Improved determination of the sample composition of dimuon events produced in {\boldmath p p ¯ } collisions at {\boldmath s √ =1.96 } TeV
aa r X i v : . [ h e p - e x ] M a y FERMILAB-PUB-11-232-E
Improved determination of the sample composition of dimuonevents produced in p ¯ p collisions at √ s = 1 .
96 TeV
T. Aaltonen, B. ´Alvarez Gonz´alez w , S. Amerio, D. Amidei, A. Anastassov, A. Annovi, J. Antos, G. Apollinari, A. Apresyan, T. Arisawa, A. Artikov, J. Asaadi, W. Ashmanskas, B. Auerbach, A. Aurisano, F. Azfar, W. Badgett, A. Barbaro-Galtieri, V.E. Barnes, B.A. Barnett, P. Barria dd , P. Bartos, M. Bauce bb , F. Bedeschi, D. Beecher, S. Behari, G. Bellettini cc , J. Bellinger, D. Benjamin, A. Beretvas, A. Bhatti, M. Binkley ∗ , D. Bisello bb , I. Bizjak hh , K.R. Bland, B. Blumenfeld, A. Bocci, A. Bodek, D. Bortoletto, J. Boudreau, A. Boveia, B. Brau a , L. Brigliadori aa , A. Brisuda, C. Bromberg, E. Brucken, M. Bucciantonio cc , J. Budagov, H.S. Budd, S. Budd, K. Burkett, G. Busetto bb , P. Bussey, A. Buzatu, C. Calancha, S. Camarda, M. Campanelli, M. Campbell, F. Canelli , B. Carls, D. Carlsmith, R. Carosi, S. Carrillo k , S. Carron, B. Casal, M. Casarsa, A. Castro aa , P. Catastini, D. Cauz, V. Cavaliere, M. Cavalli-Sforza, A. Cerri f , L. Cerrito q , Y.C. Chen, G. Chiarelli, G. Chlachidze, F. Chlebana, K. Cho, D. Chokheli, J.P. Chou, W.H. Chung, Y.S. Chung, C.I. Ciobanu, M.A. Ciocci dd , A. Clark, C. Clarke, G. Compostella bb , M.E. Convery, M.Corbo, M. Cordelli, C.A. Cox, D.J. Cox, F. Crescioli cc , C. Cuenca Almenar, J. Cuevas w , D. Dagenhart, N. d’Ascenzo u , M. Datta, P. de Barbaro, S. De Cecco, G. De Lorenzo, M. Dell’Orso cc , C. Deluca, L. Demortier, J. Deng c , M. Deninno, F. Devoto, M. d’Errico bb , A. Di Canto cc , B. Di Ruzza, J.R. Dittmann, M. D’Onofrio, S. Donati cc , P. Dong, M. Dorigo, T. Dorigo, K. Ebina, A. Elagin, A. Eppig, R. Erbacher, D. Errede, S. Errede, N. Ershaidat z , R. Eusebi, H.C. Fang, J.P. Fernandez, C. Ferrazza ee , R. Field, G. Flanagan s , R. Forrest, M.J. Frank, M. Franklin, J.C. Freeman, Y. Funakoshi, I. Furic, M. Gallinaro, J. Galyardt, J.E. Garcia, A.F. Garfinkel, P. Garosi dd , H. Gerberich, E. Gerchtein, S. Giagu ff , V. Giakoumopoulou, P. Giannetti, K. Gibson, C.M. Ginsburg, N. Giokaris, P. Giromini, M. Giunta, ∗ Deceased
1. Giurgiu, V. Glagolev, D. Glenzinski, M. Gold, D. Goldin, N. Goldschmidt, A. Golossanov, G. Gomez, G. Gomez-Ceballos, M. Goncharov, O. Gonz´alez, I. Gorelov, A.T. Goshaw, K. Goulianos, S. Grinstein, C. Grosso-Pilcher, R.C. Group , J. Guimaraes da Costa, Z. Gunay-Unalan, C. Haber, S.R. Hahn, E. Halkiadakis, A. Hamaguchi, J.Y. Han, F. Happacher, K. Hara, D. Hare, M. Hare, R.F. Harr, K. Hatakeyama, M. Herndon, S. Hewamanage, D. Hidas, A. Hocker, W. Hopkins g , S. Hou, R.E. Hughes, M. Hurwitz, U. Husemann, N. Hussain, M. Hussein, J. Huston, G. Introzzi, M. Iori ff , A. Ivanov o , D. Jang, B. Jayatilaka, E.J. Jeon, M.K. Jha, S. Jindariani, W. Johnson, M. Jones, K.K. Joo, S.Y. Jun, T.R. Junk, T. Kamon, A. Kasmi, Y. Kato n , W. Ketchum, V. Khotilovich, B. Kilminster, D.H. Kim, H.S. Kim, H.W. Kim, J.E. Kim, M.J. Kim, S.B. Kim, S.H. Kim, Y.K. Kim, N. Kimura, M. Kirby, S. Klimenko, K. Kondo, D.J. Kong, J. Konigsberg, D. Krop, N. Krumnack l , M. Kruse, V. Krutelyov d , M. Kurata, S. Kwang, A.T. Laasanen, S. Lami, S. Lammel, M. Lancaster, R.L. Lander, K. Lannon v , A. Lath, G. Latino cc , E. Lee, H.S. Lee, J.S. Lee, S.W. Lee x , S. Leo cc , S. Leone, J.D. Lewis, A. Limosani r , C.-J. Lin, J. Linacre, M. Lindgren, A. Lister, D.O. Litvintsev, C. Liu, Q. Liu, T. Liu, S. Lockwitz, A. Loginov, D. Lucchesi bb , P. Lujan, P. Lukens, G. Lungu, J. Lys, R. Lysak, R. Madrak, K. Maeshima, K. Makhoul, S. Malik, G. Manca b , A. Manousakis-Katsikakis, F. Margaroli, M. Mart´ınez, R. Mart´ınez-Ballar´ın, P. Mastrandrea, M.E. Mattson, P. Mazzanti, K.S. McFarland, P. McIntyre, R. McNulty i , A. Mehta, P. Mehtala, A. Menzione, C. Mesropian, T. Miao, D. Mietlicki, A. Mitra, H. Miyake, S. Moed, N. Moggi, M.N. Mondragon k , C.S. Moon, R. Moore, M.J. Morello, P. Movilla Fernandez, A. Mukherjee, M. Mussini aa , J. Nachtman m , Y. Nagai, J. Naganoma, I. Nakano, A. Napier, J. Nett, C. Neu, M.S. Neubauer, J. Nielsen e , O. Norniella, E. Nurse, L. Oakes, S.H. Oh, Y.D. Oh, I. Oksuzian, T. Okusawa, R. Orava, L. Ortolan, S. Pagan Griso bb , C. Pagliarone, E. Palencia f , V. Papadimitriou, J. Patrick, G. Pauletta gg , C. Paus, D.E. Pellett, A. Penzo, T.J. Phillips, G. Piacentino, J. Pilot, K. Pitts, C. Plager, L. Pondrom, K. Potamianos, O. Poukhov ∗ ,
2. Prokoshin y , A. Pronko, F. Ptohos h , E. Pueschel, G. Punzi cc , J. Pursley, A. Rahaman, V. Ramakrishnan, N. Ranjan, I. Redondo, M. Rescigno, T. Riddick, F. Rimondi aa , L. Ristori , T. Rodrigo, E. Rogers, S. Rolli, R. Roser, M. Rossi, F. Rubbo, F. Ruffini dd , A. Ruiz, J. Russ, V. Rusu, A. Safonov, W.K. Sakumoto, Y. Sakurai, L. Santi gg , L. Sartori, K. Sato, V. Saveliev u , A. Savoy-Navarro, P. Schlabach, E.E. Schmidt, M.P. Schmidt ∗ , M. Schmitt, T. Schwarz, L. Scodellaro, A. Scribano dd , F. Scuri, A. Sedov, S. Seidel, Y. Seiya, A. Semenov, F. Sforza cc , A. Sfyrla, S.Z. Shalhout, T. Shears, P.F. Shepard, M. Shimojima t , S. Shiraishi, M. Shochet, I. Shreyber, A. Simonenko, P. Sinervo, A. Sissakian ∗ , K. Sliwa, J.R. Smith, F.D. Snider, A. Soha, S. Somalwar, V. Sorin, P. Squillacioti, M. Stancari, M. Stanitzki, R. St. Denis, B. Stelzer, O. Stelzer-Chilton, D. Stentz, J. Strologas, G.L. Strycker, Y. Sudo, A. Sukhanov, I. Suslov, K. Takemasa, Y. Takeuchi, J. Tang, M. Tecchio, P.K. Teng, J. Thom g , J. Thome, G.A. Thompson, P. Ttito-Guzm´an, S. Tkaczyk, D. Toback, S. Tokar, K. Tollefson, T. Tomura, S. Torre, D. Torretta, P. Totaro, M. Trovato ee , F. Ukegawa, S. Uozumi, A. Varganov, F. V´azquez k , G. Velev, C. Vellidis, M. Vidal, I. Vila, R. Vilar, J. Viz´an, M. Vogel, G. Volpi cc , R.L. Wagner, T. Wakisaka, R. Wallny, S.M. Wang, A. Warburton, D. Waters, M. Weinberger, B. Whitehouse, A.B. Wicklund, E. Wicklund, S. Wilbur, J.S. Wilson, P. Wilson, B.L. Winer, P. Wittich g , S. Wolbers, H. Wolfe, T. Wright, X. Wu, Z. Wu, K. Yamamoto, J. Yamaoka, T. Yang, U.K. Yang p , Y.C. Yang, W.