 870 metric tons. To optimize the detector acceptance for un- and in luminosity, both the single-beam and equal beam energies, the center of the BABAR de- the luminosity-generated L1 trigger rates will in- tector is oﬀset by 370 mm in the electron beam crease and are projected to exceed 2 kHz (see Fig- direction. The principal component of the mag- ure 12). Therefore, the DCH trigger is being up- netic field, Bz , lies along the +z axis; this is also graded to improve the rejection of background the approximate direction of the electron beam. tracks originating from outside the luminous re- The backward end door is tailored to accommo- gion. In addition, the data acquisition and data date the DIRC bar boxes and to allow access processing capacity will need to be expanded to to the drift chamber electronics. Both ends al- meet the demands of higher luminosity. low space and adequate shielding for the PEP-II Overall, the occupancy in all systems, except quadrupoles. the IFR, will probably reach levels that are likely to impact the resolution and reconstruction eﬃ- 4.2. Magnetic Field Requirements ciency. For instance, the occupancy in the EMC and Design is expected to more than double. Thus, be- 4.2.1. Field Requirements yond the relatively straight forward measures cur- A solenoid magnetic field of 1.5 T was specified rently planned for BABAR system upgrades, de- in order to achieve the desired momentum res- tailed studies of the impact of higher occupancy olution for charged particles. To simplify track will be necessary for all systems. finding and fast and accurate track fitting, the magnitude of the magnetic field within the track- 4. The Solenoid Magnet and Flux Return ing volume was required to be constant within a few percent. 4.1. Overview The magnet was designed to minimize dis- The BABAR magnet system consists of a super- turbance of operation of the PEP-II beam el- conducting solenoid [19], a segmented flux return ements. The samarium-cobalt B1 dipole and and a field compensating or bucking coil. This Q1 quadrupole magnets are located inside the system provides the magnetic field which enables solenoid as shown in Figure 1. Although these charged particle momentum measurement, serves magnets can sustain the high longitudinal field of as the hadron absorber for hadron/muon separa- 1.5 T, they cannot tolerate a large radial compo- tion, and provides the overall structure and sup- nent. Specifically, the field cannot exceed 0.25 T 19 at a radius r = 200 mm (assuming a linear depen- Table 4 dence of Br on r) without degrading their field Magnet Parameters properties due to partial demagnetization. The conventional iron quadrupoles Q2, Q4, and Q5 Field Parameters are exposed to the solenoid stray fields. To avoid excessive induced skew octupole components, the Central Field 1.5 T stray field leaking into these beam elements is re- Max. Radial Field <0.25 T quired to be less than 0.01 T averaged over their at Q1 and r = 200 mm apertures. Leakage into PEP-II <0.01 T Stored Energy 27 MJ 4.2.2. Field Design Considerations Steel Parameters Saturation of the steel near the coil and the gap Overall Barrel Length 4050 mm between the coil and the steel leads to field non- Overall Door Thickness 1149 mm uniformities. To control these non-uniformities, (incl. gaps for RPCs) the current density of the coil is increased at the Overall Height 6545 mm ends relative to the center by reducing the thick- Plates in Barrel 18 ness of the aluminum stabilizer. While the re- 9 20 mm quirements on the radial field component at Q1 4 30 mm inside the solenoid can be satisfied easily at the 3 50 mm forward end, the shape of the backward plug had 2 100 mm to be specifically designed to simultaneously con- Plates in Each Door 18 trol field uniformity and unwanted radial compo- 9 20 mm nents. 4 30 mm Leakage of magnetic flux is a problem, in par- 4 50 mm ticular at the backward end. A bucking coil, 1 100 mm mounted at the face of the backward door and surrounding the DIRC strong support tube, is Main Coil Parameters designed to reduce the stray field to an accept- Mean Diameter of 3060 mm able level for the DIRC photomultipliers and the Current Sheet PEP-II quadrupoles. Current Sheet Length 3513 mm Number of layers 2 4.2.3. Magnetic Modeling Operating Current 4596 A Extensive calculations of the magnetic field Conductor Current 1.2 kA/ mm2 were performed to develop the detailed design of Density the flux return, the solenoid coil, and the bucking Inductance 2.57 H coil. To crosscheck the results of these calcula- tions the fields were modeled in detail in two and Bucking Coil Parameters three dimensions using commercial software [20]. Inner Diameter 1906 mm The shape of the hole in each end door was de- Operating Current 200 A signed by optimizing various parameters, such as Number of Turns 140 the minimum steel thickness in areas of satura- Cryostat Parameters tion. The design of the hole in the forward door was particularly delicate because the highly sat- Inner Diameter 1420 mm urated steel is very close to the Q2 quadrupole. Radial Thickness 350 mm The multiple finger design of the hole was chosen Total Length 3850 mm to control the saturation of the steel. Total Material (Al) ∼ 126 mm Most of the design work was performed in two dimensions, but some three dimensional calcula- 20 tions were necessary to assure the accuracy of from such an event. The entire detector is sup- modeling the transitions between the end doors ported on four earthquake isolators, one at each and the barrel, the leakage of field into the PEP- corner, which limit the component acceleration in II magnets, and the impact of that leakage on the horizontal plane to 0.4 g. However, these iso- the multipole purity [21,22]. The computations lators oﬀer no protection in the vertical direction. of the leakage field were done for central field of Vertical ground accelerations of 0.6 g are consid- 1.7 T instead of 1.5 T to provide some insurance ered credible and actual component accelerations against uncertainties in the modeling of complex may be considerably larger due to resonances. By steel shapes and the possible variations of the taking into account resonant frequencies and the magnetic properties of the steel. expected frequency spectra of earthquakes, the magnet and all detector components have been 4.3. Steel Flux Return designed to survive these accelerations without 4.3.1. Mechanical serious damage. Because the magnet is isolated and Magnetic Forces from the ground moving beneath it, worst case The magnet flux return supports the detector clearances to external components, e.g.,PEP-II components on the inside, but this load is not components, are provided. It is expected that a major issue. Far greater demands are placed even during a major earthquake, damage would on the structural design by the magnetic forces be modest. and the mechanical forces from a potential earth- quake. 4.3.3. Fabrication Magnetic forces are of three kinds. First, there The flux return was fabricated [23] from draw- is a symmetric magnetic force on the end doors ings prepared by the BABAR engineering team. A which was taken into consideration in their de- primary concern was the magnetic properties of sign and construction. Second, there is an ax- the steel. The need for a high saturation field ial force on the solenoid due to the forward- dictated the choice of a low carbon steel, speci- backward asymmetry of the steel. Because the fied by its chemical content (close to AISI 1006). steel is highly saturated in places, the magnitude The manufacturer supplied sample steel for crit- of the field asymmetry changes when the current ical magnetic measurements and approval. The is raised from zero, and there is no position of availability of very large tools at the factory made the solenoid at which the force remains zero at all it possible to machine the entire face of each end currents. Because it is important that this axial of the assembled barrel, thus assuring a good fit force should not change sign, which could cause of the end doors. The entire flux return was as- a quench, the superconducting solenoid was de- sembled at the factory, measured mechanically, liberately oﬀset by 30 mm towards the forward and inspected before disassembly for shipment. door. This oﬀset was chosen to accommodate a worst case scenario, including uncertainties in the 4.4. Magnet Coils calculation. Third, during a quench of the super- The design of the superconducting solenoid conducting coil, eddy currents in conducting com- is conservative and follows previous experience. ponents inside the magnetic volume could gen- The superconducting material is composed of erate sizable forces. These forces were analyzed niobium-titanium (46.5% by weight Nb) fila- for components such as the endplates of the drift ments, each less than 40 µm in diameter. These chamber and the electromagnetic calorimeter and filaments are then wound into 0.8 mm strands, were found not to be a problem. 16 of which are then formed into Rutherford cable measuring 1.4 x 6.4 mm. The final con- 4.3.2. Earthquake Considerations ductor [24] consists of Rutherford cable co- Because SLAC is located in an earthquake extruded with pure aluminum stabilizer measur- zone, considerable attention has been given to ing 4.93 x 20.0 mm for use on the outer, high protecting the detector against severe damage current density portion of the solenoid, and 21 To optimally control the stray fields and avoid a magnetization of the DIRC magnetic shield, the 3513.5 currents in the solenoid and the bucking coil are Coil Length ramped together under computer control. High 40.8 precision transducers are used to measure the 1530.2 currents and provide the feedback signals to the Coil power supplies. The values of the currents are Radius recorded in the BABAR database. A B C D E 2-2001 8583A34 +z All Dimensions in mm 4.5. Magnetic Field Map The goal of the magnetic field mapping and Figure 13. A portion of cryostat assembly. The subsequent corrections was to determine the mag- forward end is shown. Legend: (A) evacuated netic field in the tracking volume to a precision spaces filled with IR-reflective insulator; (B) su- of 0.2 mT. perconducting coil (2-layers); (C) aluminum sup- port cylinder; (D) aluminum heat shield; (E) alu- minum cryostat housing. 4.5.1. Mapping Procedure A field mapping device was built specifically for the BABAR magnet based on a design concept de- 8.49 x 20.0 mm for the central, lower current den- veloped at Fermilab [29]. The magnetic field sen- sity portion. The conductor is covered in an insu- sors were mounted on a propeller at the end of a lating dry wrap fiberglass cloth which is vacuum long cantilevered spindle which reached through impregnated with epoxy. The conductor has a the hole in the forward end door. The spindle in total length of 10.3 km. turn rode on a carriage which moved on precision- The solenoid is indirectly cooled to an oper- aligned rails. The propeller rotated to sample the ating point of 4.5K using a thermo-syphon tech- magnetic volume in φ, and the carriage moved nique. Liquid helium [25] is circulated in chan- along its axis to cover z. Measurements were ob- nels welded to the solenoid support cylinder. Liq- tained from five sets of Br and Bz and two Bφ uid helium and cold gas circulate between the Hall probes, all of which were mounted on a plate solenoid, its shields, the liquefier-refrigerator and at diﬀerent radial positions. This plate was at- a 4000 ℓ storage dewar via 60 m of coaxial, gas- tached to the propeller and its position could be screened, flexible transfer line. The solenoid coil changed to cover the desired range in the radial and its cryostat were fabricated [26] to drawings distance r from the axis. Precision optical align- prepared by the BABAR engineering team. Before ment tools were used to determine the position of shipment [27], the fully assembled solenoid was the sensors transverse to the z-axis. cooled to operating temperature and tested with The Br and Bz probes were two-element de- currents of up to 1000 A, limited by coil forces in vices with a short-term (few month) precision of the absence of the iron flux return. 0.01%, the Bφ probes were single element devices A portion of the cryostat assembly, containing with a precision of 0.1% [30]. In addition to the the solenoid coil, its support cylinder and heat Hall probes, an NMR probe [31] was mounted at shield, is shown in Figure 13. a radius of 89 mm on the propeller to provide a To reduce the leakage fields into the PEP-II very precise field reference near the z-axis as a components and the DIRC photomultipliers, an function of z for |z| < 1000 mm, where z = 0 at additional external bucking coil is installed [28]. the magnet center. The NMR measurements set This is a conventional water cooled copper coil the absolute scale of the magnetic field. consisting of ten layers. Although the nominal The magnetic field was mapped at the nomi- operating current is 200 A, a current of up to nal central operating field of 1.5 T, as well as at 575 A is attainable, if needed, to demagnetize the 1.0 T. Measurements were recorded in 100 mm DIRC shield. intervals from −1800 to +1800 mm in z, and in 22 24 azimuthal positions spaced by 15◦ for each of 1.55 three diﬀerent radial positions of the Hall probe plate. Thus for each z and φ position, the com- ponents Br and Bz were measured at 13 distinct r radii from 130 mm to 1255 mm and Bφ at six radii 0.80 1.45 between 505 mm and 1180 mm. Bz (T) 0.54 The field map was parameterized in terms of a polynomial of degree up to 40 in r and z plus ad- 0.27 0.00 ditional terms to account for expected perturba- IP tions [32]. The fit reproduced the measurements 1.35 Drift Chamber to within an average deviation of 0.2 mT through- out the tracking volume. The fitting procedure also served as a means of detecting and removing 0.05 questionable measurements. 4.5.2. Perturbations to the Field Map During the mapping process, the permanent r –0.05 0.27 Br (T) magnet dipoles (B1) and quadrupoles (Q1) were not yet installed. Their presence inside the 0.54 solenoid results in field perturbations of two 0.80 IP kinds. The first is due to the fringe fields of the –0.15 Drift Chamber B1 and Q1 permanent magnets, and of the dipole and quadrupole trim coils mounted on Q1. The -1.0 0.0 1.0 B1 field strength reaches a maximum of ∼20 mT 1-2001 close to the surface of the B1 casing and decreases 8583A2 Z (m) rapidly with increasing radius. The fields associ- Figure 14. The magnetic field components Bz ated with the trim coils were measured and pa- and Br as a function of z for various radial dis- rameterized prior to installation; they are essen- tances r (in m). The extent of the DCH and tially dipole in character. the location of the interaction point (IP) are in- The second field perturbation is due to the per- dicated. meability of the permanent magnet material. Sin- tered samarium-cobalt has a relative permeabil- Bφ component does not exceed 1 mT. The vari- ity of 1.11 to 1.13 in the z direction, and as a ation of the bend field, i.e., the field transverse result the solenoid field is modified significantly. to the trajectory, along the path of a high mo- Probes between the B1 and Q1 magnets at a ra- mentum track is at most 2.5% from maximum to dius of about 190 mm measure the eﬀect of the minimum within the tracking region, as shown in permeability. The field perturbation is obtained Figure 15. from a two-dimensional, finite element analysis which reproduces the r and z dependence of Br and Bz . The induced magnetization increases Bz 4.5.4. Field Computation by about 9 mT at the interaction point; the eﬀect In order to reduce the computation of the mag- decreases slowly with increasing radius. netic field for track reconstruction and momen- tum determination, the field values averaged over 4.5.3. Field Quality azimuth are stored in a grid of r–z space points To illustrate the quality of magnetic field, Fig- spanning the volume interior to the cryostat. Lo- ure 14 shows the field components Bz and Br as cal values are obtained by interpolation. Within a function of z for various radial distances r. In the volume of the SVT, a linear interpolation is the tracking volume the field is very uniform, the performed in a 20 mm grid; elsewhere the interpo- 23 1.02 is the eﬃcient detection of charged particles and the measurement of their momentum and angles with high precision. Among many applications, these precision measurements allow for the re- construction of exclusive B- and D-meson de- cays with high resolution and thus minimal back- 1.00 25 ˚ Bt(L)/Bt(0) ground. The reconstruction of multiple decay ver- tices of weakly decaying B and D mesons is of prime importance to the physics goals. 55˚ 140 Track measurements are also important for the ˚ ˚ 18 extrapolation to the DIRC, EMC, and IFR. At 0.98 lower momenta, the DCH measurements are more 0.0 1.0 2.0 important, while at higher momenta the SVT 1-2001 8583A1 Track Length (m) measurements dominate. Most critical are the angles at the DIRC, because the uncertainties in Figure 15. Relative magnitude of magnetic field the charged particle track parameters add to the transverse to a high momentum track as a func- uncertainty in the measurement of the Cherenkov tion of track length from the IP for various polar angle. Thus, the track errors from the combined angles (in degrees). The data are normalized to SVT and DCH measurements should be small the field at the origin. compared to the average DIRC Cherenkov angle measurements, i.e., of order of 1 mrad, particu- lation is quadratic in a 50 mm grid. Azimuthal de- larly at the highest momenta. pendence is parameterized by means of a Fourier expansion at each r–z point. The Fourier coef- 5.2. SVT Goals and Design Requirements ficients at the point of interest are obtained by The SVT has been designed to provide pre- interpolation on the r–z grid, and the average cise reconstruction of charged particle trajecto- field value is corrected using the resulting Fourier ries and decay vertices near the interaction region. series. The design choices were driven primarily by di- rect requirements from physics measurements and 4.6. Summary constraints imposed by the PEP-II interaction re- Since its successful commissioning, the magnet gion and BABAR experiment. In this section the system has performed without problems. There mechanical and electronic design of the SVT are have been no spontaneous quenches of the su- discussed, with some discussion of the point res- perconducting solenoid. In the tracking region, olution per layer and dE/dx performance. The the magnetic field meets specifications, both in tracking performance and eﬃciency of the SVT magnitude and uniformity. The field compen- alone and in combination with the DCH are de- sation and magnetic shielding work well for the scribed in Section 7. DIRC photomultiplier array and the external quadrupoles. Measurements indicate that the 5.2.1. SVT Requirements and Constraints bucking coil reduces the field at the face of Q2 The SVT is critical for the measurement of the from ∼50 mT to ∼1 mT [28], in agreement with time-dependent CP asymmetry. To avoid signif- calculations. icant impact of the resolution on the CP asym- metry measurement the mean vertex resolution 5. Silicon Vertex Tracker along the z-axis for a fully reconstructed B de- cay must be better than 80 µm [2]. The required 5.1. Charged Particle Tracking resolution in the x–y plane arises from the need The principal purpose of the BABAR charged to reconstruct final states in B decays as well as particle tracking systems, the SVT and the DCH, in τ and charm decays. For example, in decays 24 of the type B 0 → D+ D− , separating the two D vertices is important. The distance between the two D’s in the x–y plane for this decay is typi- cally ∼ 275 µm. Hence, the SVT needs to provide resolution of order ∼100 µm in the plane perpen- dicular to the beam line. Many of the decay products of B mesons have low pt . The SVT must provide standalone track- ing for particles with transverse momentum less than 120 MeV/c, the minimum that can be mea- sured reliably in the DCH alone. This feature is fundamental for the identification of slow pions from D∗ -meson decays: a tracking eﬃciency of 70% or more is desirable for tracks with a trans- Figure 16. Fully assembled SVT. The silicon sen- verse momentum in the range 50–120 MeV/c. The sors of the outer layer are visible, as is the carbon- standalone tracking capability and the need to fiber space frame (black structure) that surrounds link SVT tracks to the DCH were crucial in choos- the silicon. ing the number of layers. Beyond the standalone tracking capability, the The SVT is cooled to remove the heat gener- SVT provides the best measurement of track an- ated by the electronics. In addition, it operates gles, which is required to achieve design resolu- in the 1.5 T magnetic field. tion for the Cherenkov angle for high momentum To achieve the position resolution necessary to tracks. carry out physics analyses, the relative position Additional constraints are imposed by the stor- of the individual silicon sensors should be sta- age ring components. The SVT is located inside ble over long time periods. The assembly allows the ∼4.5 m-long support tube, that extends all for relative motion of the support structures with the way through the detector. To maximize the respect to the B1 magnets. angular coverage, the SVT must extend down to These requirements and constraints have led to 350 mrad (20◦ ) in polar angle from the beam line the choice of a SVT made of five layers of double- in the forward direction. The region at smaller sided silicon strip sensors. To fulfill the physics polar angles is occupied by the B1 permanent requirements, the spatial resolution, for perpen- magnets. In the backward direction, it is suﬃ- dicular tracks, must be 10–15 µm in the three in- cient to extend the SVT sensitive area down to ner layers and about 40 µm in the two outer lay- 30◦ . ers. The inner three layers perform the impact The SVT must withstand 2 MRad of ionizing parameter measurements, while the outer layers radiation. A radiation monitoring system capa- are necessary for pattern recognition and low pt ble of aborting the beams is required. The ex- tracking. pected radiation dose is 1 Rad/day in the hori- zontal plane immediately outside the beam pipe 5.3. SVT Layout (where the highest radiation is concentrated), and The five layers of double-sided silicon strip sen- 0.1 Rad/day on average otherwise. sors, which form the SVT detector, are organized The SVT is inaccessible during normal detec- in 6, 6, 6, 16, and 18 modules, respectively; a pho- tor operations. Hence, reliability and robustness tograph is shown in Figure 16. The strips on the are essential: all components of the SVT inside opposite sides of each sensor are oriented orthog- the support tube should have long mean-time-to- onally to each other: the φ measuring strips (φ failure, because the time needed for any replace- strips) run parallel to the beam and the z mea- ment is estimated to be 4–5 months. Redundan- suring strips (z strips) are oriented transversely cies are built in whenever possible and practical. to the beam axis. The modules of the inner three 25 580 mm Space Frame Bkwd. support cone 520 mrad Fwd. support350 mrad cone e- Front end e+ electronics Beam Pipe Figure 17. Schematic view of SVT: longitudinal section. The roman numerals label the six diﬀerent types of sensors. layers are straight, while the modules of layers 4 To satisfy the diﬀerent geometrical require- and 5 are arch-shaped (Figures 17 and 18). ments of the five SVT layers, five diﬀerent sen- This arch design was chosen to minimize the sor shapes are required to assemble the planar amount of silicon required to cover the solid angle, sections of the layers. The smallest detectors while increasing the crossing angle for particles are 43 × 42 mm2 (z × φ), and the largest are near the edges of acceptance. A photograph of 68 × 53 mm2 . Two identical trapezoidal sensors an outer layer arch module is shown in Figure 19. are added (one each at the forward and back- The modules are divided electrically into two half- ward ends) to form the arch modules. The half- modules, which are read out at the ends. modules are given mechanical stiﬀness by means of two carbon fiber/kevlar ribs, which are visible Beam Pipe 27.8mm radius in Figure 19. The φ strips of sensors in the same Layer 5a half-module are electrically connected with wire bonds to form a single readout strip. This results Layer 5b in a total strip length up to 140 mm (240 mm) in the inner (outer) layers. Layer 4b The signals from the z strips are brought to the readout electronics using fanout circuits consist- Layer 4a ing of conducting traces on a thin (50 µm) insu- lating Upilex [33] substrate. For the innermost three layers, each z strip is connected to its own preamplifier channel, while in layers 4 and 5 the Layer 3 number of z strips on a half-module exceeds the Layer 2 number of electronics channels available, requir- ing that two z strips on diﬀerent sensors be elec- Layer 1 trically connected (ganged) to a single electronics channel. The length of a z strip is about 50 mm (no ganging) or 100 mm (two strips connected). The ganging introduces an ambiguity on the z Figure 18. Schematic view of SVT: tranverse sec- coordinate measurement, which must be resolved tion. by the pattern recognition algorithms. The to- 26 Table 5 Geometric parameters for each layer and readout plane of the SVT. Floating strips refers to the number of strips between readout (R-O) strips. Note: parts of the φ sides of layers 1 and 2 are bonded at 100 µm and 110 µm pitch, respec- tively, with one floating strip. Strip length of z- strips for layers 4 and 5 includes ganging. The radial range for layers 4 and 5 includes the radial extent of the arched sections. R-O Strip Layer/ Radius pitch Floating length view (mm) ( µm) strips (mm) 1 z 32 100 1 40 1 φ 32 50-100 0-1 82 2 z 40 100 1 48 2 φ 40 55-110 0-1 88 3 z 54 100 1 70 3 φ 54 110 1 128 4 z 91-127 210 1 104 4 φ 91-127 100 1 224 5 z 114-144 210 1 104 5 φ 114-144 100 1 265 Figure 19. Photograph of an SVT arch module in an assembly jig. positioned at an angle of 350 mrad relative to the sensor for the layers 3, 4, and 5 (Figure 17). In the backward direction, the available space is larger tal number of readout channels is approximately and the inner layer electronics can be placed in 150,000. the sensor plane, allowing a simplified assembly. The inner modules are tilted in φ by 5◦ , allow- The module assembly and the mechanics are ing an overlap region between adjacent modules, quite complicated, especially for the arch mod- a feature that provides full azimuthal coverage ules, and are described in detail elsewhere [34]. and is advantageous for alignment. The outer The SVT support structure (Figure 16) is a rigid modules cannot be tilted, because of the arch ge- body made from two carbon-fiber cones, con- ometry. To avoid gaps and to have a suitable nected by a space frame, also made of carbon-fiber overlap in the φ coordinate, layers 4 and 5 are epoxy laminate. divided into two sub-layers (4a, 4b, 5a, 5b) and An optical survey of the SVT on its assembly placed at slightly diﬀerent radii (see Figure 18). jig indicated that the global error in placement of The relevant geometrical parameters of each layer the sensors with respect to design was ∼200 µm, are summarized in Table 5. FWHM. Subsequently, the detector was disas- In order to minimize the material in the sembled and shipped to SLAC, where it was re- acceptance region, the readout electronics are assembled on the IR magnets. The SVT is at- mounted entirely outside the active detector vol- tached to the B1 magnets by a set of gimbal rings ume. The forward electronics must be mounted in such a way as to allow for relative motion of in the 10 mm space between the 350 mrad stay- the two B1 magnets while fixing the position of clear space and B1 magnet. This implies that the SVT relative to the forward B1 and the orien- the hybrids carrying the front-end chip must be 27 tation relative to the axis of both B1 dipoles. The Table 6 support tube structure is mounted on the PEP- Electrical parameters of the SVT, shown for the II accelerator supports, independently of BABAR, diﬀerent layers and views. Cinput refers to the allowing for movement between the SVT and the total input capacitance, Rseries is the series re- rest of BABAR. Precise monitoring of the beam sistance. The amplifier peaking time is 200 ns for interaction point is necessary, as is described in layers 1–3 and 400 ns for layers 4–5. Section 5.5. The total active silicon area is 0.96 m2 and the Noise, material traversed by particles is ∼ 4% of a radi- Layer/ Cinput Rseries calc. meas. ation length (see Section 2). The geometrical ac- view (pF) (Ω) (elec) (elec) ceptance of SVT is 90% of the solid angle in the 1 z 6.0 40. 550 880 c.m. system, typically 86% are used in charged 1 φ 17.2 164. 990 1200 particle tracking. 2 z 7.2 48. 600 970 5.4. SVT Components 2 φ 18.4 158. 1030 1240 A block diagram of SVT components is shown 3 z 10.5 70. 700 1180 in Figure 20. The basic components of the de- 3 φ 26.8 230. 1470 1440 tector are the silicon sensors, the fanout circuits, 4 z 16.6 104. 870 1210 the Front End Electronics (FEE) and the data 4 φ 33.6 224. 1380 1350 transmission system. Each of these components 5 z 16.6 104. 870 1200 is discussed below. 5 φ 39.7 265. 1580 1600 Sensors Atom Kapton Matching resistance greater than 100 MΩ at operating bias chips HDI card Tail voltage, normally about 10 V above the depletion voltage. Typical depletion voltages are in the range 25– 35 V. The strips are biased on both sides with Fiber Optics DAQ HDI polysilicon resistors (4–20 MΩ) to ensure the re- Inside to/from DAQ link link Support Card Card Front quired radiation hardness, keeping the voltage Tube cables drop across resistors and the parallel noise as CAN low as possible. Strips are AC-coupled to the MUX Power Bus electronics via integrated decoupling capacitors, MUX module the capacitance of which depends on the sen- Power Supplies sor shape, but is always greater than 14 pF/cm. The sensors were designed to maximize the ac- Figure 20. Schematic block diagram showing the tive area, which extends to within 0.7 mm of the diﬀerent components of the SVT. physical edges. Another design goal was to con- trol the inter-strip capacitance: values between 0.7 pF/cm and 1.1 pF/cm were obtained for the 5.4.1. Silicon Sensors various sensor shapes. To achieve the required The SVT sensors [35] are 300 µm thick double- spatial resolution, while keeping the number of sided silicon strip devices. They were designed readout channels as low as possible, most of the at INFN Pisa and Trieste (Italy) and fabri- modules have a floating strip (i.e., not read out) cated commercially [36]. They are built on high- between two readout strips. resistivity (6–15 kΩ-cm) n-type substrates with The leakage currents, because of the excellent p+ strips and n+ strips on the two opposite sides. performance of the manufacturing process, were The insulation of the n+ strips is provided by in- as low as 50 nA/cm2 on average, measured at dividual p-stops, so as to achieve an inter-strip 10 V above depletion voltage. The silicon sensor 28 parameters have been measured after irradiation • signals from all strips must be retained, with 60 Co sources. Apart from an increase in the in order to improve the spatial resolution inter-strip capacitance of about 12% during the through interpolation, while keeping the first 100 krad, the main eﬀect was an increase of number of transmitted hits as low as pos- the leakage current by 0.7 µA/cm2 /MRad. How- sible. A hit refers to a deposited charge ever, in a radiation test performed in a 1 GeV/c greater than 0.95 fC, corresponding to 0.25 electron beam, an increase in leakage current of MIP; about 2 µA/cm2 /MRad and a significant shift in the depletion voltage, dependent on the initial • the amplifier must be sensitive to both neg- dopant concentration, were observed. A shift ative and positive charge; of about 8–10 V was seen for irradiation corre- • the peaking time must be programmable, sponding to a dose of approximately 1 MRad. with a minimum of 100 ns (in layers 1 and These observations indicate significant bulk dam- 2, because of the high occupancy), up to age caused by energetic electrons. As indicated 400 ns (outer layers, with high capacitance); by the change in depletion voltage, the SVT sen- sors could undergo type inversion after about 1– • capability to accept random triggers with a 3 MRad. Preliminary tests show that the sen- latency up to 11.5 µs and a programmable sors continue to function after type inversion [37]. jitter up to ±1 µs, without dead time; Studies of the behavior of SVT modules as a func- tion of radiation dose continue. • radiation hardness greater than 2.5 MRad; 5.4.2. Fanout Circuits • small dimensions: 128 channels in a 6.2 mm- The fanout circuits, which route the signals wide chip. from the strips to the electronics, have been de- signed to minimize the series resistance and the These requirements are fully satisfied by the inter-strip capacitance. As described in ref. [38], ATOM IC [39], which is depicted schematically a trace on the fanout has a series resistance about in Figure 21. 1.6 Ω/cm, an inter-strip resistance > 20 MΩ, and Thresh an inter-strip capacitance < 0.5 pF/cm. The elec- DAC trical parameters of the final assembly of sensors Readout Buffer Sparsification ToT Counter Buffer Comp Time Stamp and fanouts (referred to as Detector Fanout As- PRE Shaper Chan # AMP semblies or DFAs) are summarized in Table 6. 15 MHz Due to the diﬀerent strip lengths, there are large CAL Circular Buffer DAC diﬀerences between the inner and the outer lay- CINJ Buffer Event Time 193 Bins ers. Smaller diﬀerences are also present between and Number the forward and backward halves of the module, CAC From Silicon Serial that are of diﬀerent lengths. Data Out 5.4.3. Front End Electronics The electrical parameters of a DFA and the Figure 21. Schematic diagram of the ATOM front general BABAR requirements are the basic inputs end IC. that drove the design of the SVT front-end cus- tom IC; the ATOM (A Time-Over-Threshold Ma- The linear analog section consists of a charge- chine). In particular, the front-end IC had to sensitive preamplifier followed by a shaper. Gains satisfy the following requirements: of 200 mV/fC (low) or 300 mV/fC (high) may be selected. The channel gains on a IC are • signal to noise ratio greater than 15 for min- uniform to 5 mV/fC. Signals are presented to a imum ionizing particle (MIP) signals for all programmable-threshold comparator, designed so modules; that the output width of the pulse (Time over 29 Table 7 5.4.4. Data Transmission ATOM chip ENC parameters at diﬀerent peaking The digitized signals are transmitted from the times ATOM ICs through a thin kapton tail or ca- ble to the matching cards, from where they are Peaking ENC Noise routed to more conventional cables. Just outside time (0 pF) slope the detector, signals are multiplexed by the MUX modules, converted into optical signals and trans- 100 ns 380 e− 40.9 e− /pF mitted to the Readout Modules (ROMs). The 200 ns 280 e− 33.9 e− /pF MUX modules also receive digital signals from the 400 ns 220 e− 25.4e− /pF DAQ via a fiber optical connection. The SVT is connected to the BABAR online detector control and monitoring system via the industry standard Threshold or ToT) is a quasi-logarithmic function CAN bus. Details on SVT data transmission sys- of the collected charge. This output is sampled at tem and DAQ can be found in references [40,41]. 30 MHz and stored in a 193 location buﬀer. Upon Power to SVT modules (silicon sensor bias volt- receipt of a Level 1 (L1) trigger, the time and age and ATOM low voltages) is provided by a ToT is retrieved from this latency buﬀer, sparci- CAEN A522 power supply system [42]. fied, and stored in a four event buﬀer. Upon the receipt of an L1 Accept command from the data acquisition system, the output data (the 4 bits for 5.5. Monitoring and Calibration the ToT, 5 bits for the time stamp, and 7 bits for To identify immediately any operational prob- the strip address) are formatted, serialized, and lems, the SVT is integrated in the control and delivered to the ROM. The IC also contains a test monitoring system (see Section 12). Major charge injection circuit. The typical noise behav- concerns for SVT monitoring are temperature ior of the ATOM, as described by the Equivalent and humidity, mechanical position, and radiation Noise Charge (ENC) of the linear analog section dose. is given in Table 7. The average noise for the various module types 5.5.1. Temperature is shown in Table 6. Given that shot noise due to and Humidity Monitors sensor leakage current is negligible, the expected The total power dissipation of the SVT mod- noise may be calculated from the parameters of ules is about 350 W, mainly dissipated in the Tables 6 and 7. The results of such a calculation ATOM ICs. External cooling is provided by are also shown in Table 6. The maximum average chilled water at 8◦ C. In addition, humidity is re- noise is 1,600 electrons, leading to a signal-to- duced by a stream of dry air in the support tube. noise ratio greater than 15. Since condensation or excessive temperature The power consumption of the IC is about can permanently damage the FEE, temperature 4.5 mW/channel. Radiation hardness was stud- and humidity monitoring are very important to ied up to 2.4 MRad with a 60 Co source. At that the safe operation of the SVT. Thermistors are dose, the gain decreased 20%, and the noise in- located on the HDIs (for the measurements of creased less than 15%. FEE temperature), around the SVT, along the The ATOM ICs are mounted on thick-film cooling systems, and in the electronics (MUX) double-sided hybrid circuits (known as High crates. The absolute temperatures are monitored Density Interconnects or HDIs) based on an to 0.2◦ C and relative changes of 0.1◦ C. Addition- aluminum-nitride substrate with high thermal ally, a series of humidity sensors are employed conductivity. The electronics are powered to monitor the performance of the dry air sys- through a floating power supply system, in such a tem. The temperature and humidity monitors way as to guarantee a small voltage drop (< 1 V) also serve as an interlock to the HDI power sup- across the detector decoupling capacitors. plies. 30 0.2 the device. To date, the measured radiation ab- sorbed by the SVT is well within the allowed bud- get. 0 X (mm) The monitoring of radiation dose to the SVT is discussed in detail in Section 3. –0.2 5.5.4. Calibrations –0.4 Once a day, and each time the SVT configu- 4 5 6 ration has changed, calibrations are performed in 4-2001 Day of July 1999 8583A55 absence of circulating beams. All electronic chan- nels are tested with pulses through test capaci- Figure 22. Horizontal motion between the DCH tors, for diﬀerent values of the injected charge. and the support tube measured with the capaci- Gains, thresholds, and electronic noise are mea- tive sensors (curve) compared to the mean x coor- sured, and defective channels are identified. The dinate of the interaction point (circles) measured calibration results have proven very stable and re- with e+ e− and µ+ µ− events over a three-day pe- peatable. The main variation in time is the occa- riod in July 1999. An arbitrary oﬀset and scale sional discovery of a new defective channel. The has been applied to the beam position data. calibration procedures have also been very useful for monitoring noise sources external to the SVT. 5.5.2. Position Monitors A system of capacitive sensors was installed to identify and track changes in the position of the 5.5.5. Defects SVT with respect to the PEP-II B1 magnets and Due to a series of minor mishaps incurred dur- the position of the support tube with respect to ing the installation of the SVT, nine out of 208 the DCH. An example of the understanding that readout sections (each corresponding to one side can be achieved by this system is given in Fig- of a half-module) were damaged and are cur- ure 22, where the measured changes in the hor- rently not functioning. There is no single fail- izontal position of the SVT relative to the DCH ure mode, but several causes: defective connec- are shown for a period of six day in the summer of tors, mishandling during installation, and not- 1999. These position changes can be attributed fully-understood problems on the FEE hybrid. to local temperature variations. The sensor data There has been no module failure due to radi- are compared to measurements of the mean po- ation damage. It should be noted that due to sition of the interaction point (in the horizontal the redundancy aﬀorded by the five layers of the plane) determined with e+ e− and µ+ µ− events SVT, the presence of the defective modules has recorded over this period. While the amplitude minimal impact on physics analyses. of motion at the time was uncharacteristically In addition, there are individual channel de- large, the strong correlation between these inde- fects, of various types, at a level of about 1%. Cal- pendent measurements is quite evident. Align- ibrations have revealed an increase in the num- ment with charged particle tracks is now per- ber of defective channels at a rate of less than formed routinely to correct for relative motion 0.2%/year. of the tracking systems, as described in Section 5.6.2. 