Search for new physics in the mumu+ e/mu+ EslashT channel with a low-pT lepton threshold at the Collider Detector at Fermilab

Search for new physics in the mumu+e/mu+EslashTchannel with a low-pT lepton threshold at the ColliderDetector at Fermilab

Citation CDF Collaboration et al. “Search for new physics in themumu+e/mu+EslashT channel with a low-pT lepton threshold atthe Collider Detector at Fermilab.” Physical Review D 79.5(2009): 052004. 2009 The American Physical Society

As Published http://dx.doi.org/10.1103/PhysRevD.79.052004

Publisher American Physical Society

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Accessed Wed Jul 15 10:04:08 EDT 2015

Citable Link http://hdl.handle.net/1721.1/52309

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Search for new physics in the ��þ e=�þE6 T channel with a low-pT lepton threshold at theCollider Detector at Fermilab

T. Aaltonen,24 J. Adelman,14 T. Akimoto,56 M.G. Albrow,18 B. Alvarez Gonzalez,12 S. Amerio,44b,44a D. Amidei,35

A. Anastassov,39 A. Annovi,20 J. Antos,15 G. Apollinari,18 A. Apresyan,49 T. Arisawa,58 A. Artikov,16 W. Ashmanskas,18

A. Attal,4 A. Aurisano,54 F. Azfar,43 P. Azzurri,47d,47a W. Badgett,18 A. Barbaro-Galtieri,29 V. E. Barnes,49 B. A. Barnett,26

V. Bartsch,31 G. Bauer,33 P.-H. Beauchemin,34 F. Bedeschi,47a D. Beecher,31 S. Behari,26 G. Bellettini,47b,47a J. Bellinger,60

D. Benjamin,17 A. Beretvas,18 J. Beringer,29 A. Bhatti,51 M. Binkley,18 D. Bisello,44b,44a I. Bizjak,31,w R. E. Blair,2

C. Blocker,7 B. Blumenfeld,26 A. Bocci,17 A. Bodek,50 V. Boisvert,50 G. Bolla,49 D. Bortoletto,49 J. Boudreau,48

A. Boveia,11 B. Brau,11,b A. Bridgeman,25 L. Brigliadori,44a C. Bromberg,36 E. Brubaker,14 J. Budagov,16 H. S. Budd,50

S. Budd,25 S. Burke,18 K. Burkett,18 G. Busetto,44b,44a P. Bussey,22,l A. Buzatu,34 K. L. Byrum,2 S. Cabrera,17,u

C. Calancha,32 M. Campanelli,36 M. Campbell,35 F. Canelli,18 A. Canepa,46 B. Carls,25 D. Carlsmith,60 R. Carosi,47a

S. Carrillo,19,n S. Carron,34 B. Casal,12 M. Casarsa,18 A. Castro,6b,6a P. Catastini,47c,47a D. Cauz,55b,55a V. Cavaliere,47c,47a

M. Cavalli-Sforza,4 A. Cerri,29 L. Cerrito,31,o S. H. Chang,28 Y. C. Chen,1 M. Chertok,8 G. Chiarelli,47a G. Chlachidze,18

F. Chlebana,18 K. Cho,28 D. Chokheli,16 J. P. Chou,23 G. Choudalakis,33 S. H. Chuang,53 K. Chung,13 W.H. Chung,60

Y. S. Chung,50 T. Chwalek,27 C. I. Ciobanu,45 M.A. Ciocci,47c,47a A. Clark,21 D. Clark,7 G. Compostella,44a

M. E. Convery,18 J. Conway,8 M. Cordelli,20 G. Cortiana,44b,44a C.A. Cox,8 D. J. Cox,8 F. Crescioli,47b,47a

C. Cuenca Almenar,8,u J. Cuevas,12,s R. Culbertson,18 J. C. Cully,35 D. Dagenhart,18 M. Datta,18 T. Davies,22

P. de Barbaro,50 S. De Cecco,52a A. Deisher,29 G. De Lorenzo,4 M. Dell’Orso,47b,47a C. Deluca,4 L. Demortier,51 J. Deng,17

M. Deninno,6a P. F. Derwent,18 G. P. di Giovanni,45 C. Dionisi,52b,52a B. Di Ruzza,55b,55a J. R. Dittmann,5 M. D’Onofrio,4

S. Donati,47b,47a P. Dong,9 J. Donini,44a T. Dorigo,44a S. Dube,53 J. Efron,40 A. Elagin,54 R. Erbacher,8 D. Errede,25

S. Errede,25 R. Eusebi,18 H. C. Fang,29 S. Farrington,43 W. T. Fedorko,14 R.G. Feild,61 M. Feindt,27 J. P. Fernandez,32

C. Ferrazza,47d,47a R. Field,19 G. Flanagan,49 R. Forrest,8 M. J. Frank,5 M. Franklin,23 J. C. Freeman,18 I. Furic,19

M. Gallinaro,52a J. Galyardt,13 F. Garberson,11 J. E. Garcia,21 A. F. Garfinkel,49 K. Genser,18 H. Gerberich,25 D. Gerdes,35

A. Gessler,27 S. Giagu,52b,52a V. Giakoumopoulou,3 P. Giannetti,47a K. Gibson,48 J. L. Gimmell,50 C.M. Ginsburg,18

N. Giokaris,3 M. Giordani,55b,55a P. Giromini,20 M. Giunta,47b,47a G. Giurgiu,26 V. Glagolev,16 D. Glenzinski,18 M. Gold,38

N. Goldschmidt,19 A. Golossanov,18 G. Gomez,12 G. Gomez-Ceballos,33 M. Goncharov,54 O. Gonzalez,32 I. Gorelov,38

A. T. Goshaw,17 K. Goulianos,51 A. Gresele,44b,44a S. Grinstein,23 C. Grosso-Pilcher,14 R. C. Group,18 U. Grundler,25

J. Guimaraes da Costa,23 Z. Gunay-Unalan,36 C. Haber,29 K. Hahn,33 S. R. Hahn,18 E. Halkiadakis,53 B.-Y. Han,50

J. Y. Han,50 F. Happacher,20 K. Hara,56 D. Hare,53 M. Hare,57 S. Harper,43 R. F. Harr,59 R.M. Harris,18 M. Hartz,48

K. Hatakeyama,51 C. Hays,43 M. Heck,27 A. Heijboer,46 J. Heinrich,46 C. Henderson,33 M. Herndon,60 J. Heuser,27

S. Hewamanage,5 D. Hidas,17 C. S. Hill,11,d D. Hirschbuehl,27 A. Hocker,18 S. Hou,1 M. Houlden,30 S.-C. Hsu,29

B. T. Huffman,43 R. E. Hughes,40 U. Husemann,36 M. Hussein,36 U. Husemann,61 J. Huston,36 J. Incandela,11

G. Introzzi,47a M. Iori,52b,52a A. Ivanov,8 E. James,18 B. Jayatilaka,17 E. J. Jeon,28 M.K. Jha,6a S. Jindariani,18 W. Johnson,8

M. Jones,49 K. K. Joo,28 S. Y. Jun,13 J. E. Jung,28 T. R. Junk,18 T. Kamon,54 D. Kar,19 P. E. Karchin,59 Y. Kato,42

R. Kephart,18 J. Keung,46 V. Khotilovich,54 B. Kilminster,18 D. H. Kim,28 H. S. Kim,28 H.W. Kim,28 J. E. Kim,28

M. J. Kim,20 S. B. Kim,28 S. H. Kim,56 Y.K. Kim,14 N. Kimura,56 L. Kirsch,7 S. Klimenko,19 B. Knuteson,33 B. R. Ko,17

K. Kondo,58 D. J. Kong,28 J. Konigsberg,19 A. Korytov,19 A.V. Kotwal,17 M. Kreps,27 J. Kroll,46 D. Krop,14 N. Krumnack,5

M. Kruse,17 V. Krutelyov,11 T. Kubo,56 T. Kuhr,27 N. P. Kulkarni,59 M. Kurata,56 Y. Kusakabe,58 S. Kwang,14

A. T. Laasanen,49 S. Lami,47a S. Lammel,18 M. Lancaster,31 R. L. Lander,8 K. Lannon,40,r A. Lath,53 G. Latino,47c,47a

I. Lazzizzera,44b,44a T. LeCompte,2 E. Lee,54 H. S. Lee,14 S.W. Lee,54,t S. Leone,47a J. D. Lewis,18 C.-S. Lin,29 J. Linacre,43