-M. Yao, G.P. Yeh, K. Yi m , J. Yoh, K. Yorita, T. Yoshida j , G.B. Yu, I. Yu, S.S. Yu, J.C. Yun, A. Zanetti, Y. Zeng, and S. Zucchelli aa (CDF Collaboration † ) † With visitors from a University of MA Amherst, Amherst, MA 01003, USA, b Istituto Nazionale di FisicaNucleare, Sezione di Cagliari, 09042 Monserrato (Cagliari), Italy, c University of CA Irvine, Irvine, CA92697, USA, d University of CA Santa Barbara, Santa Barbara, CA 93106, USA, e University of CA SantaCruz, Santa Cruz, CA 95064, USA, f CERN,CH-1211 Geneva, Switzerland, g Cornell University, Ithaca,NY 14853, USA, h University of Cyprus, Nicosia CY-1678, Cyprus, i University College Dublin, Dublin4, Ireland, j University of Fukui, Fukui City, Fukui Prefecture, Japan 910-0017, k Universidad Iberoamer-icana, Mexico D.F., Mexico, l Iowa State University, Ames, IA 50011, USA, m University of Iowa, Iowa Institute of Physics, Academia Sinica,Taipei, Taiwan 11529, Republic of China Argonne National Laboratory, Argonne, Illinois 60439, USA University of Athens, 157 71 Athens, Greece Institut de Fisica d’Altes Energies, ICREA,Universitat Autonoma de Barcelona,E-08193, Bellaterra (Barcelona), Spain Baylor University, Waco, Texas 76798, USA Istituto Nazionale di Fisica Nucleare Bologna, aa University of Bologna, I-40127 Bologna, Italy University of California, Davis, Davis, California 95616, USA University of California, Los Angeles,Los Angeles, California 90024, USA Instituto de Fisica de Cantabria, CSIC-University of Cantabria, 39005 Santander, Spain Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637, USA Comenius University, 842 48 Bratislava,Slovakia; Institute of Experimental Physics, 040 01 Kosice, Slovakia Joint Institute for Nuclear Research, RU-141980 Dubna, Russia Duke University, Durham, North Carolina 27708, USA Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA University of Florida, Gainesville, Florida 32611, USA Laboratori Nazionali di Frascati, Istituto Nazionaledi Fisica Nucleare, I-00044 Frascati, Italy University of Geneva, CH-1211 Geneva 4, Switzerland
City, IA 52242, USA, n Kinki University, Higashi-Osaka City, Japan 577-8502, o Kansas State University,Manhattan, KS 66506, USA, p University of Manchester, Manchester M13 9PL, United Kingdom, q QueenMary, University of London, London, E1 4NS, United Kingdom, r University of Melbourne, Victoria3010, Australia, s Muons, Inc., Batavia, IL 60510, USA, t Nagasaki Institute of Applied Science, Nagasaki,Japan, u National Research Nuclear University, Moscow, Russia, v University of Notre Dame, Notre Dame,IN 46556, USA, w Universidad de Oviedo, E-33007 Oviedo, Spain, x Texas Tech University, Lubbock, TX79609, USA, y Universidad Tecnica Federico Santa Maria, 110v Valparaiso, Chile, z Yarmouk University,Irbid 211-63, Jordan, hh On leave from J. Stefan Institute, Ljubljana, Slovenia, Glasgow University, Glasgow G12 8QQ, United Kingdom Harvard University, Cambridge, Massachusetts 02138, USA Division of High Energy Physics, Department of Physics,University of Helsinki and Helsinki Institute of Physics, FIN-00014, Helsinki, Finland University of Illinois, Urbana, Illinois 61801, USA The Johns Hopkins University, Baltimore, Maryland 21218, USA Center for High Energy Physics: Kyungpook National University,Daegu 702-701, Korea; Seoul National University, Seoul 151-742,Korea; Sungkyunkwan University, Suwon 440-746,Korea; Korea Institute of Science and Technology Information,Daejeon 305-806, Korea; Chonnam National University, Gwangju 500-757,Korea; Chonbuk National University, Jeonju 561-756, Korea Ernest Orlando Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA University of Liverpool, Liverpool L69 7ZE, United Kingdom University College London, London WC1E 6BT, United Kingdom Centro de Investigaciones EnergeticasMedioambientales y Tecnologicas, E-28040 Madrid, Spain Massachusetts Institute of Technology,Cambridge, Massachusetts 02139, USA Institute of Particle Physics: McGill University, Montr´eal,Qu´ebec, Canada H3A 2T8; Simon Fraser University, Burnaby,British Columbia, Canada V5A 1S6; University of Toronto,Toronto, Ontario, Canada M5S 1A7; and TRIUMF,Vancouver, British Columbia, Canada V6T 2A3 University of Michigan, Ann Arbor, Michigan 48109, USA Michigan State University, East Lansing, Michigan 48824, USA Institution for Theoretical and Experimental Physics, ITEP, Moscow 117259, Russia University of New Mexico, Albuquerque, New Mexico 87131, USA Northwestern University, Evanston, Illinois 60208, USA The Ohio State University, Columbus, Ohio 43210, USA Okayama University, Okayama 700-8530, Japan Osaka City University, Osaka 588, Japan University of Oxford, Oxford OX1 3RH, United Kingdom Istituto Nazionale di Fisica Nucleare, Sezione di Padova-Trento, bb University of Padova, I-35131 Padova, Italy LPNHE, Universite Pierre et MarieCurie/IN2P3-CNRS, UMR7585, Paris, F-75252 France Istituto Nazionale di Fisica Nucleare Pisa, cc University of Pisa, dd University of Siena and ee Scuola Normale Superiore, I-56127 Pisa, Italy University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA Purdue University, West Lafayette, Indiana 47907, USA University of Rochester, Rochester, New York 14627, USA The Rockefeller University, New York, New York 10065, USA Istituto Nazionale di Fisica Nucleare, Sezione di Roma 1, ff Sapienza Universit`a di Roma, I-00185 Roma, Italy Rutgers University, Piscataway, New Jersey 08855, USA Texas A&M University, College Station, Texas 77843, USA Istituto Nazionale di Fisica Nucleare Trieste/Udine,I-34100 Trieste, gg University of Udine, I-33100 Udine, Italy University of Tsukuba, Tsukuba, Ibaraki 305, Japan Tufts University, Medford, Massachusetts 02155, USA University of Virginia, Charlottesville, Virginia 22906, USA Waseda University, Tokyo 169, Japan Wayne State University, Detroit, Michigan 48201, USA University of Wisconsin, Madison, Wisconsin 53706, USA Yale University, New Haven, Connecticut 06520, USA bstract We use a new method to estimate with 5% accuracy the contribution of pion and kaon in-flight-decays to the dimuon data set acquired with the CDF detector. Based on this improved estimate,we show that the total number and the properties of the collected dimuon events are not yetaccounted for by ordinary sources of dimuons which also include the contributions, as measured inthe data, of heavy flavor, Υ, and Drell-Yan production in addition to muons mimicked by hadronicpunchthrough.
PACS numbers: 13.85.-t, 14.65.Fy, 13.20.Fc . INTRODUCTION This article presents an improved determination of the composition of a dimuon samplerecorded in p ¯ p collisions at √ s = 1 .
96 TeV. The data sample consists of events containingtwo central ( | η | < .
7) primary (or trigger) muons, each with transverse momentum p T ≥ /c , and with invariant mass larger than 5 GeV /c and smaller than 80 GeV /c .The sample may be dominated by real muon pairs due to semileptonic decays of heavyflavor, Drell-Yan production and Υ decays, but also contains events in which one or bothmuons are produced by hadrons that decay in flight or otherwise mimic a muon signal.Although the dimuon signature can be a powerful tool with which to search for new physicsor sources of CP violation, the uncertainty of the in-flight-decay contribution makes theprecise determination of the fractions of known processes a serious experimental challenge.In particular, it remains controversial if muons originating from the decay of objects witha lifetime longer than that of heavy-flavored hadrons can be completely accounted for withordinary sources such as in-flight-decays. Earlier and recent studies estimate the fractionof this type of event to be negligible [1–3]. Other studies find it significant, suppress it byselecting muons produced close to the beamline [4], but have estimated its size with a verylarge uncertainty by using Monte Carlo simulations [5]. The present work is based on thesame Monte Carlo simulated samples, and the same analysis methods as Refs. [4, 5], butwe improve the method to estimate the number of events due to in-flight-decays achievinga 5% accuracy.Section II describes the CDF II detector. In Sec. III, we review the present experimentalsituation. Sections IV to VI describe the procedure used to tune the simulation and estimatethe contribution of ordinary sources to events in which muons are produced by objects withvery long lifetimes. Based on this results, Section VII updates the estimate of the rate ofmulti-muon events reported in Ref. [5]. Our conclusions are presented in Sec VIII. II. CDF II DETECTOR AND TRIGGER
CDF II is a multipurpose detector, equipped with a charged particle spectrometer and afinely segmented calorimeter. In this section, we describe the detector components that arerelevant to this analysis. The description of these subsystems can be found in Refs. [6–15].8wo devices inside the 1.4 T solenoid are used for measuring the momentum of chargedparticles: the silicon vertex detector (SVXII and ISL) and the central tracking chamber(COT). The SVXII detector consists of microstrip sensors arranged in six cylindrical shellswith radii between 1.5 and 10.6 cm, and with a total z coverage of 90 cm. The first SVXIIlayer, also referred to as the L00 detector, is made of single-sided sensors mounted on theberyllium beam pipe. The remaining five SVXII layers are made of double-sided sensorsand are divided into three contiguous five-layer sections along the beam direction z . Thevertex z -distribution for p ¯ p collisions is approximately described by a Gaussian functionwith a rms of 28 cm. The transverse profile of the Tevatron beam is circular and has a rmsspread of ≃ µ m in the horizontal and vertical directions. The SVXII single-hit resolutionis approximately 11 µ m and allows a track impact parameter resolution of approximately35 µ m, when also including the effect of the beam transverse size. The two additionalsilicon layers of the ISL help to link tracks in the COT to hits in the SVXII. The COT isa cylindrical drift chamber containing 96 sense wire layers grouped into eight alternatingsuperlayers of axial and stereo wires. Its active volume covers | z | ≤
155 cm and 40 to 140cm in radius. The transverse momentum resolution of tracks reconstructed using COT hitsis σ ( p T ) /p T ≃ . /c ] − . The trajectory of COT tracks is extrapolated into theSVXII detector, and tracks are refitted with additional silicon hits consistent with the trackextrapolation.The central muon detector (CMU) is located around the central electromagnetic andhadronic calorimeters, which have a thickness of 5.5 interaction lengths at normal incidence.The CMU detector covers a nominal pseudorapidity range | η | ≤ .