5.6. Data Analysis and Performance This section describes the reconstruction of 5.5.3. Radiation Monitors space points from signals in adjacent strips on Radiation monitoring is extremely important both sides of the sensors, the SVT internal and to ensure the SVT does not exceed its radiation global alignment, single hit eﬃciency, and resolu- budget, which could cause permanent damage to tion and dE/dx performance of the SVT. 31 5.6.1. Cluster and Hit Reconstruction cluster finding algorithm. First, the charge pulse Under normal running conditions, the average height (Q) of a single pulse is calculated from occupancy of the SVT in a time window of 1 µs the ToT value, and clusters are formed group- is about 3% for the inner layers, with a signif- ing adjacent strips with consistent times. In a icant azimuthal variation due to beam-induced second pass, clusters separated by just one strip backgrounds, and less than 1% for the outer lay- are merged into one cluster. The two original ers, where noise hits dominate. Figure 23 shows clusters plus the merged cluster are made avail- the typical occupancy as a function of IC index able to the pattern recognition algorithm, which (equivalent to azimuthal angle, in this case) for chooses among the three. layer 1, φ side. In the inner layers, the occupancy The position x of a cluster formed by n strips is dominated by machine backgrounds, which are is determined, with the “head-to-tail” algorithm: significantly higher in the horizontal plane, seen in the plot as the peaks near IC indices 3 and 25. (x1 + xn ) p (Qn − Q1 ) x= + , 2 2 (Qn + Q1 ) 16 where xi and Qi are the position and collected a) charge of i-th strip, respectively, and p is the read- 12 out pitch. This formula results in a cluster posi- 8 tion that is always within p/2 of the geometrical Occupancy (%) center of the cluster. The cluster pulse height 4 is simply the sum of the strip charges, while the 0 cluster time is the average of the signal times. 6 b) 5.6.2. Alignment 4 The alignment of the SVT is performed in two steps. The first step consists of determining the 2 relative positions of the 340 silicon sensors. Once this is accomplished, the next step is to align 0 the SVT as a whole within the global coordi- 0 10 20 30 40 nate system defined by the DCH. The primary 3-2001 reason for breaking the alignment procedure into 8583A39 IC Index these two steps is that the local positions are rela- tively stable in time compared to the global posi- Figure 23. Typical occupancy in percent as tion. Also, the local alignment procedure is con- a function of IC index in layer 1, φ side for siderably more complex than the global alignment a) forward half-modules and b) backward half- procedure. Thus, the global alignment can be up- modules. The IC index increases with azimuthal dated on a run-by-run basis, while the local align- angle and the higher occupancy in the horizontal ment constants are changed as needed, typically plane is visible near chip indices 3 and 25. after magnet quenches or detector access. The local alignment procedure is performed The first step of the reconstruction program with tracks from e+ e− → µ+ µ− events and cos- consists in discarding out-of-time channels. A mic rays. Well isolated, high momentum tracks time correction, i.e., the time between the pas- from hadronic events are also used to supplement sage of the particle and the time the shaper ex- di-muon and cosmic data. Data samples suﬃ- ceeds threshold, is performed, after which hits cient to perform the local alignment are collected with times more than 200 ns from the event time in one to two days of typical running conditions. (determined by the DCH) are discarded. The In µ+ µ− events, the two tracks are simulta- loss of real hits from this procedure is negligible. neously fit using a Kalman filter technique and The resulting in-time hits are then passed to the the known beam momentum. The use of tracks 32 from cosmic rays reduces any systematic distor- tion that may be introduced due to imprecise knowledge of the beam momenta. No informa- tion from the DCH is used, eﬀectively decoupling the SVT and DCH alignment. In addition to the information from tracks, data from an optical survey performed during the assembly of the SVT are included in the align- ment procedure. The typical precision of these optical measurements is 4 µm. This survey infor- mation is only used to constrain sensors relative to other sensors in the same module, but not one module to another or one layer to another. Fur- thermore, only degrees of freedom in the plane of the sensor are constrained as they are expected to Figure 24. Comparison of a local alignment of all be the most stable, given the assembly procedure. the sensors in the SVT using data from January Using the hit residuals from the aforementioned 2000 with the optical survey of the SVT made set of tracks and the optical survey information, a during assembly in February 1999 in the (a) r∆φ, χ2 is formed for each sensor and minimized with (b) ∆z and (c) ∆r coordinates. Plots (d), (e), and respect to the sensor’s six local parameters. The (f) show the diﬀerence between two local align- constraints coming from the overlapping regions ments using data from January 15-19 and March of the silicon sensors, the di-muon fit, the cosmic 6-7, 2000 for the r∆φ, ∆z, and ∆r coordinates, rays, and the optical survey result in internally respectively. In all the plots, the shaded regions consistent local alignment constants. correspond to the sensors in the first three layers. Figure 24 shows a comparison between the op- In comparing the diﬀerent alignments and optical tical alignment made during the SVT assembly in survey, a six parameter fit (three global transla- February 1999 and a local alignment using data tions and three global rotations) has been applied taken during January 2000. The alignment from between the data sets. tracking data was made without using cosmics or constraints from the optical survey. The width of step consists in determining the position of the the distributions in these plots has four contribu- SVT with respect to the DCH. Tracks with suf- tions: 1) displacement during the transfer of the ficient numbers of SVT and DCH hits are fit SVT from the assembly jig to the IR magnets, 2) two times: once using only the DCH informa- time dependent motion of the SVT after mount- tion and again using only the SVT hits. The six ing, 3) statistical errors, and 4) systematic errors. global alignment parameters, three translations The second set of plots shows the diﬀerence in two and three rotations, are determined by minimiz- alignment sets for data taken in January 2000 as ing the diﬀerence between track parameters ob- compared to March 2000. In general, the sta- tained with the SVT-only and the DCH-only fits. bility of the inner three layers is excellent, with As reported above, because of the diurnal move- slightly larger tails in the outer two layers. The ment of the SVT with respect to the DCH, this radial coordinate is less tightly constrained in all global alignment needs to be performed once per measurements because the radial location of the run (∼ every 2–3 hours). The alignment con- charge deposition is not well known, and most of stants obtained in a given run are then used to the information about the radial locations comes reconstruct the data in the subsequent run. This only from constraints in the overlap region of the procedure, known as rolling calibration, ensures sensors. that track reconstruction is always performed After the internal alignment, the SVT is con- with up-to-date global alignment constants. sidered as a rigid body. The second alignment A record of the changes in the relative posi- 33 0 on front-end chips. Actually, since most of the Y Position (mm) (a) –0.2 –0.4 –0.6 0 50 100 150 200 250 Day of Year 2000 –0.06 Y Position (mm) (b) –0.1 –0.14 –0.18 –0.22 246 248 250 252 254 256 4-2001 8583A54 Day of Year 2000 Figure 26. SVT hit reconstruction eﬃciency, as Figure 25. Global alignment of the SVT relative measured on µ+ µ− events for a) forward half- to the DCH based on e+ e− and µ+ µ− events: modules and b) backward half-modules. The changes in the relative vertical placement mea- plots show the probability of associating both a sured a) over the entire ten-month run in the year φ and z hit to a track passing through the ac- 2000, and b) a ten-day period, illustrating diurnal tive part of the detector. The horizontal axis variations. corresponds to the diﬀerent modules, with the vertical lines separating the diﬀerent layers as tion of the SVT as determined by r olling calibra- numbered. Missing values correspond to non- tions is shown in Figure 25. The position is stable functioning half-modules. to better than ±100 µm over several weeks, but changes abruptly from time to time, for instance, defects aﬀect a single channel, they do not con- during access to the detector. The calibrations tribute to the ineﬃciency, because most tracks track diurnal variations of typically ±50 µm that deposit charge in two or more strips due to track have been correlated with local changes in tem- crossing angles, and charge diﬀusion. perature of about ±2◦ C. Movements within a The spatial resolution of SVT hits is deter- single run are small compared to the size of the mined by measuring the distance (in the plane of beam. the sensor) between the track trajectory and the hit, using high-momentum tracks in two prong 5.6.3. Performance events. The uncertainty due to the track trajec- The SVT eﬃciency can be calculated for each tory is subtracted from the width of the resid- half-module by comparing the number of asso- ual distribution to obtain the hit resolution. Fig- ciated hits to the number of tracks crossing the ure 27 shows the SVT hit resolution for z and φ active area of the module. As can be seen in side hits as a function of track incident angle, for Figure 26, a combined hardware and software ef- each of the five layers. The measured resolutions ficiencies of 97% is measured, excluding defective are in excellent agreement with expectations from readout sections (9 out of 208), but employing Monte Carlo simulations. no special treatment for other defects, such as Initial studies have shown that hit reconstruc- broken AC coupling capacitors or dead channels tion eﬃciency and spatial resolution are eﬀec- 34 tively independent of occupancy for the occu- Layer 1 Layer 2 pancy levels observed so far. Measurement of the ToT value by the ATOM ICs enables one to obtain the pulse height, and hence the ionization dE/dx in the SVT sensor. z Resolution (µm) The values of ToT are converted to pulse height using a lookup table computed from the pulse Layer 3 Layer 4 shapes obtained in the bench measurements. The pulse height is corrected for track length vari- ation. The double-sided sensors provide up to ten measurements of dE/dx per track. For every track with signals from at least four sensors in Layer 5 angle (degrees) the SVT, a 60% truncated mean dE/dx is cal- culated. The cluster with the smallest dE/dx energy is also removed to reduce sensitivity to a) electronics noise. For MIPs, the resolution on the truncated mean dE/dx is approximately 14%. A angle (degrees) 2σ separation between the kaons and pions can be achieved up to momentum of 500 MeV/c, and between kaons and protons beyond 1 GeV/c. Layer 1 Layer 2 5.7. Summary and Outlook The SVT has been operating eﬃciently since its installation in the BABAR experiment in May φ Resolutiion (µm) 1999. The five layer device, based on double- sided silicon sensors, has satisfied the original de- Layer 3 Layer 4 sign goals, in particular the targets specified for eﬃciency, hit resolution, and low transverse mo- mentum track reconstruction. The radiation dose during the first 25 fb−1 of integrated luminosity is within the planned budget, and no modules have Layer 5 angle (degrees) failed due to radiation damage. The performance of the SVT modules at high radiation dose is cur- rently being studied. Early results indicate that b) the sensors will continue to function after type inversion (at 1–3 MRad), but further tests with angle (degrees) irradiated sensors and ATOM ICs need to be per- formed. A program of spare module production Figure 27. SVT hit resolution in the a) z and b) has commenced, with the goal of replacing mod- φ coordinate in microns, plotted as a function of ules that are expected to fail due to radiation track incident angle in degrees. Each plot shows damage. Beam-generated backgrounds are ex- a diﬀerent layer of the SVT. The plots in the φ pected to rise with increasing luminosity. Physics coordinate for layers 1-3 are asymmetric around studies at five times the current backgrounds lev- φ = 0 because of the “pinwheel” design of the els indicate no change in mass or vertex resolution inner layers. There are fewer points in the φ res- for the mode B 0 → J/ψ KS0 and a ∼ 20% loss of olution plots for the outer layers as they subtend resolution in the D∗+ − D0 mass diﬀerence. In smaller angles than the inner layers. this study the detector eﬃciency for both decay modes was lower by 15–20%. 35 6. Drift Chamber 6.2. Mechanical Design and Assembly 6.2.1. Overview 6.1. Purpose and Design Requirements The DCH is relatively small in diameter, but al- The principal purpose of the drift chamber most 3 m long, with 40 layers of small hexagonal (DCH) is the eﬃcient detection of charged parti- cells providing up to 40 spatial and ionization loss cles and the measurement of their momenta and measurements for charged particles with trans- angles with high precision. These high preci- verse momentum greater than 180 MeV/c. Longi- sion measurements enable the reconstruction of tudinal position information is obtained by plac- exclusive B- and D-meson decays with minimal ing the wires in 24 of the 40 layers at small angles background. The DCH complements the mea- with respect to the z-axis. By choosing low-mass surements of the impact parameter and the di- aluminum field wires and a helium-based gas mix- rections of charged tracks provided by the SVT ture, the multiple scattering inside the DCH is near the IP. At lower momenta, the DCH mea- held to a minimum, less than 0.2%X0 of mate- surements dominate the errors on the extrapola- rial. The properties of the chosen gas, a 80:20 tion of charged tracks to the DIRC, EMC, and mixture of helium:isobutane, are presented in Ta- IFR. ble 8. This mixture has a radiation length that The reconstruction of decay and interaction is five times larger than commonly used argon- vertices outside of the SVT volume, for instance based gases. The smaller Lorentz angle results in the KS0 decays, relies solely on the DCH. For this a rather uniform time-distance relationship and purpose, the chamber should be able to measure thereby improved spatial resolution. not only the transverse momenta and positions, but also the longitudinal position of tracks, with Table 8 a resolution of ∼1 mm. Properties of helium-isobutane gas mixture at at- The DCH also needs to supply information for mospheric pressure and 20◦ C. The drift velocity is the charged particle trigger with a maximum time given for operation without magnetic field, while jitter of 0.5 µs (Section 11). the Lorentz angle is stated for a 1.5 T magnetic For low momentum particles, the DCH is re- field. quired to provide particle identification by mea- surement of ionization loss (dE/dx). A resolu- Parameter Values tion of about 7% will allow π/K separation up to 700 MeV/c. This capability is complementary Mixture He : C4 H10 80:20 to that of the DIRC in the barrel region, while Radiation Length 807 m in the extreme backward and forward directions, Primary Ions 21.2/cm the DCH is the only device providing some dis- Drift Velocity 22 µm/ ns crimination of particles of diﬀerent mass. Lorentz Angle 32◦ Since the average momentum of charged parti- dE/dx Resolution 6.9% cles produced in B- and D-meson decays is less than 1 GeV/c, multiple scattering is a significant, The inner cylindrical wall of the DCH is kept if not the dominant limitation on the track pa- thin to facilitate the matching of the SVT and rameter resolution. In order to reduce this contri- DCH tracks, to improve the track resolution for bution, material in front of and inside the cham- high momentum tracks, and to minimize the ber volume has to be minimized. background from photon conversions and inter- Finally, the DCH must be operational in actions. Material in the outer wall and in the the presence of large beam-generated back- forward direction is also minimized so as not to grounds, which were predicted to generate rates degrade the performance of the DIRC and the of ∼5 kHz/cell in the innermost layers. EMC. For this reason, the HV distribution and all of the readout electronics are mounted on the 36 630 1015 1749 68 Elec– tronics 809 485 27.4 1358 Be 17.2 236 e– 464 IP e+ 469 1-2001 8583A13 Figure 28. Longitudinal section of the DCH with principal dimensions; the chamber center is oﬀset by 370 mm from the interaction point (IP). backward endplate of the chamber. This choice minum plates of 24 mm thickness. At the forward also eliminates the need for a massive, heavily end, this thickness is reduced to 12 mm beyond shielded cable plant. a radius of 46.9 cm to minimize the material in A longitudinal cross section and dimensions of front of the calorimeter endcap. For this thick- the DCH are shown in Figure 28. The DCH is ness, the estimated safety margin on the plastic bounded radially by the support tube at its in- yield point for endplate material (6061T651 alu- ner radius and the DIRC at its outer radius. The minum) is not more than a factor of two. The device is asymmetrically located with respect to maximum total deflection of the endplates under the IP. The forward length of 1749 mm is chosen loading is small, about 2 mm or 28% of the 7 mm so that particles emitted at polar angles of 17.2◦ wire elongation under tension. During installa- traverse at least half of the layers of the chamber tion of the wires, this small deflection was taken before exiting through the front endplate. In the into account by over-tensioning the wires. backward direction, the length of 1015 mm means The inner and outer cylinder cylindrical walls that particles with polar angles down to 152.6◦ are load bearing to reduce the maximum stress traverse at least half of the layers. This choice en- and deflections of the endplates. The stepped sures suﬃcient coverage for forward-going tracks, forward endplate created a complication during and thus avoids significant degradation of the in- the assembly, because the thinner forward end- variant mass resolution, while at the same time plate would deflect more than the thicker back- maintaining a good safety margin on the electri- ward endplate. The outside rim of the forward cal stability of the chamber. The DCH extends endplate had to be pre-loaded, i.e., displaced by beyond the endplate by 485 mm at the backward 2.17 mm in the forward direction, to maintain the end to accommodate the readout electronics, ca- inside and outside rims of the rear endplate at the bles, and an rf shield. It extends beyond the for- same longitudinal position after the load of the ward endplate by 68 mm to provide space for wire wires was transfered from the stringing fixture to feed-throughs and an rf shield. the outer cylinder. Prior to installation on the inner cylinder, the 6.2.2. Structural Components two endplates were inspected on a coordinate- Details of the DCH mechanical design are pre- measuring machine. All sense wire holes, as well sented in Figure 29. The endplates, which carry as 5% of the field and clearing field wire holes, an axial load of 31,800 kN, are made from alu- were measured to determine their absolute loca- 37 4 9 Honeycomb Carbon Fiber 3 12.5 R809 Outer Wall R808.5 R809 24 1 RF Shield 12 Forward Endplate R469 Backward Endplate 24 Inner Wall 5 1 3.5 Beryllium 1-2001 R236 3.5 z = –1015 z = +1749 8583A30 Figure 29. Details of the structural elements of the DCH. All components are made of aluminum, except for the 1 mm-thick inner beryllium wall and the 9 mm-thick outer composite wall. tions. The achieved accuracy of the hole place- tion, this external wall was constructed from two ment was 38 µm for both sense and field wires, half-cylinders with longitudinal and circumferen- better than the specification by more than a fac- tial joints. The gas and electrical seals for these tor of two. In addition, the diameters of the same joints were made up in situ. The main structural sample of endplate holes were checked with pre- element consists of two 1.6 mm-thick (0.006X0 ) cision gauge pins. All holes passed the diame- carbon fiber skins laminated to a 6 mm-thick hon- ter specification (4.500±0.025 0.000 for sense wires and eycomb core. The outer shell is capable of with- 2.500±0.025 0.00 for the field and guard wires). standing a diﬀerential pressure of 30 mbar and The inner cylindrical wall of the DCH, which temperature variations as large as ±20◦ C, con- carries 40% of the wire load, was made from ditions that could be encountered during ship- five sections, a central 1 mm-thick beryllium tube ping or installation. Aluminum foil, 25 µm-thick with two aluminum extensions which were in on the inside surface and 100 µm on the outside, turn electron-beam welded to two aluminum end are in good electrical contact with the endplates, flanges to form a 3 m-long cylindrical part. The thereby completing the rf shield for the chamber. central section was made from three 120◦ seg- The total thickness of the DCH at normal in- ments of rolled and brazed beryllium. The end cidence is 1.08%X0 , of which the wires and gas flanges have precision surfaces onto which the mixture contribute 0.2%X0 , and the inner wall endplates were mounted. These surfaces set the 0.28%X0 . angles of the two endplates with respect to the axis and significantly constrain the concentricity 6.2.3. Wire Feed-Throughs of the tube. The inner cylinder also provides a A total of five diﬀerent types of feed-throughs substantial rf shield down to the PEP-II bunch- were required for the chamber to accommodate gap frequency of 136 kHz. the sense, field, and clearing field wires, as well as The outer wall bears 60% of axial wire load two diﬀerent endplate thicknesses. The five types between the endplates. To simplify its installa- are illustrated in Figure 30. They incorporate crimp pins [43] of a simple design which fasten 38 and precisely locate the wires. The choice of pin material (gold-plated copper for the signal wires Copper and gold-plated aluminum for the field wires) and Field wall thickness in the crimp region was optimized Al to provide an allowable range of almost 150 µm in Sense crimp size, as a primary means for avoiding wire Cu Guard Celenex breakage. Crimp pins were either press-fit into an in- Al sulator made from a single piece of injection- molded thermoplastic reinforced with 30% silica 24 mm glass fiber [44], or swaged into a copper jacket for the field wires. The plastic insulates the sense, guard, and clearing field wires from the electri- Cu Sense cally grounded endplates, while the metal jackets provide good ground contact for field wires (< Celenex Field/ 0.1Ω) on the backward endplate. The outer diam- Guard Al eter of the field and clearing field feed-throughs 1-2001 was maintained at 2.000+0.000 −0.025 mm while the sense 12 mm 8583A11 wire feed-through had a larger (4.500+0.000 −0.025 mm) Figure 30. Design of the five DCH wire feed- outer diameter and a longer body (41.7 mm). throughs for the 24 mm-thick endplates and the This choice provided both thicker insulating walls 12 mm-thick endplate. The copper jacketed feed- and a longer projection into the gas volume to through is for grounded field wires, the other four better shield the HV from the grounded endplate. are for sense wires (4.5 mm diameter), and guard and clearing field wires (2.5 mm diameter), all 6.2.4. Assembly and Stringing made from a Celenex insulator surrounding the Assembly of the chamber components and in- crimp pins. stallation of the wires was carried out in a large clean room (Class 10,000) at TRIUMF in Vancou- ver. The wires were strung horizontally without sioned and crimped. The automated wire trans- the outer cylindrical shell in place. The endplates porters were largely built from industrial compo- were mounted and aligned onto the inner cylinder nents, employing commercial software and hard- which in turn was supported by a central shaft in ware. The semi-automatic stringing procedure a mobile fixture. The endplates were mounted on ensured the correct hole selection, accelerated the the inner cylinder at the inside rim and attached stringing rate and greatly improved the cleanli- to support rings at the outside. These rings were ness and quality of the stringing process. The in- connected by radial spiders to the central shaft stallation of a total of 28,768 wires was completed of the stringing frame. in less than 15 weeks. Two teams of two operators each worked in parallel as the wires were strung from the in- 6.3. Drift Cells ner radius outward. The two teams were each 6.3.1. Layer Arrangement assisted by an automated wire transporter [45]. The DCH consists of a total of 7,104 small drift A wire was attached to a needle which was in- cells, arranged in 40 cylindrical layers. The layers serted through one of the endplate hole, captured are grouped by four into ten superlayers, with the magnetically by one of the transporters, and then same wire orientation and equal numbers of cells transported and inserted though the appropriate in each layer of a superlayer. Sequential layers hole in the other endplate. The wire was then are staggered by half a cell. This arrangement threaded through the feed-throughs, which were enables local segment finding and left-right ambi- glued into the endplates, and the wire was ten- guity resolution within a superlayer, even if one 39 out of four signals is missing. The stereo angles of 16 0 the superlayers alternate between axial (A) and stereo (U,V) pairs, in the order AUVAUVAUVA, 15 0 as shown in Figure 31. The stereo angles vary be- 14 0 tween ±45 mrad and ±76 mrad; they have been 13 0 chosen such that the drilling patterns are identi- cal for the two endplates. The hole pattern has a 12 -57 16-fold azimuthal symmetry which is well suited 11 -55 to the modularity of the electronic readout and trigger system. Table 9 summarizes parameters 10 -54 for all superlayers. 9 -52 Table 9 8 50 The DCH superlayer (SL) structure, specifying 7 48 the number of cells per layer, radius of the inner- 6 47 most sense wire layer, the cell widths, and wire 5 45 stereo angles, which vary over the four layers in a superlayer as indicated. The radii and widths are specified at the mid-length of the chamber. 4 0 3 0 # of Radius Width Angle 2 0 SL Cells (mm) (mm) (mrad) 1 0 1 96 260.4 17.0-19.4 0 Layer Stereo 2 112 312.4 17.5-19.5 45-50 3 128 363.4 17.8-19.6 -(52-57) 4 cm 4 144 422.7 18.4-20.0 0 Sense Field Guard Clearing 5 176 476.6 16.9-18.2 56-60 6 192 526.1 17.2-18.3 -(63-57) 1-2001 7 208 585.4 17.7-18.8 0 8583A14 8 224 636.7 17.8-18.8 65-69 Figure 31. Schematic layout of drift cells for 9 240 688.0 18.0-18.9 -(72-76) the four innermost superlayers. Lines have been 10 256 747.2 18.3-19.2 0 added between field wires to aid in visualization of the cell boundaries. The numbers on the right side give the stereo angles (mrad) of sense wires in 6.3.2. Cell Design and Wires each layer. The 1 mm-thick beryllium inner wall The drift cells are hexagonal in shape, 11.9 mm is shown inside of the first layer. by approximately 19.0 mm along the radial and azimuthal directions, respectively. The hexago- make up the drift cells are given in Table 10. The nal cell configuration is desirable because approx- sense wires are made of tungsten-rhenium [46], imate circular symmetry can be achieved over a 20 µm in diameter and tensioned with a weight large portion of the cell. The choice of aspect of 30 g. The deflection due to gravity is 200 µm ratio has the benefit of decreasing the number at mid-length. Tungsten-rhenium has a substan- of wires and electronic channels, while allowing a tially higher linear resistivity (290 Ω/m), com- 40-layer chamber in a confined radial space. Each pared to pure tungsten (160 Ω/m), but it is con- cell consists of one sense wire surrounded by six siderably stronger and has better surface quality. field wires, as shown in Figure 31. The proper- While the field wires are at ground potential, a ties of the diﬀerent types of gold-coated wires that 40 positive high voltage is applied to the sense wires. An avalanche gain of approximately 5 × 104 is obtained at a typical operating voltage of 1960 V and a 80:20 helium:isobutane gas mixture. Table 10 DCH wire specifications (all wires are gold plated). Diameter Voltage Tension Type Material (µm) (V) (g) Sense W-Re 20 1960 30 Field Al 120 0 155 Guard Al 80 340 74 Sense Clearing Al 120 825 155 Guard 1-2001 Field 8583A16 The relatively low tension on the approxi- Figure 32. Drift cell isochrones, i.e., contours of mately 2.75 m-long sense wires was chosen so that equal drift times of ions in cells of layers 3 and 4 of the aluminum field wires have matching gravita- an axial superlayer. The isochrones are spaced by tional sag and are tensioned well below the elas- 100 ns. They are circular near the sense wires, but tic limit. A simulation of the electrostatic forces become irregular near the field wires, and extend shows that the cell configuration has no instabil- into the gap between superlayers. ity problems. At the nominal operating voltage of 1960 V, the wires deflect by less then 60 µm. the drift times measurements, but they contribute The field wires [47] are tensioned with 155 g to the dE/dx measurement. to match the gravitational sag of the sense wires to within 20 µm. This tension is less than one- 6.3.4. Cross Talk half the tensile yield strength of the aluminum A signal on one sense wire produces oppositely- wire. For cells at the inner or outer boundary of a charged signals on neighboring wires due to ca- superlayer, two guard wires are added to improve pacitive coupling. The cross talk is largest be- the electrostatic performance of the cell and to tween adjacent cells of adjacent layers, ranging match the gain of the boundary cells to those of from −0.5% at a superlayer boundary to −2.7% the cells in the inner layers. At the innermost for internal layers within superlayers. For adja- boundary of layer 1 and the outermost boundary cent cells in the same layer, the cross talk ranges of layer 40, two clearing wires have been added from −0.8 to −1.8%, while for cells separated by per cell to collect charges created through photon two layers it is less than 0.5%. conversions in the material of the walls. 6.4. Electronics 6.4.1. Design Requirements and Overview 6.3.3. Drift Isochrones The DCH electronic system is designed to pro- The calculated isochrones and drift paths for vide a measurement of the drift time and the inte- ions in adjacent cells of layer 3 and 4 of an ax- grated charge, as well as a single bit to the trigger ial superlayer are presented in Figure 32. The system [48] for every wire with a signal. In the isochrones are circular near the sense wires, but 80:20 helium:isobutane gas mixture, there are on deviate greatly from circles near the field wires. average some 22 primary and 44 total ionization Ions originating in the gap between superlayers clusters produced per cm. The position of the are collected by cells in the edge layers after a de- primary ionization clusters is derived from tim- lay of several µs. These lagging ions do not aﬀect ing of the leading edge of the amplified signal. 41 Signals SL 1 SL 2 SL 3 SL 4 SL 5 SL 6 SL 7 SL 8 SL 9 SL10 FEA 1 FEA 2 FEA 3 ADB ADB ADB ROIB ROIB ROIB Trigger I/0 Data I/0 Module Module FEA = Front End Assembly ADB = Amplifier/Digitizer Board G-Link G-Link ROIB = Readout Interface Board to Trigger to ROM 1-2001 SL = Superlayer 8583A12 Figure 33. Block diagram for a 1/16th wedge of the DCH readout system, showing logical organization of the three front-end assemblies and their connections to the trigger and data I/O modules The design goal was to achieve a position res- bars provide mechanical support and water cool- olution of 140 µm, averaged over the cells. To ing channel for the FEAs. The assemblies connect reduce the time jitter in the signal arrival and to the sense wires through service boards, which at the same time maintain a good signal-to-noise route the signals and HV distribution. A readout ratio, the signal threshold was set at about 2.5 interface board (ROIB) in each FEA organizes primary electrons. For the dE/dx measurement, the readout of the digitized data. Data I/O and a resolution of 7% was projected for a 40-layer trigger I/O modules multiplex serial data from chamber. the FEAs to high-speed optical fibers for transfer The small cell size and the diﬃcult access to the readout modules that are located in the through the DIRC strong support tube require a electronics building. very high density of electronics components. As a consequence, a compact and highly modular de- 6.4.2. Service Boards sign was chosen. The readout is installed in well Service boards provide the electrostatic poten- shielded assemblies that are plugged into the end- tials for signal, guard, and clearing wires, and plate and are easily removable for maintenance. pass signals and ground to the front-end read- A schematic overview of the DCH electronics out electronics. A side view of a service board is is presented in Figure 33 [49]. The 16-fold az- shown in Figure 34. The HV board contains the imuthal symmetry of the cell pattern is reflected HV buses and filtering, current limiting resistors, in the readout segmentation. The DCH amplifier and blocking capacitors. Jumpers connect adja- and digitizer electronics are installed in electron- cent boards. The stored energy is minimized by ics front-end assemblies (FEAs) that are mounted using a 220 pF HV blocking capacitors. directly onto the rear endplate. There are three The signals are connected via series resistors to FEAs in each of the 16 sectors. These sectors the upper signal board which contains the pro- are separated by brass cooling bars that extend tection diodes and standard output connectors. from the inner to the outer chamber walls. These Mounting posts, anchored into the rear endplate, also serve as ground connections. 42 Ground Post Signal SL 8-10 Board Hypertronics HV Connectors Board Feed-throughs 24 mm SL 5-7 Al Endplate Cooling Channel Sense Wires Field Wires 1-2001 8583A22 Guard Wires SL 1-4 10 cm Figure 34. Side view of service boards show- ing two-tiered structure for DCH HV distribution Amplifier Digitizer and signal collection. Board 1-2001 8583A17 6.4.3. Front-End Assemblies The FEAs plug into connectors on the back Figure 35. Layout of 1/16th of the DCH rear end- side of the service boards. These custom wedge- plate, showing three FEA boxes between water shaped crates are aluminum boxes that contain a cooled channels. ROIB and two, three, or four amplifier/digitizer boards (ADB) for superlayers 1–4, 5–7, and 8–10, The TDC is a phase-locked digital delay linear respectively, as shown in Figure 35. The crates vernier on the sample clock of 15 MHz, which are mounted with good thermal contact to the achieves a 1 ns precision for leading edge tim- water cooled radial support bars. The total heat ing. The FADC design is based on a resistor- load generated by the FEAs is 1.3 kW. divider comparator ladder that operates in bi- The ADBs are built from basic building blocks linear mode to cover the full dynamic range. The consisting of two 4-channel amplifier ICs [50] feed- digitized output signals are stored in a trigger la- ing a single 8-channel digitizer custom ASIC [51]. tency buﬀer for 12.9 µs, after which a L1 Accept The number of channels serviced by an ADB is initiates the transfer of a 2.2 µs block of data to 60, 48, or 45, for the inner, middle, and outer the readout buﬀer. In addition, trigger informa- FEA modules, respectively. tion is supplied for every channel, based either The custom amplifier IC receives the input sig- on the presence of a TDC hit during the sam- nal from the sense wire and produces a discrim- ple period or FADC diﬀerential pulse height in- inator output signal for the drift time measure- formation, should a higher discriminator level be ment and a shaped analog signal for the dE/dx desirable. measurement. Both outputs are fully diﬀerential. The ROIB interprets FCTS commands to con- The discriminator has gain and bandwidth con- trol the flow of data and trigger information. trol, and a voltage controlled threshold. The ana- Data are moved to FIFOs on the ROIBs, and then log circuit has integrator and gain control. to data and trigger I/O modules via 59.5 MHz se- The custom digitizer IC incorporates a 4-bit rial links. A total of four such links are required TDC for time measurement and a 6-bit 15 MHz per 1/16th wedge, one for each of the outer two FADC to measure the total deposited charge. FEAs and two for the innermost of the FEA. Each 43 data I/O module services all FEAs one quad- in the DCH is measured by two independent pres- rant and transmits the data to a single ROM sure gauges, one of which is connected to a regula- via one optical fiber link. The trigger stream is tor controlling the speed of the compressor. The first multiplexed onto a total of 30 serial lines per relative pressure in the chamber is controlled to wedge for transmission to the trigger I/O mod- better than ±0.05 mbar. ule. Trigger data from two wedges of FEAs are Oxygen is removed from the gas mixture us- then transmitted to the trigger system via three ing a palladium catalytic filter. The water con- optical links. Thus, a total of 28 optical fibers, tent is maintained at 3500 ± 200 ppm by passing four for the data and 24 for the charged particle an adjustable fraction of the gas through a water trigger, are required to transfer the DCH data to bubbler. This relatively high level of water vapor the readout. is maintained to prevent electrical discharge. In addition to various sensors to monitor pressure, 6.4.4. Data Acquisition temperature, and flow at several points of the sys- The data stream is received and controlled by tem, a small wire chamber with an 55 Fe source four BABAR standard readout modules. Drift continuously monitors gain of the gas mixture. chamber-specific feature extraction algorithms convert the raw FADC and TDC information into 6.6. Calibrations and Monitoring drift times, total charge, and a status word. The 6.7. Electronics Calibration time and charge are corrected channel-by-channel The front-end electronics (FEEs) are calibrated for time oﬀsets, pedestals, and gain constants. daily to determine the channel-by-channel correc- Based on measurements of the noise a thresh- tion constants and thresholds. Calibration pulses old is typically 2–3 electrons is applied to dis- are produced internally and input to the pream- criminate signals. These algorithms take about plifier at a rate of about 160 Hz. The calibration 1 µs per channel, and reduce the data volume by signals are processed in the ROM to minimize roughly a factor of four. the data transfer and fully exploit the available 6.4.5. High Voltage System processing power. The results are stored for sub- The HV bias lines on the chamber are daisy- sequent feature extraction. The entire online cal- chained together so that each superlayer requires ibration procedure takes less than two minutes. only four power supplies, except for superlayer 1 which has eight. The voltages are supplied to 6.7.1. Environmental Monitoring the sense, guard, and clearing wires by a CAEN The operating conditions of the DCH are mon- SY527 HV mainframe [42], equipped with 24- itored in realtime by a variety of sensors and read channel plug-in modules. The sense wires are sup- out by the detector-wide CAN bus system. These plied by 44 HV channels providing up to 40 µA of sensors monitor the flow rate, pressure, and gas current each that can be monitored with a reso- mixture; the voltages and currents applied to the lution of 0.1%. wires in the chamber; the voltages and currents distributed to the electronics from power sup- 6.5. Gas System plies and regulators; instantaneous and cumula- The gas system has been designed to provide a tive radiation doses; temperature and humidity stable 80:20 helium:isobutane mixture at a con- around the chamber electronics and in the equip- stant over pressure of 4 mbar [52]. The chamber ment racks. Additional sensors monitor the at- volume is about 5.2 m3 . Gas mixing and recir- mosphere in and around the detector for excess culation is controlled by precise mass flow con- isobutane, which could pose a flammability or ex- trollers; the total flow is tuned to 15 ℓ/min, of plosive hazard in the event of a leak. which 2.5 ℓ/min are fresh gas. During normal Many of the sensors are connected to hardware operation, the complete DCH gas volume is re- interlocks, which ensure that the chamber is au- circulated in six hours, and one full volume of tomatically put into a safe state in response to an fresh gas is added every 36 hours. The pressure unsafe condition. All of these systems have per- 44 formed reliably. In addition, automated software monitors raw data quality, chamber occupancies and eﬃciencies to sense variations in electronics 8 performance that might indicate more subtle op- Drift Distance (mm) erational problems. 