M. Lindgren,18 E. Lipeles,46 A. Lister,8 D.O. Litvintsev,18 C. Liu,48 T. Liu,18 N. S. Lockyer,46 A. Loginov,61

M. Loreti,44b,44a L. Lovas,15 D. Lucchesi,44b,44a C. Luci,52b,52a J. Lueck,27 P. Lujan,29 P. Lukens,18 G. Lungu,51 L. Lyons,43

J. Lys,29 R. Lysak,15 D. MacQueen,34 R. Madrak,18 K. Maeshima,18 K. Makhoul,33 T. Maki,24 P. Maksimovic,26

S. Malde,43 S. Malik,31 G. Manca,30,f A. Manousakis-Katsikakis,3 F. Margaroli,49 C. Marino,27 C. P. Marino,25

A. Martin,61 V. Martin,22,m M. Martınez,4 R. Martınez-Balların,32 T. Maruyama,56 P. Mastrandrea,52a T. Masubuchi,56

M. Mathis,26 M. E. Mattson,59 P. Mazzanti,6a K. S. McFarland,50 P. McIntyre,54 R. McNulty,30,k A. Mehta,30 P. Mehtala,24

A. Menzione,47a P. Merkel,49 C. Mesropian,51 T. Miao,18 N. Miladinovic,7 R. Miller,36 C. Mills,23 M. Milnik,27 A. Mitra,1

G. Mitselmakher,19 H. Miyake,56 N. Moggi,6a C. S. Moon,28 R. Moore,18 M. J. Morello,47b,47a J. Morlok,27

P. Movilla Fernandez,18 J. Mulmenstadt,29 A. Mukherjee,18 Th. Muller,27 R. Mumford,26 P. Murat,18 M. Mussini,6b,6a

J. Nachtman,18 Y. Nagai,56 A. Nagano,56 J. Naganoma,56 K. Nakamura,56 I. Nakano,41 A. Napier,57 V. Necula,17 J. Nett,60

PHYSICAL REVIEW D 79, 052004 (2009)

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C. Neu,46,v M. S. Neubauer,25 S. Neubauer,27 J. Nielsen,29,h L. Nodulman,2 M. Norman,10 O. Norniella,25 E. Nurse,31

L. Oakes,43 S. H. Oh,17 Y.D. Oh,28 I. Oksuzian,19 T. Okusawa,42 R. Orava,24 S. Pagan Griso,44b,44a E. Palencia,18

V. Papadimitriou,18 A. Papaikonomou,27 A.A. Paramonov,14 B. Parks,40 S. Pashapour,34 J. Patrick,18 G. Pauletta,55b,55a

M. Paulini,13 C. Paus,33 T. Peiffer,27 D. E. Pellett,8 A. Penzo,55a T. J. Phillips,17 G. Piacentino,47a E. Pianori,46 L. Pinera,19

K. Pitts,25 C. Plager,9 L. Pondrom,60 O. Poukhov,16,a N. Pounder,43 F. Prakoshyn,16 A. Pronko,18 J. Proudfoot,2

F. Ptohos,18,j E. Pueschel,13 G. Punzi,47b,47a J. Pursley,60 J. Rademacker,43,d A. Rahaman,48 V. Ramakrishnan,60

N. Ranjan,49 I. Redondo,32 P. Renton,43 M. Renz,27 M. Rescigno,52a S. Richter,27 F. Rimondi,6b,6a L. Ristori,47a

A. Robson,22 T. Rodrigo,12 T. Rodriguez,46 E. Rogers,25 S. Rolli,57 R. Roser,18 M. Rossi,55a R. Rossin,11 P. Roy,34

A. Ruiz,12 J. Russ,13 V. Rusu,18 A. Safonov,54 W.K. Sakumoto,50 O. Salto,4 L. Santi,55b,55a S. Sarkar,52b,52a L. Sartori,47a

K. Sato,18 A. Savoy-Navarro,45 P. Schlabach,18 A. Schmidt,27 E. E. Schmidt,18 M.A. Schmidt,14 M. P. Schmidt,61,a

M. Schmitt,39 T. Schwarz,8 L. Scodellaro,12 A. Scribano,47b,47a F. Scuri,47a A. Sedov,49 S. Seidel,38 Y. Seiya,42

A. Semenov,16 L. Sexton-Kennedy,18 F. Sforza,47a A. Sfyrla,25 S. Z. Shalhout,59 T. Shears,30 P. F. Shepard,48

M. Shimojima,56,q S. Shiraishi,14 M. Shochet,14 Y. Shon,60 I. Shreyber,37 A. Sidoti,47a P. Sinervo,34 A. Sisakyan,16

A. J. Slaughter,18 J. Slaunwhite,40 K. Sliwa,57 J. R. Smith,8 F. D. Snider,18 R. Snihur,34 A. Soha,8 S. Somalwar,53 V. Sorin,36

J. Spalding,18 T. Spreitzer,34 P. Squillacioti,47c,47a M. Stanitzki,61 R. St. Denis,22 B. Stelzer,34 O. Stelzer-Chilton,34

D. Stentz,39 J. Strologas,38 G. L. Strycker,35 D. Stuart,11 J. S. Suh,28 A. Sukhanov,19 I. Suslov,16 T. Suzuki,56 A. Taffard,25,g

R. Takashima,41 Y. Takeuchi,56 R. Tanaka,41 M. Tecchio,35 P. K. Teng,1 K. Terashi,51 J. Thom,18,i A. S. Thompson,22

G. A. Thompson,25 E. Thomson,46 P. Tipton,61 P. Ttito-Guzman,32 S. Tkaczyk,18 D. Toback,54 S. Tokar,15 K. Tollefson,36

T. Tomura,56 D. Tonelli,18 S. Torre,20 D. Torretta,18 P. Totaro,55b,55a S. Tourneur,45 M. Trovato,47a S.-Y. Tsai,1 Y. Tu,46

N. Turini,47c,47a F. Ukegawa,56 S. Vallecorsa,21 N. van Remortel,24,c A. Varganov,35 E. Vataga,47d,47a F. Vazquez,19,n

G. Velev,18 C. Vellidis,3 V. Veszpremi,49 M. Vidal,32 R. Vidal,18 I. Vila,12 R. Vilar,12 T. Vine,31 M. Vogel,38 I. Volobouev,29,t

G. Volpi,47b,47a P. Wagner,46 R. G. Wagner,2 R. L. Wagner,18 W. Wagner,27 J. Wagner-Kuhr,27 T. Wakisaka,42 R. Wallny,9

S.M. Wang,1 A. Warburton,34 D. Waters,31 M. Weinberger,54 J. Weinelt,27 W.C. Wester III,18 B. Whitehouse,57

D.Whiteson,46,g A. B.Wicklund,2 E.Wicklund,18 S.Wilbur,14 G.Williams,34 H.H.Williams,46 P. Wilson,18 B. L.Winer,40

P. Wittich,18,i S. Wolbers,18 C. Wolfe,14 T. Wright,35 X. Wu,21 F. Wurthwein,10 S.M. Wynne,30 S. Xie,33 A. Yagil,10

K. Yamamoto,42 J. Yamaoka,53 U.K. Yang,14,p Y. C. Yang,28 W.M. Yao,29 G. P. Yeh,18 J. Yoh,18 K. Yorita,14 T. Yoshida,42

G. B. Yu,50 I. Yu,28 S. S. Yu,18 J. C. Yun,18 L. Zanello,52b,52a A. Zanetti,55a X. Zhang,25 Y. Zheng,9,e and S. Zucchelli6b,6a

(CDF Collaboration)

1Institute of Physics, Academia Sinica, Taipei, Taiwan 11529, Republic of China2Argonne National Laboratory, Argonne, Illinois 60439, USA

3University of Athens, 157 71 Athens, Greece4Institut de Fisica d’Altes Energies, Universitat Autonoma de Barcelona, E-08193, Bellaterra (Barcelona), Spain

5Baylor University, Waco, Texas 76798, USA6aIstituto Nazionale di Fisica Nucleare Bologna, I-40127 Bologna, Italy

6bUniversity of Bologna, I-40127 Bologna, Italy7Brandeis University, Waltham, Massachusetts 02254, USA