63 relative to the centerof the detector, and is segmented into two barrels of 24 modules, each covering 15 ◦ in φ .Every module is further segmented into three submodules, each covering 4.2 ◦ in φ andconsisting of four layers of drift chambers. The smallest drift unit, called a stack, coversa 1.2 ◦ angle in φ . Adjacent pairs of stacks are combined together into a tower. A tracksegment (hits in two out of four layers of a stack) detected in a tower is referred to as a In the CDF coordinate system, θ and φ are the polar and azimuthal angles of a track, respectively,defined with respect to the proton beam direction, z . The pseudorapidity η is defined as − ln tan( θ/ p T = p sin( θ ). The rapidity is defined as y = 1 / · ln(( E + p z ) / ( E − p z )), where E and p z are the energy and longitudinal momentum of the particle associated withthe track. | η | ≤ .
54 relative to the center of the detector. Muons which producea stub in both the CMU and CMP systems are called CMUP muons. The CMX muondetector consists of eight drift chamber layers and scintillation counters positioned behindthe hadron calorimeter. The CMX detector extends the muon coverage to | η | ≤ p ¯ p collisions. The inelastic p ¯ p cross section at √ s = 1960 GeV is scaledfrom measurements at √ s = 1800 GeV using the calculations in Ref. [16]. The integratedluminosity is determined with a 6% systematic uncertainty [17].CDF uses a three-level trigger system. At Level 1 (L1), data from every beam crossingare stored in a pipeline capable of buffering data from 42 beam crossings. The L1 triggereither rejects events or copies them into one of the four Level 2 (L2) buffers. Events that passthe L1 and L2 selection criteria are sent to the Level 3 (L3) trigger, a cluster of computersrunning speed-optimized reconstruction code.For this study, we select events with two muon candidates identified by the L1 and L2triggers. The L1 trigger uses tracks with p T ≥ . /c found by a fast track processor(XFT). The XFT examines COT hits from the four axial superlayers and provides r − φ information in azimuthal sections of 1.25 ◦ . The XFT passes the track information to a set ofextrapolation units that determine the CMU towers in which a CMU stub should be foundif the track is a muon. If a stub is found, a L1 CMU primitive is generated. The L1 dimuontrigger requires at least two CMU primitives, separated by at least two CMU towers. TheL2 trigger additionally requires that at least one of the muons also has a CMP stub matchedto an XFT track with p T ≥ /c . All these trigger requirements are emulated by thedetector simulation on a run-by-run basis. The L3 trigger requires a pair of CMUP muonswith invariant mass larger than 5 GeV /c , and | δz | ≤ z is the z coordinate ofthe muon track at its point of closest approach to the beamline in the r − φ plane. Theserequirements define the dimuon trigger used in this analysis.10 II. PRESENT UNDERSTANDING OF THE DIMUON SAMPLE COMPOSI-TION
The value of σ b → µ, ¯ b → µ and σ c → µ, ¯ c → µ , the correlated cross sections for producing pairs ofcentral heavy-flavored quarks that decay semileptonically, is derived in Ref. [4] by fittingthe impact parameter [18] distribution of the primary muons with the expected shapes fromall sources believed to be significant: semileptonic heavy flavor decays, prompt quarkoniadecays, Drell-Yan production, and instrumental backgrounds due to punchthrough of promptor heavy-flavored hadrons which mimic a muon signal [19]. In the following, the sum of theseprocesses will be referred to as the prompt plus heavy flavor ( P + HF ) contribution. Thenotation K puth → µ and π puth → µ will be used to indicate muon signals mimicked bypunchthrough of kaons and pions, respectively. In order to properly model the data withthe templates of the various P + HF sources, the study in Ref. [4] has used strict selectioncriteria, referred to as tight SVX selection in the following, by requiring muon tracks withhits in the two innermost layers of the SVX detector, and in at least two of the next fourouter layers.The tight SVX requirements select events in which both muons arise from parent particlesthat have decayed within a distance of ≃ . p ¯ p interaction primary vertex inthe plane transverse to the beamline. This requirement suppresses the yield of primarymuons due to in-flight-decays of pions and kaons, in the following referred to as π ifd → µ and K ifd → µ , respectively. This type of contribution to the dimuon dataset prior to anySVX requirement was considered negligible in previous [1, 2] and recent [3] studies by theCDF and D0 collaborations.As shown by Fig. 1, the tight SVX sample is well modeled by fits using the prompt andheavy flavor contributions [4]. The sample composition determined by the fit and correctedfor the appropriate efficiency of the tight SVX requirements is listed in the first two columnsof Table I.The difference between the total number of dimuons and the P + HF component indicatesthe presence of an important source of dimuons produced beyond 1.5 cm which is suppressed The efficiency of the tight SVX selection has been measured [5] to be 0 . ± .
004 for prompt dimuonsand 0 . ± .