6.7.2. Operational Experience The design of the DCH specifies a voltage of 1960 V on the sense wires to achieve the desired 4 gain and resolution. The chamber voltage was lowered for part of the run to 1900 V out of con- cern for a small region of the chamber that was Left damaged during the commissioning phase by in- Right advertently applying 2 kV to the guard wires. 0 0 200 400 600 Wires in this region (10.4% of superlayer 5, and 1-2001 8583A18 Drift Time (ns) 4.2% of superlayer 6) were disconnected when continuous discharge was observed over extended periods of time. Figure 36. The drift time versus distance relation for left and right half of a cell. These functions 6.8. Performance are obtained from the data averaged over all cells The DCH was first operated with full mag- in a single layer of the DCH. netic field immediately after the installation into BABAR. Cosmic ray data were recorded and ex- Figure 36. tensive studies of the basic cell performance were An additional correction is made for tracks performed to develop calibration algorithms for with varying entrance angle into the drift cell. the time-to-distance and dE/dx measurements. This angle is defined relative to the radial vector These algorithms were then implemented as de- from the IP to the sense wire. The correction is scribed below for colliding beam data. Calibra- applied as a scale factor to the drift distance and tions are monitored continuously to provide feed- was determined layer-by-layer from a Garfield [53] back to the operation; some time varying parame- simulation. The entrance angle correction is im- ters are updated continuously as part of OPR. For plemented as a tenth-order Chebychev polyno- charge particle tracking the DCH and SVT infor- mial of the drift distance, with coeﬃcients which mation is combined; the performance of the com- are functions of the entrance angle. bined tracking system is described in Section 7. Figure 37 shows the position resolution as a function of the drift distance, separately for the 6.8.1. Time-to-Distance Relation left and the right side of the sense wire. The The precise relation between the measured drift resolution is taken from Gaussian fits to the dis- time and drift distance is determined from sam- tributions of residuals obtained from unbiased ples of e+ e− and µ+ µ− events. For each signal, track fits. The results are based on multi-hadron the drift distance is estimated by computing the events, for data averaged over all cells in layer 18. distance of closest approach between the track and the wire. To avoid bias, the fit does not use 6.8.2. Charge Measurement the hit on the wire under consideration. The es- The specific energy loss, dE/dx, for charged timated drift distances and measured drift times particles traversing the DCH is derived from mea- are averaged over all wires in a layer, but the data surement of total charge deposited in each drift are accumulated separately for tracks passing on cell. The charge collected per signal cell is mea- the left of a sense wire and on the right. The time- sured as part of the feature extraction algorithm distance relation is fit to a sixth-order Chebychev in the ROM. Individual measurements are cor- polynomial. An example of such a fit is shown in rected for gain variations, pedestal-subtracted 45 and integrated over a period of approximately 0.4 1.8 µs. The specific energy loss per track is computed as a truncated mean from the lowest 80% of the 0.3 individual dE/dx measurements. Various correc- tions are applied to remove sources of bias that Resolution (mm) degrade the accuracy of the primary ionization measurement. These corrections include the fol- 0.2 lowing: • changes in gas pressure and temperature, 0.1 leading to ±9% variation in dE/dx, cor- rected by a single overall multiplicative con- stant; 0 –10 –5 0 5 10 • diﬀerences in cell geometry and charge col- 1-2001 8583A19 Distance from Wire (mm) lection (±8% variation), corrected by a set of multiplicative constants for each wire; Figure 37. DCH position resolution as a function of the drift distance in layer 18, for tracks on the • signal saturation due to space charge build- left and right side of the sense wire. The data are up (±11% variation), corrected by a second- averaged over all cells in the layer. order polynomial in the dip angle, λ, of the 2 form 1/ sin λ + const; • non-linearities in the most probable energy loss at large dip angles (±2.5% variation), 104 corrected with a fourth-order Chebychev polynomial as a function of λ; and d p • variation of cell charge collection as a func- tion entrance angle (±2.5% variation), cor- K rected using a sixth-order Chebychev poly- dE/dx nomial in the entrance angle. The overall gas gain is updated continuously 103 π based on calibrations derived as part of prompt reconstruction of the colliding beam data; the e µ remaining corrections are determined once for a given HV voltage setting and gas mixture. 10–1 1 10 Corrections applied at the single-cell level can 1-2001 8583A20 Momentum (GeV/c) be large compared to the single-cell dE/dx res- olution, but have only a modest impact on the Figure 38. Measurement of dE/dx in the DCH as average resolution of the ensemble of hits. Global a function of track momenta. The data include corrections applied to all hits on a track are there- large samples of beam background triggers, as ev- fore the most important for the resolution. ident from the high rate of protons. The curves Figure 38 shows the distribution of the cor- show the Bethe-Bloch predictions derived from rected dE/dx measurements as a function of track selected control samples of particles of diﬀerent momenta. The superimposed Bethe-Bloch pre- masses. dictions for particles of diﬀerent masses have been determined from selected control samples. 46 µ+ µ− , and τ + τ − events, as well as multi-hadrons. At this time, these studies are far from complete 300 and the results represent the current status. In particular, many issues related to the intrinsic alignment of the SVT and the DCH, the varia- 200 tion with time of the relative alignment of the Tracks SVT and the DCH, and movement of the beam position relative to BABAR remain under study. 100 7.1. Track Reconstruction The reconstruction of charged particle tracks 0 -0.4 0 0.4 relies on data from both tracking systems, the 1-2001 (dE/dxmeas.– dE/dxexp.) / dE/dxexp. SVT and the DCH. Charged tracks are defined 8583A21 by five parameters (d0 , φ0 , ω, z0 , tan λ) and their associated error matrix. These parameters are Figure 39. Diﬀerence between the measured and measured at the point of closest approach to the expected energy loss dE/dx for e± from Bhabha z-axis; d0 and z0 are the distances of this point scattering, measured in the DCH at an operating from the origin of the coordinate system in the voltage of 1900 V. The curve represents a Gaus- x–y plane and along the z-axis, respectively. The sian fit to the data with a resolution of 7.5%. angle φ0 is the azimuth of the track, λ the dip an- gle relative to the transverse plane, and ω = 1/pt The measured dE/dx resolution for Bhabha is its curvature. d0 and ω are signed variables; events is shown in Figure 39. The rms resolu- their sign depends on the charge of the track. The tion achieved to date is typically 7.5%, limited track finding and the fitting procedures make use by the number of samples and Landau fluctua- of Kalman filter algorithm [54] that takes into ac- tions. This value is close to the expected resolu- count the detailed distribution of material in the tion of 7%. Further refinements and additional detector and the full map of the magnetic field. corrections are being considered to improve per- The oﬄine charged particle track reconstruc- formance. tion builds on information available from the L3 6.9. Conclusions trigger and tracking algorithm. It begins with an The DCH has been performing close to design improvement of the event start time t0 , obtained expectations from the start of operations. With from a fit to the parameters d0 , φ0 , and t0 based the exception of a small number of wires that were on the four-hit track segments in the DCH su- damaged by an unfortunate HV incident during perlayers. Next, tracks are selected by perform- the commissioning phase, all cells are fully oper- ing helix fits to the hits found by the L3 track ational. The DCH performance has proven very finding algorithm. A search for additional hits in stable over time. The design goal for the intrin- the DCH that may belong on these tracks is per- sic position and dE/dx resolution have been met. formed, while t0 is further improved by using only Backgrounds are acceptable at present beam cur- hits associated with tracks. Two more sophisti- rents, but there is concern for rising occupancies cated tracking procedures are applied which are and data acquisition capacity at the high end of designed to find tracks that either do not pass the planned luminosity upgrades. through the entire DCH or do not originate from the IP. These algorithms primarily use track seg- 7. Performance of the Charged Particle ments that have not already been assigned to Tracking Systems other tracks, and thus benefit from a progressively cleaner tracking environment with a constantly Charged particle tracking has been studied improving t0 . At the end of this process, tracks with large samples of cosmic ray muons, e+ e− , are again fit using a Kalman filter method. 47 1.0 10000 b) 8000 Efficiency Tracks/ 0.25 MeV/c2 1960 V 0.8 6000 1900 V 4000 a) 0.6 0 1 2 2000 Transverse Momentum (GeV/c) 1.0 0 0.140 0.150 5-2001 8583A46 M(Kππ) – M(Kπ) (GeV/c2) Efficiency 1960 V Figure 41. Reconstruction of low momen- 0.8 1900 V tum tracks: the mass diﬀerence, ∆M = M (K − π + π + ) − M (K − π + ), both for all detected events (data points) and for events in which the b) low momentum pion is reconstructed both in the 0.6 SVT and DCH (histogram). Backgrounds from 0.0 0.5 1.0 1.5 2.0 2.5 combinatorics and fake tracks, as well as non- 3-2001 8583A40 Polar Angle (radians) resonant data have been subtracted. Figure 40. The track reconstruction eﬃciency consistent space points from the other layers. A in the DCH at operating voltages of 1900 V and minimum of four space points are required to form 1960 V, as a function of a) transverse momentum, a good track. This algorithm is eﬃcient over a and b) polar angle. The eﬃciency is measured in wide range of d0 and z0 values. The second al- multi-hadron events as the fraction of all tracks gorithm starts with circle trajectories from φ hits detected in the SVT for which the DCH portion and then adds z hits to form helices. This al- is also reconstructed. gorithm is less sensitive to large combinatorics and to missing z information for some of the SVT The resulting tracks are then extrapolated into modules. the SVT, and SVT track segments are added, pro- Finally, an attempt is made to combine tracks vided they are consistent with the expected error that are only found by one of the two tracking in the extrapolation through the intervening ma- systems and thus recover tracks scattered in the terial and inhomogeneous magnetic field. Among material of the support tube. the possible SVT segments, those with the small- est residuals and the largest number of SVT layers 7.2. Tracking Eﬃciency are retained and a Kalman fit is performed to the The eﬃciency for reconstructing tracks in the full set of DCH and SVT hits. DCH has been measured as a function of trans- Any remaining SVT hits are then passed to verse momentum, polar and azimuthal angles in two complementary standalone track finding al- multi-track events. These measurements rely on gorithms. The first reconstructs tracks starting specific final states and exploit the fact that the with triplets of space points (matched φ and z track reconstruction can be performed indepen- hits) in layers 1, 3, and 5 of the SVT, and adding dently in the SVT and the DCH. 48 8000 a) majority of these low momentum pions the mo- Tracks/10 MeV/c Data Simulation mentum resolution is limited by multiple scat- tering, but the production angle can be deter- 4000 mined from the signals in innermost layers of the SVT. Figure 41 shows the mass diﬀerence ∆M = M (K − π + π + ) − M (K − π + ), for the to- 0 b) tal sample and the subsample of events in which 0.8 the slow pion has been reconstructed in both the Efficiency SVT and the DCH. The diﬀerence in these two distributions demonstrates the contribution from 0.4 SVT standalone tracking, both in terms of the gain of signal events and of resolution. The gain 0.0 0.0 0.1 0.2 0.3 0.4 in eﬃciency is mostly at very low momenta, and 1-2001 8583A27 Transverse Momentum (GeV/c) the resolution is impacted by multiple scattering and limited track length of the slow pions. To Figure 42. Monte Carlo studies of low momentum derive an estimate of the tracking eﬃciency for tracks in the SVT: a) comparison of data (contri- these low momentum tracks, a detailed Monte butions from combinatoric background and non- Carlo simulation was performed. Specifically, the BB events have been subtracted) with simulation pion spectrum was derived from simulation of the of the transverse momentum spectrum of pions inclusive D∗ production in BB events, and the from D∗+ → D0 π + in BB events, and b) eﬃ- Monte Carlo events were selected in the same way ciency for slow pion detection derived from simu- as the data. A comparison of the detected slow lated events. pion spectrum with the Monte Carlo prediction is presented in Figure 42. Based on this very good The absolute DCH tracking eﬃciency is deter- agreement, the detection eﬃciency has been de- mined as the ratio of the number of reconstructed rived from the Monte Carlo simulation. The SVT DCH tracks to the number of tracks detected significantly extends the capability of the charged in the SVT, with the requirement that they fall particle detection down to transverse momenta of within the acceptance of the DCH. Such stud- ∼50 MeV/c. ies have been performed for diﬀerent samples of multi-hadron events. Figure 40 shows the re- 7.3. Track Parameter Resolutions sult of one such study for the two voltage set- The resolution in the five track parameters is tings. The measurement errors are dominated by monitored in OPR using e+ e− and µ+ µ− pair the uncertainty in the correction for fake tracks events. It is further investigated oﬄine for tracks in the SVT. At the design voltage of 1960 V, in multi-hadron events and cosmic ray muons. the eﬃciency averages 98 ± 1% per track above Cosmic rays that are recorded during normal 200 MeV/c and polar angle θ > 500 mrad. The data-taking oﬀer a simple way of studying the data recorded at 1900 V show a reduction in eﬃ- track parameter resolution. The upper and lower ciency by about 5% for tracks at close to normal halves of the cosmic ray tracks traversing the incidence, indicating that the cells are not fully DCH and the SVT are fit as two separate tracks, eﬃcient at this voltage. and the resolution is derived from the diﬀerence of The standalone SVT tracking algorithms have the measured parameters for the two track halves. a high eﬃciency for tracks with low transverse To assure that the tracks pass close to the beam momentum. This feature is important for the de- interaction point, cuts are applied on the d0 , z0 , tection of D∗ decays. To study this eﬃciency, and tan λ. The results of this comparison for the decays D∗+ → D0 π + are selected by recon- coordinates of the point of closest approach and structing events of the type B ¯ → D∗+ X fol- the angles are shown in Figure 43 for tracks with lowed by D∗+ → D0 π + → K − π + π + . For the momenta above pt of 3 GeV/c. The distributions 49 800 a) b) c) d) Tracks 400 0 –0.2 0 0.2 –0.2 0 0.2 –4 0 4 –4 0 4 1-2001 8583A29 ∆d0 (mm) ∆z0 (mm) ∆Φ0 (mrad) ∆tanλ (10-3) Figure 43. Measurements of the diﬀerences between the fitted track parameters of the two halves of cosmic ray muons, with transverse momenta above 3 GeV/c, a) ∆d0 , b) ∆z0 , c) ∆φ0 , and d) ∆ tan λ. are symmetric; the non-Gaussian tails are small. The distributions for the diﬀerences in z0 and 0.4 σz0 σd0 tan λ show a clear oﬀset, attributed to residual problems with the internal alignment of the SVT. 0.3 Based on the full width at half maximum of these distributions the resolutions for single tracks can σ (mm) be parametized as 0.2 σd0 = 23µm σφ0 = 0.43 mrad σz0 = 29µm σtan λ = 0.53 · 10−3 . 0.1 The dependence of the resolution in d0 and z0 on the transverse momentum pt is presented in 0 Figure 44. The measurement is based on tracks 0 1 2 3 1-2001 in multi-hadron events. The resolution is deter- 8583A28 Transverse Momentum (GeV/c) mined from the width of the distribution of the diﬀerence between the measured parameters, d0 Figure 44. Resolution in the parameters d0 and and z0 , and the coordinates of the vertex recon- z0 for tracks in multi-hadron events as a function structed from the remaining tracks in the event. of the transverse momentum. The data are cor- These distributions peak at zero, but have a tail rected for the eﬀects of particle decays and ver- for positive values due to the eﬀect of particle de- texing errors. cays. Consequently, only the negative part of the distributions reflects the measurement error and Figure 45 shows the estimated error in the mea- is used to extract the resolution. Event shape surement of the diﬀerence along the z-axis be- cuts and a cut on the χ2 of the vertex fit are ap- tween the vertices of the two neutral B mesons, plied to reduce the eﬀect of weak decays on this one of them is fully reconstructed, the other measurement. The contribution from the vertex serves as a flavor tag. The rms width of 190 µm errors are removed from the measured resolutions is dominated by the reconstruction of the tagging in quadrature. The d0 and z0 resolutions so mea- B vertex, the rms resolution for the fully recon- sured are about 25 µm and 40 µm respectively at structed B meson is 70 µm. The data meet the pt = 3 GeV/c. These values agree well with ex- design expectation [2]. pectations, and are also in reasonable agreement While the position and angle measurements with the results obtained from cosmic rays. near the IP are dominated by the SVT measure- 50 400 2.0 σ(pt)/pt (%) Events/ 8 µm 200 1.0 0 0 0 0.2 0.4 1-2001 0 4 8 2-2001 σ∆z (mm) 8583A23 Transverse Momentum (GeV/c) 8583A37 Figure 46. Resolution in the transverse mo- Figure 45. Distribution of the error on the dif- mentum pt determined from cosmic ray muons ference ∆z between the B meson vertices for a traversing the DCH and SVT. sample of events in which one B 0 is fully recon- structed. ments, the DCH contributes primarily to the pt measurement. Figure 46 shows the resolution in the transverse momentum derived from cosmic 2000 muons. The data are well represented by a linear function, Entries/ 4MeV/c2 σpt /pt = (0.13 ± 0.01)% · pt + (0.45 ± 0.03)%, 1000 where the transverse momentum pt is measured in GeV/c. These values for the resolution param- eters are very close to the initial estimates and can be reproduced by Monte Carlo simulations. 0 3.0 3.1 3.2 More sophisticated treatment of the DCH time- 2-2001 Mass M(µ+µ−) (GeV/c2) 8583A35 to-distance relations and overall resolution func- tion are presently under study. Figure 47. Reconstruction of the decay J/ψ → Figure 47 shows the mass resolution for J/ψ µ+ µ− in selected BB events. mesons reconstructed in the µ+ µ− final state, averaged over all data currently available. The reconstructed peak is centered 0.05% below the 7.4. Summary expected value, this diﬀerence is attributed to The two tracking devices, the SVT and DCH, the remaining inaccuracies in the SVT and DCH have been performing close to design expectations alignment and in the magnetic field parameteriza- from the start of operations. Studies of track res- tion. The observed mass resolution diﬀers by 15% olution at lower momenta and as a function of for data recorded at the two DCH HV settings, polar and azimuthal angles are still under way. it is 13.0 ± 0.3 MeV/c2 and 11.4 ± 0.3 MeV/c2 at Likewise, the position and angular resolution at 1900 V and 1960 V, respectively. 51 the entrance to the DIRC or EMC are still being flection from a flat surface. Figure 48 shows a studied. Such measurements are very sensitive to schematic of the DIRC geometry that illustrates internal alignment of the SVT and relative place- the principles of light production, transport, and ment of the SVT and the DCH. A better under- imaging. The radiator material of the DIRC is standing will not only reduce the mass resolution synthetic, fused silica in the form of long, thin for the reconstruction of exclusive states, it will bars with rectangular cross section. These bars also be important for improvement of the perfor- serve both as radiators and as light pipes for the mance of the DIRC. portion of the light trapped in the radiator by total internal reflection. Fused, synthetic silica (Spectrosil [57]) is chosen because of its resistance 8. DIRC to ionizing radiation, its long attenuation length, 8.1. Purpose and Design Requirements large index of refraction, low chromatic dispersion The study of CP -violation requires the abil- within the wavelength acceptance of the DIRC, ity to tag the flavor of one of the B mesons via and because it allows an excellent optical finish the cascade decay b → c → s, while fully re- on the surfaces of the bars [58]. constructing the second B decay. The momenta In the following, the variable θc is used to des- of the kaons used for flavor tagging extend up ignate the Cherenkov angle, φc denotes the az- to about 2 GeV/c, with most of them below 1 imuthal angle of a Cherenkov photon around the GeV/c. On the other hand, pions and kaons track direction, and n represents the mean index from the rare two-body decays B 0 → π + π − and of refraction of fused silica (n = 1.473), with the B 0 → K + π − must be well-separated. They familiar relation cos θc = 1/nβ (β = v/c, v = have momenta between 1.7 and 4.2 GeV/c with a velocity of the particle, c = velocity of light). strong momentum-polar angle correlation of the For particles with β ≈ 1, some photons will al- tracks (higher momenta occur at more forward ways lie within the total internal reflection limit, angles because of the c.m. system boost) [4]. and will be transported to either one or both ends The Particle Identification (PID) system of the bar, depending on the particle incident an- should be thin and uniform in terms of radiation gle. To avoid instrumenting both ends of the bar lengths (to minimize degradation of the calorime- with photon detectors, a mirror is placed at the ter energy resolution) and small in the radial di- forward end, perpendicular to the bar axis, to mension to reduce the volume, and hence, the reflect incident photons to the backward, instru- cost of the calorimeter. Finally, for operation at mented end. high luminosity, the PID system needs fast sig- Once photons arrive at the instrumented end, nal response, and should be able to tolerate high most of them emerge into a water-filled expan- backgrounds. sion region, called the standoﬀ box. A fused silica The PID system being used in BABAR is a new wedge at the exit of the bar reflects photons at kind of ring-imaging Cherenkov detector called large angles relative to the bar axis. It thereby the DIRC [56] (the acronym DIRC stands for De- reduces the size of the required detection sur- tector of Internally Reflected Cherenkov light). It face and recovers those photons that would oth- is expected to be able to provide π/K separation erwise be lost due to internal reflection at the of ∼ 4 σ or greater, for all tracks from B-meson fused silica/water interface. The photons are decays from the pion Cherenkov threshold up to detected by an array of densely packed photo- 4.2 GeV/c. PID below 700 MeV/c relies primar- multiplier tubes (PMTs), each surrounded by re- ily on the dE/dx measurements in the DCH and flecting light catcher cones [59] to capture light SVT. which would otherwise miss the active area of the PMT. The PMTs are placed at a distance 8.2. DIRC Concept of about 1.2 m from the bar end. The expected The DIRC is based on the principle that the Cherenkov light pattern at this surface is essen- magnitudes of angles are maintained upon re- tially a conic section, where the cone opening- 52 PMT + Base 10,752 PMT's asymmetry, particles are produced preferentially forward in the detector. To minimize interfer- Standoff ence with other detector systems in the forward Light Catcher Box Purified Water region, the DIRC photon detector is placed at the 17.25 mm Thickness backward end. (35.00 mm Width) Bar Box The principal components of the DIRC are Track shown schematically in Figure 49. The bars Trajectory PMT Surface Wedge Mirror are placed into 12 hermetically sealed containers, Bar called bar boxes, made of very thin aluminum- Window hexcel panels. Each bar box, shown in Figure 50, 4.9 m 1.17 m contains 12 bars, for a total of 144 bars. Within { 4 x 1.225m Bars { a bar box the 12 bars are optically isolated by a glued end-to-end 8-2000 8524A6 ∼ 150µm air gap between neighboring bars, en- forced by custom shims made from aluminum foil. The bars are 17 mm-thick, 35 mm-wide, and 4.9 Figure 48. Schematics of the DIRC fused silica m-long. Each bar is assembled from four 1.225 m radiator bar and imaging region. Not shown is a pieces that are glued end-to-end; this length is the 6 mrad angle on the bottom surface of the wedge longest high-quality bar currently obtainable [58, (see text). 60]. The bars are supported at 600 mm intervals by angle is the Cherenkov production angle modi- small nylon buttons for optical isolation from the fied by refraction at the exit from the fused silica bar box. Each bar has a fused silica wedge glued window. to it at the readout end. The wedge, made of the The DIRC is intrinsically a three-dimensional same material as the bar, is 91 mm-long with very imaging device, using the position and arrival nearly the same width as the bars (33 mm) and a time of the PMT signals. Photons generated in trapezoidal profile (27 mm-high at bar end, and a bar are focused onto the phototube detection 79 mm at the light exit end). The bottom of the surface via a “pinhole” defined by the exit aper- wedge (see Figure 48) has a slight (∼ 6 mrad) up- ture of the bar. In order to associate the photon ward slope to minimize the displacement of the signals with a track traversing a bar, the vector downward reflected image due to the finite bar pointing from the center of the bar end to the thickness. The twelve wedges in a bar box are center of each PMT is taken as a measure of the glued to a common 10 mm-thick fused silica win- photon propagation angles αx , αy , and αz . Since dow, that provides the interface and seal to the the track position and angles are known from the purified water in the standoﬀ box. tracking system, the three α angles can be used The mechanical support of the DIRC, shown to determine the two Cherenkov angles θc and φc . in Figure 49, is cantilevered from the steel of In addition, the arrival time of the signal provides the IFR. The Strong Support Tube (SST) is a an independent measurement of the propagation steel cylinder located inside the end doors of the of the photon, and can be related to the propaga- IFR and provides the basic support for the entire tion angles α. This over-constraint on the angles DIRC. In turn, the SST is supported by a steel and the signal timing are particularly useful in support gusset that fixes it to the barrel magnet dealing with ambiguities in the signal association steel. It also minimizes the magnetic flux gap (see Section 8.6.1) and high background rates. caused by the DIRC bars extending through the flux return, and supports the axial load of the in- 8.3. Mechanical Design ner magnetic plug surrounding the beam in this and Physical Description region. The DIRC bars are arranged in a 12-sided The bar boxes are supported in the active re- polygonal barrel. Because of the beam energy gion by an aluminum tube, the Central Support 53 Figure 49. Exploded view of the DIRC mechani- Figure 52. Transverse section of the nominal cal support structure. The steel magnetic shield DIRC bar box imbedded in the CST. All dimen- is not shown. sions are given in mm. fused silica, thus minimizing the total internal re- flection at silica-water interface. Furthermore, its chromaticity index is a close match to that of fused silica, eﬀectively eliminating dispersion at the silica-water interface. The steel gusset sup- ports the standoﬀ box. A steel shield, supple- mented by a bucking coil, surrounds the standoﬀ box to reduce the field in the PMT region to be- low 1 Gauss [28]. The PMTs at the rear of the standoﬀ box lie on a surface that is approximately toroidal. Each of the 12 PMT sectors contains 896 PMTs (ETL model 9125 [61,62]) with 29 mm-diameter, in a closely packed array inside the water volume. A Figure 50. Schematics of the DIRC bar box as- double o-ring water seal is made between the sembly. PMTs and the wall of the standoﬀ box. The PMTs are installed from the inside of the standoﬀ Tube (CST), attached to the SST via an alu- box and connected via a feed-through to a base minum transition flange. The CST is a thin, mounted outside. The hexagonal light catcher double-walled, cylindrical shell, using aircraft- cone is mounted in front of the photocathode of type construction with stressed aluminum skins each PMT, which results in an eﬀective active sur- and bulkheads having riveted or glued joints. The face area light collection fraction of about 90%. CST also provides the support for the DCH. The geometry of the DIRC is shown in Figures 51 The standoﬀ box is made of stainless steel, and 52. consisting of a cone, cylinder, and 12 sectors of The DIRC occupies 80 mm of radial space in PMTs. It contains about 6,000 liters of purified the central detector volume including supports water. Water is used to fill this region because and construction tolerances, with a total of about it is inexpensive and has an average index of re- 17% radiation length thickness at normal inci- fraction (n ≈ 1.346) reasonably close to that of dence. The radiator bars subtend a solid angle corresponding to about 94% of the azimuth and 54 Figure 51. Elevation view of the nominal DIRC system geometry. For clarity, the end plug is not shown. All dimensions are given in mm. 83% of the c.m. polar angle cosine. tained after multiple bounces along the bars (365 The distance from the end of the bar to the bounces in the example of Figure 53). The ultra- PMTs is ∼ 1.17 m, which together with the size violet cut-oﬀ is at ∼ 300 nm, determined by the of the bars and PMTs, gives a geometric contri- epoxy (Epotek 301-2 [63]) used to glue the fused bution to the single photon Cherenkov angle res- silica bars together. The dominant contributor olution of ∼ 7 mrad. This value is slightly larger to the overall detection eﬃciency is the quantum than the rms spread of the photon production eﬃciency of the PMT. Taking into account ad- (dominated by a ∼ 5.4 mrad chromatic term) and ditional wavelength independent factors, such as transmission dispersions. The overall single pho- the PMT packing fraction and the geometrical ton resolution is estimated to be about 10 mrad. eﬃciency for trapping Cherenkov photons in the fused silica bars via total internal reflection, the 8.3.1. Cherenkov Photon Detection expected number of photoelectrons (Npe ) is ∼ 28 Eﬃciency for a β = 1 particle entering normal to the sur- Figure 53 shows the contribution of various op- face at the center of a bar, and increases by over tical and electronic components of the DIRC to a factor of two in the forward and backward di- the Cherenkov photon detection eﬃciency as a rections. function of wavelength. The data points per- tain to a particle entering the center of the bar 8.3.2. DIRC Water System at 90◦ . A typical design goal for the photon The DIRC water system is designed to main- transport in the bar was that no single compo- tain good transparency at wavelengths as small as nent should contribute more than 10–20% loss of 300 nm. One way to achieve this is to use ultra- detection eﬃciency. Satisfying this criterion re- pure, de-ionized water, close to the theoretical quired an extremely high internal reflection coef- limit of 18 MΩcm resistivity. In addition, the wa- ficient of the bar surfaces (greater than 0.9992 per ter must be de-gassed and the entire system kept bounce), so that about 80% of the light is main- free of bacteria. Purified water is aggressive in at- 55 Water transmission (1.2m) pH-value, temperature, and flow. A gravity feed Mirror reflectivity return system prevents overpressure. The entire Internal reflection coeff. (365 bounces) standoﬀ box water volume can be recirculated up Epotek 301-2 transmission (25µm) to four times a day. EMI PMT 9215B quantum efficiency (Q.E.) The operating experience with the water sys- PMT Q.E. ⊗ PMT window transmission tem so far has been very good. The water volume Final Cherenkov photon detection efficiency is exchanged every ten hours and the resistivity of the water is typically 18 MΩcm in the sup- Transmission or Reflectivity or Q.E. ply line and 8–10 MΩcm in the return line at a 1 temperature of about 23–26◦ C. The pH-value is about 6.5 and 6.6-6.7 in the supply and return 0.8 water, respectively. The water transparency is routinely measured using lasers of three diﬀer- ent wavelengths. The transmission is better than 0.6 92% per meter at 266 nm and exceeds 98% per meter at 325 nm and 442 nm. 0.4 Potential leaks from the water seals between the bar boxes and the standoﬀ box are detected by a water leak detection system of 20 custom wa- 0.2 ter sensors in and about the bar box slots. Two commercial ultrasonic flow sensors are used to 0 monitor water flow in two (normally dry) drain 200 300 400 500 600 700 lines in addition to the 12 humidity sensors on a nitrogen gas output line from each bar box (see Wavelength [nm] below). Should water be detected, a valve in a 100 mm diameter drain line is opened, and the Figure 53. Transmission, reflectivity and quan- entire system is drained in about 12 minutes. tum eﬃciency for various components of the All elements inside the standoﬀ box (PMT, DIRC as a function of wavelength for a β = 1 plastic PMT housing, gaskets, light catchers) particle at normal incidence to the center of a were tested at normal and elevated temperatures bar [64]. to withstand the highly corrosive action of ultra- pure water and to prevent its pollution. For in- tacking many materials, and those in contact with stance, rhodium-plated mirrors on ULTEM sup- the water were selected based on known compata- port had to be used for the light catchers [59]. bility with purified water. To maintain the neces- sary level of water quality, most plumbing compo- 8.3.3. DIRC Gas System nents are made of stainless steel or polyvinylidene Nitrogen gas from liquid nitrogen boil-oﬀ is fluoride. used to prevent moisture from condensing on the The system contains an input line with six bars, and used also to detect water leaks. The mechanical filters (three 10 µm, two 5 µm, and gas flows through each bar box at the rate of one 1 µm), a reverse osmosis unit, de-ionization 100–200 cm3 /min, and is monitored for humid- beds, a Teflon microtube de-gasser and various ity to ensure that the water seal around the bar pumps and valves. To prevent bacteria growth, box remains tight. The gas is filtered through a it is equipped with a UV lamp (254 nm wave- molecular sieve and three mechanical filters to re- length) and filters (two 1 µm, two 0.2 µm, and move particulates (7 µm, 0.5 µm, and 0.01 µm). charcoal filters). Sampling ports are provided to Dew points of the gas returned from the bar boxes check the water quality and to monitor resistivity, are about -40◦ C. Approximately one third of the input nitrogen gas leaks from the bar boxes and 56 keeps the bar box slots in the mechanical support pulse is output by a zero-crossing discriminator, structure free of condensation. as well as a pulse shaped by a CR-RC filter with 80 ns peaking time, which was chosen to allow 8.4. Electronics for the ADC multiplexing. The multiplexer se- 8.4.1. DIRC PMT Electronics lects the channel to be digitized by the FADC for The DIRC PMT base contains a single printed calibration. circuit board, equipped with surface mounted The TDC IC [67] is a 16-channel TDC with components. The operating high voltage (HV) 0.5 ns binning, input buﬀering, and selective read- of the PMTs is ∼ 1.14 kV, with a range between out of the data in time with the trigger. To cope 0.9 and 1.3 kV. Groups of 16 tubes are selected with the L1 maximum trigger latency of 12 µs for uniformity of gain to allow their operation at and jitter of 1 µs, the selective readout process ex- a common HV provided from a single distribution tracts data in time with the trigger within a pro- board. grammable time window. The acceptance win- The HV is provided by a CAEN SY-527 high dow width is programmable between 64 ns and voltage distribution system. Each of the 12 sec- 2 µs and is typically set at 600 ns. The twelve tors receives HV through 56 high voltage chan- DIRC Crate Controllers (DCCs) that form the nels, distributed through a single cable bundle. interface to the VME front-end crates are con- Each voltage can be set between 0 and 1.6 kV. nected to six ROMs via 1.2 Gbit/s optical fibers. 