8University of California, Davis, Davis, California 95616, USA9University of California, Los Angeles, Los Angeles, California 90024, USA

10University of California, San Diego, La Jolla, California 92093, USA11University of California, Santa Barbara, Santa Barbara, California 93106, USA

12Instituto de Fisica de Cantabria, CSIC-University of Cantabria, 39005 Santander, Spain13Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA

14Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637, USA15Comenius University, 842 48 Bratislava, Slovakia; Institute of Experimental Physics, 040 01 Kosice, Slovakia

16Joint Institute for Nuclear Research, RU-141980 Dubna, Russia17Duke University, Durham, North Carolina 27708, USA

18Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA19University of Florida, Gainesville, Florida 32611, USA

20Laboratori Nazionali di Frascati, Istituto Nazionale di Fisica Nucleare, I-00044 Frascati, Italy21University of Geneva, CH-1211 Geneva 4, Switzerland

22Glasgow University, Glasgow G12 8QQ, United Kingdom23Harvard University, Cambridge, Massachusetts 02138, USA

24Division of High Energy Physics, Department of Physics, University of Helsinki and Helsinki Institute of Physics,FIN-00014, Helsinki, Finland

T. AALTONEN et al. PHYSICAL REVIEW D 79, 052004 (2009)

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25University of Illinois, Urbana, Illinois 61801, USA26The Johns Hopkins University, Baltimore, Maryland 21218, USA

27Institut fur Experimentelle Kernphysik, Universitat Karlsruhe, 76128 Karlsruhe, Germany28Center 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, Korea29Ernest Orlando Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA

30University of Liverpool, Liverpool L69 7ZE, United Kingdom31University College London, London WC1E 6BT, United Kingdom

32Centro de Investigaciones Energeticas Medioambientales y Tecnologicas, E-28040 Madrid, Spain33Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA

34Institute of Particle Physics: McGill University, Montreal, Quebec, Canada H3A 2T8; Simon Fraser University, Burnaby, BritishColumbia, Canada V5A 1S6; University of Toronto, Toronto, Ontario, Canada M5S 1A7; TRIUMF,

Vancouver, British Columbia, Canada V6T 2A335University of Michigan, Ann Arbor, Michigan 48109, USA

36Michigan State University, East Lansing, Michigan 48824, USA37Institution for Theoretical and Experimental Physics, ITEP, Moscow 117259, Russia

38University of New Mexico, Albuquerque, New Mexico 87131, USA39Northwestern University, Evanston, Illinois 60208, USA40The Ohio State University, Columbus, Ohio 43210, USA

41Okayama University, Okayama 700-8530, Japan42Osaka City University, Osaka 588, Japan

43University of Oxford, Oxford OX1 3RH, United Kingdom44aIstituto Nazionale di Fisica Nucleare, Sezione di Padova-Trento, I-35131 Padova, Italy

44bUniversity of Padova, I-35131 Padova, Italy45LPNHE, Universite Pierre et Marie Curie/ IN2P3-CNRS, UMR7585, Paris, F-75252 France

46University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA47aIstituto Nazionale di Fisica Nucleare Pisa, I-56127 Pisa, Italy

47bUniversity of Pisa, I-56127 Pisa, Italy47cUniversity of Siena, I-56127 Pisa, Italy

47dScuola Normale Superiore, I-56127 Pisa, Italy48University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA

49Purdue University, West Lafayette, Indiana 47907, USA50University of Rochester, Rochester, New York 14627, USA

51The Rockefeller University, New York, New York 10021, USA52aIstituto Nazionale di Fisica Nucleare, Sezione di Roma 1, I-00185 Roma, Italy

52bSapienza Universita di Roma, I-00185 Roma, Italy53Rutgers University, Piscataway, New Jersey 08855, USA

54Texas A&M University, College Station, Texas 77843, USA55aIstituto Nazionale di Fisica Nucleare Trieste/Udine, I-34100 Trieste, Italy

vVisitor from the University of Virginia, Charlottesville, VA22904, USA.

uVisitor from IFIC (CSIC-Universitat de Valencia), 46071Valencia, Spain.

tVisitor from Texas Tech University, Lubbock, TX 79409,USA.

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wOn leave from J. Stefan Institute, Ljubljana, Slovenia.lVisitor from the Royal Society of Edinburgh.

sVisitor from the University de Oviedo, E-33007 Oviedo,Spain.

aDeceased.

dVisitor from the University of Bristol, Bristol BS8 1TL,United Kingdom.

bVisitor from the University of Massachusetts Amherst,Amherst, MA 01003, USA.

iVisitor from Cornell University, Ithaca, NY 14853, USA.

cVisitor from the Universiteit Antwerpen, B-2610 Antwerp,Belgium.

eVisitor from the Chinese Academy of Sciences, Beijing100864, China.

fVisitor from the Istituto Nazionale di Fisica Nucleare,Sezione di Cagliari, 09042 Monserrato (Cagliari), Italy.

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pVisitor from the University of Manchester, Manchester M139PL, England.

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SEARCH FOR NEW PHYSICS IN THE ��þ e=�þ E6 T . . . PHYSICAL REVIEW D 79, 052004 (2009)

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55bUniversity of Trieste/Udine, I-33100 Udine, Italy56University of Tsukuba, Tsukuba, Ibaraki 305, Japan57Tufts University, Medford, Massachusetts 02155, USA

58Waseda University, Tokyo 169, Japan59Wayne State University, Detroit, Michigan 48201, USA

60University of Wisconsin, Madison, Wisconsin 53706, USA61Yale University, New Haven, Connecticut 06520, USA(Received 20 October 2008; published 18 March 2009)

A search for new physics using three-lepton (trilepton) data collected with the CDF II detector and

corresponding to an integrated luminosity of 976 pb�1 is presented. The standard model predicts a low

rate of trilepton events, which makes some supersymmetric processes, such as chargino-neutralino

production, measurable in this channel. The ��þ ‘ signature is investigated, where ‘ is an electron

or a muon, with the additional requirement of large missing transverse energy. In this analysis, the lepton

transverse momenta with respect to the beam direction (pT) are as low as 5 GeV=c, a selection that

improves the sensitivity to particles that are light as well as to ones that result in leptonically decaying tau

leptons. At the same time, this low-pT selection presents additional challenges due to the non-negligible

heavy-quark background at low lepton momenta. This background is measured with an innovative

technique using experimental data. Several dimuon and trilepton control regions are investigated, and

good agreement between experimental results and standard-model predictions is observed. In the signal

region, we observe one three-muon event and expect 0:4� 0:1��þ ‘ events from standard-model

processes.

DOI: 10.1103/PhysRevD.79.052004 PACS numbers: 11.30.Pb, 12.60.Jv, 13.85.Hd, 13.85.Rm

I. INTRODUCTION

The standard model (SM) of particle physics is enor-mously successful in describing known particles and theirinteractions. However, strong motivation from experimen-tal data as well as important theoretical considerationspoint to new physics beyond the SM. Astrophysical obser-vations that have resulted in the ‘‘concordance’’ model ofcosmology [1] require a source of dark matter that does notexist in the SM. Theoretically, the SM has well-knownlimitations in explaining the origin of mass and solvingthe hierarchy problem. Moreover, it does not satisfy ourdesire for the unification of the strong and electroweakinteractions and the integration of gravity in a uniquetheory [2].

A powerful strategy for discovering new physics is tosearch in event topologies where the SM predicts ex-tremely low production rates. One of these topologies isthree leptons (trileptons) in hadronic collisions. The leptoncandidates we observe at the Tevatron collider resultmainly from QCD processes or the decays of massivegauge bosons (W or Z), or photon conversions. The leptonsrarely appear in multiplicity greater than two. The trileptonsignature is favored by a large class of models of super-symmety (SUSY) [3,4], in which the lightest supersym-metric particles are the gauginos, the supersymmetricpartners of the gauge bosons. The corresponding observ-able SUSY particles are two charginos (~��

1;2) and four

neutralinos (~�01;2;3;4), which result from the mixing of the

gauginos and the supersymmetric partners of the Higgsbosons, the higgsinos. The associated production of char-ginos and neutralinos may have a detectable cross section

[5] at the Tevatron and may give rise to trilepton events asshown in Figs. 1 and 2. The most common decays arethrough off-shell vector bosons or scalar leptons (sleptons),with branching fractions that depend on the chargino,neutralino, and slepton masses.Under the assumption of R-parity [6] conservation,

SUSY particles cannot yield only SM particles in theirdecay; the lightest SUSY particle (LSP) will be stableand escape detection. Therefore, SUSY events would becharacterized by large transverse momentum imbalance(‘‘missing transverse energy,’’ or E6 T). In many SUSYscenarios the lightest neutralino is either the LSP or itdecays to the LSP resulting in E6 T in both cases.Additional E6 T results from the undetected final-state neu-

FIG. 1. Chargino-neutralino production through an s-channelW boson (a) and a t-channel squark propagator (b). The t channelis suppressed in scenarios with very massive squarks.