001 for dimuons produced by heavy flavor decays by using control samples of data fromvarious sources (
J/ψ → µ + µ − , B ± → µ + µ − K ± , B → µD , and Υ → µ + µ − ). ABLE I: Number of events attributed to the different dimuon sources by the fit to the muonimpact-parameter distribution. The fit parameters BB , CC , and P P represent the b ¯ b , c ¯ c , andprompt dimuon contributions, respectively. The component BC represents events containing b and c quarks. The fit parameter BP ( CP ) estimates the number of events in which there is onlyone b ( c ) quark in the detector acceptance and the second muon is produced by prompt hadronsin the recoiling jet that mimic a muon signal. Real muons are muons from semileptonic decay ofheavy flavors, Drell-Yan production or quarkonia decays. The data correspond to an integratedluminosity of 742 pb − . The dimuon data set consists of 743006 events.Component No. of Events No. of real µ − µ No. and type of misidentified µBB ± R bb × BB R bb × [7902 K puth → µ +8145 π puth → µ ] CC ± R cc × CC R cc × [17546 K puth → µ + 9535 π puth → µ ] P P ± ±
649 4400 K puth → µ K puth → µ + DY = 54200 ± π puth → µ π puth → µ + 23000 K puth → µ π puth → µBP ± K puth → µ + 29253 π puth → µCP ± K puth → µ + 35275 π puth → µBC ± P + HF ± by the tight SVX requirements. Because unnoticed by previous experiments, this source waswhimsically referred to as the ghost contribution.The relative size of the ghost and P + HF contributions depends upon the type of SVXrequirement applied to the trigger muons. Reference [5] shows that neglecting the presence ofghost events affected previous measurements of σ b → µ, ¯ b → µ [1, 20] and of ¯ χ [2] at the Tevatron.Finally, the ghost sample is shown to be the source of the dimuon invariant mass discrepancyobserved in Ref. [21].Reference [5] has studied a number of potential sources of muons originating beyond12 d (cm) M uon s / ( . c m ) FIG. 1: The projection of the two-dimensional impact parameter distribution of muon pairs ontoone of the two axes is compared to the fit result (histogram). the beam pipe. Contrary to what assumed by previous experiments, the one source foundto contribute significantly arises from in-flight-decays of pions and kaons. Based upon ageneric QCD simulation, that study estimates a contribution of 57000 events. A smallercontribution (12052 ±
466 events) from K S and hyperon decays in which the punchthroughof a hadronic prong mimics a muon signal was estimated using the data. Secondary inelasticinteractions in the tracking volume were found to be a negligible source of ghost events.The final estimate of the size of possible sources of ghost events underpredicts the observednumber by approximately a factor of two (154000 observed and 69000 accounted for), butthe difference was not considered significant because of the simulation uncertainty.The present study uses events selected with the tight SVX requirements to tune the QCDsimulation. Since these data are well modeled by the impact parameter templates of the P + HF components, misidentified muons can only arise from the punchthrough of prompthadrons or hadrons produced by heavy-flavor decays. The numbers of misidentified muonsin the data are derived by subtracting the expected number of real muons, listed in the thirdcolumn of Table I, from the corresponding components in the second column. We then com-pare these differences to the rate of K puth → µ and π puth → µ misidentifications predictedby the simulation, and listed on the fourth column of the same table. The simulation is13uned by adjusting the predicted rate of pions and kaons to reproduce the observed numberof muon misidentifications. Then, the tuned simulation is used to predict the number ofmuons due to in-flight-decays with 5% accuracy. IV. RATES OF MISIDENTIFIED MUONS IN THE DATA AND SIMULATION
We make use of three different samples of simulated events generated with the herwig parton-shower Monte-Carlo program [22], the settings of which are described in Appendix Aof Ref. [4]. We use option 1500 of the herwig program to generate final states producedby hard scattering of partons with transverse momentum larger than 3 GeV /c (sampleA=generic QCD). Hadrons with heavy flavors are subsequently decayed using the evtgen Monte Carlo program [23]. The detector response to particles produced by the above gen-erators is modeled with the CDF II detector simulation that in turn is based on the geant
Monte Carlo program [24]. The values of the heavy flavor cross sections predicted by the gen-erator are scaled to the measured values σ b → µ, ¯ b → µ = 1549 ±
133 pb and σ c → µ, ¯ c → µ = 624 ± b + single c ) is extracted from A byrequiring the presence of at least a trigger muon generated from heavy-flavor semileptonicdecays. The simulated sample C= b ¯ b + c ¯ c is extracted from B by requiring the presence ofat least two trigger muons generated from heavy flavor decays. This sample has been usedto construct impact parameter templates and estimate kinematic acceptances in Ref. [4].In the various simulations, we evaluate the number of dimuons from heavy flavor decaysand the number of pairs of tracks of different type that pass the same kinematic selection.The ratios of these numbers are listed in Tables II to IV. The rate of pairs of tracks ofdifferent type predicted by the simulation are normalized to the data by multiplying theseratios by the number of dimuons from b ¯ b or c ¯ c production observed in the data. TABLE II: Ratio of the numbers of ππ , KK , and Kπ pairs to that of primary dimuons from b ¯ b decays (221096 pairs) in the generic QCD simulation (sample A).Process R KK R Kπ R ππ generic QCD 867 8935 22913 ABLE III: Ratio of the numbers of µ − K ( π ) combinations to that of primary dimuons from b ¯ b and c ¯ c production in the single- b and single- c simulated samples (221096 and 83590 dimuons,respectively).Process R K R π single b c µ − K ( π ) combinations to that of primary dimuons from heavyflavor production in the b ¯ b and c ¯ c simulated samples (221096 and 83590 dimuons, respectively).Process R K R π b ¯ b c ¯ c The probability P puthK ( π ) that a kaon (pion) is not contained by the calorimeter and mimics amuon signal has been measured in Ref. [4] by using kaons and pions from D ∗± → π ± D with D → K + π − decays. The probability that kaon (pion) in-flight-decays mimic a trigger muon, P ifdK ( π ) , has been derived in Ref. [5] by using the simulated sample C. These probabilitiesdepend on the particle transverse momentum. Table V lists the average probabilities thatkaons (pions) mimic a primary muon when applying the P puthK ( π ) and P ifdK ( π ) probabilities tosimulated kaon (pion) tracks with p T ≥ /c and | η | < . TABLE V: Average probabilities (%) that punchthroughs or in-flight decays result into a primarymuon. The p T distribution of kaons and pions in the different simulations are almost indistinguish-able. < P puthK > < P puthπ > < P ifdK > < P ifdπ > . ± .
003 0 . ± . .
004 0 . ± .