8.4.2. DIRC Front-End Electronics 8.4.3. DAQ Feature Extraction The DIRC front-end electronics (FEE) is de- Raw data from the DFBs are processed in signed to measure the arrival time of each the ROMs by a feature extraction algorithm be- Cherenkov photon detected by the PMT ar- fore being transmitted to the segment and event ray [65] to an accuracy that is limited by the in- builder. This software algorithm reduces the trinsic 1.5 ns transit time spread of the PMTs. data volume by roughly 50% under typical back- The design contains a pipeline to deal with the ground conditions. DFB data that contain er- L1 trigger latency of 12µ s, and can handle ran- rors are flagged and discarded. The only data dom background rates of up to 200 kHz/PMT errors seen to date have been traced to dam- with zero dead time. In addition, the pulse height aged DFBs that were replaced immediately. Be- spectra can be measured to ensure that the PMTs cause the dataflow system can reliably transmit operate on the HV plateau. However, because the at most 32 kBytes/crate, the feature extraction ADC information is not needed to reconstruct must sometimes truncate data to limit the event events, 64 PMTs are multiplexed onto a single size. Event data are replaced with a per-DFB ADC for monitoring and calibration. occupancy summary when a ROM’s hit occu- The DIRC FEE is mounted on the outside of pancy exceeds 56%, which occurs about once in the standoﬀ box and is highly integrated in or- 104 events. An appropriate flag is inserted into der to minimize cable lengths and to retain the the feature extraction output whenever trunca- required single photoelectron sensitivity. Each tion or deletion occurs. Errors, truncation, and of the 168 DIRC Front-end Boards (DFBs) pro- feature extraction performance are continuously cesses 64 PMT inputs, containing eight custom monitored online, and exceptions are either im- analog chips along with their associated level mediately corrected or logged for future action. translators, four custom-made TDC ICs, one 8- bit flash ADC (FADC), two digitally controlled 8.4.4. DIRC Calibration calibration signal generators, multi-event buﬀers, The DIRC uses two independent approaches for and test hardware. a calibration of the unknown PMT time response The PMT signals are amplified, and pulse- and the delays introduced by the FEE and the shaped by an eight-channel analog IC [66]. A fast control system. The first is a conventional digital pulse timed with the peak of the input pulser calibration. The second uses reconstructed 57 tracks from collision data. timing resolution than the pulser calibration. The The pulser calibration is performed online us- time delay values per channel are typically stable ing a light pulser system which generates precisely to an rms of less than 0.1 ns over more than one timed 1 ns duration light pulses from twelve, blue year of daily calibrations. LEDs, one per sector. The LEDs are triggered by the global fast control calibration strobe com- 8.4.5. DIRC Environmental Monitoring mand sent to the DCCs. The DCC triggers an System individual LED for each sector upon receipt of The DIRC environmental monitoring system is calibration strobe. Pulses in adjacent sectors are divided into three parts, corresponding to three staggered by 50 ns to prevent light crosstalk be- separate tasks. The first deals with the control tween sectors. The pulser is run at roughly 2 kHz and monitoring of the HV system for the PMTs. for the time delay calibration. The LED light is The second is devoted to monitoring low voltages transmitted through approximately 47 m-long op- related to the FEE. The third controls a variety tical fibers to diﬀusers mounted on the inner sur- of other detector parameter settings. An inter- face of the standoﬀ box wall opposite the PMTs. lock system, based on a standard VME module This light produces about 10% photoelectron oc- (SIAM), is provided. For the purposes of the cupancy nearly uniformly throughout the stand- DIRC, three dedicated VME CPUs run the appli- oﬀ box. cation code. The communication between the HV Histograms of TDC times for each PMT are ac- mainframes and the monitoring crate is achieved cumulated in parallel in the ROMs, and then fit by a CAENET controller (V288). The HV mon- to an asymmetric peak function. About 65,000 itor task controls the step sizes for ramping the light pulses are used to determine the mean time HV up or down, as well as the communication of delay of each of the PMTs in the standoﬀ box to alarm conditions, and the values and limits for a statistical accuracy of better than 0.1 ns. The the HV and current of each channel. LED pulser is also used to monitor the photo- The purpose of FEE monitoring is to con- tube gains using the ADC readout. As with the trol and monitor parameters related to the TDC calibration, histograms and fits of the ADC FEE. For each DIRC sector, a custom multi- spectrum are accumulated and fit in the ROM. purpose board, the DCC, equipped with a micro- A calibration run including both TDC and ADC controller [68] incorporating the appropriate com- information for all PMTs requires a few minutes, munication protocol (CANbus), is situated in the and is run once per day. Daily calibrations not same crate as the DFB. All monitoring and con- only verify the time delays, but allow the detec- trol tasks are implemented on this card. The pa- tion of hardware failures. rameters monitored are the low voltages for the The data stream calibration uses reconstructed DFBs and DCCs, the status of the optical link tracks from the collision data. For calibration of (Finisar), the temperature on supply boards, and the global time delay, the observed, uncalibrated the VME crate status. times minus the expected arrival times, ∆tγ , are The third part of the monitoring system is collected during the online prompt reconstruction based on a custom ADC VME board (VSAM), processing. To calculate individual channel cali- used to monitor various type of sensors: magnetic brations, ∆tγ values for each DIRC channel are field sensors, an ensemble of 12 beam monitoring accumulated until statistics equivalent to 100,000 scalers, 16 CsI radiation monitors, the water level tracks are collected. The distribution for each in the standoﬀ box as well as its pH-value, resis- channel is fit to extract the global time oﬀset cal- tivity, and temperature. ibration. The data stream and online pulser calibrations 8.5. Operational Issues of the electronic delays, and of the PMT time The DIRC was successfully commissioned and response and gain yield fully consistent results, attained performance close to that expected from although the data stream results in 15% better Monte Carlo simulation. The DIRC has been 58 Figure 54. Display of an e+ e− → µ+ µ− event reconstructed in BABAR with two diﬀerent time cuts. On the left, all DIRC PMTs with signals within the ±300 ns trigger window are shown. On the right, only those PMTs with signals within 8 ns of the expected Cherenkov photon arrival time are displayed. robust and stable, and, indeed, serves also as is associated with a loss of sodium and boron from a background detector for PEP-II tuning. Fig- the surface of the glass [69]. For most tubes, the ure 54 shows a typical di-muon event (e+ e− → leaching rate is a few microns per year, and is ex- µ+ µ− ). In addition to the signals caused by the pected to be acceptable for the full projected ten Cherenkov light from the two tracks, about 500 year lifetime of the experiment. However, for the background signals can be seen in the readout ∼ 50 tubes, the incorrect glass was used by the window of ±300 ns. This background is dom- manufacturer. That glass does not contain zinc, inated by low energy photons from the PEP-II making it much more susceptible to rapid leach- machine hitting the standoﬀ box. Some care in ing. This leaching may eventually lead to either machine tuning is required to stay under a noise a loss of performance, or some risk of mechanical limit of about 200 kHz/tube imposed by limited failure of the face plates for these tubes. Direct DAQ throughput. Lead shielding has been in- measurements of the number of Cherenkov pho- stalled around the beam line components just tons observed in di-muon events as a function of outside the backward endcap, and has substan- time suggest that the total loss of photons from tially reduced this background. all sources is less than 2%/year. After about two years of running, approxi- mately 99.7% of PMTs and electronic channels 8.6. Data Analysis and Performance are operating with nominal performance. Figure 54 shows the pattern of Cherenkov Some deterioration of the PMT front glass win- photons in a di-muon event, before and after dows (made of B53 Borosilicate glass) that are im- reconstruction. The time distribution of real mersed in the ultra-pure water of the standoﬀ box Cherenkov photons from a single event is of order has been observed. For most of the tubes, the ob- ∼ 50 ns wide, and during normal data-taking they servable eﬀect is typically a slight cloudiness, but are accompanied by hundreds of random photons for about 50 tubes, it is much more pronounced. in a flat background within the trigger acceptance Extensive R&D has demonstrated that the eﬀect window. Given a track pointing at a particular 59 fused silica bar and a candidate signal in a PMT didate signal in the PMT and the photon prop- within the optical phase space of that bar, the agation time within the bar and the water filled Cherenkov angle is determined up to a 16-fold standoﬀ box. The time information and the re- ambiguity: top or bottom, left or right, forward quirement of using only physically possible pho- or backward, and wedge or no-wedge reflections. ton propagation paths reduce the number of am- The goal of the reconstruction program is to as- biguities from 16 to typically 3. Applying the sociate the correct track with the candidate PMT time information also substantially improves the signal, with the requirement that the transit time correct matching of photons with tracks and re- of the photon from its creation in the bar to its duces the number of accelerator induced back- detection at the PMT be consistent with the mea- ground signals by approximately a factor 40, as surement error of ∼ 1.5 ns. illustrated in Figure 54. The reconstruction routine currently provides a 8.6.1. Reconstruction likelihood value for each of the five stable particle An unbinned maximum likelihood formalism is types (e,µ,π,K,p) if the track passes through the used to incorporate all information provided by active volume of the DIRC. These likelihoods are the space and time measurements from the DIRC. calculated in an iterative process by maximizing The emission angle and the arrival time of the likelihood value for the entire event while test- the Cherenkov photons are reconstructed from ing diﬀerent hypotheses for each track. If enough the observed space-time coordinates of the PMT photons are found, a fit of θc and the number of signals, transformed into the Cherenkov coordi- observed signal and background photons are cal- nate system (θc , φc , and δt) as follows: The culated for each track. known spatial position of the bar through which the track passed and the PMTs whose signal 8.6.2. Results times lie within the readout window of ±300 ns The parameters of expected DIRC performance from the trigger are used to calculate the three- were derived from extensive studies with a va- dimensional vector pointing from the center of the riety of prototypes, culminating with a full-size bar end to the center of each tube. This vector prototype in a beam at CERN [70]. The test is then extrapolated into the radiator bar (using beam results were well-described by Monte Carlo Snell’s law). This procedure defines, up to the simulations of the detector. The performance of 16-fold ambiguity described above, the angles θc the full detector is close to expectations, and ad- and φc of a photon. ditional oﬄine work, particularly on geometrical The DIRC time measurement represents the alignment, is expected to lead to further improve- third dimension of the photomultiplier hit recon- ments. struction. The timing resolution is not competi- In the absence of correlated systematic errors, tive with the position information for Cherenkov the resolution (σC,track ) on the track Cherenkov angle reconstruction, but timing information is angle should scale as used to suppress background hits from the beam σC,γ induced background and, more importantly, ex- σC,track = , (5) Npe clude other tracks in the same event as the source of the photon. Timing information is also used to where σC,γ is the single photon Cherenkov an- resolve the forward-backward and wedge ambigu- gle resolution, and Npe is the number of photons ities in the hit-to-track association. detected. Figure 55(a) shows the single photon The relevant observable to distinguish between angular resolution obtained from di-muon events. signal and background photons is the diﬀerence There is a broad background of less than 10% between the measured and expected photon ar- relative height under the peak, that originates rival time, ∆tγ . It is calculated for each photon mostly from track-associated sources, such as δ using the track time-of-flight (assuming it to be rays, and combinatorial background. The width a charged pion), the measured time of the can- of the peak translates to a resolution of about 60 entries per mrad 80000 80 (a) 60000 40000 60 Data Monte Carlo Simulation 20000 〈 Nγ〉 0 40 -100 -50 0 50 100 ∆ θC,γ (mrad) 20 entries per 0.2ns 80000 0 (b) -1 -0.5 0 0.5 1 60000 cosθtrack 40000 20000 Figure 56. Number of detected photons versus 0 track polar angle for reconstructed tracks in di- -5 0 5 muon events compared to Monte Carlos simula- tion. The mean number of photons in the simu- ∆ tγ (ns) lation has been tuned to match the data. Figure 55. The diﬀerence between (a) the mea- shown in Figure 57. The width of the fitted Gaus- sured and expected Cherenkov angle for single sian distribution is 2.5 mrad compared to the de- photons, ∆θc,γ , and (b) the measured and ex- sign goal of 2.2 mrad. From the measured sin- pected photon arrival time, for single muons in gle track resolution versus momentum in di-muon µ+ µ− events. events and the diﬀerence between the expected Cherenkov angles of charged pions and kaons, the 10.2 mrad, in good agreement with the expected pion-kaon separation power of the DIRC can be value. The measured time resolution (see Fig- inferred. As shown in Figure 58, the expected ure 55(b)) is 1.7 ns, close to the intrinsic 1.5 ns separation between kaons and pions at 3 GeV/c transit time spread of the PMTs. is about 4.2σ, within 15% of the design goal. The number of photoelectrons shown in Fig- Figure 59 shows an example of the use of the ure 56 varies between 20 for small polar angles at DIRC for particle identification. The Kπ invari- the center of the barrel and 65 at large polar an- ant mass spectra are shown with and without the gles. This variation is well reproduced by Monte use of the DIRC for kaon identification. The peak Carlo simulation, and can be understood from the corresponds to the decay of the D0 particle. geometry of the DIRC. The number of Cherenkov The eﬃciency for correctly identifying a photons varies with the pathlength of the track charged kaon that traverses a radiator bar and in the radiator, it is smallest at perpendicular in- the probability to wrongly identify a pion as a cidence at the center and increases towards the kaon are determined using D0 → K − π + decays ends of the bars. In addition, the fraction of pho- selected kinematically from inclusive D∗ produc- tons trapped by total internal reflection rises with tion and are shown as a function of the track larger values of | cos θtrack |. This increase in the momentum in Figure 60 for a particular choice number of photons for forward going tracks is a of particle selection criteria. The mean kaon se- good match to the increase in momentum and lection eﬃciency and pion misidentification are thus benefits the DIRC performance. 96.2 ± 0.2% (stat.) and 2.1 ± 0.1% (stat.), respec- With the present alignment, the track tively. Cherenkov angle resolution for di-muon events is 61 + – + – Without DIRC e e →µ µ x 10 2 60000 1500 2 entries per 5 MeV/c Tracks 40000 1000 20000 500 With DIRC 0 -10 0 10 ∆θC,track (mrad) 0 Figure 57. The diﬀerence between the measured 1.75 1.8 1.85 1.9 1.95 and expected Cherenkov angle, ∆θc,track , for sin- Kπ mass (GeV/c ) 2 gle muons in µ+ µ− events. The curve represents a Gaussian distribution fit to the data with a width Figure 59. Invariant Kπ inclusive mass spectrum of 2.5 mrad. with and without the use of the DIRC for kaon identification. The mass peak corresponds to the decay of the D0 particle. 10 Expected π-K Separation (σ) π Mis-ID as K Kaon Efficiency 8 1 0 + – 0.9 B →π π 6 0.8 0.7 4 0.6 0.2 2 0.1 0 0 2 3 4 1 2 3 Momentum (GeV/c ) Track Momentum (GeV/c) Figure 58. Expected π-K separation in B 0 → Figure 60. Eﬃciency and misidentification prob- π + π − events versus track momentum inferred ability for the selection of charged kaons as a from the measured Cherenkov angle resolution function of track momentum, determined using and number of Cherenkov photons per track in D0 → K − π + decays selected kinematically from di-muon events. inclusive D∗ production. 62 8.7. Summary 2 GeV, the π 0 mass resolution is dominated by The DIRC is a novel ring-imaging Cherenkov the energy resolution. At higher energies, the an- detector that is well-matched to the hadronic PID gular resolution becomes dominant, and therefore requirements of BABAR. The DIRC has been ro- it is required to be of the order of a few mrad. bust and stable and, two years after installation, Furthermore, the EMC has to be compatible about 99.7% of all PMTs and electronic channels with the 1.5 T field of the solenoid and operate re- are operating with nominal performance. Addi- liably over the anticipated ten-year lifetime of the tional shielding in the standoﬀ box tunnel region experiment. To achieve excellent resolution, sta- should reduce the sensitivity to beam-induced ble operating conditions have to be maintained. backgrounds, as should faster FEE, both installed Temperatures and the radiation exposure must during the winter 2000-2001 shutdown. At lumi- be closely monitored, and precise calibrations of nosities around 1 × 1034 cm−2 s−1 , the TDC IC the electronics and energy response over the full will have to be replaced with a faster version and dynamic range must be performed frequently. deeper buﬀering. The design process for this is underway. 9.1.2. Design Considerations The detector performance achieved is rather The requirements stated above lead to the close to that predicted by the Monte Carlo sim- choice of a hermetic, total-absorption calorime- ulations. Alignment and further code develop- ter, composed of a finely segmented array of ments are expected to further improve perfor- thallium-doped cesium iodide (CsI(Tl)) crystals. mance. The crystals are read out with silicon photodi- odes that are matched to the spectrum of scin- tillation light. Recent experience at CLEO [71] 9. Electromagnetic Calorimeter has demonstrated the suitability of this choice for 9.1. Purpose and Design physics at the Υ (4S) resonance. The electromagnetic calorimeter (EMC) is de- The energy resolution of a homogeneous crystal signed to measure electromagnetic showers with calorimeter can be described empirically in terms excellent eﬃciency, and energy and angular res- of a sum of two terms added in quadrature olution over the energy range from 20 MeV to σE a = ⊕ b, (6) 9 GeV. This capability allows the detection of E 4 E( GeV) photons from π 0 and η decays as well as from elec- tromagnetic and radiative processes. By identify- where E and σE refer to the energy of a photon ing electrons, the EMC contributes to the flavor and its rms error, measured in GeV. The energy tagging of neutral B mesons via semi-leptonic de- dependent term a arises primarily from the fluctu- cays, to the reconstruction of vector mesons like ations in photon statistics, but it is also impacted J/ψ , and to the study of semi-leptonic and rare by electronic noise of the photon detector and decays of B and D mesons, and τ leptons. The electronics. Furthermore, beam-generated back- upper bound of the energy range is set by the need ground will lead to large numbers of additional to measure QED processes, like e+ e− → e+ e− (γ) photons that add to the noise. This term is dom- and e+ e− → γγ, for calibration and luminos- inant at low energies. The constant term, b, is ity determination. The lower bound is set by dominant at higher energies (> 1 GeV). It arises the need for highly eﬃcient reconstruction of B- from non-uniformity in light collection, leakage or meson decays containing multiple π 0 s and η 0 s. absorption in the material between and in front of the crystals, and uncertainties in the calibrations. 9.1.1. Requirements Most of these eﬀects can be influenced by design The measurement of extremely rare decays of choices, and they are stable with time. Others B mesons containing π 0 s (e.g., B 0 → π 0 π 0 ) poses will be impacted by changes in the operating con- the most stringent requirements on energy reso- ditions, like variations in temperature, electronics lution, namely of order 1–2%. Below energies of gain, and noise, as well as by radiation damage 63 caused by beam-generated radiation. Table 11 The angular resolution is determined by the Properties of CsI(Tl) . transverse crystal size and the distance from the interaction point. It can also be empirically pa- Parameter Values rameterized as a sum of an energy dependent and Radiation Length 1.85 cm a constant term, Moli`ere Radius 3.8 cm c Density 4.53 g/cm3 σθ = σ φ = + d, (7) Light Yield 50,000 γ/ MeV E( GeV) Light Yield Temp. Coeﬀ. 0.28%/◦ C where the energy E is measured in GeV. The de- Peak Emission λmax 565 nm sign of the EMC required a careful optimization Refractive Index (λmax ) 1.80 of a wide range of choices, including the crystal Signal Decay Time 680 ns (64%) material and dimensions, the choice of the photon 3.34 µs (36%) detector and readout electronics, and the design of a calibration and monitoring system. These choices were made on the basis of extensive stud- radius allow for excellent energy and angular res- ies, prototyping and beam tests [72], and Monte olution, while the short radiation length allows Carlo simulation, taking into account limitations for shower containment at BABAR energies with a of space and the impact of other BABAR detector relatively compact design. Furthermore, the high systems. light yield and the emission spectrum permit eﬃ- Under ideal conditions, values for the energy cient use of silicon photodiodes which operate well resolution parameters a and b close to 1–2% could in high magnetic fields. The transverse size of the be obtained. A position resolution of a few mm crystals is chosen to be comparable to the Moli`ere will translate into an angular resolution of a few radius achieving the required angular resolution mrad; corresponding parameter values are c ≈ at low energies while appropriately limiting the 3 mrad and d ≈ 1 mrad. total number of crystals (and readout channels). However in practice, such performance is very diﬃcult to achieve in a large system with a small, 9.2. Layout and Assembly but unavoidable amount of inert material and 9.2.1. Overall Layout gaps, limitations of electronics, and background The EMC consists of a cylindrical barrel and in multi-particle events, plus contributions from a conical forward endcap. It has full coverage in beam-generated background. azimuth and extends in polar angle from 15.8◦ Though in CsI(Tl) the intrinsic eﬃciency for to 141.8◦ corresponding to a solid-angle cover- the detection of photons is close to 100% down age of 90% in the c.m. system (see Figure 61 to a few MeV, the minimum measurable en- and Table 12). The barrel contains 5,760 crystals ergy in colliding beam data is expected to be arranged in 48 distinct rings with 120 identical about 20 MeV, a limit that is largely determined crystals each. The endcap holds 820 crystals ar- by beam- and event-related background and the ranged in eight rings, adding up to a total of 6,580 amount of material in front of the calorimeter. crystals. The crystals have a tapered trapezoidal Because of the sensitivity of the π 0 eﬃciency to cross section. The length of the crystals increases the minimum detectable photon energy, it is ex- from 29.6 cm in the backward to 32.4 cm in the tremely important to keep the amount of material forward direction to limit the eﬀects of shower in front of the EMC to the lowest possible level. leakage from increasingly higher energy particles. To minimize the probability of pre-showering, 9.1.3. CsI(Tl) Crystals the crystals are supported at the outer radius, Thallium-doped CsI meets the needs of BABAR with only a thin gas seal at the front. The barrel in several ways. Its properties are listed in Ta- and outer five rings of the endcap have less than ble 11. The high light yield and small Moli`ere 0.3–0.6X0 of material in front of the crystal faces. 64 2359 1555 2295 External Support 1375 1127 1801 26.8˚ 920 38.2˚ 558 15.8˚ 22.7˚ Interaction Point 1-2001 1979 8572A03 Figure 61. A longitudinal cross section of the EMC (only the top half is shown) indicating the arrangement of the 56 crystal rings. The detector is axially symmetric around the z-axis. All dimensions are given in mm. Table 12 9.2.2. Crystal Fabrication and Assembly Layout of the EMC, composed of 56 axially sym- The crystals were grown in boules from a melt metric rings, each consisting of CsI crystals of of CsI salt doped with 0.1% thallium [73]. They identical dimensions. were cut from the boules, machined into tapered trapezoids (Figure 62) to a tolerance of ±150 µm, θ Interval Length # Crystals and then polished [74]. The transverse dimen- (radians) (X0 ) Rings /Ring sions of the crystals for each of the 56 rings vary to Barrel achieve the required hermetic coverage. The typi- 2.456 − 1.214 16.0 27 120 cal area of the front face is 4.7×4.7 cm2 , while the 1.213 − 0.902 16.5 7 120 back face area is typically 6.1×6.0 cm2 . The crys- 0.901 − 0.655 17.0 7 120 tals act not only as a total-absorption scintillating 0.654 − 0.473 17.5 7 120 medium, but also as a light guide to collect light at the photodiodes that are mounted on the rear Endcap surface. At the polished crystal surface light is 0.469 − 0.398 17.5 3 120 internally reflected, and a small fraction is trans- 0.397 − 0.327 17.5 3 100 mitted. The transmitted light is recovered in part 0.326 − 0.301 17.5 1 80 by wrapping the crystal with two layers of diﬀuse 0.300 − 0.277 16.5 1 80 white reflector [75,76], each 165 µm thick. The uniformity of light yield along the wrapped crys- tal was measured by recording the signal from a highly collimated radioactive source at 20 points The SVT support structure and electronics, as along the length of the crystal. The light yield well as the B1 dipole shadow the inner three rings was required to be uniform to within ±2% in the of the endcap, resulting in up to 3.0X0 for the front half of the crystal; the limit increased lin- innermost ring. The principal purpose of the two early up to a maximum of ±5% at the rear face. innermost rings is to enhance shower containment Adjustments were made on individual crystals to for particles close to the acceptance limit. meet these criteria by selectively roughing or pol- 65 Output thick polysterene substrate that, in turn, is glued Cable Preamplifier Board to the center of the rear face of the crystal by Fiber Optical Cable to Light Pulser an optical epoxy [77] to maximize light transmis- Diode sion [78]. The surrounding area of the crystal face Carrier is covered by a plastic plate coated with white Plate reflective paint [79]. The plate has two 3 mm- Aluminum diameter penetrations for the fibers of the light Frame pulser monitoring system. Silicon As part of the quality control process, the Photo-diodes 1.836 MeV photon line from a 88 Y radioactive source was used to measure the light yield of every crystal-diode assembly, employing a preamplifier TYVEK with 2 µs Gaussian shaping. The resulting signal (Reflector) distribution had a mean and rms width of 7300 Aluminum and 890 photoelectrons/MeV, respectively; none CsI(Tl) Crystal Foil of the crystals had a signal of less than 4600 pho- (R.F. Shield) toelectrons/MeV [78,80]. Each of the diodes is directly connected to a low-noise preamplifier. The entire assembly is en- Mylar closed by an aluminum fixture as shown in Fig- (Electrical ure 62. This fixture is electrically coupled to the Insulation) aluminum foil wrapped around the crystal and CFC thermally coupled to the support frame to dissi- Compartments (Mechanical pate the heat load from the preamplifiers. 11-2000 Support) 8572A02 Extensive aging tests were performed to ascer- tain that the diodes and the preamplifiers met the Figure 62. A schematic of the wrapped ten-year lifetime requirements. In addition, daily CsI(Tl) crystal and the front-end readout package thermal cycles of ±5◦ C were run for many months mounted on the rear face. Also indicated is the to assure that the diode-crystal epoxy joint could tapered, trapezoidal CFC compartment, which is sustain modest temperature variations. open at the front. This drawing is not to scale. 9.2.4. Crystal Support Structure ishing the crystal surface to reduce or increase its The crystals are inserted into modules that are reflectivity. supported individually from an external support Following these checks, the crystals were fur- structure. This structure is built in three sec- ther wrapped in 25 µm thick aluminum foil which tions, a cylinder for the barrel and two semi- was electrically connected to the metal housing of circular structures for the forward endcap. The the photodiode-preamplifier assembly to provide barrel support cylinder carries the load of the bar- a Faraday shield. The crystals were covered on rel modules plus the forward endcap to the mag- the outside with a 13 µm-thick layer of mylar to net iron through four flexible supports. These assure electrical isolation from the external sup- supports decouple and dampen any acceleration port. induced by movements of the magnet iron during a potential earthquake. The modules are built from tapered, trape- 9.2.3. Photodiodes zoidal compartments made from carbon-fiber- and Preamplifier Assembly epoxy composite (CFC) with 300 µm-thick walls The photon detector consists of two 2 × 1 cm2 (Figure 63). Each compartment loosely holds silicon PIN diodes glued to a transparent 1.2 mm- a single wrapped and instrumented crystal and 66 Detail of Module Detail of Mini–Crate Bulkhead Mounting Features Fan Out Board Cableway Cooling Channels ADBs IOB Aluminum Cu Heat Sink Carbon Strongback Fiber Tubes 0.9 m Electronic 0.48 m Aluminum Mini–Crates RF Shield 4m Aluminum Support Cylinder 3-2001 8572A06 Figure 63. The EMC barrel support structure, with details on the modules and electronics crates (not to scale). thus assures that the forces on the crystal sur- Each module was installed into the 2.5 cm-thick, faces never exceed its own weight. Each module 4 m-long aluminum support cylinder, and subse- is surrounded by an additional layer of 300 µm quently aligned. On each of the thick annular CFC to provide additional strength. The mod- end-flanges this cylinder contains access ports for ules are bonded to an aluminum strong-back that digitizing electronics crates with associated cool- is mounted on the external support. This scheme ing channels, as well as mounting features and minimizes inter-crystal materials while exerting alignment dowels for the forward endcap. minimal force on the crystal surfaces; this pre- The endcap is constructed from 20 identical vents deformations and surface degradation that CFC modules (each with 41 crystals), individu- could compromise performance. By supporting ally aligned and bolted to one of two semi-circular the modules at the back, the material in front of support structures. The endcap is split vertically the crystals is kept to a minimum. into two halves to facilitate access to the central The barrel section is divided into 280 sepa- detector components. rate modules, each holding 21 crystals (7 × 3 in The entire calorimeter is surrounded by a dou- θ × φ). After the insertion of the crystals, the ble Faraday shield composed of two 1 mm-thick aluminum readout frames, which also stiﬀen the aluminum sheets so that the diodes and pream- module, are attached with thermally-conducting plifiers are further shielded from external noise. epoxy to each of the CFC compartments. The en- This cage also serves as the environmental bar- tire 100 kg-module is then bolted and again ther- rier, allowing the slightly hygroscopic crystals to mally epoxied to an aluminum strong-back. The reside in a dry, temperature controlled nitrogen strong-back contains alignment features as well atmosphere. as channels that couple into the cooling system. 67 9.2.5. Cooling System 9.3.1. Photodiode Readout The EMC is maintained at constant, accurately and Preamplifiers monitored temperature. Of particular concern The ENE is minimized by maximizing the light are the stability of the photodiode leakage current yield and collection, employing a highly eﬃcient which rises exponentially with temperature, and photon detector, and a low-noise electronic read- the large number of diode-crystal epoxy joints out. The PIN silicon photodiodes [82] have a that could experience stress due to diﬀerential quantum eﬃciency of 85% for the CsI(Tl) scin- thermal expansion. In addition, the light yield tillation light [83]. At a depletion voltage of of CsI(Tl) is weakly temperature dependent. 70 V, their typical dark currents were measured The primary heat sources internal to to be 4 nA for an average capacitance of 85 pF; the calorimeter are the preamplifiers the diodes are operated at a voltage of 50 V. (2 × 50 mW/crystal) and the digitizing elec- The input capacitance to the preamplifier is min- tronics (3 kW per end-flange). In the barrel, the imized by connecting the diodes to the preampli- preamplifier heat is removed by conduction to the fier with a very short cable. The preamplifier is a module strong backs which are directly cooled low-noise charge-sensitive amplifier implemented by Fluorinert (polychlorotrifluoro-ethylene) [81]. as a custom application specific integrated cir- The digitizing electronics are housed in 80 cuit (ASIC) [84]. It shapes the signal and acts mini-crates, each in contact with the end-flanges as a band-pass filter to remove high- and low- of the cylindrical support structure. These crates frequency noise components. The optimum shap- are indirectly cooled by chilled water pumped ing time for the CsI(Tl)-photodiode readout is through channels milled into the end-flanges 2–3 µs, but a shorter time was chosen to reduce close to the inner and outer radii. A separate the probability of overlap with low-energy pho- Fluorinert system in the endcap cools both the tons from beam background. The commensurate 20 mini-crates of digitizing electronics and the degradation in noise performance is recovered by preamplifiers. implementing a realtime digital signal-processing algorithm following digitization. To achieve the required operational reliabil- ity [85] for the inaccessible front-end readout com- ponents, two photodiodes were installed, each connected to a preamplifier. In addition, all com- 9.3. Electronics ponents were carefully selected and subjected to The EMC electronics system, shown schemat- rigorous tests, including a 72-hour burn-in of the ically in Figure 64, is required to have negligible preamplifiers at 70◦ C to avoid infant mortality. impact on the energy resolution of electromag- The dual signals are combined in the postam- netic showers from 20 MeV to 9 GeV, while ac- plification/digitization circuits, installed in mini- commodating the use of a 6.13 MeV radioactive crates at the end-flanges, a location that is acces- source for calibration. These requirements set a sible for maintenance. limit of less than 250 keV equivalent noise energy (ENE) per crystal and define an 18-bit eﬀective 9.3.2. Postamplification, Digitization dynamic range of the digitization scheme. For and Readout source calibrations, the least significant bit is set The two preamplifiers on each crystal, A and B, to 50 keV, while for colliding beam data it is set each provide amplification factors of 1 and 32 and to 200 keV. To reach the required energy reso- thus reduce the dynamic range of the signal that lution at high energies, the coherent component is transmitted to the mini-crates to 13-bits. A has to be significantly smaller than the incoher- custom auto-range encoding (CARE) circuit [84] ent noise component. In addition, the impact of further amplifies the signal to arrive at a total high rates of low energy (<5 MeV) beam-induced gain of 256, 32, 4 or 1 for four energy ranges, photon background needs to be minimized. 0–50 MeV, 50–400 MeV, 0.4–3.2 GeV, and 3.2– 68 on Crystal on-detector off-detector Amplifier Rangebits ROM x1 CARE A x32 Chip x1 Link Mem Crystal with x4 Event ADC Link two x1 select, x32 10-bit Optical Data Diodes B x32 A,B, or Fiber Sum ABavg x256 11-2000 Trigger 8572A04 Data Figure 64. Schematic diagram of the EMC readout electronics. 13.0 GeV, respectively. The appropriate range is identified by a comparator and the signal is dig- Filtered itized by a 10-bit, 3.7 MHz ADC. Data from 24 Channels / 0.01 (MeV) 800 crystals are multiplexed onto a fiber-optic driver and sent serially at a rate of 1.5 Gbytes/s across a 30 m-long optical fiber to the ROM. In the ROM, the continuous data stream is entered into a dig- Unfiltered ital pipeline. A correction for pedestal and gain 400 is applied to each sample. The pipeline is then tapped to extract the input to the calorimeter trigger. Upon receipt of the L1 Accept signal, data samples within a time window of ±1 µs are se- lected for the feature extraction. Up to now, the 0 calorimeter feature extraction algorithm performs 0 0.4 0.8 1-2001 a parabolic fit to the peak of the signal waveform 8583A9 Electronics Noise (MeV) to derive its energy and time. In the future, it is planned to employ a digital filter prior to the sig- nal fit to further reduce noise. For this filter algo- Figure 65. The distribution of equivalent noise rithm, the frequency decomposition of an average energy (ENE) or all channels of the EMC with signal pulse and the typical noise spectrum are and without digital filtering. The data were measured for all channels and subsequently used recorded in the absence of beams by a random to derive an optimum set of weights that maxi- trigger. mizes the signal-to-noise ratio. These weights are then applied to individual samples to obtain a filtered waveform. function indicate that the coherent noise com- The magnitude of the electronic noise is mea- ponent is negligible compared to the incoherent sured as the rms width of the pedestal distribu- noise, except for regions where the preamplifiers tion as shown in Figure 65. The observed dis- saturate (see below). tribution for all channels translates to an ENE During data-taking, the data acquisition im- of 230 keV and 440 keV with and without digital poses a single-crystal readout threshold in order filtering; this result is comparable to design ex- to keep the data volume at an acceptable level. pectations. Measurements of the auto-correlation 69 This energy threshold is currently set to 1 MeV ergy deposited. Second, the energy deposited in and during stable colliding beam conditions on a shower spreading over several adjacent crystals average 1,000 crystals are read out (measured has to be related to the energy of the incident with 600 mA of e− and 1100 mA of e+ and a ran- photon or electron by correcting for energy loss dom clock trigger), corresponding to an average mostly due to leakage at the front and the rear, occupancy of 16%. The electronic noise accounts and absorption in the material between and in for about 10%, while the remaining signals orig- front of the crystals, as well as shower energy not inate from beam-generated background (see Sec- associated with the cluster. tion 3). A typical hadronic event contributes sig- The oﬄine pattern recognition algorithm that nals in 150 crystals. groups adjacent crystals into clusters is described in detail in Section 9.6. 