Experimental Discrimination between Charge
2
e=
3
Top Quark and Charge
4
e=
3
Exotic Quark
Production Scenarios
V. M. Abazov,36B. Abbott,76M. Abolins,66B. S. Acharya,29M. Adams,52T. Adams,50M. Agelou,18S. H. Ahn,31 M. Ahsan,60G. D. Alexeev,36G. Alkhazov,40A. Alton,65G. Alverson,64G. A. Alves,2M. Anastasoaie,35T. Andeen,54 S. Anderson,46B. Andrieu,17M. S. Anzelc,54Y. Arnoud,14M. Arov,53A. Askew,50B. A˚ sman,41A. C. S. Assis Jesus,3 O. Atramentov,58C. Autermann,21C. Avila,8C. Ay,24F. Badaud,13A. Baden,62L. Bagby,53B. Baldin,51D. V. Bandurin,60
P. Banerjee,29S. Banerjee,29E. Barberis,64P. Bargassa,81P. Baringer,59C. Barnes,44J. Barreto,2J. F. Bartlett,51 U. Bassler,17D. Bauer,44A. Bean,59M. Begalli,3M. Begel,72C. Belanger-Champagne,5L. Bellantoni,51A. Bellavance,68 J. A. Benitez,66S. B. Beri,27G. Bernardi,17R. Bernhard,42L. Berntzon,15I. Bertram,43M. Besanc¸on,18R. Beuselinck,44 V. A. Bezzubov,39P. C. Bhat,51V. Bhatnagar,27M. Binder,25C. Biscarat,43K. M. Black,63I. Blackler,44G. Blazey,53 F. Blekman,44S. Blessing,50D. Bloch,19K. Bloom,68U. Blumenschein,23A. Boehnlein,51O. Boeriu,56T. A. Bolton,60 G. Borissov,43K. Bos,34T. Bose,78A. Brandt,79R. Brock,66G. Brooijmans,71A. Bross,51D. Brown,79N. J. Buchanan,50 D. Buchholz,54M. Buehler,82V. Buescher,23S. Burdin,51S. Burke,46T. H. Burnett,83E. Busato,17C. P. Buszello,44 J. M. Butler,63P. Calfayan,25S. Calvet,15J. Cammin,72S. Caron,34W. Carvalho,3B. C. K. Casey,78N. M. Cason,56 H. Castilla-Valdez,33D. Chakraborty,53K. M. Chan,72A. Chandra,49F. Charles,19E. Cheu,46F. Chevallier,14D. K. Cho,63
S. Choi,32B. Choudhary,28L. Christofek,59D. Claes,68B. Cle´ment,19C. Cle´ment,41Y. Coadou,5M. Cooke,81 W. E. Cooper,51D. Coppage,59M. Corcoran,81M.-C. Cousinou,15B. Cox,45S. Cre´pe´-Renaudin,14D. Cutts,78M. C´ wiok,30
H. da Motta,2A. Das,63M. Das,61B. Davies,43G. Davies,44G. A. Davis,54K. De,79P. de Jong,34S. J. de Jong,35 E. De La Cruz-Burelo,65C. De Oliveira Martins,3J. D. Degenhardt,65F. De´liot,18M. Demarteau,51R. Demina,72 P. Demine,18D. Denisov,51S. P. Denisov,39S. Desai,73H. T. Diehl,51M. Diesburg,51M. Doidge,43A. Dominguez,68 H. Dong,73L. V. Dudko,38L. Duflot,16S. R. Dugad,29D. Duggan,50A. Duperrin,15J. Dyer,66A. Dyshkant,53M. Eads,68
D. Edmunds,66T. Edwards,45J. Ellison,49J. Elmsheuser,25V. D. Elvira,51S. Eno,62P. Ermolov,38H. Evans,55 A. Evdokimov,37V. N. Evdokimov,39S. N. Fatakia,63L. Feligioni,63A. V. Ferapontov,60T. Ferbel,72F. Fiedler,25 F. Filthaut,35W. Fisher,51H. E. Fisk,51I. Fleck,23M. Ford,45M. Fortner,53H. Fox,23S. Fu,51S. Fuess,51T. Gadfort,83 C. F. Galea,35E. Gallas,51E. Galyaev,56C. Garcia,72A. Garcia-Bellido,83J. Gardner,59V. Gavrilov,37A. Gay,19P. Gay,13
D. Gele´,19R. Gelhaus,49C. E. Gerber,52Y. Gershtein,50D. Gillberg,5G. Ginther,72N. Gollub,41B. Go´mez,8 A. Goussiou,56P. D. Grannis,73H. Greenlee,51Z. D. Greenwood,61E. M. Gregores,4G. Grenier,20Ph. Gris,13 J.-F. Grivaz,16S. Gru¨nendahl,51M. W. Gru¨newald,30F. Guo,73J. Guo,73G. Gutierrez,51P. Gutierrez,76A. Haas,71 N. J. Hadley,62P. Haefner,25S. Hagopian,50J. Haley,69I. Hall,76R. E. Hall,48L. Han,7K. Hanagaki,51P. Hansson,41