005 0 . ± . By weighting simulated pion (kaon) tracks that pass the muon kinematic selection with15he corresponding P puthK ( π ) probability, we obtain the prediction of misidentified primary muonsfor the various P + HF components that is listed in the fourth column of Table I. The thirdcolumn of the same table lists the number of real muons for the various P + HF contributions.The sum of real plus misidentified muon pairs is in general agreement with the data listedin the second column of the table. Therefore, it is reasonable to use the observed rate ofdimuons, the knowledge of the fraction of real dimuons due to semileptonic decay of heavyflavors, Drell-Yan or Υ mesons, and the knowledge of the P puthK ( π ) probabilities to normalizethe absolute yields of pions and kaons predicted by the simulation. The simulation fitted tothe data is then used to predict the rate of events due to in-flight-decay misidentificationsby weighting simulated tracks with the P ifdK ( π ) probabilities, the average of which is listed inTable V. In addition, the total rate of K → µ = K puth → µ + K ifd → µ misidentificationspredicted by the simulation can be further constrained with data. This is done in the nextsection by using the number of primary muons due to misidentification of K ∗ , K ∗± , and K S decays.We first describe the evaluation of the content of real muons in the various P + HF components and the function used to fit the simulation to the data. Reference [4] estimatesthat the fraction R bb = 0 . ± .
04 of the BB component is due to real muons from b -quark semileptonic decays whereas the remaining 4% is due to muons mimicked by thepunchthrough of hadrons produced by heavy flavor decays. Similarly, the fraction R cc =0 . ± .
09 of the CC component is due to real muons from c -quark semileptonic decayswhereas the remaining 19% is due to muons mimicked by the punchthrough of hadronsproduced by heavy flavor decays. The uncertainty of the fraction of real muons due to b ¯ b ( c ¯ c ) production is accounted for by multiplying R bb ( R cc ) by the fit parameter f bb ( f cc )constrained to 1 with a 4% (11%) Gaussian error.The number of Υ mesons contributing to the P P component (Υ = 51680 ±
649 candi-dates) has been determined in Ref. [4] by fitting the dimuon invariant mass spectrum withthree Gaussian functions to model the signal and a straight line to model the combinatorialbackground. The Drell-Yan contribution is evaluated as DY = Υ × σ DY /σ Υ . The crosssection σ DY in the 5 −
80 GeV /c mass range is evaluated with a NLO calculation [25], andwe use the measured value of σ Υ [26]. The ratio σ DY /σ Υ is 1.05 with a 10% error mostlydue to the measurement in Ref. [26]. To account for the uncertainty, we weight the DY contribution with the fit parameter f dy constrained to 1 with a 10% Gaussian error.16he magnitude of the BP ( CP ) component, predicted with the single- b (single- c ) simula-tion, with respect to that of the BB ( CC ) contribution depends on the ratio of NLO to LOterms evaluated by the herwig generator. Because of the dependence on the renormaliza-tion and factorization scales, the uncertainty of the single- b ( c ) cross section to that of the b ¯ b ( c ¯ c ) cross section is estimated [27] to be ≃
20 (30)% . We account for this uncertaintyby weighting the rate of pion and kaon tracks predicted by the single- b (single- c ) simulationwith the additional fit parameter f sb ( f sc ) constrained to 1 with a 20% (30%) Gaussian error.The simulation prediction of the number of muons mimicked by the punchthrough ofpions (kaons) is weighted with the fit free parameter f π ( f K ). These fit parameters providethe absolute normalization of the pion (kaon) rate predicted by the simulation including theuncertainties of the punchthrough probabilities. V. MEASUREMENT OF THE K → µ CONTRIBUTION
The small rate of K → µ = K puth → µ + K ifd → µ misidentifications is measured usinga higher statistics sample of dimuon events corresponding to an integrated luminosity of 3.9fb − . The number of K → µ misidentification, N K , is derived from N K ∗ , the number ofidentified K ∗ → K + π − decays with K + → µ + (and charge-conjugate states). The number N K ∗ is related to N K by N K ∗ = N K · ǫ · R ( K ∗ ) , where R ( K ∗ ) is the fraction of kaons that result from K ∗ → K + π − decays and ǫ is theefficiency to reconstruct the pion.We also select K S → π + π − with π → µ candidates and reconstruct K ∗± → K S π ± decays.The number of K ∗± is related to that of K S by N K ∗± = N K S · ǫ · R ( K ∗± ) , where R ( K ∗± ) is the fraction of K S resulting from K ∗± → K S π ± decays and ǫ is theefficiency to reconstruct the additional pion. We use isospin invariance to set R ( K ∗± ) = R ( K ∗ ). Since the additional pion used to search for the K ∗± and K ∗ candidates is selected However, the study in Ref. [28] shows that the herwig generator predicts the observed single andcorrelated heavy-flavor cross sections to better than 10%. ǫ = ǫ . It follows that N K = N K S /N K ∗± × N K ∗ . We search for K ∗ decays by combining primary muons, assumed to be kaons, with allopposite charge tracks, assumed to be pions, with p T ≥ . /c and in an angular conewith cos θ ≥ . K ∗ → K + π − candidatesis shown in Fig. 2. We fit the invariant mass distribution with a Breit-Wigner function ) M (GeV/c ) C and i da t e s / ( M e V / c *0 K FIG. 2: Invariant mass distribution of K ∗ → K + π − candidates passing our selection criteria. Theline represents the fit described in the text. smeared by the detector mass resolution to model the signal. We fix the mass and width ofthe Breit-Wigner function to 896 and 51 MeV/ c [29], respectively . We use a fourth orderpolynomial to model the combinatorial background under the signal, and the fitted rangeof invariant mass is conveniently chosen to yield a fit with 50% probability. The size of thesignal is not affected by the arbitrary choice of the function used to model the combinatorial The mass resolution due to the track reconstruction in simulated events which include kaon in-flight-decays is 4.9 MeV/ c , and is negligible compared to the resonance width. N K ∗ = 87471 ± K ∗ mesons.We search for K S → π + π − with a π → µ misidentification by combining primary muonswith tracks passing the same requirements as those used in the K ∗ search. In this case,both tracks are assumed to be pions. As in the previous case, we select pairs consistent witharising from a common three-dimensional vertex. We take advantage of the K S long lifetimeto suppress the combinatorial background. We further require that the distance between the K S vertex and the event primary vertex, corrected by the K S Lorentz boost, corresponds to ct > . K S candidates is shown in Fig. 3. ) M (GeV/c ) C and i da t e s / ( . M e V / c S0 K FIG. 3: Invariant mass distribution of K S candidates passing our selection criteria. The linerepresents the fit described in the text. We fit the signal with two Gaussian functions and the combinatorial background with astraight line in the mass range 0 . − . /c . Having fixed the peak of the Gaussianfunctions at 0.497 GeV /c [29], the fit returns an averaged σ of 8.4 MeV/ c , consistent withwhat is expected from simulated events , and a signal of 32445 ± K S mesons in themass range 0 . − .