9.3.3. Electronics Calibration and Linearity 9.4.1. Individual Crystal Calibration To measure pedestal oﬀsets, determine the In spite of the careful selection and tuning of overall gain, and to remove non-linearities the the individual crystals, their light yield varies sig- FEE are calibrated by precision charge injection nificantly and is generally non-uniform. It also into the preamplifier input. Initially, residual changes with time under the impact of beam- non-linearities of up to 12% in limited regions generated radiation. The absorbed dose is largest near each of the range changes were observed and at the front of the crystal and results in increased corrected for oﬄine [86]. These non-linearities attenuation of the transmitted scintillation light. were traced to oscillations on the ADC cards that The light yield must therefore be calibrated at have since been corrected. The correction re- diﬀerent energies, corresponding to diﬀerent av- sulted in markedly improved energy resolution erage shower penetration, to track the eﬀects of at high energies. Residual non-linearities (typi- the radiation damage. cally 2–4%) arise primarily from cross-talk, im- The calibration of the deposited energies is pacting both the electronics calibrations and the performed at two energies at opposite ends of colliding-beam data. The eﬀect is largest at about the dynamic range, and these two measurements 630 MeV (950 MeV) in a high (low) gain preampli- are combined by a logarithmic interpolation. A fier channel, inducing a 2 MeV (6 MeV) cross-talk 6.13 MeV radioactive photon source [87] provides signal in an adjacent channel. The implemen- an absolute calibration at low energy, while at tation of an energy dependent correction is ex- higher energies (3–9 GeV) the relation between pected to significantly reduce this small, remain- polar angle and energy of e± from Bhabha events ing eﬀect, and lead to a further improvement of is exploited [88]. the energy resolution. A flux of low-energy neutrons (4×108 /s) is used 9.3.4. Electronics Reliability to irradiate Fluorinert [81] to produce photons of With the exception of minor cable damage 6.13 MeV via the reaction 19 F + n →16 N + α, 16 during installation (leaving two channels inop- N →16 O∗ + β, 16 O∗ →16 O + γ. The activated 16 erative), the system of 13,160 readout channels N has a half-life of 7 seconds and thus does not has met its reliability requirements. After the cause radiation damage or long-term activation. replacement of a batch of failing optical-fiber The fluid is pumped at a rate of 125 ℓ/s from the drivers, the reliability of the digitizing electronics neutron generator to a manifold of thin-walled improved substantially, averaging channel losses (0.5 mm) aluminum pipes that are mounted im- of less than 0.1%. mediately in front of the crystals. At this loca- tion, the typical rate of photons is 40 Hz/crystal. 9.4. Energy Calibration Figure 66 shows a typical source spectrum that The energy calibration of the EMC proceeds was derived from the raw data by employing a in two steps. First, the measured pulse height in digital filter algorithm. For a 30-minute exposure, each crystal has to be translated to the actual en- a statistical error of 0.35% is obtained, compared 70 Events / 0.047 MeV 9.4.2. Cluster Energy Correction The correction for energy loss due to shower leakage and absorption is performed as a function of cluster energy and polar angle. At low energy 200 (E < 0.8 GeV), it is derived from π 0 decays [90]. The true energy of the photon is expressed as a product of the measured deposited energy and a correction function which depends on ln E and cos θ. The algorithm constrains the two-photon 100 mass to the nominal π 0 mass and iteratively finds the coeﬃcients of the correction function. The typical corrections are of order 6 ± 1%. The un- certainty in the correction is due to systematic uncertainties in the background estimation and the fitting technique. 0 At higher energy (0.8 < E < 9 GeV) the correc- 4 6 8 tion is estimated from single-photon Monte Carlo Energy (MeV) simulations. A second technique using radiative Bhabha events [91] is being developed. The beam Figure 66. A typical pulse-height spectrum energy and the precise track momenta of the e+ recorded with the radioactive source to calibrate and e− , together with the direction of the radia- the single-crystal energy scale of the EMC. The tive photon, are used to fit the photon energy. spectrum shows the primary 6.13 MeV peak and This fitted value is compared to the measured two associated escape peaks at 5.62 MeV and photon energy to extract correction coeﬃcients, 5.11 MeV. The solid line represents a fit to the again as a function of ln E and cos θ. total spectrum, the dotted lines indicate the con- tributions from the three individual photon spec- 9.5. Monitoring tra. 9.5.1. Environmental Monitoring The temperature is monitored by 256 thermal to a systematic uncertainty of less than 0.1%. sensors that are distributed over the calorimeter, This calibration is performed weekly. and has been maintained at 20 ± 0.5◦ C. Dry ni- At high energies, single crystal calibration trogen is circulated throughout the detector to is performed with a pure sample of Bhabha stabilize the relative humidity at 1 ± 0.5%. events [88]. As a function of the polar angle of the e± , the deposited cluster energy is constrained to 9.5.2. Light-Pulser System equal the prediction of a GEANT-based Monte The light response of the individual crystals is Carlo simulation [89]. For a large number of en- measured daily using a light-pulser system [92, ergy clusters, a set of simultaneous linear equa- 93]. Spectrally filtered light from a xenon flash tions relates the measured to the expected energy lamp is transmitted through optical fibers to the and thus permits the determination of a gain con- rear of each crystal. The light pulse is similar in stant for each crystal. In a 12-hour run at a lu- spectrum, rise-time and shape to the scintillation minosity of 3 × 1033 cm−2 s−1 some 200 e± per light in the CsI(Tl) crystals. The pulses are var- crystal can be accumulated, leading to a statisti- ied in intensity by neutral-density filters, allowing cal error of 0.35%. This calibration has been per- a precise measurement of the linearity of light col- formed about once per month, and will be fully lection, conversion to charge, amplification, and automated in the future. digitization. The intensity is monitored pulse-to- pulse by comparison to a reference system with two radioactive sources, 241 Am and 148 Gd, that 71 200 9.5.3. Radiation Monitoring and Damage Forward Barrel The radiation exposure is monitored by 56 Backward Barrel and 60 realtime integrating dosimeters (Rad- 160 FETs) [18] placed in front of the barrel and end- Endcap cap crystals. In Figure 67, the accumulated dose Dose (Rad) 120 is compared to the observed loss in scintillation light, separately for the endcap, the forward, and the backward barrel. The dose appears to follow 80 the integrated luminosity, approximately linearly. The light loss is greatest in the forward region corresponding to the area of highest integrated 40 radiation dose. The size of the observed light loss is close to expectations, based on extensive irra- 0 diation tests. 9.6. Reconstruction Algorithms A typical electromagnetic shower spreads over many adjacent crystals, forming a cluster of en- Gain Change (%) ergy deposits. Pattern recognition algorithms have been developed to eﬃciently identify these –4 clusters and to diﬀerentiate single clusters with one energy maximum from merged clusters with more than one local energy maximum, referred to as a bumps. Furthermore, the algorithms deter- Forward Barrel mine whether a bump is generated by a charged Backward Barrel or a neutral particle. Endcap Clusters are required to contain at least one –8 seed crystal with an energy above 10 MeV. Sur- 0 10 20 rounding crystals are considered as part of the 1-2001 8583A10 Integrated Luminosity (fb-1) cluster if their energy exceeds a threshold of 1 MeV, or if they are contiguous neighbors (in- Figure 67. Impact of beam-generated radiation cluding corners) of a crystal with at least 3 MeV. on the CsI(Tl) crystals: a) the integrated dose The value of the single crystal threshold is set by measured with RadFETs placed in front of the the data acquisition system in order to keep the crystals, b) the degradation in light yield mea- data volume at an acceptable level, given the cur- sured with the radioactive-source calibration sys- rent level of electronics noise and beam-generated tem. background. It is highly desirable to reduce this threshold since fluctuations in the eﬀective energy are attached to a small CsI(Tl) crystal that is loss at the edges of a shower cause a degradation read out by both a photodiode and a photomul- in resolution, particularly at low energies. tiplier tube. The system is stable to 0.15% over Local energy maxima are identified within a a period of one week and has proven to be very cluster by requiring that the candidate crys- valuable in diagnosing problems. For example, tal have an energy, ELocalMax , which exceeds the ability to accurately vary the light intensity the energy of each of its neighbors, and sat- led to the detection of non-linear response in the isfy the following condition: 0.5(N − 2.5) > electronics [92]. ENMax /ELocalMax , where ENMax is the highest en- ergy of any of the neighboring N crystals with an energy above 2 MeV. 72 Clusters are divided into as many bumps as 500 there are local maxima. An iterative algorithm is used to determine the energy of the bumps. 400 Each crystal is given a weight,wi , and the bump Entries/0.007 energy is defined as Ebump = i wi Ei , where the 300 sum runs over all crystals in the cluster. For a cluster with a single bump, the result is wi ≡ 1. For a cluster with multiple bumps, the crystal 200 weight for each bump is calculated as exp(−2.5ri /rM ) 100 wi = Ei , (8) j Ej exp(−2.5rj /rM ) 0 where the index j runs over all crystals in the 0.6 0.8 1.0 1-2001 cluster. rM refers to the Moli`ere radius, and ri 8583A32 EMeasured/EExpected is the distance of the ith crystal from the cen- troid of the bump. At the outset, all weights are Figure 68. The ratio of the EMC measured en- set to one. The process is then iterated, whereby ergy to the expected energy for electrons from the centroid position used in calculating ri is de- Bhabha scattering of 7.5 GeV/c. The solid line termined from the weights of the previous itera- indicates a fit using a logarithmic function. tion, until the bump centroid position is stable to within a tolerance of 1 mm. The position of a bump is calculated us- ing a center-of-gravity method with logarithmic, rather than linear weights [94,95], Wi = 4.0 + π0 → γ γ Bhabhas ln Ei /Ebump , where only crystals with positive 0.06 χ c → J/ψ γ weights, i.e., Ei > 0.0184×Ebump , are used in the MonteCarlo calculation. This procedure emphasizes lower- energy crystals, while utilizing only those crystals 0.04 σE / E that make up the core of the cluster. A system- atic bias of the calculated polar angle originates from the non-projectivity of the crystals. This 0.02 bias is corrected by a simple oﬀset of −2.6 mrad for θ > 90◦ and +2.6 mrad for θ < 90◦ . A bump is associated with a charged particle 0.02 10–1 1.0 10.0 by projecting a track to the inner face of the 3-2001 Photon Energy (GeV) 8583A41 calorimeter. The distance between the track im- pact point and the bump centroid is calculated, Figure 69. The energy resolution for the ECM and if it is consistent with the angle and momen- measured for photons and electrons from various tum of the track, the bump is associated with processes. The solid curve is a fit to Equation 6 this charged particle. Otherwise, it is assumed to and the shaded area denotes the rms error of the originate from a neutral particle. fit. On average, 15.8 clusters are detected per hadronic event, of which 10.2 are not associated energies above 10 MeV (see Section 3). with charged particle tracks. At current oper- ating conditions, beam-induced background con- 9.7. Performance tributes on average 1.4 neutral clusters with en- 9.7.1. Energy Resolution ergies above 20 MeV. This number is significantly At low energy, the energy resolution of the smaller than the average number of crystals with EMC is measured directly with the radioactive 73 Entries / 0.001 GeV 12000 π0 → γ γ 12 MonteCarlo 10000 8000 σθ (mrad) 8 6000 4000 4 2000 0 0 0 1 2 3 0.05 0.1 0.15 0.2 0.25 3-2001 Photon Energy (GeV) mγ γ (GeV) 8583A42 Figure 70. The angular resolution of the EMC Figure 71. Invariant mass of two photons in BB for photons from π 0 decays. The solid curve is a events. The energies of the photons and the π 0 fit to Equation 7. are required to exceed 30 MeV and 300 MeV, re- spectively. The solid line is a fit to the data. source yielding σE /E = 5.0 ± 0.8% at 6.13 MeV (see Figure 66). At high energy, the resolution is photons of approximately equal energy. The re- derived from Bhabha scattering, where the energy sult is presented in Figure 70. The resolution of the detected shower can be predicted from the varies between about 12 mrad at low energies and polar angle of the e± . The measured resolution is 3 mrad at high energies. A fit to an empirical pa- σE /E = 1.9 ± 0.07% at 7.5 GeV (see Figure 68). rameterization of the energy dependence results Figure 69 shows the energy resolution extracted in from a variety of processes as a function of en- ergy. Below 2 GeV, the mass resolution of π 0 and σθ = σφ η mesons decaying into two photons of approx- 3.87 ± 0.07 imately equal energy is used to infer the EMC = ( + 0.00 ± 0.04) mrad. (10) E( GeV) energy resolution [90]. The decay χc1 → J/ψ γ provides a measurement at an average energy of These fitted values are slightly better than would about 500 MeV, and measurements at high energy be expected from detailed Monte Carlo simula- are derived from Bhabha scattering. A fit to the tions. energy dependence results in σE (2.32 ± 0.30)% 9.7.3. π 0 Mass and Width = ⊕ (1.85 ± 0.12)%. (9) E 4 E( GeV) Figure 71 shows the two-photon invariant mass Values of these fitted parameters are higher than in BB events. The reconstructed π 0 mass is mea- the somewhat optimistic design expectations, but sured to be 135.1 MeV/c2 and is stable to better they agree with detailed Monte Carlo simulations than 1% over the full photon energy range. The which include the contributions from electronic width of 6.9 MeV/c2 agrees well with the predic- noise and beam background, as well as the impact tion obtained from detailed Monte-Carlo simula- of the material and the energy thresholds. tions. In low-occupancy τ + τ − events, the width is slightly smaller, 6.5 MeV/c2 , for π 0 energies be- 9.7.2. Angular Resolution low 1 GeV. A similar improvement is also ob- The measurement of the angular resolution is served in analyses using selected isolated photons based on the analysis of π 0 and η decays to two in hadronic events. 74 9.7.4. Electron Identification 1.0 0.010 Electrons are separated from charged hadrons a) primarily on the basis of the shower energy, lat- 0.8 0.008 eral shower moments, and track momentum. In addition, the dE/dx energy loss in the DCH and Efficiency e± the DIRC Cherenkov angle are required to be 0.6 0.006 consistent with an electron. The most impor- π± tant variable for the discrimination of hadrons is the ratio of the shower energy to the track 0.4 0.004 momentum (E/p). Figure 72 shows the eﬃ- ciency for electron identification and the pion mis- 0.2 0.002 identification probability as a function of momen- tum for two sets of selection criteria. The elec- tron eﬃciency is measured using radiative Bhab- 0.0 0.000 has and e+ e− → e+ e− e+ e− events. The pion 0 1 2 misidentification probability is measured for se- Momentum (GeV/c) lected charged pions from KS0 decays and three- 1.0 0.010 prong τ decays. A tight (very tight) selector re- sults in an eﬃciency plateau at 94.8% (88.1%) b) 0.8 0.008 in the momentum range 0.5 < p < 2 GeV/c. The pion misidentification probability is of order 0.3% e± Efficiency (0.15%) for the tight (very tight) selection crite- 0.6 0.006 ria. π± 9.8. Summary 0.4 0.004 The EMC is presently performing close to de- sign expectations. Improvements in the energy 0.2 0.002 resolution are expected from the optimization of the feature-extraction algorithms designed to fur- ther reduce the electronics noise. Modifications to 0.0 0.000 the electronics should allow for more precise cal- 0.0 40 80 120 ibrations. The expected noise reduction should 3-2001 8583A43 Polar Angle (degrees) permit a lower single-crystal readout threshold. However, this decrease in noise might be oﬀset Figure 72. The electron eﬃciency and pion mis- by an increase in the beam background that is identification probability as a function of a) the expected for higher luminosities and beam cur- particle momentum and b) the polar angle, mea- rents. sured in the laboratory system. leptonic decays, for the reconstruction of vector 10. Detector for Muons and Neutral mesons, like the J/ψ , and for the study of semi- Hadrons leptonic and rare decays involving leptons of B 10.1. Physics Requirements and Goals and D mesons and τ leptons. KL0 detection al- The Instrumented Flux Return (IFR) was de- lows the study of exclusive B decays, in particular signed to identify muons with high eﬃciency and CP eigenstates. The IFR can also help in vetoing good purity, and to detect neutral hadrons (pri- charm decays and improve the reconstruction of marily KL0 and neutrons) over a wide range of neutrinos. momenta and angles. Muons are important for The principal requirements for IFR are large tagging the flavor of neutral B mesons via semi- solid angle coverage, good eﬃciency, and high 75 background rejection for muons down to mo- Aluminum X Strips H.V. menta below 1 GeV/c. For neutral hadrons, high Foam Insulator eﬃciency and good angular resolution are most important. Because this system is very large and Graphite diﬃcult to access, high reliability and extensive Bakelite 2 mm Gas 2 mm monitoring of the detector performance and the Bakelite 2 mm associated electronics plus the voltage distribu- Graphite tion are required. Insulator Foam Y Strips Spacers 10.2. Overview and RPC Concept Aluminum The IFR uses the steel flux return of the mag- 8-2000 8564A4 net as a muon filter and hadron absorber. Sin- gle gap resistive plate chambers (RPCs) [96] with two-coordinate readout have been chosen as de- Figure 74. Cross section of a planar RPC with the tectors. schematics of the high voltage (HV) connection. The RPCs are installed in the gaps of the finely segmented steel (see Section 4) of the barrel and edge by a 7 mm wide frame. The gap width is the end doors of the flux return, as illustrated in kept uniform by polycarbonate spacers (0.8 cm2 ) Figure 73. The steel segmentation has been cho- that are glued to the bakelite, spaced at dis- sen on the basis of Monte Carlo studies of muon tances of about 10 cm. The bulk resistivity of penetration and charged and neutral hadron in- the bakelite sheets has been especially tuned to teractions. The steel is segmented into 18 plates, 1011 –1012 Ω cm. The external surfaces are coated increasing in thickness from 2 cm for the inner with graphite to achieve a surface resistivity of ∼ nine plates to 10 cm for the outermost plates. The 100 kΩ/square. These two graphite surfaces are nominal gap between the steel plates is 3.5 cm connected to high voltage (∼ 8 kV) and ground, in the inner layers of the barrel and 3.2 cm else- and protected by an insulating mylar film. The where. There are 19 RPC layers in the barrel bakelite surfaces facing the gap are treated with and 18 in the endcaps. In addition, two layers of linseed oil. The RPCs are operated in limited cylindrical RPCs are installed between the EMC streamer mode and the signals are read out ca- and the magnet cryostat to detect particles exit- pacitively, on both sides of the gap, by external ing the EMC. electrodes made of aluminum strips on a mylar RPCs detect streamers from ionizing particles substrate. via capacitive readout strips. They oﬀer several The cylindrical RPCs have resistive electrodes advantages: simple, low cost construction and the made of a special plastic composed of a conduct- possibility of covering odd shapes with minimal ing polymer and ABS plastic. The gap thickness dead space. Further benefits are large signals and and the spacers are identical to the planar RPCs. fast response allowing for simple and robust front- No linseed oil or any other surface treatments end electronics and good time resolution, typi- have been applied. The very thin and flexible cally 1–2 ns. The position resolution depends on electrodes are laminated to fiberglass boards and the segmentation of the readout; a value of a few foam to form a rigid structure. The copper read- mm is achievable. out strips are attached to the fiberglass boards. The construction of the planar and cylindrical RPCs diﬀer in detail, but they are based on the 10.3. RPC Design and Construction same concept. A cross section of an RPC is shown The IFR detectors cover a total active area of schematically in Figure 74. about 2,000 m2 . There are a total of 806 RPC The planar RPCs consist of two bakelite (phe- modules, 57 in each of the six barrel sectors, 108 nolic polymer) sheets, 2 mm-thick and separated in each of the four half end doors, and 32 in the by a gap of 2 mm. The gap is enclosed at the two cylindrical layers. The size and the shape of 76 Figure 73. Overview of the IFR: Barrel sectors and forward (FW) and backward (BW) end doors; the shape of the RPC modules and their dimensions are indicated. the modules are matched to the steel dimensions cal readout strips. with very little dead space. More than 25 diﬀer- The readout strips are separated from the ent shapes and sizes were built. Because the size ground aluminum plane by a 4 mm-thick foam of a module is limited by the maximum size of sheet and form strip lines of 33 Ω impedance. The the material available, i.e., 320×130 cm2 for the strips are connected to the readout electronics at bakelite sheets, two or three RPC modules are one end and terminated with a 2 kΩ resistor at joined to form a gap-size chamber. The modules the other. Even and odd numbered strips are of each chamber are connected to the gas system connected to diﬀerent front-end cards (FECs), so in series, while the high voltage is supplied sepa- that a failure of a card does not result in a to- rately to each module. tal loss of signal, since a particle crossing the gap In the barrel sectors, the gaps between the steel typically generates signals in two or more adja- plates extend 375 cm in the z direction and vary cent strips. in width from 180 cm to 320 cm. Three modules The cylindrical RPC is divided into four sec- are needed to cover the whole area of the gap, as tions, each covering a quarter of the circumfer- shown in Figure 73. Each barrel module has 32 ence. Each of these sections has four sets of two strips running perpendicular to the beam axis to single gap RPCs with orthogonal readout strips, measure the z coordinate and 96 strips in the or- the inner with helical u–v strips that run paral- thogonal direction extending over three modules lel to the diagonals of the module, and the outer to measure φ. with strips parallel to φ and z. Within each sec- Each of the four half end doors is divided into tion, the strips of the four sets of RPCs in a given three sections by steel spacers that are needed readout plane are connected to form long strips for mechanical strength. Each of these sections is extending over the whole chamber. Details of the covered by two RPC modules that are joined to segmentation and dimensions can be found in Ta- form a larger chamber with horizontal and verti- ble 13. 77 Table 13 IFR Readout segmentation. The total number of channels is close to 53,000. # of # of readout # strips strip length strip width total # section sectors coordinate layers layer/sect (cm) (mm) channels barrel 6 φ 19 96 350 19.7-32.8 ≈ 11, 000 z 19 96 190-318 38.5 ≈ 11, 000 endcap 4 y 18 6x32 124-262 28.3 13,824 x 18 3x64 10-180 38.0 ≈ 15, 000 cylinder 4 φ 1 128 370 16.0 512 z 1 128 211 29.0 512 u 1 128 10-422 29.0 512 v 1 128 10-423 29.0 512 Prior to shipment to SLAC, all RPC modules custom-built switching devices with load and line were tested with cosmic rays. The single rates, regulation of better than 1%. Additional features dark currents, and eﬃciency were measured as a are precision shunts to measure output currents function of HV. In addition, detailed studies of and TTL logic to inhibit output. the eﬃciency, spatial resolution, and strip multi- The HV power system is a custom adaptation plicity were performed [97,98]. by CAEN [100]. Each HV mainframe can hold After the assembly of RPC modules into gap- up to ten cards, each carrying two independent size chambers, a new series of cosmic rays tests 10 kV outputs at 1 mA and 2 mA. The RPC mod- was performed to assure stable and eﬃcient oper- ules are connected via a distribution box to the ation. Before the installation of the steel flux re- HV supplies. Each distribution box services six turn, the planar chambers were inserted into the RPC modules and up to six distribution boxes are gaps. The cylindrical chambers were inserted af- daisy-chained to one HV output. Provisions are ter the installation of the solenoid and the EMC. made for monitoring the currents drawn by each For each module, test results and conditions module. To reduce noise, the RPC ground plane are retained in a database, together with records is decoupled from the HV power supply ground of the critical parameters of the components, the by a 100 kΩ resistor. assembly and cabling. In addition, operational The RPCs operate with a non-flammable gas data are stored, such as the results of the weekly mixture, typically 56.7% Argon, 38.8% Freon eﬃciency measurements that are used in the re- 134a (1,1,1,2 tetrafluoroethane), and 4.5% isobu- construction and simulation software. tane. This mixture is drawn from a 760 liter tank that is maintained at an absolute pressure of 10.4. Power and Utilities 1500–1600 Torr. The mixing tank is filled on de- Once the return flux assembly was completed, mand with the three component gases under con- the FECs [99] were installed and the low (LV) trol of mass-flow meters. Samples are extracted and high voltage (HV), and the gas system were from the mixing tank periodically and analyzed connected. There are approximately 3,300 FECs, to verify the correct mixture. most placed inside the steel gaps, while the re- The mixed gas is distributed at a gauge pres- mainder was installed in custom crates mounted sure of approximately 6.5 Torr through a parallel on the outside of the steel. manifold system of 12.7 mm-diameter copper tub- Each FEC is individually connected to the LV ing. Each chamber is connected to the manifold power distribution. The total power required by through several meters of 6 mm-diameter plastic the entire system is about 8 kW at +7.0 V and tubing (polyamide or Teflon). The flow to each of 2.5 kW at -5.2 V. The LV power is supplied by 78 Inside the IFR Iron Outside the Detector Counting Signals from 3,300 FECs are transmitted to or in Minicrates House (1 - 4 m) (1 - 8 m) eight custom IFR front-end crates that are lo- FEC FIFO Board cated near the detector. Each front-end crate Data ITB ITB IFB Calibration houses up to 16 data handling cards, four trigger Clock ITB ITB ICB Board cards and a crate controller card (ICC) that col- ITB ITB TDC Readout Module lects data from the DAQ cards and forwards them Data ITB ICC to a ROM. There are three kinds of data cards: ROM the FIFO boards (IFBs) that buﬀer strip hits, the PDB TDC boards (ITBs) that provide time informa- IFR Crate tion, and the calibration boards (ICBs) that in- Controller ject test pulses into the FECs. To deliver the data Front End Cards Front End Crate DAQ and clock signals to all the boards in the front-end Discrimination and Fast Buffering Zero Suppression crate, a custom backplane (PDB) for the standard Noise Reduction on Trigger Basis and Encoding 8-2000 6U Eurocard crate was designed using 9-layer 8564A3 strip line technology. Each board is connected to the ICC via three point-to-point lines for three Figure 75. Block diagram of the IFR electronics. single-end signals (data-in, data-out and clock), all of the same length and impedance (50 Ω). The IFB reads the digital hit patterns from the these is adjusted individually with a small multi- FECs in less than 2.2 ms, stores the data into FI- turn metering valve. Protection against overpres- FOs and transfers the FIFO contents into one of sure is provided by an oil bubbler to atmosphere the ROMs. Each IFB handles 64 FECs, acting as in parallel with each chamber, limiting the gauge an acquisition master. It receives commands via pressure in the chamber to a maximum of about the PDB, and transmits and receives data pat- 1 Torr. Return flow of gas from each chamber terns from the ROM (via G-Link and ICC). This is monitored by a second oil bubbler which cre- card operates with the system clock frequency of ates a back pressure of about 0.2 Torr. The total 59.5 MHz. flow through the entire system is approximately The ICB is used for front-end tests and cali- 5 ℓ/minute and corresponds on average to two gas brations. A signal with programmable amplitude exchanges per day. and width is injected into the FEC input stage. To provide timing calibration and to determine 10.5. Electronics the correct readout delay, the board is also used A block diagram of the IFR electronics system together with the TDCs. [101] is shown in Figure 75. It includes the FECs, The ICC interfaces the crate backplane with the data acquisition, and the trigger. the G-Link. The physical interface is the Finisar The FECs service 16 channels each. They transceiver, a low cost and highly reliable data shape and discriminate the input signals and set link for applications up to 1.5 Gbytes/s. a bit for each strip with a signal above a fixed The TDC boards exploit the excellent time res- threshold. The input stage operates continuously olution of the RPCs. Each board has 96 ECL and is connected directly to the strips which act diﬀerential input channels for the fast OR sig- as transmission lines. A fast OR of all FEC in- nals from the FECs. Time digitization is achieved put signals provides time information and is also by three custom TDCs, designed at CERN [102]. used for diagnostic purposes. Two types of FECs Upon receipt of a L1 Accept, data are selected and are employed to handle inputs of diﬀerent polar- stored until readout by the ROM. The 59.5 MHz ity for signals from the opposite sides of the gap. clock signal is synchronized with the data and Because of the very low occupancy there is no distributed to the 16 boards. High performance provision for buﬀering during the trigger latency drivers provide a reliable clock distribution with [99]. a jitter of less than 0.5 ns. 79 10.6. Slow Controls 400 and Online Monitoring # of Modules The IFR is a system with a large number of components and electronics distributed all over 200 the BABAR detector. To assure safe and stable operation, an extensive monitoring and control system was installed. The IFR Online Detector Control (IODC) monitors the performance of the 0 RPCs by measuring the singles counting rate and 0.0 0.5 1.0 1-2001 the dark current of every module. It also controls 8583A4 Efficiency and monitors the operation of the electronics, the Figure 76. Distribution of the eﬃciency for all DAQ and trigger, as well as the LV, the HV, and RPC modules measured with cosmic rays in June the gas system. The total number of hardware 1999. Some 50 modules were not operational at channels is close to 2,500 [103]. that time. The system has been easy to operate. HV trips are rare. Temperature monitoring in the steel one-dimensional clusters (of the same readout structure and the electronics crates has proven coordinate) in diﬀerent layers. In each sector, very useful for the diagnosis of operational prob- two-dimensional clusters in diﬀerent coordinates lems. The occupancy is extremely low every- are combined into three-dimensional clusters pro- where, except in layer 18 of the forward end door vided there are fewer than three layers missing in which lacks adequate shielding from machine- one of the two coordinates. The second algorithm generated background. On average, there are extrapolates charged tracks reconstructed by the about 100–150 strip signals per event. DCH. IFR clusters which are less than 12 cm from the extrapolated track are combined to form 10.7. Eﬃciency Measurements three-dimensional or two-dimensional clusters. A and Performance detailed discussion of the clustering algorithm can The eﬃciency of the RPCs is evaluated both for be found elsewhere [104]. normal collision data and for cosmic ray muons The residual distributions from straight line fits recorded with the IFR trigger. Every week, cos- to two-dimensional clusters typically have an rms mic ray data are recorded at diﬀerent voltage set- width of less than 1 cm. An RPC is considered tings and the eﬃciency is measured chamber-by- eﬃcient if a signal is detected at a distance of less chamber as a function of the applied voltage. The than 10 cm from the fitted straight line in either absolute eﬃciency at the nominal working voltage of the two readout planes. Following the instal- (typically 7.6 kV) is stored in the database for use lation and commissioning of the IFR system, all in the event reconstruction software. RPC modules were tested with cosmic rays and To calculate the eﬃciency in a given chamber, their eﬃciency was measured. The results are nearby hits in a given layer and hits in diﬀerent presented in Figure 76. Of the active RPC mod- layers are combined to form clusters. Two diﬀer- ules, 75% exceed an eﬃciency of 90%. ent algorithms are used. The first is based solely Early tests indicated that the RPC dark cur- on the IFR information and uses data recorded rent was very temperature dependent, specifi- with a dedicated IFT trigger; the second matches cally, the current increases 14–20% per ◦ C. Be- the IFR clusters with the tracks reconstructed in cause the IR experimental hall does not have tem- the DCH. Both these algorithms start from one- perature regulation this presents a serious prob- dimensional IFR clusters defined as a group of ad- lem. The FECs that are installed in the steel jacent hits in one of the two readout coordinates. gaps dissipate 3 W each, generating a total power The cluster position is defined as the centroid of of 3.3 kW in the barrel and 1.3 kW in the forward the strips in the cluster. In the first algorithm, end door. two-dimensional clusters are formed by joining 80 32 1.0 Hall Current ( µA ) Temp. ( °C ) 0.5 24 a) 0 a) 1.0 16 IFR 0.5 10000 b) 0 1.0 b) 2000 0.5 8-2000 0 50 100 150 200 c) 8564A1 Day of Year 2000 0 Jun Aug Oct Dec Feb Apr Jun Figure 77. History of the temperature and dark 12-2000 current in the RPC modules since January 2000. 8577A8 a) temperature in the IR-2 hall and in the back- Figure 78. Eﬃciency history for 12 months start- ward end door; b) total dark current in the 216 ing in June 1999 for RPC modules showing dif- modules of the backward end door. ferent performance: a) highly eﬃcient and stable; b) continuous slow decrease in eﬃciency; c) more During the first summer of operation, the daily recent, faster decrease in eﬃciency. average temperature in the IR hall was 28◦ C and the maximum hall temperature frequently very low eﬃciency in these modules, but no clear exceeded 31◦ C. The temperature inside the steel pattern was identified. rose to more than 37◦ C and the dark currents The cause of the eﬃciency loss remains under in many modules exceeded the capabilities of the investigation. Several possible causes have been HV system and some RPCs had to be temporarily excluded as the primary source of the problem, disconnected. such as a change in the bakelite bulk resistivity, To overcome this problem, water cooling was loosened spacers, gas flow, or gas composition. installed on the barrel and end door steel, remov- A number of prototype RPCs developed similar ing ≈10 kW of heat and stabilizing the tempera- eﬃciency problems after being operated above a ture at 20–21◦ C in the barrel, 22◦ C in the back- temperature 36◦ C for a period of two weeks. In ward and 24◦ C in the forward end doors. Fig- some of these modules, evidence was found that ure 77 shows the history of temperature in the the linseed oil had failed to cure and had accumu- hall and temperature and total dark current in lated at various spots under the influence of the the backward end door. While the current closely electric field. follows the temperature variations, the range of change is now limited to a few degrees. 10.8. Muon Identification During operation at high temperatures, a large While muon identification relies almost entirely fraction of the RPCs (>50%) showed not only on the IFR, other detector systems provide com- very high dark currents, but also some reduc- plementary information. Charged particles are tion in eﬃciency compared to earlier measure- reconstructed in the SVT and DCH and muon ments [105]. After the cooling was installed and candidates are required to meet the criteria for the RPCs were reconnected, some of them contin- minimum ionizing particles in the EMC. Charged ued to deteriorate while others remained stable, tracks that are reconstructed in the tracking sys- some of them (> 30%) at full eﬃciency. (see Fig- tems are extrapolated to the IFR taking into ac- ure 78). Detailed studies revealed large regions of count the non-uniform magnetic field, multiple 81 scattering, and the average energy loss. The pro- 1.0 0.5 jected intersections with the RPC planes are com- a) puted and for each readout plane all clusters de- 0.8 0.4 tected within a predefined distance from the pre- dicted intersection are associated with the track. Efficiency A number of variables are defined for each IFR 0.6 0.3 cluster associated with a charged track to discrim- inate muons from charged hadrons: 1) the to- 0.4 0.2 tal number of interaction lengths traversed from the IP to the last RPC layer with an associated 0.2 0.1 cluster, 2) the diﬀerence between this measured number of interaction lengths and the number of interaction lengths predicted for a muon of the 0.0 0.0 same momentum and angle, 3) the average num- 1 2 3 0 Momentum (GeV/c) ber and the rms of the distribution of RPC strips per layer, 4) the χ2 for the geometric match be- 1.0 0.5 tween the projected track and the centroids of b) clusters in diﬀerent RPC layers, and 5) the χ2 of 0.8 0.4 a polynomial fit to the two-dimensional IFR clus- ters. Selection criteria based on these variables Efficiency 0.6 0.3 are applied to identify muons. The performance of muon identification has been tested on samples of muons from µµee and 0.4 0.2 µµγ final states and pions from three-prong τ de- cays and KS → π + π − decays. The selection of 0.2 0.1 these control samples is based on kinematic vari- ables, and not on variables used for muon identifi- cation. As illustrated in Figure 79, a muon detec- 0.0 0.0 tion eﬃciency of close to 90% has been achieved 0 40 80 120 160 3-2001 in the momentum range of 1.5 < p < 3.0 GeV/c 8583A44 Polar Angle (degrees) with a fake rate for pions of about 6–8%. Decays Figure 79. Muon eﬃciency (left scale) and pion in flight contribute about 2% to the pion misiden- misidentification probability (right scale) as a tification probability. The hadron misidentifica- function of a) the laboratory track momentum, tion can be reduced by a factor of about two by and b) the polar angle (for 1.5 < p < 3.0 GeV/c tighter selection criteria which lower the muon momentum), obtained with loose selection crite- detection eﬃciency to about 80%. ria. 10.9. KL0 and Neutral Hadron Detection KL0 ’s and other neutral hadrons interact in the showers that spread into adjacent sectors of the steel of the IFR and can be identified as clus- barrel, several sections of the end doors and/or ters that are not associated with a charged track. the cylindrical RPC. This procedure also com- Monte Carlo simulations predict that about 64% bines multiple clusters from large fluctuations in of KL ’s above a momentum of 1 GeV/c produce a the hadronic showers. The direction of the neu- cluster in the cylindrical RPC, or a cluster with tral hadron is determined from the event vertex hits in two or more planar RPC layers. and the centroid of the neutral cluster. No infor- Unassociated clusters that have an angular sep- mation on the energy of the cluster can be ob- aration of ≤ 0.3 rad are combined into a com- tained. posite cluster, joining clusters that originate from 82 MC 160 Data 80 Background Neutral Clusters 120 Events 40 80 40 0 0.96 0.98 1.00 3-2001 0 8583A45 cos ∆ξ -100 0 100 Figure 80. Angular diﬀerence, cos ∆ξ, between 1-2001 8583A6 ∆φ (Degrees) the direction of the missing momentum and the closest neutral IFR cluster for a sample of φ Figure 81. Diﬀerence between the direction of mesons produced in the reaction e+ e− → φγ with the reconstructed neutral hadron cluster and the φ → KL0 KS0 . missing transverse momentum in events with a reconstructed J/ψ decay. The Monte Carlo simu- Since a significant fraction of hadrons interact lation is normalized to the luminosity of the data; before reaching the IFR, information from the the background is obtained using neutral hadrons EMC and the cylindrical RPCs is combined with and the missing momentum from diﬀerent events. the IFR cluster information. Neutral showers in the EMC are associated with the neutral hadrons momentum range from 1 GeV/c to 4 GeV/c (EMC detected in the IFR, based on a match in produc- and IFR combined). tion angles. For a good match, a χ2 probability of ≥ 1% is required. 