052004-4

http://dx.doi.org/10.1103/PhysRevD.79.052004

trinos, as shown in Fig. 2. The trileptonþ E6 T topologyinvestigated here is the ‘‘golden’’ signature for the discov-ery of SUSY at the Tevatron [7–11].

The LSP is a candidate for the cold dark matter of theuniverse [12,13]. In addition, SUSYoffers a solution to thehierarchy problem [14–16] and the possibility for unifica-tion of interactions at high energies [17].

Searches for chargino and neutralino production havebeen previously performed by the LEP [18,19] andTevatron experiments [20,21]. In this paper we present atrilepton analysis that utilizes increased luminosity andimproved kinematic acceptance. We search for new phys-ics in the final state with two muons and an additionalelectron or muon using data collected with the CDF IIdetector from March 2002 to February 2006 from proton-antiproton collisions at

ffiffiffis

p ¼ 1:96 TeV. The integratedluminosity of our sample is 976 pb�1. To increase thesensitivity to new light particles and tau leptons that decayleptonically, we use a very low pT threshold (5 GeV=c) forthe identified leptons, where pT is the transverse momen-tum with respect to the beam direction.

We define several dimuon and trilepton SM-dominatedcontrol regions, in which we verify our understanding ofthe backgrounds. In order to avoid bias, we complete thevalidation of the background—both in event yields andkinematic shapes—in the control regions before investigat-ing the events in the signal region. Finally, our result iscombined with other trilepton searches at CDF [22] to set astronger limit on chargino-neutralino production. Althoughthis search is inspired by SUSY-predicted chargino-neutralino production, the analysis is generic enough tobe sensitive to any new physics that would enhance theproduction of prompt trileptons and E6 T .

This paper is organized as follows: In Sec. II, we de-scribe the CDF II detector. In Sec. III, we define theexperimental dataset and present an event selection thatreduces the SM background expectation while acceptingevents from possible new-physics signals. Section IV de-scribes how the SM background rates are estimated, andSec. V discusses two SUSY-model scenarios we consider.In Sec. VI, we present the determination of the systematicuncertainties on the signal and background event-yieldpredictions. In Sec. VII, we present the event yields andkinematic distributions in our control regions that increasethe confidence in our understanding of the SM background.Finally, in Sec. VIII, we present the results in the signalregion.

II. THE CDF II DETECTOR

The CDF II detector [23] is a multipurpose cylindricaldetector with projective-tower calorimeter geometry andexcellent lepton-identification capability. It operates at theTevatron collider where protons and antiprotons collidewith a center-of-mass energy of 1.96 TeV. In our coordi-nate system, the positive z-axis is defined by the protonbeam direction and the positive y-axis by the verticalupward direction. The detector is approximately symmet-ric in the � and� coordinates, where the pseudorapidity �is defined as � ¼ � lnðtanð�=2ÞÞ, � is the polar angle withrespect to ~z, and � is the azimuthal angle. We brieflypresent here the CDF components that are most criticalto this analysis.In the center of the apparatus, near the beam collision

point, a silicon detector of inner radius of 1.35 cm andouter radius of 25.6 cm provides detailed tracking in thej�j< 2 region, necessary for the accurate determination ofthe proton-antiproton interaction points (primary vertices)and impact parameters of particle trajectories with respectto these points.A cylindrical 96-layer open-cell argon-ethane (50%-

50%) drift chamber (COT) of inner radius of 44 cm andouter radius of 132 cm provides tracking for chargedparticles with �100% detection efficiency in the central(j�j< 1:1) region. The central tracking system is locatedin a magnetic field of 1.4 T provided by a superconductingsolenoidal magnet. The relative resolution in tracking mo-mentum provided by the COT is �pT=pT ¼0:0017pTðGeV=cÞ�1.Surrounding the central tracker, and outside the sole-

noid, a central electromagnetic calorimeter (CEM) and acentral hadronic calorimeter (CHA) measure the energy ofelectrons, photons, and hadrons. The CEM is composed oflayers of lead and scintillator whereas the CHA is com-posed of layers of steel and scintillator. The relative energyresolution is 13:5%=

ffiffiffiffiffiffiET

p � 2% for the CEM and75%=


p � 3% for the CHA, where the transverse energyET ¼ E sin� is quoted in GeV units. A strip chamber,placed inside the electromagnetic calorimeter at the posi-

FIG. 2. Chargino and neutralino decays through gauge bosons(a,b) and through sleptons (c,d). The branching fractions dependon the masses of the sleptons, which always decay to chargedleptons, unlike the gauge bosons. The leptonic signature consistsof three leptons in both cases.


052004-5

tion of maximum development of the electromagneticshower (six radiation lengths), is used for shower shapedetermination and for matching the calorimeter energydepositions with COT tracks. In the forward region, aplug electromagnetic calorimeter (1:1< j�j< 2:4) has arelative resolution of 16%=


p � 0:7% and a plug had-ronic calorimeter (1:3< j�j< 2:4) a resolution of130%=


p � 4%. The raw missing transverse energy vec-

tor is defined as�ðPi~EiTÞ, where ~EiT has magnitude equal

to the energy deposited in the ith calorimeter tower anddirection perpendicular to the beam axis and pointing tothat calorimeter tower.

Outside the calorimeters, the central muon system con-sists of drift chambers. The central muon chambers (CMU)detect muons in the pseudorapidity range j�j< 0:6, whilethe central muon extension (CMX) chambers detect muonsin the 0:6< j�j< 1:0 range, both with a detection effi-ciency of almost 100% for muons above 3 GeV=c. Toreduce the hadron punch-through contamination, extrachambers (CMP) are installed outside the CMU chambers,with extra steel absorber added between them. The muonsthat are detected by both CMU and CMP chambers arelabeled ‘‘CMUP muons,’’ and their detector signaturescannot be easily caused by hadrons.

The instantaneous luminosity is measured withCherenkov counters located close to the beam line at 3:7<j�j< 4:7.

The CDF trigger system [24] has a three-level pipelinedand buffered architecture; each level provides a rate reduc-tion sufficient to allow for processing at the next level withminimal deadtime. The first level consists of special-purpose processors that accept events at rate of 25 kHz,with an average event size of 170 kB, counts main trigger-ing objects. The second level is also based on hardware andperforms a partial event reconstruction before passing theevents to the next level at a rate of 350 Hz. Finally, asoftware-based third level uses a fast version of the offlineevent reconstruction to reduce the event rate to 75 Hz,appropriate for writing to tape. The track-based triggersaccount for approximately 75% of the trigger bandwidthand are used in this analysis. For a muon trigger, the mainrequirement is that a COT track is geometrically matchedto a track segment in a muon detector.