522 GeV /c . Because of the K S long decay path, reconstructed track segments may be shorter than the availabletracking detector length. When K S mesons decay before entering the COT volume, the mass resolutionis 4 MeV/ c .
19e search for K ∗± by combining K S candidates with mass between 0.474 and 0.522GeV /c and ct > . K ∗ candidates. We constrain the K S mass to0.497 GeV /c and require that the K S candidate and the pion track are consistent witharising from a common three-dimensional vertex. The invariant mass distribution of K ∗± candidates is shown in Fig. 4. ) M (GeV/c ) C and i da t e s / ( M e V / c – * K FIG. 4: Invariant mass distribution of K ∗± → K S π ± candidates passing our selection criteria. Theline represents the fit described in the text. We fit the invariant mass distribution with a Breit-Wigner function to model the signaland a fourth order polynomial to model the combinatorial background. We fix the massand width of the Breit-Wigner function to 892 and 51 MeV/ c [29], respectively . The fitreturns a signal of 3326 ± K ∗± mesons.The signals obtained by analyzing the 3.9 fb − sample are rescaled to estimate the numberof K → µ misidentifications present in the 742 pb − dataset. After rescaling, we obtain N K = N K S /N K ∗± × N K ∗ = 164769 ± N sim K of K → µ misidentifications predicted by the simulation. In simulated events, when constraining the K S mass to the PDG value, the mass-constrained K S mo-mentum is measured as accurately as that of a track corresponding to a K → µ decay. The resulting K ∗± mass resolution is approximately 5 MeV/ c . I. FIT OF THE SIMULATION PREDICTION TO THE DATA
We fit the simulation prediction with a χ -minimization method [30]. The χ function isdefined as χ = X i =1 ( D [ i ] − P [ i ]) /ED [ i ] + ( N sim K − / , where D [ i ] and ED [ i ] are the size and error of the component listed in the second columnand i-th row of Table I. The term P [ i ] is the sum of the real muon contribution and thepunchthrough contribution predicted by the simulation in the third and forth columns of thesame table, respectively. These contributions are weighted with the fit parameters describedin the previous section, and the terms read P [1] = f bb (0 . · f k f π ,P [2] = f cc (0 . · f k f π ,P [3] = Υ + f dy DY + f K f π f K f π ,P [4] = f sb ( f K f π , and P [5] = f sc ( f K f π . In addition, the sum P i =1 D [ i ] is constrained to the observed number of P + HF − BC events within its error. The fit results are shown in Tables VI to VIII. In Table VI, the fitparameters that tune the various cross sections predicted by the herwig generator are veryclose to their nominal values indicating that the default simulation provides a quite accuratemodeling of the data.The fit returns 163501 K → µ candidates (164769 ± K → µ decays in identified K ∗ → K + π − decays that pass the tightSVX requirements. The efficiencies of the tight SVX requirement applied to primary muonsare 0 . ± .
002 for muons due to punchthrough of prompt and heavy-flavored hadronsand 0 . ± .
005 for muons arising from in-flight decays . Based on the kaon composition In Ref. [5], which uses 0.74 fb − of data, these efficiencies have been measured to be 0.45 and 0.21,respectively. In 3.9 fb − of data, by using Υ candidates, we measure a smaller efficiency of the tight SVXselection. The efficiency loss comes from periods of data taking in which the pedestals of the L00 channelswere miscalibrated. ABLE VI: Parameter values returned by the fit described in the text. The fit yields χ = 2 . f bb . ± . f cc . ± . f dy . ± . f π . ± . f K . ± . f sb . ± . f sc . ± . f bb f cc f dy f π f K f sb f cc − . f dy .
17 0 . f π − . − . − . f K − . − .
17 0 . − . f sb − . − .
03 0 . − . − . f sc − . − .
21 0 . − . − .
12 0 . returned by the fit, we estimate the efficiency of the tight SVX requirement applied to K → µ misidentifications to be 0 . ± . K ∗ candidates after applying the tight SVX requirement.We fit the invariant mass distribution with the same function used to fit the K ∗ massdistribution in Fig. 2. The fit returns 22689 ± K ∗ candidates to be compared with87741 ± K ∗ candidates before applying the tight SVX requirement. The resultingefficiency of the tight SVX requirement is 0 . ± . . ± . ABLE VIII: Number of events due to different production mechanisms are compared to the resultof the present fit.Component No. of Events Fit result BB ± CC ± P P ± BP ± CP ± P + HF ± ) M (GeV/c ) C and i da t e s / ( M e V / c *0 K FIG. 5: Invariant mass distribution of K ∗ candidates in which the K → µ misidentification passesthe tight SVX requirement. The line represents the fit described in the text. Continuing with the analysis of the results returned by the fit, the total number of π → µ misidentifications is 240915, 64% of which are due to punchthrough and 36% toin-flight-decays. The fractional composition of the K → µ and π → µ misidentifications issummarized in Table IX.The total fraction of misidentified muons in the dataset is 27%. The number of misidenti-fied muons due in-flight-decays of pions and kaons (ghost events) is 113613 ± ABLE IX: Contributions (%) of various processes to pion or kaon misidentifications.Type K puth → µ π puth → µ All sample 51 64generic QCD 20 33single b + b ¯ b +single c + c ¯ c
31 31Type K ifd → µ π ifd → µ All sample 49 36generic QCD 27 27single b + b ¯ b +single c + c ¯ c
22 9 number of muons from in-flight-decays is derived from that of muons mimicked by hadronpunchthrough using the fake probabilities listed in Table V, the uncertainty of these proba-bilities yields an additional error of 3845 events.After adding the 12052 ±
466 events from K S and hyperon decays, we predict 125665 ± ± ± . ± . b ¯ b production and(18 . ± . VII. REVISED ESTIMATE OF THE RATE OF ADDITIONAL REAL MUONS INGHOST EVENTS
As a cross-check of its b ¯ b content, Reference [5] has investigated the rate of sequentialsemileptonic decays of single b quarks in the dimuon sample. We provide here a summary ofthat study and its conclusions. That study searches for additional muons with p T ≥ /c and | η | ≤ . − .The sample of 1426571 events contains 1131090 ± P + HF events in which both muonsoriginate inside the beam pipe and 295481 ± OS ) and invariant mass smaller than 5 GeV /c .In the case of Drell-Yan or quarkonia production, which was not simulated, the rateof same-charge pairs ( SS ) is a measure of the fake muon contribution since misidentifiedmuons arise from the underlying event which has no charge correlation with primary muons.The rate of additional muons mimicked by hadronic punchthrough is also estimated witha probability per track derived by using kaons and pions from D ∗± → π ± D with D → K + π − decays. This misidentification probability is approximately ten times larger thanthat for primary muons that have to penetrate twice as many interaction lengths . Thepunchthrough probabilities for pions and kaons differ by a factor of two. In addition, insimulated events due to heavy flavor production, the pion to kaon ratio depends on theinvariant mass and the charge of the muon-hadron pairs. Therefore, for P + HF events,the rate of OS − SS pairs is compared to that predicted by the heavy flavor simulationin which pions and kaons are weighted with the corresponding probabilities of mimicking amuon signal. In P + HF events, the number of sequential semileptonic decay-candidates(29262 ± b -quarks predictedby the simulation (29190 ± P + HF total contributionand (6 . ± . b ¯ b contribution (424506 ± ± θ ≥ . ± ± ± . ± . K S and K ∗ candidates due to π → µ and K → µ misidentification, respectively. As shown in Sec. V, there are 32445 ±