10.10. Summary and Outlook An estimate of the angular resolution of the The IFR is the largest RPC system built to neutral hadron cluster can be derived from a sam- date. It provides eﬃcient muon identification and ple of KL0 ’s produced in the reaction e+ e− → allows for the detection of KL0 ’s interacting in the φγ → KL0 KS0 γ. The KL0 direction is inferred from steel and the calorimeter. During the first year of the missing momentum computed from the mea- operation, a large fraction of the RPC modules sured particles in the final state. The data in have suﬀered significant losses in eﬃciency. This Figure 80 indicate that the angular resolution of eﬀect appears to be correlated with high temper- the KL0 derived from the IFR cluster information atures, but the full extent of the problem and its is of the order of 60 mrad. For KL0 ’s interacting cause remain under study. Thanks to the large in the EMC, the resolution is better by about a number of RPC layers, this problem has not yet factor of two. impacted the overall performance severely. But For multi-hadron events with a reconstructed present extrapolations, even after installation of J/ψ decay, Figure 81 shows the angular diﬀer- water cooling on the steel, indicate a severe prob- ence, ∆φ, between the missing momentum and lem for the future operation. Recently, 24 end the direction of the nearest neutral hadron clus- door modules have been replaced by new RPCs ter. The observed peak demonstrates clearly that with a substantially thinner coating of linseed oil the missing momentum can be associated with a and improved treatment of the bakelite surfaces. neutral hadron, assumed to be a KL0 . The KL0 de- Results with these new RPCs and other tests will tection eﬃciency increases roughly linearly with need to be evaluated before decisions on future momentum; it varies between 20% and 40% in the improvements of the IFR can be made. Further- 83 more, it is planned to reduce the contamination Fast Control and Timing System (FCTS). Data from hadron decays and punch through by in- used to form the trigger decision are preserved creasing the absorber thickness, i.e., adding more with each event for eﬃciency studies. steel on the outside and replacing a few of the The L3 receives the output from L1, performs a RPCs with absorber plates. second stage rate reduction for the main physics sources, and identifies and flags the special cat- egories of events needed for luminosity determi- 11. Trigger nation, diagnostic, and calibration purposes. At 11.1. Trigger Requirements design luminosity, the L3 filter acceptance for The basic requirement for the trigger system is physics is ∼90 Hz, while ∼30 Hz contain the other the selection of events of interest (see Table 14) special event categories. The L3 algorithms com- with a high, stable, and well-understood eﬃciency ply with the same software conventions and stan- while rejecting background events and keeping dards used in all other BABAR software, thereby the total event rate under 120 Hz. At design lumi- simplifying its design, testing, and maintenance. nosity, beam-induced background rates are typi- cally about 20 kHz each for one or more tracks 11.3. Level 1 Trigger System in the drift chamber with pt > 120 MeV/c or at The L1 trigger decision is based on charged least one EMC cluster with E > 100 MeV. Ef- tracks in the DCH above a preset transverse mo- ficiency, diagnostic, and background studies re- mentum, showers in the EMC, and tracks de- quire prescaled samples of special event types, tected in the IFR. Trigger data are processed such as those failing the trigger selection criteria, by three specialized hardware processors. As de- and random beam crossings. scribed below, the drift chamber trigger (DCT) The total trigger eﬃciency is required to exceed and electromagnetic calorimeter trigger (EMT) 99% for all BB events and at least 95% for contin- both satisfy all trigger requirements indepen- uum events. Less stringent requirements apply to dently with high eﬃciency, and thereby provide other event types, e.g., τ + τ − events should have a high degree of redundancy, which enables the a 90-95% trigger eﬃciency, depending on the spe- measurement of trigger eﬃciency. The instru- cific τ ± decay channels. mented flux return trigger (IFT) is used for trig- The trigger system must be robust and flexible gering µ+ µ− and cosmic rays, mostly for diagnos- in order to function even under extreme back- tic purposes. ground situations. It must also be able to operate The overall structure of the L1 trigger is illus- in an environment with dead or noisy electronics channels. The trigger should contribute no more Table 14 than 1% to dead time. Cross sections, production and trigger rates for the principal physics processes at 10.58 GeV for a 11.2. Trigger Overview luminosity of 3 × 1033 cm−2 s−1 . The e+ e− cross The trigger is implemented as a two-level hier- section refers to events with either the e+ , e− , or archy, the Level 1 (L1) in hardware followed by both inside the EMC detection volume. the Level 3 (L3) in software. It is designed to ac- commodate up to ten times the initially projected Cross Production Level 1 [3] PEP-II background rates at design luminos- Event section Rate Trigger ity and to degrade slowly for backgrounds above type (nb) (Hz) Rate (Hz) that level. Redundancy is built into the system to measure and monitor trigger eﬃciencies. bb 1.1 3.2 3.2 During normal operation, the L1 is configured other qq 3.4 10.2 10.1 to have an output rate of typically 1 kHz. Triggers e+ e− ∼53 159 156 are produced within a fixed latency window of 11– µ+ µ− 1.2 3.5 3.1 τ +τ − 0.9 2.8 2.4 12 µs after the e+ e− collision, and delivered to the 84 3 bits/134 ns basis. IFR FEE IFR Muon Trigger Global Level 1 Synch Board (IFS) Trigger (GLT) The L1 hardware is housed in five 9U VME FastOR [1] 90 bits/ 134 ns [1] crates. The L1 trigger operates in a continuous sampling mode, generating trigger information at Calorimeter Trigger 24 bits/67 ns regular, fixed time intervals. The DCH front-end EMC ROM Processor Board (TPB) [10] electronics (FEEs) and the EMC untriggered per- sonality cards (UPCs) send raw data to the DCT 32 bits/134 ns 16 bits/134 ns and EMT about 2 µs after the e+ e− collision. DCH FEE Drift Chamber Track The DCT and EMT event processing times are Segment Finder (TSF) Fast Control 4–5 µs, followed by another ∼3 µs of processing [24] in the GLT to issue a L1 trigger. The L1 trigger takes approximately 1 µs to propagate through 320 bits/134 ns the FCTS and the readout modules (ROMs) to Drift Chamber Binary initiate event readout. These steps are all ac- Link Tracker (BLT) [1] complished within the 12.8 µs FEE buﬀer capac- 176x16 bits/269 ns ity limit. Drift Chamber PT The DCT, EMT and GLT each maintain a four- Discriminator (PTD) event buﬀer to hold information resulting from [8] the various stages of the L1 trigger. These data are read out by the normal data acquisition sys- Figure 82. Simplified L1 trigger schematic. Indi- tem. cated on the figure are the number of components (in square brackets), and the transmission rates between components in terms of total signal bits. 11.3.1. Level 1 Drift Chamber Trigger trated in Figure 82. Each of the three L1 trigger The input data to the DCT consist of one bit processors generates trigger primitives, summary for each of the 7104 DCH cells. These bits convey data on the position and energy of particles, that time information derived from the sense wire sig- are sent to the global trigger (GLT) every 134 ns. nal for that cell. The DCT output primitives are The DCT and EMT primitives sent to the GLT candidate tracks encoded in terms of three 16-bit are φ-maps. An individual φ-map consists of an φ-maps as listed in Table 15. n-bit word representing a particular pattern of The DCT algorithms are executed in three trigger objects as distributed in fixed-width φ re- types of modules [106]. First, track segments, gions from 0 to 2π. A trigger object is a quantity their φ positions and drift time estimates are indicating the presence of a particle, such as a found using a set of 24 Track Segment Finder drift chamber track or a calorimeter energy de- (TSF) modules [107]. These data are then posit. The IFT primitive is a three-bit pattern passed to the Binary Link Tracker (BLT) mod- representing the hit topology in the IFR. The ule [108], where segments are linked into complete meaning of the various trigger primitive inputs tracks. In parallel, the φ information for segments to the GLT are summarized in Table 15. found in axial superlayers is transmitted to eight The GLT processes all trigger primitives to transverse momentum discriminator (PTD) mod- form specific triggers and then delivers them to ules [109], which search for tracks above a set pt the FCTS. The FCTS can optionally mask or threshold. prescale any of these triggers. If a valid trigger Each of the three DCT modules (TSF, BLT, remains, a L1 Accept is issued to initiate event and PTD) relies heavily on multiple FPGA’s [110] readout. The trigger definition logic, masks, and which perform the control and algorithmic func- prescale values are all configurable on a per run tions. All cabling is handled by a small (6U) back- of-crate interface behind each main board. 85 Table 15 Trigger primitives for the DCT and EMT. Most energy thresholds are adjustable; those listed are typical values. Description Origin No. of bits Threshold B Short track reaching DCH superlayer 5 BLT 16 120 MeV/c A Long track reaching DCH superlayer 10 BLT 16 180 MeV/c A′ High pt track PTD 16 800 MeV/c M All-θ MIP energy TPB 20 100 MeV G All-θ intermediate energy TPB 20 250 MeV E All-θ high energy TPB 20 700 MeV X Forward endcap MIP TPB 20 100 MeV Y Backward barrel high energy TPB 10 1 GeV Track Segment Finder Track The TSF modules are responsible for find- 6 2 ing track segments in 1776 overlapping eight-cell pivot groups. A pivot group is a contiguous set Super 4 Pivot cell layer of cells that span all four layers within a super- layer 5 1 layer. The pivot group shape is such that only reasonably straight tracks originating from the 7 3 0 interaction point can produce a valid segment. Figure 83 shows the arrangement of cells within 8 Cell Template a pivot group. Cell 4 is called the pivot cell ; the TSF algorithm is optimized to find track seg- ments that pivot about this cell. Figure 83. Track Segment Finder pivot group. The DCH signals are sampled every 269 ns. The passage of a single particle through the DCH cell occupancies, forms the basis of data sent to will produce ionization that drifts to the sense the BLT and PTD. The TSF algorithm uses the wires in typically no more than four of these time-variation of the look-up-table weights to re- clock ticks. Each cell is associated with a two-bit fine both the event time and its uncertainty, thus counter that is incremented at every clock tick for enabling it to output results to the BLT every which a signal is present. In this way, a short time 134 ns. history of each cell is preserved. For each clock The position resolution as measured from the tick, the collection of two-bit counters for each data after calibration, is typically ∼600 µm for a pivot group forms a 16-bit value used to address four-layer segment and ∼900 µm for a three-layer a look-up-table. This look-up-table contains two- segment. For tracks originating from the IP, the bit weights indicating whether there is no accept- eﬃciency for finding TSF segments is 97%, and able segment, a low-quality segment, a three-layer the eﬃciency for high-quality three-layer or four- segment (allowing for cell ineﬃciencies), or a four- layer TSF segments is 94%. layer segment. When an acceptable segment is found, that pivot group is examined to determine Binary Link Tracker which of three subsequent clock ticks produce the The BLT receives segment hit information from highest weight or best pattern. all 24 TSF’s at a rate of 320 bits every 134 ns The look-up-table also contains position and and links them into complete tracks. The seg- time information which, along with a summary of ment hits are mapped onto the DCH geometry in 86 Efficiency terms of 320 supercells, 32 sectors in φ and ten radial superlayers. Each bit indicates whether a 1 segment is found in that supercell or not. The BLT input data are combined using a logical OR with a programmable mask pattern. The mask- , 0.75 B A A ing allows the system to activate track segments DCT corresponding to dead or highly ineﬃcient cells to prevent eﬃciency degradation. The linking al- 0.5 gorithm uses an extension of a method developed for the CLEO-II trigger [111]. It starts from the innermost superlayer, A1, and moves radially out- 0.25 ward. Tracks that reach the outer layer of the DCH (superlayer A10) are classified as type A. Tracks that reach the middle layer (superlayer U5) are 0 0 0.25 0.5 0.75 1 classified as type B. An A track is found if there is a segment in at least eight superlayers and if Track Transverse Momentum (GeV/c) the segments in two consecutive superlayers fall Figure 84. DCT track eﬃciency versus trans- azimuthally within three to five supercells of each verse momentum for A, B, and A′ tracks. The other (depending on the superlayer type). This A′ threshold is set to 800 MeV/c. allows for track curvature and dip angle varia- tions. The data are compressed and output to and consequently define the eﬀective pt discrim- the GLT in the form of two 16-bit φ-maps, one ination threshold. The resulting pt threshold for each for A and B tracks. the PTD A′ tracks is shown in Figure 84 together PT Discriminator with the BLT A, B track eﬃciency. The eight PTD modules receive φ information 11.3.2. Level 1 Calorimeter Trigger of high quality track segments in the axial super- For trigger purposes, the EMC is divided into layers (A1, A4, A7 and A10), and determine if the 280 towers, 7 × 40 (θ × φ). Each of the barrel’s segments are consistent with a track pt greater 240 towers is composed of 24 crystals in a 8 × than a configurable minimum value. An envelope 3 (θ × φ) array. The endcap is divided into 40 for tracks above the minimum pt is defined using towers, each forming a wedge in φ containing 19– the IP, and a track segment position in one of the 22 crystals. For each tower, all crystal energies seed superlayers, A7 or A10. A high pt candidate, above a threshold of 20 MeV are summed and sent denoted as A′ , is identified when suﬃcient track to the EMT every 269 ns. segments with accurate φ information from the The conversion of the tower data into the GLT other axial superlayers lie within this envelope. φ-maps is performed by ten Trigger Processor Each PTD module searches for seed segments Boards (TPBs). The TPBs determine energies in superlayers A7 and A10, and within a 45- in the 40 φ sectors, summing over various ranges degree azimuthal wedge of the DCH. This search of θ, compare these energies against thresholds region spans eight supercells, and the processing for each of the trigger primitives (see Table 15), for each supercell is performed by its own process- estimate the time of energy deposition, correct for ing engine on the PTD. The principal components timing jitter, and then transmit the result to the in each engine are an algorithmic processor and GLT. look-up-tables containing the limits for each in- Each TPB receives data from 28 towers, corre- dividual seed position. The contents of the look- sponding to an array of 7 × 4 in θ × φ, or four φ- up-tables specify the allowed track segment posi- sectors. Each of the 40 φ-sectors is summed inde- tions for each of the three other axial superlayers pendently. To identify energy deposits that span 87 EMT Efficiency two adjacent φ-sectors, the energy of each sector is also made available to the summing circuit for 1 a single adjoining sector in such a way that all possible pairs of adjacent φ-sectors are summed. These energy sums are compared against thresh- 0.75 olds to form trigger objects. Each sum is also sent to an eight-tap finite impulse response (FIR) dig- ital filter which is used to estimate the energy 0.5 deposition time. A look-up-table is used to make an energy-dependent estimate of the timing jit- ter which, along with the FIR output, is used to 0.25 time the transmission of any trigger objects to the GLT. Pairs of φ-sectors are ORed to form 20-bit φ-maps for the M, G, E, and X primitives, while 0 for the Y primitive, groups of four are ORed to 0.0 0.05 0.1 0.15 0.2 form a 10-bit φ-map. The complete algorithm is EMC Cluster Energy (GeV) implemented in one FPGA [112] for each φ-sector, Figure 85. EMT M eﬃciency vs. EMC cluster with four identical components per TPB. Further energy for an M threshold setting of 120 MeV. details of the EMT system can be found in [113]. The basic performance of the EMT can be ex- pressed in terms of the eﬃciency and timing jit- ter of the trigger primitives. The eﬃciency of the primitives can be measured by the number of Table 16 times a trigger bit is set for a specific energy re- IFR trigger pattern (U) definition, where µ refers constructed oﬄine in events from a random trig- to a signal within a sector. ger. Figure 85 shows this eﬃciency for energies near the M threshold. The eﬃciency changes U Trigger condition from 10% to 90% in the range of 110 to 145 MeV, 1 ≥ 2µ topologies other than U = 5 − 7 and reaches 99% at 180 MeV, close to the average 2 1 µ in backward endcap energy deposition of a minimum ionizing particle 3 1 µ in forward endcap at normal incidence. 4 1 µ in barrel The EMT time jitter is measured by comparing 5 2 back-back µ’s in barrel +1 forward µ the time centroid of φ-strip M hits in µ+ µ− events 6 1 µ in barrel +1 forward µ with the DCH track start time, t0 . The diﬀerence 7 2 back-back µ’s in barrel has an rms width of 90 ns with >99.9% of the matching M hits within a ±500 ns window. dow of 134 ns. The IFR trigger synchronization module processes the trigger objects from the ten 11.3.3. Level 1 IFR Trigger sectors and generates the three-bit trigger word The IFT is used for triggering on µ+ µ− and (U) encoding seven exclusive trigger conditions, cosmic rays. For the purposes of the trigger, the as defined in Table 16. The trigger U ≥ 5, for IFR is divided into ten sectors, namely the six example, covers all µ+ µ− topologies of interest. barrel sextants and the four half end doors. The The eﬃciency of the IFT has been evaluated us- inputs to the IFT are the Fast OR signals of all ing cosmic rays triggered by the DCT and cross- φ readout strips in eight selected layers in each ing the detector close to the IP. For these events, sector. 98% were triggered by the IFT as events with A majority logic algorithm defines trigger ob- at least one track, and 73% as events with two jects for every sector in which at least four of the tracks, inside the geometrical region of the IFR. eight trigger layers have hits within a time win- 88 Most of the IFT ineﬃciency is concentrated at plemented as an array of 16 memory chips with the boundaries between sectors. 8 Mbytes of configuration data. 11.3.4. Global Trigger 11.4. Level 1 Trigger Performance The GLT receives the eight trigger primitives and Operational Experience in the form of φ-maps as listed in Table 15 along The L1 trigger configuration consists of DCT- with information from the IFT (Table 16) to form only, EMT-only, mixed and prescaled triggers, specific triggers that are then passed to the FCTS aimed not only for maximum eﬃciency and back- for the final trigger decision. Due to the diﬀerent ground suppression, but also for the convenience latencies associated with the production of these of trigger eﬃciency determination. primitives, the GLT forms a time alignment of Although most triggers target a specific physics these input data using configurable delays. source, they often also select other processes. For The GLT then forms some additional combined example, two-track triggers are not only eﬃcient φ-maps from the DCT and EMT data. These for Bhabha, µ+ µ− , and τ + τ − events, but are also maps include matched objects such as BM for B useful for selecting jet-like hadronic events and tracks matched to an M cluster in φ, back-to-back some rare B decays. objects, B∗ and M∗ , which require a pair of φ The eﬃciencies and rates of selected L1 trig- bits separated by a configurable angle of typically gers for various physics processes are listed in ∼ 120◦ , and an EM∗ object for back-to-back EM Table 17. Although triggering on generic BB pairs. events is relatively easy, it is essential to en- All 16 φ-maps are then used to address indi- sure high eﬃciencies for the important rare low- vidual GLT look-up-tables which return three-bit multiplicity B decays. For this reason, eﬃciencies counts of trigger objects contained within those for B 0 → π 0 π 0 and B − → τ − ν are also listed in maps, e.g., the number of B tracks or number of Table 17. M clusters. To count as distinct trigger objects, The eﬃciencies listed for the hadronic events the map bits are typically required to have a sep- are absolute and include acceptance losses based aration of more than one φ bin. The resulting 16 on Monte Carlo simulation, and local ineﬃciency counts plus the IFT hit pattern are then tested in eﬀects. The eﬃciencies for τ -pair events are for logical operations. The permissible operations in- fiducial events, i.e., events with two or more clude: always-pass; or a comparison (≥, =, or <) tracks with pt > 120 MeV/c and polar angle θ to with a configurable selection parameter. A trig- reach at least DCH superlayer U5. The Bhabha ger line is then set as the logical AND of these 17 and µ-pair eﬃciencies are determined from the operations. This process is performed for each of data, for events with two high momentum parti- the 24 trigger lines. cles, which are back-to-back in the c.m. system, The GLT derives the L1 trigger time from the and within the EMC fiducial volume. The data in centroid of the timing distribution of the highest Table 17 demonstrate that the DCT and the com- priority trigger, binned in the 134 ns interval and bined EMT/IFT provide fully eﬃcient, indepen- spanning about 1 µs. Other trigger signals com- dent triggers for most physics processes, although patible with this time are retained and cached. independent triggers for µ+ µ− and τ + τ − are not The average time is calculated to the nearest 67 ns individually fully eﬃcient. The eﬃciencies pre- and the 24-bit GLT output signal is sent to the dicted by the Monte Carlo simulation are gener- FCTS every 67 ns. The achieved timing resolu- ally in good agreement with data when tested us- tion for hadronic events has an rms width of 52 ns; ing events passing typical analysis selections and and 99% of the events are within 77 ns. based on orthogonal triggers. Prescaled triggers The GLT hardware consists of a single 9U with a very open acceptance of physics events, VME module. Most of the logic, including di- such as (B≥2 & A≥1) or (M≥2) are also used to agnostic and DAQ memories, are implemented in measure the trigger eﬃciencies. FPGA’s [110]. The look-up-table section is im- The trigger rates listed in Table 17 are for a 89 Table 17 Level 1 Trigger eﬃciencies (%) and rates (Hz) at a luminosity of 2.2 × 1033 cm−2 s−1 for selected triggers applied to various physics processes. The symbols refer to the counts for each object. Level 1 Trigger εBB εB→π0 π0 εB→τ ν εcc εuds εee εµµ ετ τ Rate ∗ A≥3 & B ≥1 97.1 66.4 81.8 88.9 81.1 — — 17.7 180 A≥1 & B∗ ≥1 & A′ ≥1 95.0 63.0 83.2 89.2 85.2 98.6 99.1 79.9 410 Combined DCT (ORed) 99.1 79.7 92.2 95.3 90.6 98.9 99.1 80.6 560 M≥3 & M∗ ≥1 99.7 98.6 93.7 98.5 94.7 — — 53.7 160 EM∗ ≥1 71.4 94.9 55.5 77.1 79.5 97.8 — 65.8 150 Combined EMT (ORed) 99.8 99.2 95.5 98.8 95.6 99.2 — 77.6 340 B≥3 & A≥2 & M≥2 99.4 81.2 90.3 94.8 87.8 — — 19.7 170 M∗ ≥1 & A≥1 & A′ ≥1 95.1 68.8 83.7 90.1 87.0 97.8 95.9 78.2 250 E≥1 & B≥2 & A≥1 72.1 92.4 60.2 77.7 79.2 99.3 — 72.8 140 M∗ ≥1 & U≥5 (µ-pair) — — — — — — 60.3 — 70 Combined Level 1 triggers >99.9 99.8 99.7 99.9 98.2 >99.9 99.6 94.5 970 No. of Tracks typical run with HER (LER) currents at 650 mA 8000 (1350 mA) and a luminosity of 2.2×1033 cm−2 s−1 . These rates are stable to within 20% for the same PEP-II configuration, but they are impacted by 6000 changes in vacuum conditions, beam currents, and orbits. There are occasional background spikes which can double the L1 rate. However, 4000 due to the 2 kHz capability of the data acquisi- tion, these spikes do not induce significant dead time. 2000 For a typical L1 rate of 1 kHz, Bhabha and annihilation physics events contribute ∼130 Hz. There are also 100 Hz of cosmic ray and 20 Hz of random beam crossing triggers. The remaining 0 -80 -40 0 40 80 triggers are due to lost particles interacting with L3 Track z 0 (cm) the beam pipe or other components. The distri- bution of single track z0 values as reconstructed Figure 86. Single track z0 for all L1 tracks, re- by L3 for all L1 triggers is shown in Figure 86. constructed by L3. The most prominent peaks at z = ±20 cm cor- respond to a flange of the beam pipe. The peak suppress noisy channels in the EMC electronics. at z0 = −55 cm corresponds to a step in the syn- chrotron mask. 11.5. Level 3 Trigger System The L1 trigger hardware operation has been The L3 trigger software comprises event recon- very stable. For the first one and half years of op- struction and classification, a set of event selec- eration, there have been only four hardware fail- tion filters, and monitoring. This software runs ures in the L1 system, mainly auxiliary or com- on the online computer farm. The filters have ac- munication boards. Occasional adjustments to cess to the complete event data for making their the EMT tower mask were used to temporarily decision, including the output of the L1 trigger 90 processors and FCTS trigger scalers. L3 operates 11.5.1. Level 3 Drift Chamber by refining and augmenting the selection meth- Tracking Algorithm ods used in L1. For example, better DCH track- Many events which pass L1 but must be re- ing (vertex resolution) and EMC clustering filters jected by L3 are beam-induced charged particle allow for greater rejection of beam backgrounds background that are produced in material close and Bhabha events. to the IP. L1 does not currently have suﬃcient The L3 system runs within the Online Event tracking resolution to identify these background Processing (OEP) framework (see Section 12). tracks. The DCH-based algorithm, L3Dch, per- OEP delivers events to L3, then prescales and forms fast pattern recognition (track finding) and logs those which pass the L3 selection criteria. track fitting, which determines the five helix track To provide optimum flexibility under diﬀerent parameters for tracks with pt above 250 MeV/c. running conditions, L3 is designed according to a To speed up the process of pattern recognition, general logic model that can be configured to sup- L3Dch starts with the track segments from the port an unlimited variety of event selection mech- TSF system and improves the resolution by mak- anisms. This provides for a number of diﬀerent, ing use of the actual DCH information. independent classification tests, called scripts, For those TSF segments that have a simple so- that are executed independently, together with lution to the left-right ambiguity, a track t0 is a mechanism for combining these tests into the determined. The t0 values for each segment in an final set of classification decisions. event are binned and the mean produced from the The L3 trigger has three phases. In the first values in the most populated bin is used as the phase, events are classified by defining L3 input estimated event t0 . All events which pass L1 typi- lines, which are based on a logical OR of any num- cally have enough segments to form a t0 estimate. ber of the 32 FCTS output lines. Any number of The measured rms resolution on this estimate is L3 input lines may be defined. 1.8 ns for Bhabha events and 3.8 ns for hadronic The second phase comprises a number of events. scripts. Each script executes if its single L3 input The pattern recognition for L3Dch is done with line is true and subsequently produces a single a look-up-table. For this track table, the DCH is pass–fail output flag. Internally, a script may ex- divided into 120 φ-sectors, corresponding to the ecute one or both of the DCH or EMC algorithms, number of cells in the innermost layers. The track followed by one or more filters. The algorithms table is populated with the hit patterns of Monte construct quantities of interest, while the filters Carlo generated tracks with a pt above 250 MeV/c determine whether or not those quantities satisfy and originating within 2 cm of the IP in the x–y the specific selection criteria. plane, and within 10 cm in z. The pattern recog- In the final phase, the L3 output lines are nition algorithm searches the table entries looking formed. Each output line is defined as the log- for matches to segments found by the TSFs. The ical OR of selected script flags. L3 can treat matched set of segments for a given track is then script flags as vetoes, thereby rejecting, for exam- passed to the track fitting algorithm. The track ple, carefully selected Bhabha events which might table allows for up to two missing DCH TSF seg- otherwise satisfy the selection criteria. ments per track. L3 utilizes the standard event data analysis The track fitting algorithm is provided with framework and depends crucially on several of its both the track segments found in pattern recog- aspects. Any code in the form of modules can be nition and the individual hits within those seg- included and configured at run time. A sequence ments. From this information the five helix pa- of these software modules compose a script. The rameters are fitted. The fit is then iterated, same instance of a module may be included in adding segments close to the initially fitted track, multiple scripts yet it is executed only once, thus and dropping hits with large residuals. The final avoiding significant additional CPU overhead. fit does not demand that the track originate from the IP. 91 2500 ing the shower shape for particle identification are No. of Events 2000 calculated. 1500 11.5.3. Level 3 Filters 1000 Based on the L3 tracks and clusters, a variety 500 of filters perform event classification and back- 0 ground reduction. The logging decision is pri- -1 0 1 -8 0 8 marily made by two orthogonal filters, one based ∆d0 (cm) ∆z0 (cm) exclusively on DCH data and the other based only on EMC data. Figure 87. Transverse and longitudinal miss dis- The drift chamber filters select events with one tances between the two tracks in Bhabha events. tight (high pt ) track or two loose tracks originat- ing from the IP, respectively. To account for the The two-track miss distances for Bhabha events fact that the IP is not exactly at the origin, track are plotted in Figure 87. The resolutions for indi- selection is based on its x–y closest approach dis- vidual tracks are 0.80 mm and 6.1 mm for d0 and tance to the IP, dIP IP 0 , and z0 , the corresponding z0 , respectively. Similarly, the 1/pt diﬀerence be- z coordinate for that point. The IP position is tween the two tracks in µ-pair events yields a pt a fixed location close to the average beam po- resolution of δpt /pt ∼ 0.019·pt , with pt in GeV/c. sition over many months. The high pt track is required to have a transverse momentum of pt > 11.5.2. Level 3 Calorimeter 600 MeV/c and to satisfy a vertex condition de- Clustering Algorithm fined as |dIP IP 0 | < 1.0 cm, and |z0 − zIP | < 7.0 cm. The all-neutral trigger for L3 is based on infor- Two tracks are accepted with pt >250 MeV/c and mation from the EMC. In addition, calorimeter a somewhat looser vertex condition defined as information is a vital complement to the DCH |dIP IP 0 | < 1.5 cm, |z0 − zIP | < 10.0 cm. data for the identification of Bhabha events. Two calorimeter cluster filters select events The L3 EMC-based trigger, L3Emc, identifies with either high energy deposits or high cluster energy clusters with a sensitivity suﬃcient for multiplicity. Each filter also requires a high eﬀec- finding minimum ionizing particles. EMC data tive mass calculated from the cluster energy sums are processed in two steps: first, lists of crystals and the energy weighted centroid positions of all with significant energy deposits are formed; and clusters in the event assuming massless particles. second, clusters are identified. The EMC typi- The first filter requires at least two clusters of cally sends data for ∼1400 crystals (of 6580 total). ECM > 350 MeV (c.m. system energy) and event The majority of these are caused by electronics mass greater than 1.5 GeV; the second filter re- noise and beam-induced background. For each quires at least four clusters, and an event mass crystal, these data include the peak energy and greater than 1.5 GeV. time of the crystal waveform. To filter out noise, At current luminosities, the output of both the L3Emc rejects individual crystal signals below an DCH and EMC filters is dominated by Bhabha energy threshold of 20 MeV or which lie outside events, which need to be rejected. This is ac- a 1.3 µs time window around the event time. For complished by a Bhabha veto filter that selects the remaining crystals, raw energies and times are one-prong (with only a positron in the backward converted into physical units and added to the part of the detector) and two-prong events (with L3Emc crystal list. Clusters are formed using an both e+ and e− detected). Stringent criteria on optimized look-up-table technique requiring only EMC energy deposits are imposed, relying on the a single pass over the crystal list. Clusters with a track momenta and on E/p. The two-prong veto total energy above 100 MeV are retained, and the requires either colinearity between the tracks in energy weighted centroid and average time, the the c.m. system or an acolinearity that is consis- number of crystals, and a lateral moment describ- tent with initial state radiation (ISR). 92 For purposes of calibration and oﬄine luminos- Table 19 ity measurements, Bhabha, radiative Bhabha, γγ Composition of the L3 output at a luminosity of final state, and cosmic ray events are flagged. The 2.6×1033 cm−2 s−1 . output rate of flagged Bhabha events is adjusted to generate an approximately flat distribution of Event type Rate (Hz) events in polar angle. Radiative Bhabha events Hadrons, τ τ , and µµ 16 are identified by selecting two-prong events with Other QED, 2-photon events 13 missing energy and requiring an EMC cluster in Unidentified Bhabha backgrounds 18 the direction of the missing momentum. Events Beam-induced backgrounds 26 with two high energy clusters, back-to-back in the c.m. system select the e+ e− → γγ process. The Total physics accept 73 cosmic ray selection is DCH-based and requires Calibration Bhabhas (e+ e− ) 30 two back-to-back tracks in the laboratory frame γγ, Radiative Bhabhas (e+ e− γ) 10 with nearly equal impact parameters and curva- Random triggers and cosmic rays 2 ture. A significant background from ISR Bhabha L1,L3 pass through diagnostics 7 events faking this topology is removed using the same kinematic constraints used in the two-prong Total calibration/diagnostics 49 veto. The online luminosity monitoring and energy but ignored due to prescale factor (-1). The right scale monitoring are performed in L3. A track- column shows the same information for the L3 based lepton-pair selection with a well known ef- trigger lines. ficiency monitors the luminosity. Hadronic filters For a typical run on the Υ (4S) peak with an for selection of continuum and BB-enriched sam- average luminosity of 2.6×1033 cm−2 s−1 , the L3 ples monitor the energy scale. The latter two cat- event composition is tabulated in Table 19. The egories are distinguished by an event shape selec- desired physics events contribute 13% of the total tion using a ratio of Fox-Wolfram moments [114]. output while the calibration and diagnostic sam- The ratio of the BB-enriched sample to the lu- ples comprise 40%. minosity is a sensitive measure of relative posi- The L3 executable currently takes an average tion on the Υ (4S) peak and thereby monitors the processing time of 8.5 ms per event per farm com- beam energies. puter. A Level 1 input rate of 2700 Hz saturates the Level 3 processors, well above the 2 kHz de- 11.6. Level 3 Performance sign requirement. At this input rate the L3 pro- and Operational Experience cess consumes ∼72% of the CPU time, the rest is The L3 trigger eﬃciency for Monte Carlo sim- spent in OEP, including the network event builder ulated events are tabulated in Table 18 for events and in the operating system kernel. passing Level 1. High eﬃciencies are indepen- dently achieved for the DCH and EMC based fil- 11.7. Summary and Outlook ters applied to simulated hadronic events. The Both the L1 and L3 trigger systems have met comparison between data and Monte Carlo L3 their original design goals at a luminosity of trigger pass fractions for the various filters also 3 × 1033 cm−2 s−1 . The triggering eﬃciencies for show good agreement when requiring tracking, BB events generally meet the 99% design goal for and EMC based hadronic event selections in turn. both L1 and L3. The orthogonal triggers based on An example of the event display used for online DCH-only and EMC-only information have suc- trigger monitoring is shown in Figure 88. L3 re- cessfully delivered stable and measurable overall constructed tracks and EMC clusters are shown trigger eﬃciency. The current system also pro- together with the L1 and L3 trigger line states vides a solid foundation for an upgrade path to for the event. The left column lists the L1 trig- luminosities of 1034 cm−2 s−1 or more. ger lines and their states: on (1); oﬀ (0); or on Short-term L1 trigger improvements will pri- 93 Figure 88. A Level 3 event display. The small circles and small crosses in the DCH volume are DCH hits and TSF segment hit wires respectively. The filled EMC crystals represent energy deposit (full crystal depth = 2 GeV) from Level 3 EMC clusters while the small triangles just inside the EMC indicate the location of the cluster centroid. marily come from further background rejection, Future improvements for L3 will also empha- aﬀorded by algorithm refinements and upgrades size background rejection. Improvements in the of the DCT. This is essential for reducing the load L3 IP track filter are expected to further reduce on the DAQ and L3. The new PTD algorithm will beam-induced background to about one third of eﬀectively narrow the track d0 acceptance win- current levels. The physics filter algorithms will dow, while a new BLT algorithm will narrow the be tuned and improved, primarily for rejecting track z0 acceptance. Bhabha, QED, and two-photon events. Improve- For the longer term future, a major DCT up- ments in the L3 tracking algorithms are expected grade is planned. By adding the stereo layer in- to lower the pt thresholds below 250 MeV/c. A formation, a z0 resolution of 4 cm is expected, al- moderate CPU upgrade for the L3 online farm lowing for an eﬃcient rejection of beam-induced will be suﬃcient to keep up with luminosities of background beyond z = ±20 cm. ∼ 1034 cm−2 s−1 . 