III. THE CDF DATASET AND SIGNAL-REGIONEVENT SELECTION

In order to include in our analysis muons and electronsthat come from tau decays, we use a low transverse mo-mentum requirement (pT > 5 GeV=c) for these leptons.For this reason, we analyze data collected with the CDFlow-pT dimuon triggers (pT nominally above 4 GeV=c forboth muons). These muons are central in the detector(CMUP or CMX). We measure the trigger efficiency usingJ=c , � and Z-boson events collected with single-muon

triggers. In these samples, we remove hadronic back-grounds using the mass-spectra sidebands, and count thefrequency that a second muon fired the trigger of interest.The plateau value of the trigger efficiency’s pT dependencefor single muons is �0:95 and it is reached at pT �5 GeV=c.After the collected events are processed by the offline

reconstruction software, additional requirements are ap-plied for the definition of the dimuon sample. We requirethat each event has a primary vertex within 60 cm from thenominal center of the detector in the z direction and that atleast two muons with transverse momenta above 5 GeV=coriginate from that primary vertex and pass the CDFstandard muon tracking and calorimetry requirementsand track-chamber matching requirements [25]. In eventswith more than one reconstructed primary vertex, we usethe primary vertex that is closest to the tracks of the twohighest-pT muons that satisfy all other event requirements.We specifically require that two good-quality COT tracksare geometrically matched with respective reconstructedtrack segments in the CMX orCMUþ CMP detectors, thatthe energies deposited in the electromagnetic and hadroniccalorimeters are consistent with that expected from mini-mum ionizing particles, and that the two muons are iso-lated. We define the isolation I as the energy deposited in

the calorimeters in a cone of �R ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffið��Þ2 þ ð��Þ2p ¼0:4 around the muon without counting the energy depos-ited by the muon. We require that I < 0:1� pTc if pT >20 GeV=c or I < 2 GeV otherwise, where pT is the trans-verse momentum of the muon. The selected two muons arealso �R> 0:4 apart. A critical requirement is that themuons are prompt as measured by the impact parameter(d0), defined as the distance of closest approach of a trackto the primary vertex in the transverse plane. We requirethat jd0j< 0:02 cm if the muon leaves tracking signals inthe silicon detector (silicon hits) and that jd0j< 0:2 cm ifthe muon leaves no silicon hits. We expect that most muonswith large impact parameters come from heavy flavor(bottom- or charm-hadron semileptonic decays), fakemuons (light-flavor hadrons such as pions and kaons thatdecay in flight or punch through to the muon detectors),and cosmic rays. The heavy flavor (HF) and fake-muonbackgrounds dominate at low dimuon masses. Residualcosmic-ray background, not removed by the cosmic filtersdescribed in [25], is reduced by requiring that the three-dimensional angular separation (�’) of the two highest-pT

muons is less than 178 degrees. After including the selec-tion criteria discussed above, the total muon-identificationefficiency, as measured with J=c and Z boson CDF data, is(90–96)%, rising with increasing muon pT .For the trilepton selection, we require the presence of a

third muon satisfying the same selection requirements asthe first two, or an electron satisfying the CDF standardelectron calorimeter, tracking, and track-calorimetermatching identification requirements [25]. The transverseenergy and momentum of an electron is required to exceed


052004-6

5 GeV. Tracks associated with electrons should match hitsin the strip chamber wires. We require that I < 0:1� ET ifET > 20 GeV or I < 2 GeV otherwise, where I is now theenergy-based isolation of the electron, and ET is its trans-verse energy. Electrons originating from photons that con-vert into eþe� pairs are identified with an algorithm [25]that seeks nearby tracks with a common vertex and direc-tion. These electrons are removed from the observed datasample. The electron identification efficiency is (75–83)%[25], rising with increasing electron transverse energy, asmeasured with Drell-Yan (DY) [26] electrons. The thirdlepton is required to be �R> 0:4 away from the leadingtwo muons.

We define the signal region by the following additionalrequirements: the dimuon mass (constructed using the twohighest-pT muons) is greater than 15 GeV=c2, for removalof low-mass resonances, and outside a Z mass window of76<M�� < 106 GeV=c2. In addition, we require the

missing transverse energy (E6 T) to exceed 15 GeV, in orderto select events with undetected new particles while reject-ing DY, HF, and fake-muon backgrounds. Finally, we countthe number of jets Njets with energy above 15 GeV and we

require that Njets � 1, in order to reduce the t�t background.

In this analysis, we use jets defined by a fixed-cone algo-rithm [25] with a cone size of �R ¼ 0:4. We require thatjets deposit less than 90% of their measured energy in theelectromagnetic calorimeter, in order to avoid countingelectrons or photons as jets. Jet energies are corrected[27] to represent better the energy of the final-state had-rons. Global and local corrections are applied as well asinclusion of corrections for the effects of multiple inter-actions. These corrections are also applied to the rawmissing transverse energy for the calculation of E6 T , whichis also corrected for the presence of muons in our events.We check the consistency of the observed data comparedwith the SM predictions in the control regions that aredescribed in Sec. VII.

IV. STANDARD-MODEL BACKGROUNDS

To determine the significance of any incompatibilitybetween prediction and observation, and also to set limitson production cross sections and masses of new particles,we need a reliable background estimation. The major SMsource of dimuons is the DY process and, in events withlow dimuon mass, HF production and the fake-leptonbackground. In the trilepton regions, the dominant back-grounds are DY (accompanied by a fake or conversionlepton), dibosons (WW, ZZ, and WZ), and HF. BecauseHF and fake leptons are difficult to model withMonte Carlo (MC) simulations due to sizable higher-orderQCD effects and the imperfect modeling of the leptonisolation in a high particle-multiplicity hadronic environ-ment, we estimate these backgrounds using CDF data. Allother backgrounds are estimated with MC simulation.

A. MC-estimated backgrounds

We use the PYTHIA [28] generator to model the DY,WW,ZZ, and t�t background, and MADEVENT [29] for the WZbackground [30]. The DY background includes the decaysto tau leptons that subsequently decay to muons [31]. Weuse the CTEQ5L [32] parton distribution functions (PDF)throughout. For the trilepton predictions we require thereconstructed electrons and muons to be kinematicallymatched with the generator-level leptons, in order not todouble count some of the fake-lepton contribution. Toestimate the trilepton background from DYþ �, we relaxthis matching requirement, demand that the electron isidentified at the event-simulation level as a photon-conversion product, and normalize the surviving eventusing a scale factor [33]. This scale factor accounts forthe difference in conversion-removal inefficiency betweenthe observed data and the MC simulation. In the remainderof the paper we add the DYþ � background to the rest ofthe diboson contribution (WW, ZZ, and WZ). We processeach generated event with the CDF detector simulation,based on GEANT [34]. We normalize all samples using theleading-order theoretical cross sections multiplied by theappropriate scale (‘‘K-factor’’) to correct for next-to-lead-ing-order effects [35,36]. Scale factors that correct for theknown differences in lepton identification and reconstruc-tion efficiencies between the observed data and the MCsimulation are also applied.

B. Data-estimated backgrounds

We first estimate the fake-lepton background, using anindependent CDF data sample. Subsequently, we use thisfake-lepton background and the MC-estimated DY contri-bution in our HF-estimation method.

1. Fake leptons

‘‘Fake’’ leptons are reconstructed lepton candidates thatare either not real leptons or are real leptons but neitherprompt nor do they originate from semileptonic decays ofHF quarks. In the case of muons, the fakes can be light-flavored hadrons, such as pions and kaons or part ofhadronic showers, that penetrate (‘‘punch through’’) thecalorimeters and reach the muon detectors or decay tomuons in flight. In the case of electrons, fakes are jetsthat are misreconstructed as electrons, often due to neutralpions that decay to photons, which shower in the electro-magnetic calorimeter. We can thus associate the fake lep-tons with light-flavor partons. Using multijet CDF datasetscollected with jet-based triggers, we measure the ‘‘fakerate,’’ i.e., the probability for an isolated track to be mis-reconstructed as a muon or the probability for a jet to bemisreconstructed as an electron. The fake rate is measuredas a function of the track’s (jet’s) transverse momentum(energy) and pseudorapidity. The fake rate is of the order of10�2 for isolated tracks to be incorrectly reconstructed as


052004-7

muons and increases with the pT of the muon candidate’strack. The fake rate of a jet being reconstructed as anelectron is of the order of 10�4 and falls with increasingET . The fake rates increase for higher pseudorapidityleptons [33].

To determine the background coming from a real muonand a misidentified hadron (i.e., fake dimuon background),we use single-muon low-pT-triggered CDF data. For eachevent, we require one good muon candidate that passes therequirements of our analysis. We then apply the fake rateon all other tracks in the event, except on the track of asecond muon (to remove DY contamination of the fakebackground). We remove events in which the ‘‘muon+track’’ mass is within the Z boson window (76<M�� <

106 GeV=c2) and also 35< E6 T < 55 GeV. These eventsare associated with decays of real Z bosons produced atrest, where one decay muon is not detected, resulting in E6 T

equal to about half the mass of the Z boson. We investigatethe heavy flavor contamination in this light-flavor-dominated background, caused by a real muon comingfrom a heavy-quark semileptonic decay, which is misre-constructed as an isolated track instead of a muon. Thiscontamination is negligible (approximately 0.2% of thebackground), mainly because of the background-rejectionpower of our muon isolation requirement. Because thesingle-muon low-pT trigger was not always present duringdata taking, the dimuon fakes extracted using this triggerare normalized to the default dimuon CDF data luminosity,in order to represent the size of fake dimuon contaminationin our analysis CDF dataset. In CDF data with no E6 T or jetmultiplicity cuts applied (‘‘inclusive’’ dataset),�ð9� 5Þ%of the dimuons are fake. In the signal region, the dimuonfake contamination is �ð16� 8Þ%.