421 and 87471 ± Therefore, the contribution of in-flight-decays to additional muons is negligible in comparison with thepunchthrough contribution. θ ≥ . K S ( K ∗ ) candidates when at leastone primary muon is accompanied by an additional muon or a predicted misidentified muon.As previously done, we fit the K S distributions with two Gaussian functions to model the ) M (GeV/c ) C and i da t e s / ( . M e V / c ) M (GeV/c ) C and i da t e s / ( . M e V / c FIG. 6: Invariant mass distribution of K S candidates accompanied by (left) an additional muonand (right) a misidentified muon. Lines represent the fits described in the text. signal and a straight line to model the background. The K ∗ distribution is fitted with aBreit-Wigner function plus a fourth order polynomial.The fits return 4572 ±
91 and 1954 ±
109 events in which a K S meson is accompaniedby an additional muon and by a fake muon, respectively. The fits return 10176 ±
739 and5230 ±
493 events in which a K ∗ meson is accompanied by an additional muon and by afake muon, respectively.As shown in Fig. 8 for events triggered by K S misidentifications, sometimes the additionalmuon is contributed by the second prong of the K S decay. Figure 8 shows the invariant massdistribution of primary and additional muons that pass the analysis selection. The usual fit26 M (GeV/c ) C and i da t e s / ( M e V / c ) M (GeV/c ) C and i da t e s / ( M e V / c FIG. 7: Invariant mass distribution of K ∗ candidates accompanied by (left) an additional muonand (right) a misidentified muon. Lines represent the fits described in the text. yields 403 ±
33 events in which the additional muon is mimicked by the second leg of the K S decay that also produced the primary muon. We remove this contribution to evaluatethe fraction of real muons accompanying K → µ misidentifications. We will add it for thefraction of events triggered by misidentified K S decays. ) M (GeV/c ) C and i da t e s / ( M e V / c S0 K FIG. 8: Invariant mass distribution of K S candidates reconstructed using primary and additionalmuons. The line represents the fit described in the text. K S meson that contain additional real muons is (6 . ± . . ± . K ∗ meson. The average of the two fractions is (6 . ± . ± . ± . K → µ misidentifications are due to events with heavy flavors,whereas (51 . ± . ± TABLE X: Number of additional muons in ghost events are compared to the number of expectedfake muons and real muons from heavy flavor decays. The data correspond to an integratedluminosity of 1426 pb − .Data Fakes K S second leg Heavy Flavor49142 ±
519 20902 ±
284 147 ±
15 15924 ± VIII. CONCLUSIONS
This article reports an improved undestanding of the dimuon samples acquired by theCDF experiment. One dataset, corresponding to an integrated luminosity of 742 pb − ,consists of 743006 events containing two central ( | η | < .
7) primary (or trigger) muons,each with transverse momentum p T ≥ /c , and with invariant mass larger than 5GeV /c and smaller than 80 GeV /c . These data are split into two subsets: one, referredto as P + HF , consisting of 589015 ± ± P + HF events are correctly modeled by theexpected contributions of semileptonic heavy flavor decays, prompt quarkonia decays, Drell-28an production, and instrumental backgrounds due to punchthrough of prompt or heavy-flavored hadrons which mimic a muon signal. A previous study [5] has investigated significantsources of ghost events, such as in-flight-decays of pions and kaons and hyperon decays. Thatstudy could account for approximately half of the ghost events but was unable to asses theuncertainty of the in-flight-decay prediction. The present study shows that the herwig parton-shower generator provides an accurate model of the data. The large discrepancy inthe previous study was generated by not including the contribution of final states in which a b ( c ) hadron decays semileptonically and the second muon is produced by the in-flight-decayof a particle in the recoiling jet. After tuning by a few percent the pion and kaon ratespredicted by the simulation with a fit to the data, we show that ordinary sources, mostlyin-flight-decays, account for 125665 ± ± . ± .
26 stat ± .
53 syst)% to the fraction of muons produced at a distance larger than1.6 cm from the beamline including pion and kaon in-flight-decays. This appears to be incontradiction with the present result, and also with a recent estimate [31] of the fraction of K → µ and π → µ contributions in the D0 subset of same-charge dimuons ( ≃ p T ≥ /c and | η | ≤ . ± b ¯ b events thatcontain additional muons due to sequential semileptonic decays is (6 . ± . K → µ and π → µ misidentifications with a similar method. Afterremoving this background, the remaining muon pairs with same charge are attributed to b ¯ b production. The present study shows that, after removing this type of misidentified muons,the data set still contains an additional component that cannot be accounted for with ordi-nary sources. The size of this component, equally split in opposite and same sign pairs [5],is (12 . ± . b ¯ b production.29 X. ACKNOWLEDGMENTS
We thank the Fermilab staff and the technical staffs of the participating institutions fortheir vital contributions. This work was supported by the U.S. Department of Energy andNational Science Foundation; the Italian Istituto Nazionale di Fisica Nucleare; the Ministryof Education, Culture, Sports, Science and Technology of Japan; the Natural Sciences andEngineering Research Council of Canada; the National Science Council of the Republic ofChina; the Swiss National Science Foundation; the A.P. Sloan Foundation; the Korean WorldClass University Program, the National Research Foundation of Korea; the Science andTechnology Facilities Council and the Royal Society, UK; the Institut National de PhysiqueNucleaire et Physique des Particules/CNRS; the Russian Foundation for Basic Research;the Ministerio de Ciencia e Innovaci´on, and Programa Consolider-Ingenio 2010, Spain; theSlovak R&D Agency; the Academy of Finland; and the Australian Research Council (ARC). [1] F. Abe et al.,
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