94 Table 18 L3 trigger eﬃciency (%) for various physics processes, derived from Monte Carlo simulation. L3 Trigger εBB εB→π0 π0 εB→τ ν εcc εuds ετ τ 1 track filter 89.9 69.9 86.5 89.2 88.2 94.1 2 track filter 98.9 84.1 94.5 96.1 93.2 87.6 Combined DCH filters 99.4 89.1 96.6 97.1 95.4 95.5 2 cluster filter 25.8 91.2 14.5 39.2 48.7 34.3 4 cluster filter 93.5 95.2 62.3 87.4 85.5 37.8 Combined EMC filters 93.5 95.7 62.3 87.4 85.6 46.3 Combined DCH+EMC filters >99.9 99.3 98.1 99.0 97.6 97.3 Combined L1+L3 >99.9 99.1 97.8 98.9 95.8 92.0 12. The Online Computing System quired by the system are subjected to monitoring. Such monitoring is configurable by experts and 12.1. Overview designed to detect anomalies in the detector sys- The BABAR online computing system comprises tems which, if present, are reported to operators the data acquisition chain from the common FEE, for rapid assessment and, if necessary, corrective through the embedded processors in the data ac- action. quisition system and the L3 trigger, to the logging Environmental conditions of the detector, such of event data. It also includes those components as the state of low and high voltage power, the pu- required for detector and data acquisition control rity of gas supplies, and the operating conditions and monitoring, immediate data quality monitor- of the accelerator, such as beam luminosity and ing, and online calibration. currents, are measured and recorded in a fash- 12.1.1. Design Requirements ion that permits the association with the event The data acquisition chain was designed to data logged. Conditions relevant to data quality meet the following basic performance require- are monitored for consistency with specified stan- ments. It must support a L1 trigger accept rate dards. Operators are alerted if these are not met. of up to 2 kHz, with an average event size of Data-taking is inhibited or otherwise flagged if ∼32 kbytes and a maximum output (L3 trigger conditions are incompatible with maintaining the accept) rate of 120 Hz. While performing these quality of the data. functions it should not contribute more than Operational configurations, calibration results, a time-averaged 3% to deadtime during normal active software version numbers, and routine mes- data acquisition. sages and error messages are also recorded. Dur- The online system is also required to be capa- ing data analysis or problem diagnosis, these data ble of performing data acquisition simultaneously help in reconstructing the detailed operating con- on independent partitions—sets of detector sys- ditions. tem components—to support calibrations and di- 12.1.2. System Components agnostics. The online computing system is designed as a Normal detector operation, data acquisition set of subsystems using elements of a common and routine calibrations are performed eﬃciently software infrastructure running on a dedicated and under the control of a simple user interface collection of hardware. with facilities for detecting, diagnosing, and re- The major subsystems are: covering from common error conditions. Following standard practice, the event data ac- • Online Dataflow (ODF)—responsible for 95 communication with and control of the de- them for event building to 32 commercial Unix tector systems’ front-end electronics, and workstations [115] which are part of the online the acquisition and building of event data farm. Other farm machines perform data moni- from them; toring and calibrations. The crates and farm ma- chines communicate via full-duplex 100 Mbits/s • Online Event Processing (OEP)— Ethernet, linked by a network switch—the event responsible for processing of complete builder switch [116]. The ROMs are supported by events, including L3 (software) triggering, a boot server providing core and system-specific data quality monitoring, and the final code and configuration information [117]. stages of calibrations; The thirty-two online farm machines host the • Logging Manager (LM)—responsible for re- OEP and L3 trigger software. The events ac- ceiving selected events sent from OEP and cepted by the trigger are logged via TCP/IP to a writing them to disk files for use as input to logging server [117] and written to a disk buﬀer the Online Prompt Reconstruction process- for later reconstruction and archival storage. Var- ing; ious data quality monitoring processes run on farm machines not used for data acquisition. • Online Detector Control (ODC)— Several additional file servers hold the online responsible for the control and monitoring databases and production software releases. A of environmental conditions of the detector further set of application servers host the central systems; functions of the various online subsystems. Op- erator displays are supported by a group of ten • Online Run Control (ORC)—ties together console servers [118]. all the other components, and is responsi- An additional set of 15 VME crates, each with ble for sequencing their operations, inter- an embedded processor, contain the data acquisi- locking them as appropriate, and providing tion hardware for the detector control subsystem. a graphical user interface (GUI) for opera- All VME crates, the online farm, and all the tor control. application and console servers are connected via a switched 100 Mbits/s Ethernet network distinct Each of these components, as well as a selection from that used for event building, with 1 Gbits/s of the common tools which tie them together are fiber Ethernet used for the file servers and inter- described below. switch links. The entire system is coded primarily in the C++ language, with some use of Java for graph- ical user interfaces. Object-oriented analysis and 12.1.4. User interaction design techniques have been used throughout. Operator control of the online system is This has been an important feature, enhancing achieved primarily through a custom Motif GUI development speed, maintainability, and extensi- for run control and an extensive hierarchy of dis- bility. plays for detector control, including control pan- els, strip charts and an alarm handler. An elec- 12.1.3. Hardware Infrastructure tronic logbook is made available through a Web The hardware infrastructure for the online sys- browser interface. These and other GUIs are or- tem is shown schematically in Figure 89. ganized across seventeen displays for the use of The data from the FEEs of the various detector the experiment’s operators. This operator envi- systems are routed via optical fiber links to a set ronment provides for basic control of data acquisi- of 157 custom VME Readout Modules (ROMs). tion, the overall state of the detector, and certain These ROMs are grouped by detector system and calibration tasks. housed in 23 data acquisition VME crates that Each detector system has developed a set of are controlled by the ODF software. One ROM specialized calibration and diagnostic applica- in each crate aggregates the data and forwards tions using the tools provided in the online sys- 96 BABAR detector VME FEE VME dataflow detector [24] control [15] logging server [1] x24 x16 720 GB file and DB event servers [5] network application builder switch switches x10 servers [1] 740 GB [10] [2] x78 x78 x10 console farm nodes [78] servers [10] 1.2 Gbps G-Links various analog/digital links 1 Gbps ethernet ... 100 Mbps ethernet computer monitors [17] center Figure 89. Physical infrastructure of the BABAR online system, including VME crates, computers, and networking equipment. tem. A subset of these calibrations has been spec- the L1 trigger. L1 Accepts are distributed, in ified to be run once per day, during a ten-minute the full detector configuration, to the 133 ROMs scheduled beam-oﬀ period. The run control logic, connected via optical fibers to the detector sys- combined with the capability for creating parti- tem FEE. These ROMs read and process the data tions, allows calibrations for all detector systems from the FEE. One to ten such ROMs from a sin- to be run in parallel and provides the operator gle detector system are located in each of the data with basic feedback on the success or failure of acquisition VME crates. ODF builds complete each. events from these ROMs, first collecting the data in each crate into an additional dedicated ROM, 12.2. Online Dataflow and then collecting the data from the 23 of these, ODF handles data acquisition and processing across the event builder network switch, into the from the detector systems’ FEE through the de- online farm. livery of complete events to the online farm [119]. The operation of the system is controlled by The ODF subsystem receives the L1 trigger out- ODF software running on one of the application puts, filters and distributes them to the FEE, servers, under the direction of run control. A sin- reads back the resulting data and assembles them gle ROM in the VME crate containing the central into events. It provides interfaces for control of FCTS hardware supports the software interface data acquisition, processing and calibration of de- to ODF. The distribution of ROMs by detector tector system data, and FEE configuration. Mul- system is shown in Table 20. The numbers of tiple independent partitions of the detector may ROMs is shown as a sum of those connected di- be operated simultaneously. rectly to the detector FEE, and those used for Event data acquisition proceeds from a trig- event building. ger decision formed in the Fast Control and Tim- All of the ROM CPUs boot via NFS over the ing System (FCTS) [120] based on inputs from 97 Table 20 as an additional, idempotent state transition, L1 VME crates and ROMs used by ODF Accept, and are treated uniformly with the others wherever possible. Detector VME Readout Segment The ROMs connected to the detec- System Crates Modules tor FEE, with their ODF and detector system- SVT 5 14+5 specific software. Each segment level ROM re- DCH 2 4+2 ceives state transition messages from the source DIRC 2 6+2 level and runs appropriate core and detector EMC 10 100+10 system-specific tasks in response. These tasks in- IFR 1 4+1 clude the acquisition of raw data from the FEE EMT 1 1+1 in response to L1 Accepts, and feature extraction. DCT 1 3+1 Output data resulting from this processing is at- GLT 1 1+1 tached to the transition messages, which are then FCTS 1 1 forwarded over the VME backplane to the frag- ment level ROM in each crate. TotalTotal 24 157 Fragment The per-crate event builder ROMs and software. The single fragment level ROM in each crate aggregates the messages from the event building network from the boot server de- crate’s segment level ROMs—the first stage of scribed above. About 1.5 Mbytes of core ODF event building—and forwards the combined mes- code plus another ∼4 Mbytes of detector-specific sage to one of the event level Unix nodes. code are loaded into each ROM. This, along with Event The processes on the online farm nodes the booting process, takes about 40 seconds. receiving complete events and handing them over The ODF software allows all the components to OEP for filtering and logging. The ODF event of this heterogeneous system to be represented in level code aggregates messages, with their at- a uniform object-oriented application framework. tached data, from all the crates in a partition— These components are organized into five levels the second and final stage of event building. The which map closely onto the physical structure. resulting data may be further processed by user For each component at each level, its operation code in the event level, but are normally just is abstracted as a finite state machine. The com- passed on to OEP. The control level is noti- plete set of these machines is kept coherent by fied of the completion of processing of all tran- passing messages and data regarding state tran- sitions other than L1 Accept. Both the fragment sitions along the chain of levels. The basic flow and event level event builders use a data-driven of control and data is shown in Figure 90. The “push” model, with a back pressure mechanism mapping of levels to components is as follows: to signal when they are unable to accept more Control The Unix-based process controlling data. the operation of each partition and the source Test stands of varying complexity are sup- of all state transitions except for L1 Accept. It ported. The simplest possible consists of a single transmits state transition messages over the net- Unix machine which runs both control and event work to the source level, waiting for acknowledge- level code, with two FCTS modules and a sin- ment of their successful processing by all levels. gle ROM, running source, segment, and fragment Source The FCTS hardware and the software level code, in one VME crate. Configuration is running in the ROM located in the FCTS VME detected at run-time, so the same code that runs crate. For each partition, its source level receives in the full system can also run in test stand sys- control level transitions and L1 trigger outputs tems. and distributes them via the FCTS hardware to all ROMs in the VME crates included in the par- tition. L1 triggers are modeled in the subsystem 98 Run Control L1 Trigger Detector FEEs control control & data Source Segment Fragment Control Event (FCTS) (1-10/crate) (Crate) 3-2001 8583A47 x1 Unix x1 FCTS ROM x133 ROMs x23 Slot- 1 ROMs x32 Unix farm Figure 90. Schematic of the ODF levels, their mapping onto physical components, and the flow of control signals and data between them. 12.2.1. Control and Source Levels subsystem. The first arises from the minimum The control level sends state transition mes- 2.7 µs spacing between L1 Accept transitions. sages for a partition over the network, using This restriction simplifies the logic design of the the User Datagram Protocol, UDP [121], to the FEE readout, because each signal in the silicon source level in the single ROM inside the FCTS tracker and drift chamber is thus associated with crate. In the source level, the transition message only one L1 Accept. The FCTS hardware enforces is sent over VME to an FCTS module which for- this minimum separation between transitions, in- wards it as a 104-bit 59.5 MHz serial word to all troducing an irreducible, yet minimal dead time VME crates in the relevant partition. This serial of 0.54% at 2 kHz. word contains a 56-bit event time stamp (count- The second type of deadtime arises when all ing at 59.5 MHz), a 32-bit transition-specific word FEE buﬀers are full and thus unable to accept and additional control bits. L1 Accept transitions another event. In a time required to be less than and calibration sequences, however, originate in the inter-command spacing, each VME crate in the source level and the same mechanism is used a partition may send back a full signal indicat- to transmit them through the system. ing that it is no longer able to process further The FCTS hardware receives the 24 L1 trigger L1 Accept transitions. The FCTS hardware de- output lines and eight additional external trigger tects these signals and disables triggering until lines. The FCTS crate is a 9U VME crate, with a the FEE are once again prepared to accept data. custom P3 backplane on which all the trigger lines An actual L1 Accept signal is only generated are bussed. For each partition, an FCTS module from a partition’s trigger decision when neither receives these lines. It is configurable with a bit form of dead time is asserted. mask specifying the trigger lines enabled for its partition, and an optional prescale factor for each 12.2.2. Segment and Fragment Levels line. A trigger decision is formed for the partition The segment and fragment levels reside in the by taking the logical OR of the enabled prescaled 23 detector system VME crates. These are stan- lines. Twelve of these modules are installed in the dard 9U crates with a custom P3 backplane. full system, thus setting its maximum number of The 104-bit serial transition messages that partitions. A detector system can belong to only leave the source level are received by a FCTS one partition at a time. module in each VME crate in a partition. This The FCTS crate receives two timing signals module in turn forwards these messages to the from the accelerator: a 476 MHz clock tied to ROMs in the crate over the custom backplane, the RF structure of PEP-II and a 136 kHz fidu- along with the 59.5 MHz system clock. cial that counts at the beam revolution fre- A ROM consists of four components (see Fig- quency. The former is divided by eight to create ure 91), a commercial single-board computer a 59.5 MHz system clock. The fiducial is used to (SBC) [122] and three custom boards. The cus- start timing counters and to check the synchro- tom boards include: a controller card for receiv- nization of the clocks. ing FCTS commands and supporting FEE reads There are two types of deadtime in the ODF and writes; a personality card that transmits com- 99 Table 21 Typical event sizes from detector systems Personality Card Single-board Computer (SBC) P1 Hit Size Total Size Overhead Detector (bytes) (kB) (kB) SVT 2 4.9 0.4 Two bers to FEE DCH 10 4.8 0.2 i960 Card P2 DIRC 4 3.1 0.3 Two bers from FEE EMC 4 9.1 3.0 IFR 8 1.2 0.2 EMT — 1.2 < 0.1 Controller Card DCT — 2.7 0.1 P3 GLT — 0.9 < 0.1 Total 27.9 4.2 Front Panel FULL 59.5 MHz Clock FCTS Transition Messages ware and then refined oﬄine in the course of full event reconstruction. FEE commands are sent and data received Figure 91. A ROM with a triggered personality by the personality cards over uni-directional card (TPC) 1.2 Gbits/s serial optical fiber links [123]. All FEEs provide zero suppression in hardware ex- mands to and receives data from the FEE; and a cept in the EMC and IFR. Data are transferred PCI mezzanine card with a 33 MHz Intel i960 I/O from the personality card to the SBC memory us- processor. The SBCs run the VxWorks [8] oper- ing the i960 as a direct memory access (DMA) ating system with custom code written in C++ engine. This DMA runs at nearly the ideal and assembly language. 133 Mbytes/s rate of the PCI bus. There are two styles of personality cards in the The FEEs for various systems are able to buﬀer system: triggered (TPC) and untriggered (UPC). data for three to five L1 Accept transitions. The UPCs are used only in the EMC system. UPCs ROM keeps track of the buﬀer occupancy and accept data continuously from the FEE into a sends, when necessary, a full signal as previously buﬀer pipeline, at a rate of 3.7 MHz. From these described. The full condition is removed when samples EMC trigger information is derived and event reading by the ROM frees suﬃcient buﬀer sent over a dedicated serial link to the trigger space. This mechanism handles back pressure hardware, providing it with a continuous data from any stage of the data acquisition through stream. An L1 Accept causes up to 256 samples to data logging by OEP. of the raw data stream to be saved to an inter- The ODF application framework provides uni- mediate memory on the UPC. form software entry points for the insertion of user A TPC (used in all other systems) reads out code at each level of the system. This capability FEE data only when an L1 Accept signal is re- is used primarily at the segment level, for FEE ceived, again saving it into an intermediate mem- configuration and feature extraction. Table 21 ory. Each detector reads out data in a time win- presents typical data contributions from each de- dow around the trigger signal, large enough to tector system and the trigger. allow for trigger jitter and detector time resolu- Data from the segment level ROMs in a crate tion. For instance, this window is about 500 ns are gathered by the fragment level ROM using wide for the SVT. The actual event time within a chained sequence of DMA operations. The this window is estimated in the L3 trigger soft- maximum throughput of the fragment level event builder is about 31 Mbits/s. 100 In calibrations, ODF may be operated in a To ensure that the data from the correct event mode in which L1 Accept data are not transferred is retrieved from the FEEs, a five-bit counter is in- out of the segment level ROMs. This allows for cremented and sent from the FCTS to the FEEs calibration data accumulation at high rates inside with each L1 Accept. These bits are stored in the ROMs, not limited by the throughput of the the FEEs along with the data and are compared event builders or any downstream software. Com- on read-back. If they disagree, a special clear- pleted calibration results are computed, read out, readout command is sent which resynchronizes and written to a database. ROM buﬀer pointers with FEE buﬀer pointers. All transitions, including L1 Accept, are logged 12.2.3. Event Level in a 4 kbytes-deep by 20 byte-wide FIFO as they For each L1 Accept transition passing through pass through the FCTS crate. The transition the ODF subsystem, all fragment ROM data are type, the event time stamp, a bit list of the trigger sent to one of the farm machines. The destination lines contributing to the decision, and the current is chosen by a deterministic calculation based on full bit list from all VME crates are recorded in the L1 Accept’s 56-bit time stamp, available from this FIFO. There are also scalers which record the FCTS in each ROM. This technique produces delivered and accepted luminosity, deadtime due a uniform quasi-random distribution and intro- to the 2.7 µs minimum inter-command spacing, duces no detectable ineﬃciency. Events sent to a deadtime caused by VME crates being full and farm machine still busy with a previous event are triggers on each line. These FIFOs and scalers are held in a buﬀer to await processing. read out by the FCTS ROM, which then trans- All fragment data for an event are sent over mits the data to monitoring programs that calcu- the switched 100 Mbits/s Ethernet event building late quantities such as luminosity, deadtime and network to the selected farm machine. The con- trigger rates. The UDP multicast protocol [125] is nectionless UDP was chosen as the data trans- used to allow eﬃcient simultaneous transmission port protocol [124], allowing a flow control mech- of data to multiple clients. anism to be tailored specifically to this applica- To provide diagnostics, a system which multi- tion. Dropped packets are minimized by the net- casts additional performance information on de- work’s purely point-to-point, full duplex switched mand from each CPU, typically at 1 Hz, is used. architecture, and by careful tuning of the buﬀer- This information is currently received by a single ing in the network switch and other parameters. client on one of the Unix application servers and The rare instance of packet loss is detected by the archived. It can be retrieved subsequently to in- event builder and the resulting incomplete event vestigate any unusual behaviour observed in the is flagged. system. The event level provides the standard software entry points for user code. During normal oper- 12.3. Online Event Processing ation, these are used only to transfer events via The online event processing (OEP) subsys- shared memory to the OEP subsystem for L3 trig- tem provides a framework for the processing of gering, monitoring, and logging. complete events delivered from the ODF event builder [126]. The L3 software trigger operates 12.2.4. System Monitoring in this framework, along with event-based data It is critical that the clocks of the FEEs stay quality monitoring and the final stages of online synchronized with the rest of the system. Each calibrations. Figure 92 shows the basic flow of FEE module maintains a time counter which is data in the OEP subsystem. compared to the time stamp of each L1 Accept in The OEP subsystem serves as an adapter be- order to ensure that the system remains synchro- tween the ODF event builder interface and the ap- nized. If it becomes unsynchronized, a special plication framework originally developed for the synch command can be sent through the FCTS, oﬄine computing system. Raw data delivered causing all systems to reset their clocks. from the ODF subsystem are put into an object- 101 oriented form and made available through the ing”). A dedicated operator console supports the standard event data analysis interface. JAS-based data quality monitoring system. This The use of this technique permits the L3 trig- console is used to display histograms from Fast ger and most of the data quality monitoring soft- Monitoring and the L3 trigger processes, along ware to be written and debugged within the of- with any error conditions detected by the auto- fline environment. This software is decomposed matic histogram analysis facility. into small, reusable units—modules, pluggable software components in the framework—many of 12.4. Data logging which are shared among multiple applications. Events selected by the L3 trigger algorithms The OEP interfaces allow user applications to in OEP are retained for subsequent full recon- append new data blocks to the original raw data struction. The events are sent from the 32 OEP from ODF. The results of L3 event analysis are nodes via TCP/IP to a single multithreaded pro- stored in this manner so that the trigger decision cess, the Logging Manager (LM), running on the and the tracks and calorimeter clusters on which logging server. The LM writes these data to it is based may be used in later processing, such as RAID storage arrays in a format specific to OEP. reconstruction and trigger performance studies. Data from all 32 nodes are combined into a sin- Histograms and other monitoring data are ac- gle file for each data-taking run (typically two to cumulated across the farm. A distributed his- three hours of data acquisition, resulting in files tograming package (DHP) [127] was developed to of about 15–20 Gbytes in size). provide networked clients with a single view of Completed data files are copied to the SLAC histograms and time history data. This data is High Performance Storage System (HPSS) [131] summed across all nodes via CORBA-based com- system for archiving to tape. Within eight hours munication protocols [128,129]. of data acquisition these files are retrieved from The fast monitoring system provides auto- HPSS for event reconstruction. The data files are mated comparisons of monitoring data against also retrievable for other tasks such as detector defined references. Statistical comparisons of live system hardware diagnostics and oﬄine tests of histograms, or the results of fits to reference his- the L3 trigger algorithms. tograms, analytic spectra, or nominal values of fit parameters may be performed at configurable 12.5. Detector Control time intervals. Comparison failures, tagged with 12.5.1. Design Principles configurable severity levels based on the confi- The Experimental Physics and Industrial Con- dence levels of the comparisons, are displayed to trol System, EPICS [7], was selected to provide operators and logged in the common occurrence the basis for the ODC subsystem. This provides database, described below. direct connection to the electrical signals of the The Java Analysis Studio (JAS) package [130] power supplies and other hardware, with suﬃ- previously developed at SLAC was enhanced with cient monitoring and control to allow commission- the ability to serve as a DHP client. It is used for ing, fault diagnosis, and testing. A summary of viewing of monitoring data. This feature was im- monitoring and control points is presented in Ta- plemented by devising a Java server that adapts ble 22. the DHP protocol to the native JAS data proto- Beyond the writing of custom drivers, only mi- col. nor additions or changes were required to EPICS. In addition to the primary triggering and mon- EPICS and the additional BABAR-specific soft- itoring functions carried out on 32 online farm ware are written in the C language. machines, OEP provides a “trickle stream” pro- Detector-wide standard hardware was adopted tocol that allows networked clients to subscribe to to ease development and maintenance. The stan- a sampling of the event data. This scheme pro- dard ODC crate is a 6U VME chassis contain- vides support for event displays and additional ing a single board computer [132] serving as an detailed data quality monitoring (“Fast Monitor- EPICS input/output controller (IOC). Fifteen 102 Appl. Servers DHP JAS EL L3 Req Disp JAS Srvr EL L3 JAS Disp DataFlow 32 OEP farm nodes Farm (crates) FM Node Evt EL L3 Disp Logging LM Server Trickle stream event data Consoles DHP CORBA protocol JAS RMI protocol Disk array 3-2001 (various event data protocols) 8583A48 Figure 92. Flow of data in the OEP subsystem: ODF event level (EL) and L3 trigger processes on each OEP node; the Logging Manager (LM) on the logging server; the DHP “requestor” process that combines histograms from all 32 L3 processes; one instance of a Fast Monitoring (FM) process with DHP histograms; the Java server that makes DHP histograms available to JAS clients; two such clients, and one event display for the L3 trigger. OEP-specific data transport protocols are identified. Table 22 ODC Distribution by system of approximately 12,000 recorded monitor channels SVT DCH DIRC EMC IFR Central Radiation Dose 12 8 22 116 — — Data Rate — — 12 — 1612 20 Temperature 208 87 36 506 146 100 Humidity 4 5 12 7 — — 2 Magnetic Field — — 8 — — — 50 Position 30 22 — — — — Gas System 4 115 12 1 32 — Fluid System 20 2 18 18 20 90 Liquid Source — — — 12 — 3 HV System 2080 1299 672 24 1574 — LV System (non-VME) — 62 1080 442 94 — VME Crates 5 2 12 12 8 1500 CAN Micro Controller 48 40 3 47 45 — Finisar Monitor 28 12 12 — — — System Totals 2439 1654 1899 1185 3531 1765 103 such crates are used in the experiment. EPICS systems, aggregated from the ∼ < 105 individual is fully distributed. For example, each IOC sup- EPICS records. plies its own naming service, notify-by-exception The CPs present a simple finite state ma- semantics, and processing. The IOCs boot from chine model as their interface to Run Control. a dedicated server. The most important actions available are Config- Analog data are either digitized by modules ure, on which the CP accesses the configuration within the crates or, more commonly, on digitizer database, retrieving set points for its component’s boards located directly on the detector. In the channels, and Begin Run, which puts the CP into latter case, the CANbus standard [133] is used the Running state, in which setpoint changes are for the transport of signals to and from the de- prohibited and readbacks are required to match tector. A custom “general monitoring board” settings. While in the Running state, the CP (GMB) [134] was developed to interface CANbus maintains a Runnable flag which reflects that re- to the on-detector electronics. The GMB contains quirement and allows Run Control to ensure that a microcontroller, an ADC, multiplexors, and op- data acquisition is performed only under satisfac- erational amplifiers. It can digitize up to 32 sig- tory conditions. nals. The CP’s other principal function is to provide an interface for the rest of the online system to 12.5.2. User Interface the ambient data collected by ODC on the state The operator view of this part of the control of the detector hardware and its environment. It system is via screens controlled by the EPICS dis- is the task of the archiver processes, each paired play manager (DM). Dedicated control and dis- with a CP, to collect the ambient data, aggregate play panels were developed using DM for each of them and write out histories approximately ev- the detector systems, using common color rules ery hour to the ambient database. These recorded to show the status of devices. A top-level panel data are associated with times so that they may for ODC summarizes the status of all systems and be correlated with the time stamps of the event provides access to specialized panels. data. Data from the archiver processes or from The EPICS alarm handler with some BABAR- the database may be viewed with a custom graph- specific modifications is used to provide oper- ical browser. ators with audible and color-coded alarms and Ambient data typically vary only within a nar- warnings in a hierarchical view of all the systems row noise range or dead-band. The storage of and components. Conditions directly relevant to unnecessary data is avoided by recording only personnel or detector safety are further enforced those monitored quantities which move outside of by hardware interlocks, the status of which are a per-channel dead-band range or across an alarm themselves displayed in a set of uniform EPICS threshold. screens, in the alarm handler, and on an alarm annunciator panel. 12.5.4. Integration With the Accelerator Close integration between the BABAR detector 12.5.3. Interfaces to Other and the PEP-II accelerator is essential for safe BABAR Software and eﬃcient data collection. Data from the accel- A custom C++ layer above EPICS consisting erator control system are transferred via EPICS of Component Proxies and Archivers provides for channel access to BABAR for display and storage, device-oriented state management and archival managed by a dedicated CP. In turn, background data collection. This is ODC’s interface to the signals from the detector are made available to rest of the online system. PEP-II to aid in injection and tuning, minimiz- The 27 component proxies (CPs), running on ing backgrounds, and optimizing integrated lu- a Unix application server, each define a logical minosity. An important component of this com- component representing some aspect of a detec- munication is the “injection request” handshake. tor system or the experiment’s central support When the PEP-II operator requests a significant 104 change in the beam conditions, such as injection, The system is highly automated; user input is the request can only procede following confirma- generally required only to initialize the system, tion from BABAR. This procedure complements start and stop runs, and handle unusual error the safety interlocks based on radiation dose mon- conditions. The user communicates with ORC itors. via a configurable Motif-based GUI included in SMI++. 12.5.5. Operational Experience The states and behavior of ORC objects repre- The ODC subsystem has been operational since senting external systems are provided by a spe- the initial cosmic-ray commissioning of the detec- cial class of intermediate software processes called tor and the beginning of data-taking with col- proxies. A proxy monitors its system, provides liding beams. The core EPICS infrastructure an abstraction of it to ORC, and receives state has proven to be very robust. The large size transition commands. These commands are in- of the subsystem, with its 15 IOCs and ∼ < 105 terpreted and applied to the underlying hardware records, produces heavy but manageable traﬃc or software components, implementing the tran- on the experiment’s network. The rate of data sitions’ actions. The control level of an ODF par- into the ambient database averages 4.6 Mbytes/hr tition is an example of such a proxy. or 110 Mbytes/day. Communication between the various proxies and the ORC engines is provided by DIM [136], 12.6. Run Control a fault tolerant “publish and subscribe” commu- The ORC subsystem is implemented as an ap- nications package based on TCP/IP sockets, al- plication of SMI++, a toolkit for designing dis- lowing ORC to be distributed transparently over tributed control systems [135]. Using this soft- a network. ware, the BABAR experiment is modeled as a col- Essential to the operation of the online sys- lection of objects behaving as finite state ma- tem is the notion of the Runnable status of its chines. These objects represent both real entities, various ODC and data acquisition components, such as the ODF subsystem or the drift chamber indicating that they are in a state suitable for high voltage controller, and abstract subsystems production-quality data-taking. The ORC logic such as the “calibrator,” a supervisor for the co- interlocks data-taking to the logical AND of all ordination of online components during detector components’ Runnable status. Whenever this calibration. The behavior of the objects are de- condition is not satisfied, data-taking may not scribed in a specialized language (SML) which is start and any existing run will be paused with interpreted by a generic logic engine to implement an alert sent to the operator. the control system. The SML descriptions of the objects which 12.7. Common Software Infrastructure make up the experiment simply specify their own 12.7.1. Databases states and transitions as well as the connections Five major databases are used by the online between the states of diﬀerent objects. Ob- system: jects perform actions on state transitions, which 1. Configuration Database: This database, im- may include explicitly commanding transitions in plemented using the commercial object-oriented other objects; objects may also be programmed database management system Objectivity [6], al- to monitor and automatically respond to changes lows the creation of hierarchical associations of of state in other objects. Anticipated error con- system-specific configuration data with a single ditions in components of the online system are numeric configuration key. This key is distributed reflected in their state models, allowing many er- to all online components, which can then use it rors to be handled automatically by the system. to retrieve from the database all the configuration To reduce complexity, logically related objects are information they require. Convenient mnemonics grouped together into a hierarchy of cooperating are associated with the keys for currently rele- domains. vant configurations, and may be selected for use 105 via the ORC GUI [137]. 12.7.2. Software Release Control 2. Conditions Database: The Conditions and Configuration Management Database is used to record calibration and align- All of the online software is maintained in the ment constants, and the configuration keys in common BABAR code repository, based on the force during data-taking runs. It has the addi- freely available Concurrent Versions System soft- tional feature that the data for a given time in- ware, CVS [141]. terval may be updated as they are refined in the The online’s Unix and VxWorks applications course of improved understanding of the appara- are built and maintained with an extension of the tus [138]. standard BABAR software release tools [142]. At The Configuration and Conditions Databases the start of every data-taking run, the identities are both made available for reconstruction and of the current production software release and any physics analysis. installed patches are recorded; thus it is possible 3. Ambient Database: The Ambient Database at a later date to reconstruct the versions of online is used principally by the ODC subsystem to software used to acquire data. record detector parameters and environmental data at the time they are measured [137]. 