For the determination of the background coming from areal muon pair and a misidentified hadron (i.e., fake tri-lepton background), we use our dimuon low-pT-triggeredCDF dataset, require two good muons, and model the fakethird lepton by applying the fake rate to the extra tracks andjets. We assume that the number of events with two fakeleptons is negligible, given the low value of the fake rates.In order not to over count the trilepton fakes, we require thethree leptons in our signal MC and background MCsamples to be kinematically matched with the generatedones. The fake trilepton background is determined to be�ð50� 25Þ% of the total background in both the inclusivedataset and the signal region.

2. Heavy flavor

One of the most significant challenges of this analysis isthe consideration of muons with transverse momentum aslow as 5 GeV=c. This low pT requirement increases ouracceptance, but at the same time contaminates our samplewith HF and fake-lepton events.

We present here an innovative technique for the deter-mination of the amount of the HF background using theobserved data. We construct an HF-rich (HFR) CDF data-

set by reversing the impact parameter requirement for atleast one of the observed muons, so that the absolute valueof the muon impact parameter is above 0.02 cm if there aresilicon hits associated with the muon track, or above 0.2 cmif there are no silicon hits. We also require the dimuon massto be less than 35 GeV=c2. Monte Carlo studies show thatabove that value we expect mainly DYand a negligible HFbackground. We investigated the expected dimuon massspectrum of DYand fake-lepton in the HFR sample, and wedetermined that the effect of the contamination isnegligible.We subsequently use the HFR dimuon mass shape com-

bined with the absolute fake dimuon mass distribution plusthe absolute DY dimuon mass distribution from MC simu-lation in order to fit the observed data. All data samplesother than HFR include the low impact parameter require-ment. Because we observe negligible DY in the same-charge dimuon channel, we perform the fit for same-chargeand opposite-charge dimuons separately. This helps usvalidate our HF-estimation method in the HF-rich same-charge dimuon environment. The only free parameter ofthe fits is the HF normalization—the DY contribution isfixed based on the theoretical cross section and the inte-grated luminosity of the observed data, and the fake-leptonbackground distribution is fixed based on the absoluteexpectation, as described in Sec. IVB1. The results ofthe fits can be seen in Figs. 3 and 4, for opposite-chargeand same-charge muons, respectively. From the two fits,we extract two HF normalization factors that are applied asweights to the original unweighted same-charge andopposite-charge dimuon HFR events, in order to describethe HF background in the observed data. The same weights

FIG. 3 (color online). Fit of HFþDYþ fakes dimuon massdistribution to the observed data for opposite-charge dimuons.The HF normalization is the only free parameter of the fit. Theblue (light gray) filled histogram is the HF and the red (darkgray) is the fakes. The thick line represents the total background,which is almost exclusively DY, HF, and fakes at the opposite-charge dimuon level. The hatched areas indicate the total uncer-tainty on the prediction.


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are used in all kinematic control regions. The weight forthe opposite-charge HF dimuons is 1:94� 0:04 and for thesame-charge HF dimuons is 1:12� 0:05, where the un-certainties come from the fits. This tells us that we expectalmost twice as many opposite-charge HF events in the lowimpact parameter region compared with the high impactparameter one. Overall, the ratio of opposite-charge HF tosame-charge HF events in the inclusive observed data afterthe normalization is�4:1, a value that is also verified withb �b=c �c MC simulation and is a result of the conservedcharge in the underlying quark-pair production and therates of cascade semileptonic decays of b hadrons and chadrons. In regions with no HFR events, we estimate thesize of this background by extrapolating the HF predictionfrom neighboring dimuon control regions that containsufficient numbers of events.

The trilepton HF background is estimated by requiringthat the normalized HFR sample has a third lepton. If thereare no events satisfying this requirement, then we extrapo-late from either neighboring dimuon or trilepton controlregions with sufficient statistics. For example, we have noHFR data in the trilepton signal region. We estimate the HFbackground there by extrapolating from the low-E6 T region,where we have trilepton HFR events. For the extrapolationwe use the dimuon E6 T distribution, using the fact the E6 T

distribution is similar for dimuon and trilepton events. Weverify this fact with the use of MC-simulated b �b=c �c events.

For the determination of the systematic uncertainty as-sociated with the HF-estimation method, we re-estimatethe HF background by redefining the HFR dataset usingeither a requirement on the number of silicon detector hitsfor the muon that has large impact parameter (at least two

silicon hits), and/or applying a requirement on the impactparameter significance (jd0j=�jd0j> 5). These cuts favorHF events but reduce our HFR dataset statistics. The HF-estimation method systematic uncertainty is about 25% inthe signal region.Although the HF normalization is extracted from the

inclusive analysis sample, with dimuon mass greater than10:5 GeV=c2 (to avoid the� resonances) and no additionalE6 T or jet-multiplicity requirements , the agreement of ourHF predictions in both event yields and kinematic distri-butions for all our dimuon and trilepton control regions isexcellent, as we show in Sec. VII.

V. SUSY SIGNAL SCENARIOS

This analysis is a generic search for trilepton events inwhich we focus on minimizing the SM background. Wenevertheless consider two mSUGRA [37] SUSY signalscenarios, ‘‘SIG1’’ and ‘‘SIG2’’, defined by the value ofthe common sfermion mass (m0) and common gauginomass (m1=2) at unification scale, the trilinear coupling

(A0), the ratio of the two Higgs fields vacuum expectationvalues ( tan�), and the sign of the higgsino mixing parame-ter (signð�Þ):(i) SIG1: m0 ¼ 100 GeV=c2, m1=2 ¼ 180 GeV=c2,

A0 ¼ 0, tan� ¼ 5, �> 0. The expected cross sec-tion ðp �p ! ~��

1 ~�02Þ times the branching ratio B to

leptons is �B ¼ 0:642� 0:25 pb. The cross sec-tion was obtained using the next-to-leading-ordercalculation of PROSPINO [38] and the branching ratiousing PYTHIA. The corresponding chargino and light-est neutralino masses would be �116 GeV=c2 and�65 GeV=c2, respectively.

(ii) SIG2: m0 ¼ 74 GeV=c2, m1=2 ¼ 168 GeV=c2,A0 ¼ 0, tan� ¼ 3, �> 0. The expected cross sec-tion times the branching ratio to leptons is �B ¼1:023� 0:5 pb, as given by PROSPINO and PYTHIA.The corresponding chargino and lightest neutralinomasses would be �103 GeV=c2 and �57 GeV=c2,respectively.

These two signal scenarios serve as benchmarks ofpossible SUSY signal and were used for the optimizationof the minimum E6 T requirement in the signal region, whichis set at 15 GeV [39]. The mass spectrum of the super-symmetric particles was obtained with ISAJET [40] and theevents are generated with PYTHIA. The SIG1 scenario leadsto three-body decays (Figs. 2(a) and 2(b)) of the lightestchargino (~��

1 ) and the next-to-lightest neutralino (~�02) with

branching ratios to electrons and muons suppressed, due tothe low branching ratio of the gauge bosons to leptons. Onthe other hand, the SIG2 scenario leads exclusively to two-body decays (Fig. 2(d)) of ~�0

2. Our analysis is more sensi-

tive to SIG2, due to the higher cross section and our abilityto select events with low momentum final-state leptons,originating from tau decays.

FIG. 4 (color online). Fit of HFþDYþ fakes dimuon massdistribution to the observed data for same-charge dimuons. TheHF normalization is the only free parameter of the fit. The blue(light gray) filled histogram is the HF and the red (dark gray) isthe fakes. The thick line represents the total background, whichis constituted almost exclusively by HF and fakes for same-charge dimuon pairs. The hatched areas indicate the total uncer-tainty on the prediction.