K. Harder,60A. Harel,72R. Harrington,64J. M. Hauptman,58R. Hauser,66J. Hays,54T. Hebbeker,21D. Hedin,53 J. G. Hegeman,34J. M. Heinmiller,52A. P. Heinson,49U. Heintz,63C. Hensel,59K. Herner,73G. Hesketh,64 M. D. Hildreth,56R. Hirosky,82J. D. Hobbs,73B. Hoeneisen,12H. Hoeth,26M. Hohlfeld,16S. J. Hong,31R. Hooper,78 P. Houben,34Y. Hu,73Z. Hubacek,10V. Hynek,9I. Iashvili,70R. Illingworth,51A. S. Ito,51S. Jabeen,63M. Jaffre´,16S. Jain,76 K. Jakobs,23C. Jarvis,62A. Jenkins,44R. Jesik,44K. Johns,46C. Johnson,71M. Johnson,51A. Jonckheere,51P. Jonsson,44 A. Juste,51D. Ka¨fer,21S. Kahn,74E. Kajfasz,15A. M. Kalinin,36J. M. Kalk,61J. R. Kalk,66S. Kappler,21D. Karmanov,38 J. Kasper,63P. Kasper,51I. Katsanos,71D. Kau,50R. Kaur,27R. Kehoe,80S. Kermiche,15N. Khalatyan,63A. Khanov,77 A. Kharchilava,70Y. M. Kharzheev,36D. Khatidze,71H. Kim,79T. J. Kim,31M. H. Kirby,35B. Klima,51J. M. Kohli,27
J.-P. Konrath,23M. Kopal,76V. M. Korablev,39J. Kotcher,74B. Kothari,71A. Koubarovsky,38A. V. Kozelov,39 J. Kozminski,66D. Krop,55A. Kryemadhi,82T. Kuhl,24A. Kumar,70S. Kunori,62A. Kupco,11T. Kurcˇa,20,*J. Kvita,9
S. Lammers,71G. Landsberg,78J. Lazoflores,50A.-C. Le Bihan,19P. Lebrun,20W. M. Lee,53A. Leflat,38F. Lehner,42 V. Lesne,13J. Leveque,46P. Lewis,44J. Li,79Q. Z. Li,51J. G. R. Lima,53D. Lincoln,51J. Linnemann,66V. V. Lipaev,39 R. Lipton,51Z. Liu,5L. Lobo,44A. Lobodenko,40M. Lokajicek,11A. Lounis,19P. Love,43H. J. Lubatti,83M. Lynker,56
A. L. Lyon,51A. K. A. Maciel,2R. J. Madaras,47P. Ma¨ttig,26C. Magass,21A. Magerkurth,65A.-M. Magnan,14 N. Makovec,16P. K. Mal,56H. B. Malbouisson,3S. Malik,68V. L. Malyshev,36H. S. Mao,6Y. Maravin,60M. Martens,51
R. McCarthy,73D. Meder,24A. Melnitchouk,67A. Mendes,15L. Mendoza,8M. Merkin,38K. W. Merritt,51A. Meyer,21 J. Meyer,22M. Michaut,18H. Miettinen,81T. Millet,20J. Mitrevski,71J. Molina,3N. K. Mondal,29J. Monk,45R. W. Moore,5
T. Moulik,59G. S. Muanza,16M. Mulders,51M. Mulhearn,71L. Mundim,3Y. D. Mutaf,73E. Nagy,15M. Naimuddin,28 M. Narain,63N. A. Naumann,35H. A. Neal,65J. P. Negret,8P. Neustroev,40C. Noeding,23A. Nomerotski,51S. F. Novaes,4
G. J. Otero y Garzo´n,52M. Owen,45P. Padley,81N. Parashar,57S.-J. Park,72S. K. Park,31J. Parsons,71R. Partridge,78 N. Parua,73A. Patwa,74G. Pawloski,81P. M. Perea,49E. Perez,18K. Peters,45P. Pe´troff,16M. Petteni,44R. Piegaia,1
J. Piper,66M.-A. Pleier,22P. L. M. Podesta-Lerma,33V. M. Podstavkov,51Y. Pogorelov,56M.-E. Pol,2A. Pomposˇ,76 B. G. Pope,66A. V. Popov,39C. Potter,5W. L. Prado da Silva,3H. B. Prosper,50S. Protopopescu,74J. Qian,65A. Quadt,22
B. Quinn,67M. S. Rangel,2K. J. Rani,29K. Ranjan,28P. N. Ratoff,43P. Renkel,80S. Reucroft,64M. Rijssenbeek,73 I. Ripp-Baudot,19F. Rizatdinova,77S. Robinson,44R. F. Rodrigues,3C. Royon,18P. Rubinov,51R. Ruchti,56V. I. Rud,38 G. Sajot,14A. Sa´nchez-Herna´ndez,33M. P. Sanders,62A. Santoro,3G. Savage,51L. Sawyer,61T. Scanlon,44D. Schaile,25
R. D. Schamberger,73Y. Scheglov,40H. Schellman,54P. Schieferdecker,25C. Schmitt,26C. Schwanenberger,45 A. Schwartzman,69R. Schwienhorst,66J. Sekaric,50S. Sengupta,50H. Severini,76E. Shabalina,52M. Shamim,60V. Shary,18