12.8. Summary and Outlook Both the Ambient and Conditions databases, The online system has exceeded its data acqui- are implemented using Objectivity, and are based sition performance goals. It is capable of acquir- on the notion of time histories of various data ing colliding beam events, with an average size associated with the experiment. The history for of 28 kbytes, at a ∼ 2500 Hz L1 trigger rate and each item is divided into intervals over which a reducing this rate in L3 to the required ∼ 120 Hz specific value is consistent. limit. This provides comfortable margins, since 4. Occurrence (Error) Log: Informational and under normal beam conditions the L1 trigger rate error messages generated in the online system is 800–1000 Hz. are routed through the CMLOG system [139] to The system is capable of logging data at much a central database, from which they are avail- higher rates; the nominal 120 Hz figure represents able for operators’ realtime viewing or historical a compromise between data volume and its con- browsing, using a graphical tool, as well as for sequential load on downstream processing and subscription by online components which may re- archival storage, and trigger eﬃciency for low quire notification of certain occurrences. multiplicity final states. 5. Electronic Logbook: An Oracle-based [140] During normal data-taking, the online system logbook is used to maintain the history of the routinely achieves an eﬃciency of over 98%, tak- data-taking, organized by runs. It contains infor- ing both data acquisition livetime and the sys- mation on beam parameters—instantaneous and tem’s overall reliability into account. integrated luminosity, currents, and energies— There are several hardware options for enhanc- as well as records of data acquisition parame- ing ODF capacity. Currently most ROMs receive ters such as trigger rates, data volumes, and dead more than one fiber from the FEE. These fibers times, and the detector configuration used for a could be distributed over more ROMs to add pro- run. The logbook also contains text comments cessing power. There are also commercial up- and graphics added by the operations staﬀ. grade paths for the ROMs’ SBC boards available. A number of other databases are used in the Crates can be split (up to a maximum total of 32) online system for various tasks such as indexing to create more VME event building bandwidth, as logged data files, the repair history of online hard- well as more fragment level CPU power and net- ware and spares, and software problem reports. work bandwidth. Gigabit Ethernet connections could also be installed to improve the network event builder’s bandwidth. Various software upgrade options are being investigated, including optimizing the VxWorks 106 network drivers and grouping sets of events to- also improve the matching of tracks with signals gether in order to reduce the impact of per-event in the DIRC and EMC. Detailed studies and the overhead. full integration of all available information per- Current background projections indicate that tinent to the identification of charged and neu- fragment level CPU, segment level memory bus tral particles are expected to result in better un- bandwidth, and network event building band- derstanding and improved performance of various width are the most likely bottlenecks for future techniques. running. Beyond routine maintenance, minor upgrades Increases in the L1 trigger rate or in the back- and a few replacements of faulty components are ground occupancy and complexity of events are currently planned. They include the replacement expected to necessitate additional capacity for of SVT modules that are expected to fail in the OEP, principally for L3 triggering. The online next few years due to radiation damage, plus a farm machines could be replaced with faster mod- few others that cannot be correctly read out due els. More machines could be added, at the ex- to broken connections. A large fraction of the pense of increases in coherent loading on various RPCs are showing gradually increasing losses in servers and of additional management complex- eﬃciency and plans are being developed for the ity. replacement of the RPC modules over the next No significant capacity upgrades to the data few years. Furthermore, 20-25 cm of absorber will logging subsystem or to ODC are foreseen at this be added to the flux return to reduce the hadron time. misidentification rates. With the expected increase in luminosity, machine-induced backgrounds will rise. Measures 13. Conclusions are being prepared to reduce the sources and the During the first year of operation, the BABAR impact of such backgrounds on BABAR. Apart detector has performed close to expectation with from the addition of shielding against shower de- a high degree of reliability. In parallel, the PEP-II bris, upgrades to the DCH power supply system storage rings gradually increased its performance and to the DIRC electronics are presently under and towards the end of the first year of data- way. Most important are upgrades to the trigger, taking routinely delivered close to design luminos- both at levels L1 and L3. Specifically, the DCH ity. In fact, the best performance surpassed the stereo layer information will be added to allow for design goals, both in terms of instantaneous as a more eﬃcient suppression of background tracks well as integrated luminosity per day and month. from outside the luminous region of the beam. Of the total luminosity of 23.5 fb−1 delivered by The L3 processing will be refined so as to reject PEP-II during the first ten months of the year both backgrounds and high rate QED processes 2000, BABAR logged more than 92%. The data with higher eﬃciency. In addition, data acquisi- are fully processed with a delay of only a few tion and processing capacity will be expanded to hours. They are of very high quality and have meet the demands of higher luminosity. been extensively used for physics analysis. In summary, the BABAR detector is perform- A large variety of improvements to the ing very well under current conditions and is well event reconstruction and detailed simulation are suited to record data at significantly higher than presently being pursued. They include improve- design luminosity. ments in many aspect of the calibration and re- construction procedures and software, for exam- Acknowledgements ple, the calibration and noise suppression in the EMC, and the development of techniques for pre- The authors are grateful for the tremendous cision alignment of the SVT and DCH. The latter support they have received from their home in- eﬀort will not only benefit the overall eﬃciency stitutions and supporting staﬀ over the past six and precision of the track reconstruction, it will years. They also would like to commend their 107 PEP-II colleagues for their extraordinary achieve- Systems, Inc., Alameda, CA, USA. ment in reaching the design luminosity and high 9. T. Glanzman et al., The BABAR Prompt reliability in a remarkably short time. The col- Reconstruction System, Proceedings of the laborating institutions wish to thank SLAC for International Conference on Computing in its support and kind hospitality. High Energy Physics, Chicago, USA (1998). This work has been supported by the US De- F. Safai Tehrani, The BABAR Prompt Recon- partment of Energy and the National Science struction Manager: A Real Life Example of a Foundation, the Natural Sciences and Engineer- Constructive Approach to Software Develop- ing Research Council (Canada), the Institute of ment, submitted to Computer Physics Com- High Energy Physics (P.R. China), le Commis- munications (2000). ariat a` l’Energie Atomique and Institut National 10. J. Seeman et al., The PEP-II Storage Rings, de Physique Nucl´eaire et de Physique des Par- SLAC-PUB-8786 (2001), submitted to Nucl. ticules (France), Bundesministerium f¨ ur Bildung Instr. and Methods . und Forschung (Germany), Istituto Nazionale di 11. J. Seeman et al., Status Report on PEP-II Fisica Nucleare (Italy), the Research Council of Performance, Proceeedings of the 7th Euro- Norway, the Ministry of Science and Technol- pean Particle Accelerator Conference (EPAC ogy of the Russian Federation, and the Par- 2000), Vienna, Austria (2000). ticle Physics and Astronomy Research Council 12. M. Sullivan, B-Factory Interaction Region (United Kingdom). In addition, individuals have Designs, Proceedings of the IEEE Particle received support from the Swiss National Foun- Accelerator Conference (PAC97), Vancouver, dation, the A.P. Sloan Foundation, the Research B.C., Canada (1997), SLAC-PUB-7563. Corporation, and the Alexander von Humboldt 13. S.E. Csorna et al., (CLEO Collaboration), Foundation. Phys. Rev. 61 (2000) 111101. 14. T. Mattison et al., Background Measure- ments during PEP-II Commissioning, Pro- REFERENCES ceedings of the IEEE Particle Accelerator 1. PEP-II - An Asymmetric B Factory, Con- Conference (PAC99), New York, NY, USA ceptual Design Report, SLAC-418, LBL-5379 (1999). (1993). 15. W. Kozanecki, Nucl. Instr. Methods A446 2. The BABAR Collaboration, Letter of Intent for (2000) 59. the Study of CP Violation and Heavy Flavor 16. C. Hast et al., Report of the High-Luminosity Physics at PEP-II, SLAC-443 (1994). Background Task Force, BABAR Note 522 3. BABAR Technical Design Report, SLAC-R- (2000). 457 (1995). 17. T.I. Meyer et al., Contribution to DPF 2000, 4. The BABAR Physics Book, Physics at Meeting of the Division of Particles and Fields an Asymmetric B Factory, P.F. Harrison, of the American Physical Society, Columbus, H.R. Quinn, Editors, SLAC-R-504 (1998). OH, USA (2000). 5. The Physics Program of a High-Luminosity 18. B. Camanzi, et al., Nucl. Instr. Methods A Asymmetric B Factory at SLAC, SLAC- 457 (2001) 476. 353 (1989); Proceedings of the Workshop on 19. F.Kircher, et al., IEEE Transactions on Ap- Physics and Detector Issues for a High Lu- plied Superconductivity, 9 #2 (1999) 847. minosity Asymmetric B Factory at SLAC, 20. Opera-2D, Vector Fields, Inc., Aurora, IL, SLAC-373 (1991). USA; ANSYS by ANSYS Inc., Canonsburg, 6. Objectivity, Inc., Mountain View, CA, USA. c PA, USA; Mermaid ( 1994) by Sim Limited, 7. L. Dalesio et al., Nucl. Instr. Methods A352 Novosibirsk, Russia. (1994) 179. 21. A. Onuchin et al., Magnetic Field Calcula- 8. The VxWorks realtime operating system and tion in the BABAR Detector, BABAR Note 344 Tornado Development interface, Wind River (1996). 108 22. L. Keller et al., Magnetic Field Calculation in 46. Luma Metall AB, Kalmar, Sweden. the BABAR Detector, BABAR Note 370 (1997). 47. California Fine Wire, Grover Beach, CA, 23. Kawasaki Heavy Industries (KHI), Kobe, USA. Japan. 48. A. Berenyi et al., IEEE Trans. Nucl. Sci. 46 24. Europa Metali SpA, Fornaci di Barga, Italy. (1999) 348; ibid. 46 (1999) 928. Alcatel Swiss Cable (now Nexans), Cortail- 49. J. Albert et al., IEEE Trans. Nucl. Sci. 46 lod, Switzerland. (1999) 2027; A. Bouchan et al., Nucl. Instr. 25. Solenoid cool-down and cryogenic Helium is Methods A409 (1998) 46; G. Sciolla et al., supplied by a modified Linde TCF-200 lique- Nucl. Instr. Methods A419 (1998) 310. fier/refrigerator. 50. D. Dorfan et al., Nucl. Instr. Methods A409 26. Ansaldo Energia, Genova, Italy. (1998) 310. 27. The solenoid was shipped from Genova on a 51. S.F. Dow et al., IEEE Trans. Nucl. Sci. 46 C5-B transport plane of the US Air Force. (1999) 785. 28. E. Antokhin et al., Nucl. Instr. Methods A432 52. Y. Karyotakis, D. Boutigny, LAPP Annecy, (1999) 24. France, private communication. 29. C. Newman-Holmes, E.E. Schmidt, R. Ya- 53. Garfield: Simulation of Gaseous Detectors, mada, Nucl. Instr. Methods A274 (1989) 443. CERN Program Library (1992). 30. Sentron model 2MR-4A/3B-14B25-20 (2- 54. P. Billoir, Nucl. Instr. Methods A225 (1984) element) and, AMR-3B-14B25-20 (1-element) 225. Hall Probes, GMW Associates, San Carlos, 55. D.N. Brown, E.A. Charles, D.A. Roberts, The CA, USA. BABAR Track Fitting Algorithm, Proceedings 31. Metrolab model PT 2025 Telsameter with of CHEP 2000, Padova, Italy (2000). model 1060 NMR probe, Metrolab Instru- 56. B.N. Ratcliﬀ, SLAC-PUB-5946 (1992); B. N. ments, SA, CH-1228, Geneva, Switzerland. Ratcliﬀ, SLAC-PUB-6067 (1993); P. Coyle et 32. A. Boyarski et al., Field Measurements in the al., Nucl. Instr. Methods A343 (1994) 292. BABAR Solenoid, BABAR Note 514 (2000). 57. Spectrosil is a trademark of TSL Group PCL, 33. UBE Industries, Japan. see also [34] Wallsend, Tyne on Wear, NE28 6DG, Eng- 34. C. Bozzi et al., Nucl. Instr. Methods A447 land; Sold in the USA by Quartz Products (2000) 20. Co., Louisville, KY, USA. 35. D. Barbieri et al., Nuo. Cim. A112 (1999) 113. 58. I. Adam et al., IEEE Trans. Nucl. Sci. 36. MICRON Semiconductor Ltd., Lancing, U.K. 45 No. 3 (June) 657; I. Adam et al., 37. L. Bosisio, INFN Trieste, Italy, private com- ibid 450; J. Cohen-Tanugi, M. C. Convery, munication. B. N. Ratcliﬀ, X. Sarazin, J. Schwiening, 38. G. Della Ricca et al., Nucl. Instr. Methods J. Va’vra, SLAC-JOURNAL-ICFA-21, ICFA A409 (1998) 258. Instrumentation Bulletin, Fall 2000 Issue. 39. V. Re et al., Nucl. Instr. Methods A409 59. M. Benkebil et al., Nucl. Instr. Methods A442 (1998) 354. (2000) 364. 40. J. Beringer et al., The Data Transmission Sys- 60. Boeing Optical Fabrication, Albuquerque, tem for the BABAR Silicon Vertex Tracker, NM, USA. BABAR Note 518 (2000). 61. Electron Tubes Limited (formerly: Thorn 41. R. Claus et al., SLAC-PUB-8134 (1999). EMI Electron Tubes), Ruislip, Middlesex, 42. CAEN, Costruzioni Apparecchiature Elet- England. troniche Nucleari, Viareggio, Italy. 62. P. Bourgeois, M. Karolak, G. Vasseur, Nucl. 43. Medelec Mechanical, Puidoux, Switzerland. Instr. Methods A442 (2000) 105. 44. Celenex 3300-2 polyester thermoplastic with 63. Epoxy Technology, Inc., Billerica, MA, USA. 30% silica glass fiber. 64. J. Va’vra, Nucl. Instr. Methods A453 (2000) 45. E. Borsato et al., Nucl. Instr. Methods A451 262. (2000) 414. 65. P. Bailly et al., Nucl. Instr. Methods A432 109 (1999) 157. 81. Fluorinert (polychlorotrifluoro-ethylene) is 66. J. Ardelean et al., LAL-RT-97-04 (1997). manufactured by 3M Corporation, St. Paul, 67. P. Bailly et al., Nucl. Instr. Methods A433 MN, USA. (1999) 432. 82. S-2744-08 PIN diode by Hamamatsu Photon- 68. The micro-controller is a MC68HC05x32, Mo- ics, K. K., Hamamatsu City, Japan. Dark cur- torola Inc., Schaumburg, IL, USA. rent < 5 nA, capacitance < 105 pF at the 69. I. Adam et al., SLAC-PUB-8783 (2001), to be depletion voltage of 70 V. published in IEEE Trans. Nucl. Sci. 83. J. Harris, C. Jessop, Performance Tests 70. R. Aleksan et al., Nucl. Instr. Methods A397 of Hamamatsu 2774-08 Diodes for the (1997) 261. BABAR Electromagnetic Calorimeter Front 71. T. Swarnicki, Performance of the CLEO-II End Readout and Proposal for Reliability Is- CsI(Tl) Calorimeter, Proceedings of Work- sues, BABAR Note 236 (1995). shop on B Factories, Stanford, CA, USA 84. G. Haller, D Freytag, IEEE Trans. Nucl. Sci. (1992). 43 (1996) 1610. 72. R.J. Barlow et al., Nucl. Instr. Methods A420 85. C. Jessop, Reliability Issues for the BABAR (1999) 162. CsI(Tl) Calorimeter Front End Readout, 73. Aldrich-APL, Urbana, IL, USA. Chemetall BABAR Note 217 (1995). GmbH, Frankfurt, Germany. 86. S. Menke, Oﬄine Correction of Non- 74. Shanghai Institute of Ceramics, Shanghai, Linearities in the BABAR Electromagnetic P.R.China; Beijing Glass Research Institute, Calorimeter, BABAR Note 527 (2000). Beijing, P.R.China; Hilger Analytical, Mar- 87. F. Gaede, D. Hitlin, M. Weaver, The Radioac- gate, Kent, UK; Crismatec, Nemours, France; tive Source Calibration of the BABAR Elec- Amcrys-H, Kharkov, Ukraine. tromagnetic Calorimeter, BABAR Note 531 75. TYVEK, registered trademark of E.I. DuPont (2001); de Nemours & Co., Wilmington, DE, USA. J. Button-Shafer et al., Use of Radioactive 76. G. Dahlinger, Aufbau und Test eines Photon Sources with the BABAR Electromag- Kalorimeter-Prototyps aus CsI(Tl) zur netic Calorimeter, BABAR Note 322 (1996). Energie- und Ortsmessung hochenergetis- 88. R. M¨ uller-Pfeﬀerkorn, Die Kalibration des cher Photonen, Ph.D Thesis, Technische elektromagnetischen CsI(Tl)-Kalorimeters Universit¨at Dresden, Germany, (1998). des BABAR-Detektors mit Ereignissen der 77. EPILOX A17-01 manufactured by Leunaer Bhabha-Streuung, Ph.D Thesis, TUD- Harze GmbH, Leuna, Germany. IKTP/01-01, Technische Universit¨at Dres- 78. J. Brose, G. Dahlinger, K.R. Schubert, Nucl. den, Germany (2001). Instr. Methods A417 (1998) 311; 89. GEANT Detector Description and Simulation C. Jessop et al., Development of Front End tool, CERN Program Library, Long Writeup Readout for the BABAR CsI(Tl) Calorimeter, W5013 (1994). BABAR Note 216 (1995); 90. S. Menke et al., Calibration of the BABAR C. Jessop et al., Development of Direct Read- Electromagnetic Calorimeter with π 0 s, out for CsI Calorimeter, BABAR Note 270 BABAR Note 528 (2000). (1995). 91. J. Bauer, Kinematic Fit for the EMC Ra- 79. NE-561 manufactured by Nuclear Enter- diative Bhabha Calibration, BABAR Note 521 prises, Sighthill, Edinburgh, Scotland. (2000). 80. The calibration procedure employed in this 92. M. Kocian, Das Lichtpulsersystem des measurement introduces a dependency of the elektromagnetischen CsI(Tl)-Kalorimeters light yield on the shaping time of the pream- des BABAR-Detektors, Ph.D Thesis, TUD- plifier. When connected to the actual front- IKTP/00-03, Technische Universit¨at Dres- end electronics in the BABAR detector, the den, Germany (2000). signal is reduced by a factor 1.29. 93. P.J. Clark, The BABAR Light Pulser System, 110 Ph.D Thesis, University of Edinburgh, UK 107.A. Berenyi et al., IEEE Trans. Nucl. Sci. 46 (2000); (1999) 348. B. Lewandowski, Entwicklung und Aufbau 108.A. Berenyi et al., IEEE Trans. Nucl. Sci. 46 eines Lichtpulsersystems f¨ ur das Kalorime- (1999) 928. ter des BABAR-Detektors, Ph.D Thesis, Ruhr- 109.A. Berenyi et al., Nuclear Science Sympo- Universit¨at Bochum, Germany (2000). sium, Conference Record 2 (1998) 988. 94. B. Brabson et al., Nucl. Instr. Methods A332 110.ORCA 2C series, Lucent Technologies, Berke- (1993) 419. ley Heights, NJ, USA. 95. S. Otto, Untersuchungen zur Ortsrekonstruk- 111.K. Kinoshita, Nucl. Instr. Methods A276 tion electromagnetischer Schauer, Diplomar- (1989) 242. beit, Technische Universit¨ at Dresden, Ger- 112.Xilinx 4020E, Xilinx Inc., Mountain View, many (2000). CA, USA. 96. R. Santonico, R. Cardarelli, Nucl. Instr. 113.P. D. Dauncey et al., Design and Performance Methods A187 (1981) 377. of the Level 1 Calorimeter Trigger for the 97. F. Anulli et al., Nucl. Instr. Methods A409 BABAR Detector, to be published in IEEE (1998) 542. Trans. Nucl. Sci. (2001). 98. A. Calcaterra et al., Performance of the 114.G.C. Fox, S. Wolfram, Nucl. Phys. B149 BABAR RPCs in a Cosmic Ray Test, Proceed- (1979) 413. ings of the International Workshop on Resis- 115.Sun Ultra 5, with single 333 MHz tive Plate Chambers and Related Detectors, UltraSPARC-IIi CPUs and 512 Mbytes Naples, Italy (1997). of RAM, Sun Microsystems, Inc., Palo Alto, 99. N.Cavallo et al., Nucl. Phys. B(Proc. Suppl.) CA, USA. 61B (1998) 545; N. Cavallo et al., Nucl. Instr. 116.Cisco model 6500, Cisco Systems, Inc., San Methods A409 (1998) 297. Jose, CA, USA. 100.HV system SY-127, Pod Models A300-P and 117.A single Sun Microsystems Ultra Enter- A300-N by CAEN, Viareggio, Italy. prise 450, with four 300 MHz CPUs, 2 Gbytes 101.G. Crosetti et al., Data Acquisition System of RAM, and 720 Gbytes of RAID-3 disk, acts for the RPC Detector of BABAR Experiment, as the data logging server as well as the ODF Proceedings of the International Workshop on boot server. Beginning in 2001 all its respon- Resistive Plate Chambers and Related Detec- sibilities other than data logging were moved tors, Naples, Italy (1997). to a new Sun Ultra 220R machine with dual 102.J. Christiansen, 32-Channel TDC with On- 450 MHz UltraSPARC-II CPUs, 1 Gbytes of chip Buﬀering and Trigger Matching, Pro- RAM, and an additional 200 Gbytes RAID ar- ceedings of Electronics for LHC experiments, ray. London, UK (1997) 333. S. Minntoli and 118.These file and database servers (presently E. Robutti, IEEE Trans. Nucl. Sci. 47 (2000) five) are primarily additional Sun Ul- 147. tra 220Rs, each with about 200 Gbytes of 103.P. Paolucci, The IFR Online Detector Control RAID disk. The ten application servers are System, SLAC PUB-8167 (1999). a mix of Sun Ultra 60s, with dual 360 MHz 104.L. Lista, Object Oriented Reconstruction UltraSPARC-II CPUs and 1 Gbytes of RAM, Software for the IFR Detector of BABAR Ex- and Ultra 5s; the console servers are all Ul- periment, Proceedings of the Conference on tra 5s. Computing in High Energy Physics, Padova, 119.R. Claus et al., Development of a Data Italy (2000). Acquisition System for the BABAR CP 105.A. Zallo et al., Nucl. Instr. Methods A456 Violation Experiment, Proceedings of (2000) 117. the 11th IEEE NPSS RealTime Con- 106.A. Berenyi et al., IEEE Trans. Nucl. Sci. 46 ference, Santa Fe, NM, USA (1999), (1999) 2006. http://strider.lansce.lanl.gov/- 111 rt99/index11.html. International Business Machines, Inc., Ar- 120.P. Grosso et al., The BABAR Fast Control Sys- monk, NY, USA. tem, Proceedings of the International Confer- 132.The ODC IOCs are model MVME177 boards ence on Computing in High-Energy Physics, with MC68060 CPUs from Motorola Com- Chicago, IL, USA (1998). puter Group, op. cit.. 121.J. Postel, User Datagram Protocol, RFC-0768 133.Controller area network (CAN) for high- (1980); Internet documentation including all speed communication (1993); ISO standards: RFCs (Requests for Comment) are available 11519 Road vehicles (Low-speed serial data online from the Internet Engineering Task communication) (1994); 11898, Road ve- Force, http://www.ietf.org. hicles (Interchange of digital information), 122.Motorola model MVME2306 boards, each http://www.iso.ch/. with a 300 MHz PowerPC 604 CPU, 134.T. Meyer, R. McKay, The BABAR General 32 Mbytes of RAM, 5 Mbytes of non-volatile Monitoring Board, BABAR Note 366 (1998). flash memory, two PCI mezzanine card slots 135.B. Franek, C. Gaspar, IEEE Trans. Nucl. Sci. (only one of which is currently used), a 45 (1998) 1946. 100 Mbits/s Ethernet interface, and a Tundra 136.C. Gaspar, M. Donszelman, DIM - A Dis- Universe II VME interface, Motorola Inc., tibuted Information Management System for Tempe, AZ, USA. the DELPHI experiment at CERN, Pro- 123.G-Link, Giga-bit Transmit/Receive Chip Set ceedings of the IEEE Eighth Conference on HDMP-1012/HDMP-1014, Hewlett-Packard, Real-Time Computer Applications in Nu- Inc., Palo Alto, CA, USA. clear, Particle and Plasma Physics, Vancou- 124.T.J. Pavel et al., Network Performance Test- ver, Canada (1993). ing for the BABAR Event Builder, Proceedings 137.G. Zioulas, et al., Ambient and Configuration of the CHEP Conference, Chicago, IL, USA databases for the BABAR online system, Pro- (1998). ceedings of the IEEE Real Time Conference, 125.S. Deering, Host Extensions for IP Multicas- Santa Fe, NM, USA (1999). ting, RFC-1112 (1989). 138.I. Gaponenko, et al., An Overview of the 126.G. P. Dubois-Felsmann, E. Chen, Yu. BABAR Conditions Database, Proceedings of Kolomensky et al., IEEE Trans. Nucl. Sci. 47 the International Conference on Computing (2000) 353. in High Energy Physics, Padova, Italy (2000). 127.S. Metzler, et al., Distributed Histogram- 139.cdev and CMLOG are software facilities pro- ming, Proceedings of the International Con- duced by the Accelerator Controls group ference on Computing in High Energy of the Thomas Jeﬀerson National Accelera- Physics, Chicago, IL, USA (1998). tor Facility. Documentation is available from 128.CORBA (Common Object Request Broker their Web site, http://www.cebaf.gov/. Architecture) Standards, Object Manage- 140.Oracle Corporation, Redwood Shores, CA, ment Group, http://www.corba.org/. USA. 129.ACE/TAO CORBA implementation, Dis- 141.The Concurrent Versions System is an open- tributed Object Computing Group, Wash- source distributed version control system, ington University, St. Louis, Missouri, http://www.cvshome.org/. USA and University of California, Irvine, 142.BABAR Software Release Tools, CA, USA, http://www.cs.wustl.edu/- http://www.slac.stanford.edu/- schmidt/TAO.html. BFROOT/www/doc/workbook/srt1/srt1.html. 130.T.Johnson, Java Analysis Studio, Proceedings of the International Conference on Comput- ing in High Energy Physics, Padova, Italy (2000). 131.High Performance Storage System (HPSS),

References (145)

PEP-II -An Asymmetric B Factory, Con- ceptual Design Report, SLAC-418, LBL-5379 (1993).
The BABAR Collaboration, Letter of Intent for the Study of CP Violation and Heavy Flavor Physics at PEP-II, SLAC-443 (1994).
BABAR Technical Design Report, SLAC-R- 457 (1995).
The BABAR Physics Book, Physics at an Asymmetric B Factory, P.F. Harrison, H.R. Quinn, Editors, SLAC-R-504 (1998).
The Physics Program of a High-Luminosity Asymmetric B Factory at SLAC, SLAC- 353 (1989); Proceedings of the Workshop on Physics and Detector Issues for a High Lu- minosity Asymmetric B Factory at SLAC, SLAC-373 (1991).
L. Dalesio et al., Nucl. Instr. Methods A352 (1994) 179.
The VxWorks realtime operating system and Tornado Development interface, Wind River Systems, Inc., Alameda, CA, USA.
T. Glanzman et al., The BABAR Prompt Reconstruction System, Proceedings of the International Conference on Computing in High Energy Physics, Chicago, USA (1998). F. Safai Tehrani, The BABAR Prompt Recon- struction Manager: A Real Life Example of a Constructive Approach to Software Develop- ment, submitted to Computer Physics Com- munications (2000).
J. Seeman et al., The PEP-II Storage Rings, SLAC-PUB-8786 (2001), submitted to Nucl. Instr. and Methods .
J. Seeman et al., Status Report on PEP-II Performance, Proceeedings of the 7th Euro- pean Particle Accelerator Conference (EPAC 2000), Vienna, Austria (2000).
M. Sullivan, B-Factory Interaction Region Designs, Proceedings of the IEEE Particle Accelerator Conference (PAC97), Vancouver, B.C., Canada (1997), SLAC-PUB-7563.
S.E. Csorna et al., (CLEO Collaboration), Phys. Rev. 61 (2000) 111101.
T. Mattison et al., Background Measure- ments during PEP-II Commissioning, Pro- ceedings of the IEEE Particle Accelerator Conference (PAC99), New York, NY, USA (1999).
W. Kozanecki, Nucl. Instr. Methods A446 (2000) 59.
C. Hast et al., Report of the High-Luminosity Background Task Force, BABAR Note 522 (2000).
T.I. Meyer et al., Contribution to DPF 2000, Meeting of the Division of Particles and Fields of the American Physical Society, Columbus, OH, USA (2000).
B. Camanzi, et al., Nucl. Instr. Methods A 457 (2001) 476.
F.Kircher, et al., IEEE Transactions on Ap- plied Superconductivity, 9 #2 (1999) 847.
A. Onuchin et al., Magnetic Field Calcula- tion in the BABAR Detector, BABAR Note 344 (1996).
L. Keller et al., Magnetic Field Calculation in the BABAR Detector, BABAR Note 370 (1997).
Europa Metali SpA, Fornaci di Barga, Italy. Alcatel Swiss Cable (now Nexans), Cortail- lod, Switzerland.
Solenoid cool-down and cryogenic Helium is supplied by a modified Linde TCF-200 lique- fier/refrigerator.
Ansaldo Energia, Genova, Italy.
The solenoid was shipped from Genova on a C5-B transport plane of the US Air Force.
E. Antokhin et al., Nucl. Instr. Methods A432 (1999) 24.
C. Newman-Holmes, E.E. Schmidt, R. Ya- mada, Nucl. Instr. Methods A274 (1989) 443.
Sentron model 2MR-4A/3B-14B25-20 (2- element) and, AMR-3B-14B25-20 (1-element)
Hall Probes, GMW Associates, San Carlos, CA, USA.
Metrolab model PT 2025 Telsameter with model 1060 NMR probe, Metrolab Instru- ments, SA, CH-1228, Geneva, Switzerland.
A. Boyarski et al., Field Measurements in the BABAR Solenoid, BABAR Note 514 (2000).
UBE Industries, Japan. see also [34]
C. Bozzi et al., Nucl. Instr. Methods A447 (2000) 20.
D. Barbieri et al., Nuo. Cim. A112 (1999) 113.
MICRON Semiconductor Ltd., Lancing, U.K.
L. Bosisio, INFN Trieste, Italy, private com- munication.
G. Della Ricca et al., Nucl. Instr. Methods A409 (1998) 258.
V. Re et al., Nucl. Instr. Methods A409 (1998) 354.
J. Beringer et al., The Data Transmission Sys- tem for the BABAR Silicon Vertex Tracker, BABAR Note 518 (2000).
R. Claus et al., SLAC-PUB-8134 (1999).
CAEN, Costruzioni Apparecchiature Elet- troniche Nucleari, Viareggio, Italy.
Medelec Mechanical, Puidoux, Switzerland.
Celenex 3300-2 polyester thermoplastic with 30% silica glass fiber.
E. Borsato et al., Nucl. Instr. Methods A451 (2000) 414.
Luma Metall AB, Kalmar, Sweden.
California Fine Wire, Grover Beach, CA, USA.
A. Berenyi et al., IEEE Trans. Nucl. Sci. 46 (1999) 348; ibid. 46 (1999) 928.
J. Albert et al., IEEE Trans. Nucl. Sci. 46 (1999) 2027;
A. Bouchan et al., Nucl. Instr. Methods A409 (1998) 46; G. Sciolla et al., Nucl. Instr. Methods A419 (1998) 310.
D. Dorfan et al., Nucl. Instr. Methods A409 (1998) 310.
S.F. Dow et al., IEEE Trans. Nucl. Sci. 46 (1999) 785.
Y. Karyotakis, D. Boutigny, LAPP Annecy, France, private communication.
Garfield: Simulation of Gaseous Detectors, CERN Program Library (1992).
P. Billoir, Nucl. Instr. Methods A225 (1984) 225.
D.N. Brown, E.A. Charles, D.A. Roberts, The BABAR Track Fitting Algorithm, Proceedings of CHEP 2000, Padova, Italy (2000).
B.N. Ratcliff, SLAC-PUB-5946 (1992);
B. N. Ratcliff, SLAC-PUB-6067 (1993);
P. Coyle et al., Nucl. Instr. Methods A343 (1994) 292.
Spectrosil is a trademark of TSL Group PCL, Wallsend, Tyne on Wear, NE28 6DG, Eng- land; Sold in the USA by Quartz Products Co., Louisville, KY, USA.
I. Adam et al., IEEE Trans. Nucl. Sci. 45 No. 3 (June) 657; I. Adam et al., ibid 450; J. Cohen-Tanugi, M. C. Convery, B. N. Ratcliff, X. Sarazin, J. Schwiening, J. Va'vra, SLAC-JOURNAL-ICFA-21, ICFA Instrumentation Bulletin, Fall 2000 Issue.
M. Benkebil et al., Nucl. Instr. Methods A442 (2000) 364.
Boeing Optical Fabrication, Albuquerque, NM, USA.
Electron Tubes Limited (formerly: Thorn EMI Electron Tubes), Ruislip, Middlesex, England.
P. Bourgeois, M. Karolak, G. Vasseur, Nucl. Instr. Methods A442 (2000) 105.
J. Va'vra, Nucl. Instr. Methods A453 (2000) 262.
P. Bailly et al., Nucl. Instr. Methods A432 (1999) 157.
J. Ardelean et al., LAL-RT-97-04 (1997).
P. Bailly et al., Nucl. Instr. Methods A433 (1999) 432.
The micro-controller is a MC68HC05x32, Mo- torola Inc., Schaumburg, IL, USA.
I. Adam et al., SLAC-PUB-8783 (2001), to be published in IEEE Trans. Nucl. Sci.
R. Aleksan et al., Nucl. Instr. Methods A397 (1997) 261.
T. Swarnicki, Performance of the CLEO-II CsI(Tl) Calorimeter, Proceedings of Work- shop on B Factories, Stanford, CA, USA (1992).
R.J. Barlow et al., Nucl. Instr. Methods A420 (1999) 162.
Aldrich-APL, Urbana, IL, USA. Chemetall GmbH, Frankfurt, Germany.
Shanghai Institute of Ceramics, Shanghai, P.R.China; Beijing Glass Research Institute, Beijing, P.R.China; Hilger Analytical, Mar- gate, Kent, UK; Crismatec, Nemours, France; Amcrys-H, Kharkov, Ukraine.
TYVEK, registered trademark of E.I. DuPont de Nemours & Co., Wilmington, DE, USA.
G. Dahlinger, Aufbau und Test eines Kalorimeter-Prototyps aus CsI(Tl) zur Energie-und Ortsmessung hochenergetis- cher Photonen, Ph.D Thesis, Technische Universität Dresden, Germany, (1998).
EPILOX A17-01 manufactured by Leunaer Harze GmbH, Leuna, Germany.
J. Brose, G. Dahlinger, K.R. Schubert, Nucl. Instr. Methods A417 (1998) 311;
C. Jessop et al., Development of Front End Readout for the BABAR CsI(Tl) Calorimeter, BABAR Note 216 (1995);
C. Jessop et al., Development of Direct Read- out for CsI Calorimeter, BABAR Note 270 (1995).
NE-561 manufactured by Nuclear Enter- prises, Sighthill, Edinburgh, Scotland.
The calibration procedure employed in this measurement introduces a dependency of the light yield on the shaping time of the pream- plifier. When connected to the actual front- end electronics in the BABAR detector, the signal is reduced by a factor 1.29.
Fluorinert (polychlorotrifluoro-ethylene) is manufactured by 3M Corporation, St. Paul, MN, USA.
S-2744-08 PIN diode by Hamamatsu Photon- ics, K. K., Hamamatsu City, Japan. Dark cur- rent < 5 nA, capacitance < 105 pF at the depletion voltage of 70 V.
J. Harris, C. Jessop, Performance Tests of Hamamatsu 2774-08 Diodes for the BABAR Electromagnetic Calorimeter Front End Readout and Proposal for Reliability Is- sues, BABAR Note 236 (1995).
G. Haller, D Freytag, IEEE Trans. Nucl. Sci. 43 (1996) 1610.
C. Jessop, Reliability Issues for the BABAR CsI(Tl) Calorimeter Front End Readout, BABAR Note 217 (1995).
S. Menke, Offline Correction of Non- Linearities in the BABAR Electromagnetic Calorimeter, BABAR Note 527 (2000).
F. Gaede, D. Hitlin, M. Weaver, The Radioac- tive Source Calibration of the BABAR Elec- tromagnetic Calorimeter, BABAR Note 531 (2001);
J. Button-Shafer et al., Use of Radioactive Photon Sources with the BABAR Electromag- netic Calorimeter, BABAR Note 322 (1996).
R. Müller-Pfefferkorn, Die Kalibration des elektromagnetischen CsI(Tl)-Kalorimeters des BABAR-Detektors mit Ereignissen der Bhabha-Streuung, Ph.D Thesis, TUD- IKTP/01-01, Technische Universität Dres- den, Germany (2001).
GEANT Detector Description and Simulation tool, CERN Program Library, Long Writeup W5013 (1994).
S. Menke et al., Calibration of the BABAR Electromagnetic Calorimeter with π 0 s, BABAR Note 528 (2000).
J. Bauer, Kinematic Fit for the EMC Ra- diative Bhabha Calibration, BABAR Note 521 (2000).
M. Kocian, Das Lichtpulsersystem des elektromagnetischen CsI(Tl)-Kalorimeters des BABAR-Detektors, Ph.D Thesis, TUD- IKTP/00-03, Technische Universität Dres- den, Germany (2000).
P.J. Clark, The BABAR Light Pulser System, Ph.D Thesis, University of Edinburgh, UK (2000);
B. Lewandowski, Entwicklung und Aufbau eines Lichtpulsersystems für das Kalorime- ter des BABAR-Detektors, Ph.D Thesis, Ruhr- Universität Bochum, Germany (2000).
B. Brabson et al., Nucl. Instr. Methods A332 (1993) 419.
S. Otto, Untersuchungen zur Ortsrekonstruk- tion electromagnetischer Schauer, Diplomar- beit, Technische Universität Dresden, Ger- many (2000).
R. Santonico, R. Cardarelli, Nucl. Instr. Methods A187 (1981) 377.
F. Anulli et al., Nucl. Instr. Methods A409 (1998) 542.
A. Calcaterra et al., Performance of the BABAR RPCs in a Cosmic Ray Test, Proceed- ings of the International Workshop on Resis- tive Plate Chambers and Related Detectors, Naples, Italy (1997).
N.Cavallo et al., Nucl. Phys. B(Proc. Suppl.) 61B (1998) 545;
N. Cavallo et al., Nucl. Instr. Methods A409 (1998) 297.
HV system SY-127, Pod Models A300-P and A300-N by CAEN, Viareggio, Italy.
G. Crosetti et al., Data Acquisition System for the RPC Detector of BABAR Experiment, Proceedings of the International Workshop on Resistive Plate Chambers and Related Detec- tors, Naples, Italy (1997).
J. Christiansen, 32-Channel TDC with On- chip Buffering and Trigger Matching, Pro- ceedings of Electronics for LHC experiments, London, UK (1997) 333. S. Minntoli and E. Robutti, IEEE Trans. Nucl. Sci. 47 (2000) 147.
P. Paolucci, The IFR Online Detector Control System, SLAC PUB-8167 (1999).
L. Lista, Object Oriented Reconstruction Software for the IFR Detector of BABAR Ex- periment, Proceedings of the Conference on Computing in High Energy Physics, Padova, Italy (2000).
A. Zallo et al., Nucl. Instr. Methods A456 (2000) 117.
A. Berenyi et al., IEEE Trans. Nucl. Sci. 46 (1999) 2006.
A. Berenyi et al., IEEE Trans. Nucl. Sci. 46 (1999) 348.
A. Berenyi et al., IEEE Trans. Nucl. Sci. 46 (1999) 928.
A. Berenyi et al., Nuclear Science Sympo- sium, Conference Record 2 (1998) 988.
ORCA 2C series, Lucent Technologies, Berke- ley Heights, NJ, USA.
K. Kinoshita, Nucl. Instr. Methods A276 (1989) 242.
Xilinx 4020E, Xilinx Inc., Mountain View, CA, USA.
P. D. Dauncey et al., Design and Performance of the Level 1 Calorimeter Trigger for the BABAR Detector, to be published in IEEE Trans. Nucl. Sci. (2001).
G.C. Fox, S. Wolfram, Nucl. Phys. B149 (1979) 413.
Sun Ultra 5, with single 333 MHz UltraSPARC-IIi CPUs and 512 Mbytes of RAM, Sun Microsystems, Inc., Palo Alto, CA, USA.
A single Sun Microsystems Ultra Enter- prise 450, with four 300 MHz CPUs, 2 Gbytes of RAM, and 720 Gbytes of RAID-3 disk, acts as the data logging server as well as the ODF boot server. Beginning in 2001 all its respon- sibilities other than data logging were moved to a new Sun Ultra 220R machine with dual 450 MHz UltraSPARC-II CPUs, 1 Gbytes of RAM, and an additional 200 Gbytes RAID ar- ray.
These file and database servers (presently five) are primarily additional Sun Ul- tra 220Rs, each with about 200 Gbytes of RAID disk. The ten application servers are a mix of Sun Ultra 60s, with dual 360 MHz UltraSPARC-II CPUs and 1 Gbytes of RAM, and Ultra 5s; the console servers are all Ul- tra 5s.
R. Claus et al., Development of a Data Acquisition System for the BABAR CP Violation Experiment, Proceedings of the 11th IEEE NPSS RealTime Con- ference, Santa Fe, NM, USA (1999), http://strider.lansce.lanl.gov/- rt99/index11.html.
P. Grosso et al., The BABAR Fast Control Sys- tem, Proceedings of the International Confer- ence on Computing in High-Energy Physics, Chicago, IL, USA (1998).
J. Postel, User Datagram Protocol, RFC-0768 (1980); Internet documentation including all RFCs (Requests for Comment) are available online from the Internet Engineering Task Force, http://www.ietf.org.
Motorola model MVME2306 boards, each with a 300 MHz PowerPC 604 CPU, 32 Mbytes of RAM, 5 Mbytes of non-volatile flash memory, two PCI mezzanine card slots (only one of which is currently used), a 100 Mbits/s Ethernet interface, and a Tundra Universe II VME interface, Motorola Inc., Tempe, AZ, USA.
G-Link, Giga-bit Transmit/Receive Chip Set HDMP-1012/HDMP-1014, Hewlett-Packard, Inc., Palo Alto, CA, USA.
T.J. Pavel et al., Network Performance Test- ing for the BABAR Event Builder, Proceedings of the CHEP Conference, Chicago, IL, USA (1998).
S. Deering, Host Extensions for IP Multicas- ting, RFC-1112 (1989).
G. P. Dubois-Felsmann, E. Chen, Yu. Kolomensky et al., IEEE Trans. Nucl. Sci. 47 (2000) 353.
S. Metzler, et al., Distributed Histogram- ming, Proceedings of the International Con- ference on Computing in High Energy Physics, Chicago, IL, USA (1998).
CORBA (Common Object Request Broker Architecture) Standards, Object Manage- ment Group, http://www.corba.org/.
ACE/TAO CORBA implementation, Dis- tributed Object Computing Group, Wash- ington University, St. Louis, Missouri, USA and University of California, Irvine, CA, USA, http://www.cs.wustl.edu/- schmidt/TAO.html.
T.Johnson, Java Analysis Studio, Proceedings of the International Conference on Comput- ing in High Energy Physics, Padova, Italy (2000).
High Performance Storage System (HPSS), International Business Machines, Inc., Ar- monk, NY, USA.
The ODC IOCs are model MVME177 boards with MC68060 CPUs from Motorola Com- puter Group, op. cit..
Controller area network (CAN) for high- speed communication (1993); ISO standards: 11519 Road vehicles (Low-speed serial data communication) (1994); 11898, Road ve- hicles (Interchange of digital information), http://www.iso.ch/.
T. Meyer, R. McKay, The BABAR General Monitoring Board, BABAR Note 366 (1998).
B. Franek, C. Gaspar, IEEE Trans. Nucl. Sci. 45 (1998) 1946.
C. Gaspar, M. Donszelman, DIM -A Dis- tibuted Information Management System for the DELPHI experiment at CERN, Pro- ceedings of the IEEE Eighth Conference on Real-Time Computer Applications in Nu- clear, Particle and Plasma Physics, Vancou- ver, Canada (1993).
G. Zioulas, et al., Ambient and Configuration databases for the BABAR online system, Pro- ceedings of the IEEE Real Time Conference, Santa Fe, NM, USA (1999).
I. Gaponenko, et al., An Overview of the BABAR Conditions Database, Proceedings of the International Conference on Computing in High Energy Physics, Padova, Italy (2000).
cdev and CMLOG are software facilities pro- duced by the Accelerator Controls group of the Thomas Jefferson National Accelera- tor Facility. Documentation is available from their Web site, http://www.cebaf.gov/.
The Concurrent Versions System is an open- source distributed version control system, http://www.cvshome.org/.
BABAR Software Release Tools, http://www.slac.stanford.edu/- BFROOT/www/doc/workbook/srt1/srt1.html.

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