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VI. SYSTEMATIC UNCERTAINTIES

The sensitivity of our search to signals of new physicsand the significance of a potential excess of events areinfluenced by the uncertainties on our background esti-mates. Because we perform a counting experiment, weconcentrate on the uncertainties on the expected numberof background and signal events. The event-yield system-atic uncertainty is naturally different for MC-simulated andCDF-data-estimated physical processes. We first discussthe systematic uncertainty on the MC-estimated back-grounds and SUSY signals and then we treat the systematicuncertainty on the CDF-data-based background from HFand fake leptons.

The sources of systematic uncertainty in the signalregion, with their effect on signal and MC-estimated back-ground event yields are

(i) the luminosity uncertainty (6%) [41,42],(ii) the lepton-identification scale factors uncertainty

(� 10%),(iii) the trigger efficiency uncertainty (� 1%),(iv) the jet-energy scale uncertainty (� 1%) [27]; this

source of systematic uncertainty is responsible formigrating events from one control or signal region toanother, since variations in jet energies affect boththe corrections to the E6 T and the jet multiplicity,

(v) the PDF uncertainty (1%–2%) [32],(vi) the uncertainty from the theoretical cross-sections

estimates (5–12% depending on the process)[35,36],

(vii) the uncertainty on the initial- and final-state QCD-induced radiation (ISR/FSR) [43], which has aneffect of 4% and 12% for background and signalMC samples, respectively, and

(viii) the uncertainty induced from the limited MC statis-tics: for the SIG2 MC it is�2% for the dimuons and�6% for the trileptons; for the standard-model back-groundMC it is�3% for the dimuons and�40% forthe trileptons (the latter mainly due to the DYþ �limited MC statistics).

All of the above sources of systematic uncertainty arecorrelated among the different physics processes (DY,diboson, t�t), with the exceptions of the cross section sys-tematic uncertainties and the MC samples’ statistical un-certainties. Still, the sources of systematic uncertainties areuncorrelated with each other and the respective uncertain-ties are summed in quadrature with each other to give thetotal yield uncertainties in the control and signal regions.

The systematic uncertainty associated with the HF-estimation method consists of a part that is anticorrelatedwith the DYþ fakes systematic uncertainty (because theHF weights are given by the fit of the DYþ fakesþ HF tothe observed data, and a varied level of DYþ fakes affectsthese weights) and an uncorrelated part (from the fit un-certainty of about 2–4% for the fixedDYþ fakes level andfrom the HF-estimation method systematic uncertainty, as

described in Sec. IVB 2). The correlated DYþ HFþfakes systematic uncertainty is about 22%. The fake-leptonuncertainty is set to a conservative maximum-envelope50% level, which is determined by studying different jet-triggered CDF samples [33].For the total systematic uncertainty of the predicted SM

event yield in all control regions we take into account allcorrelations among physics processes. For each source ofsystematic uncertainty affecting the MC samples, we varyall MC samples (including DY) in a correlated manner andredo the fit of DYþ fakesþ HF to the observed data toextract a new HF estimation. The total variation gives usthe total effect of the systematic uncertainty. The sameprocedure is followed when we include the fake-leptonestimation uncertainty and its effect on HF due to the abovefit.

TABLE II. The dimuon and trilepton event-yield systematicuncertainties in the signal region, for the SIG1 and SIG2 SUSYscenarios.

Source Dimuons Trileptons

Electron scale factors — �6%Muon scale factors �11% �15%Luminosity �6% �6%Trigger efficiency �0:9% �0:5%PDF �1% �1%ISR �12% �12%Theoretical cross sections �10% �10%Jet-energy scale �0:3% �0:6%MC statistics �2% �6%Total �20% �24%

TABLE I. The dimuon and trilepton event-yield systematicuncertainties for the backgrounds in the signal region. Theuncertainties are summed based on the contributions of theseparate backgrounds taking into account all correlations. Theupper part of the table shows the MC-related systematic uncer-tainties, whereas the lower part shows the systematic uncertain-ties for the CDF-data-estimated backgrounds. The uncertaintiesdue to the MC statistics are not shown.

Source Dimuons Trileptons

Electron scale factors — �2%Muon scale factors �8% �5%Luminosity �3% �2%Trigger efficiency �0:5% �0:2%PDF �2% �1%ISR/FSR �2% �1%Theoretical cross sections �3% �2%Jet-energy scale �0:5% �0:02%Total MC syst. �9% �6%Fakes estimation �8% �25%HF estimation �5% �2%Total (with correlations) �10% �24%


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The effect of the systematic uncertainties on the back-ground and SUSY signal expected event yields in thesignal region can be found in Tables I and II, respectively.

VII. CONTROL REGIONS

We investigate control regions defined by the dimuonmass E6 T , and jet multiplicity, as shown in Fig. 5. Overall,20 dimuon and trilepton control regions are defined, the 11most significant of which are presented here. Most controlregions we investigate are naturally SM-dominated withlittle expectation of SUSY signal. Low E6 T and M�� re-

gions are dominated by the HF background, whereas the76<M�� < 106 GeV=c2 region is almost exclusively

populated with Z bosons. The 5 GeV gap in the E6 T cutsbetween the signal and the control regions ensures that thelow E6 T control regions contain a negligible amount ofsignal. We compare the SM event-yield predictions withobserved events in the control regions (along with kine-matic plots) before looking at the signal region.

We present here a Z boson resonance control region(‘‘Control Z’’), a low E6 T control region (‘‘Control A’’),and three control regions (‘‘Control B, C, and D’’) thatresult from the inversion of one of the three signal-regionrequirements at a time [dimuon mass (M��), or E6 T , or jet

multiplicity (Njets), respectively]. In region Z, we require

that the muons have opposite charge and that the dimuonmass lie between 76 and 106 GeV=c2. We use this controlregion for validating the luminosity, the trigger efficien-cies, and muon-identification scale factors. In region A, werequire that E6 T < 10 GeV and M�� > 10:5 GeV=c2. This

region is used for verifying our knowledge of HF and fake-lepton backgrounds. In region B, we require E6 T > 15 GeV,dimuon mass within the Z-mass region, and low jet multi-plicity (at most one jet). This region helps us verify ourbackground prediction in a low-yield region as most Zevents are characterized by low E6 T . In region C, we requireE6 T < 10 GeV, exclusion of the Z mass region, M�� >

15 GeV=c2, and low jet multiplicity. This region alongwith region A are the ones with the highest population ofHF events. In region D, we require E6 T > 15 GeV, exclu-sion of the Zmass region,M�� > 15 GeV=c2, and high jet

multiplicity (more than one jet). This region is expected tobe the most sensitive to t�t production. We finally study thedimuon events with all signal-region kinematic cuts ap-plied, but before the requirement for a third lepton. This isa critical control region as the trilepton signal is a subset ofthis region.Table III shows the expected and observed number of

dimuon events in our control regions, and Table IV showsthe expected and observed number of trilepton events.After requiring the presence of a third electron or muon,only control regions Z, A, and C are populated with experi-mental data. Region Z trilepton event yields establish ourunderstanding of the electron fakes, since the third electronin Z boson events is almost exclusively a nonpromptelectron. On the other hand, trilepton regions A and C

confirm our understanding of the HF and fake backgroundsfor trileptons. Figures 6 and 7 show the dimuon mass andE6 T distributions for the dimuon control regions. The agree-ment between observed data and prediction in the controlregions is satisfactory, both in event yields and kinematicdistributions.

jets(if N >1) jets(if N >1)Control D

Control A

Control D

Control C

Control B

Signal

10

15

Control Z76 10610.5 15

ET

M

(GeV)

µµ(GeV/c )2

Signal

Control C

FIG. 5 (color online). The control and signal regions used inour analysis are defined in the dimuon mass vs E6 T plane, with theextra requirement of low ( � 1) or high (> 1) jet multiplicity. Inthis paper we show results for the control regions that result fromthe inversion of one of the three main kinematic selections(dimuon mass, missing transverse energy, and jet multiplicity),with the addition of a Z-mass control region (Control Z) and alow-E6 T control region (Control A). The control regions above aredefined for low jet multiplicity, unless otherwise stated.

TABLE III. Expected and observed dimuon event yields, in all control regions and the signal region. The expected SUSY signalevent yield is for the SIG2 mSUGRA scenario. Combined statistical and systematic uncertainties are shown and correlations amongsources of systematic uncertainty are included. The signal region without a requirement for a third lepton is a dimuon control region.

Region DY HF Fakes Diboson t�t Total SM expected SUSY expected Observed

Control Z 6419� 709 — 10� 11 2:4� 0:2 1:18� 0:14 6433� 712 0:30� 0:07 6347

Control A 14820� 2242 9344� 1612 2294� 1148 1:03� 0:09 0:12� 0:03 26459� 1429 0:9� 0:2 26295

Control B 217� 25 — 9� 7 1:7� 0:2 0:27� 0:05 227� 26 0:5� 0:1 253

Control C 5770� 1043 2238� 384 466� 234 0:49� 0:07 0:02� 0:01 8474� 857 0:7� 0:2 8205

Control D 7:8� 1:5 9� 4 0:3� 0:3 0:21� 0:07 4:1� 0:4 22� 5 1:8� 0:4 23

Signal Reg. 169� 30 90� 20 49� 25 6:5� 0:4 0:96� 0:11 315� 37 17� 3 297


052004-11

TABLE IV. Expected and observed trilepton event yields, in all control regions and the signal region. The expected SUSY signalevent yield is for the SIG2 mSUGRA scenario. Combined statistical and systematic uncertainties are shown and correlations amongsources of systematic uncertainty are included.

Region DY HF Fakes Diboson t�t Total SM expected SUSY expected Observed

Control Z 0:2� 0:2 - 2:5� 1:2 0:26� 0:06 - 3� 1 0:06� 0:01 4

Control A 0:3� 0:2 6� 3 7:6� 3:8 0:25� 0:08 - 14� 4 0:08� 0:02 16

Control B - - 0:2� 0:1 0:094� 0:009 - 0:3� 0:1 0:10� 0:03 0

Control C 0:2� 0:2 3� 2 2� 1 0:10� 0:06 - 5� 2 0:06� 0:02 8

Control D - - 0:02� 0:01 0:003� 0:002 0:011� 0:008 0:03� 0:01 0:04� 0:02 0

Signal Reg. - 0:06� 0:04 0:2� 0:1 0:15� 0:06 - 0:4� 0:1 1:7� 0:4 1

CDF DATA

Drell-Yan

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Dibosons

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(GeV)TE15 20 25 30 35 40 45 50

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FIG. 6 (color online). Dimuon mass and E6 T distributions for the SM background in the dimuon control regions Z (a,b), A (c,d), and B

(e,f). The background histograms are stacked. The CDF data are indicated by points with error bars.


052004-12

VIII. SIGNAL-REGION RESULT

After observing satisfactory agreement between experi-mental data and SM predictions in both the dimuon andtrilepton control regions, we look at the CDF data in thetrilepton signal region. We observe one event containingthree muons (trimuon event). The event is characterized bylow track activity and three well-identified muons that areproduced within �40 degrees in �. Two of the muons areenergetic, with transverse momenta of 45 and 21 GeV=c,and the third one is a soft muon with pT of 8 GeV=c.Table V shows the main properties of this event. It isinteresting to note the close values of all three dimuonmasses. The event includes one hadronic jet and twoenergy clusters of mostly electromagnetic energy withtransverse energy of �41, �9, and �4 GeV, respectively.

CDF DATA

Drell-Yan

Heavy flavor

Dibosons

Fake lepton

tt


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FIG. 7 (color online). Dimuon mass and E6 T for the SM background in the dimuon control regions C (a,b), D (c,d), and dimuon signalregion (e,f). The background histograms are stacked. The CDF data are indicated by points with error bars.

TABLE V. Observed trimuon event properties.

Kind of muons CMUP-CMX-CMX

pT of muons (GeV=c) 45.0, 21.1, 7.8

� of muons �0:2, �0:9, 0.8� of muons (deg.) 359, 321, 340

Isolation of muons (GeV) 2.4, 0.2, 1.1

Charge of muons �1, 1, �1Dimuon masses (GeV=c2) 29.3(1&2), 21.7(1&3), 25.7(2&3)

Transverse mass (muonþ E6 T) 86.4, 51.4, 34.2

3-d �’ (leading muons) [deg.] 46.3

E6 T (GeV) 43.8

E6 T� (deg.) 205.6

Number of Jets 1

ET of jet (GeV) 41.1

� of jet -1.6

� of jet (deg.) 102.9


052004-13

The jet and the muons originate from the same and onlyhigh-quality primary vertex. If the electromagnetic energyclusters correspond to real photons, the event would also beinteresting in the gauge-mediated supersymmetry breaking

(GMSB [44]) scenario, where the lightest neutralino de-cays to a photon and a gravitino, which is the LSP. In thatcase, the final leptonic signature of the chargino-neutralinoproduction would be three leptons, two photons, and E6 T .

CDF DATA

Heavy flavor

Dibosons

Fake lepton

γDrell-Yan +

SUSY Signal

)2

Dimuon mass (GeV/c

20 40 60 80 100 120 140

)2N

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ber

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GeV

/c

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1a) Signal Region, trileptons

)2

Dimuon mass (GeV/c

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rees

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FIG. 8 (color online). Kinematic variables for the SM background and the SIG2 SUSY signal, in the trilepton signal region. Thebackground histograms are stacked; the signal histogram is not. The CDF data are indicated by points with error bars.


052004-14

Figure 8 shows where the one trimuon event observed inthe signal region appears in the expected distributions ofkinematic variables for the signal and the backgrounds.Kinematic distributions include the three-dimensionalopening angle between the leading muons, �’. Figures 9and 10 show the transverse and lego detector displays,respectively, for this trimuon event. The Poisson probabil-ity to see one event or more, when we expect 0:4� 0:1, is32.6%.

It is interesting also to interpret this event in the contextof a search only for trimuon events. The diboson back-grounds remain, but the large source of fakes in thedimuonþ e sample is reduced. The total trimuon back-ground estimation is 0:16� 0:04 events. The Poissonprobability to observe one event or more, when we expect0:16� 0:04, is 14.7%. We conclude that our event yield isstatistically consistent with the SM prediction, noting thatmost of the kinematics of the event—especially the three-dimensional opening angle of the leading muons �’—areconsistent with new physics expectation, as can be seen inFig. 8.

We have combined the results of this analysis with otherCDF trilepton analyses to set exclusion limits in severalmodels. For mSUGRA with no slepton mixing, we set alower limit for the chargino mass of 129 GeV=c2, whichcorresponds to an upper limit in �B of about 0.25 pb atthe 95% confidence level [22].

ACKNOWLEDGMENTS

We thank the Fermilab staff and the technical staff of theparticipating institutions for their vital contributions. Thiswork was supported by the U.S. Department of Energy andNational Science Foundation; the Italian Istituto Nazionaledi Fisica Nucleare; the Ministry of Education, Culture,Sports, Science, and Technology of Japan; the NaturalSciences and Engineering Research Council of Canada;the National Science Council of the Republic of China; theSwiss National Science Foundation; the A. P. SloanFoundation; the Bundesministerium fur Bildung undForschung, Germany; the Korean Science andEngineering Foundation, and the Korean ResearchFoundation; the Science and Technology FacilitiesCouncil and the Royal Society, UK; the Institut Nationalde Physique Nucleaire et Physique des Particules/CNRS;the Russian Foundation for Basic Research; the Ministeriode Ciencia e Innovacion, and Programa Consolider-Ingenio 2010, Spain; the Slovak R&D Agency; andthe Academy of Finland.

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[6] The R-parity is defined as ð�1Þ3ðB�LÞþ2S, where B is thebaryon number, L is the lepton number, and S is the spin

of the particle.

-3

-2

-1

0

1

2

3

0100

200300

0

20

40

ET(GeV)

φ

η

µµ

µ E1E2 jet

FIG. 10 (color online). The trimuon event in the �� view.Calorimeter transverse energies above 1 GeV are shown. Thelonger bars correspond to the track momenta of the three muons(high �). The calorimeter energy depositions E1 and E2 aremainly electromagnetic and could be associated with two pho-tons.

=45 GeV/cTµp

=8 GeV/cTµp

=21 GeV/cTµp

=41 GeVTjetE

=44 GeVTE

FIG. 9 (color online). The trimuon event in the transverse viewof the central CDF detector. Tracks with transverse momentaabove 1 GeV=c are shown.


052004-15

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052004-16

Search for new physics in the mumu+ e/mu+ EslashT channel with a low-pT lepton threshold at the Collider Detector at Fermilab

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