A. A. Shchukin,39W. D. Shephard,56R. K. Shivpuri,28D. Shpakov,51V. Siccardi,19R. A. Sidwell,60V. Simak,10 V. Sirotenko,51P. Skubic,76P. Slattery,72R. P. Smith,51G. R. Snow,68J. Snow,75S. Snyder,74S. So¨ldner-Rembold,45
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L. S. Vertogradov,36M. Verzocchi,51F. Villeneuve-Seguier,44P. Vint,44J.-R. Vlimant,17E. Von Toerne,60 M. Voutilainen,68,†M. Vreeswijk,34H. D. Wahl,50L. Wang,62M. H. L. S Wang,51J. Warchol,56G. Watts,83M. Wayne,56
M. Weber,51H. Weerts,66N. Wermes,22M. Wetstein,62A. White,79D. Wicke,26G. W. Wilson,59S. J. Wimpenny,49 M. Wobisch,51J. Womersley,51D. R. Wood,64T. R. Wyatt,45Y. Xie,78N. Xuan,56S. Yacoob,54R. Yamada,51M. Yan,62 T. Yasuda,51Y. A. Yatsunenko,36K. Yip,74H. D. Yoo,78S. W. Youn,54C. Yu,14J. Yu,79A. Yurkewicz,73A. Zatserklyaniy,53
C. Zeitnitz,26D. Zhang,51T. Zhao,83B. Zhou,65J. Zhu,73M. Zielinski,72D. Zieminska,55A. Zieminski,55 V. Zutshi,53and E. G. Zverev38
(D0 Collaboration)
1Universidad de Buenos Aires, Buenos Aires, Argentina 2LAFEX, Centro Brasileiro de Pesquisas Fı´sicas, Rio de Janeiro, Brazil
3
Universidade do Estado do Rio de Janeiro, Rio de Janeiro, Brazil
4Instituto de Fı´sica Teo´rica, Universidade Estadual Paulista, Sa˜o Paulo, Brazil 5
University of Alberta, Edmonton, Alberta, Canada, Simon Fraser University, Burnaby, British Columbia, Canada,
York University, Toronto, Ontario, Canada, and McGill University, Montreal, Quebec, Canada
6Institute of High Energy Physics, Beijing, People’s Republic of China 7University of Science and Technology of China, Hefei, People’s Republic of China
8Universidad de los Andes, Bogota´, Colombia 9
Center for Particle Physics, Charles University, Prague, Czech Republic
10Czech Technical University, Prague, Czech Republic 11
Center for Particle Physics, Institute of Physics, Academy of Sciences of the Czech Republic, Prague, Czech Republic
12Universidad San Francisco de Quito, Quito, Ecuador
13Laboratoire de Physique Corpusculaire, IN2P3-CNRS, Universite´ Blaise Pascal, Clermont-Ferrand, France 14Laboratoire de Physique Subatomique et de Cosmologie, IN2P3-CNRS, Universite de Grenoble 1, Grenoble, France
15CPPM, IN2P3-CNRS, Universite´ de la Me´diterrane´e, Marseille, France 16
IN2P3-CNRS, Laboratoire de l’Acce´le´rateur Line´aire, Orsay, France
17LPNHE, IN2P3-CNRS, Universite´s Paris VI and VII, Paris, France 18
DAPNIA/Service de Physique des Particules, CEA, Saclay, France
19IPHC, IN2P3-CNRS, Universite´ Louis Pasteur, Strasbourg, France
and Universite´ de Haute Alsace, Mulhouse, France
20Institut de Physique Nucle´aire de Lyon, IN2P3-CNRS, Universite´ Claude Bernard, Villeurbanne, France 21III. Physikalisches Institut A, RWTH Aachen, Aachen, Germany
24Institut fu¨r Physik, Universita¨t Mainz, Mainz, Germany 25Ludwig-Maximilians-Universita¨t Mu¨nchen, Mu¨nchen, Germany 26Fachbereich Physik, University of Wuppertal, Wuppertal, Germany
27Panjab University, Chandigarh, India 28
Delhi University, Delhi, India
29Tata Institute of Fundamental Research, Mumbai, India 30
University College Dublin, Dublin, Ireland
31Korea Detector Laboratory, Korea University, Seoul, Korea 32
SungKyunKwan University, Suwon, Korea
33CINVESTAV, Mexico City, Mexico
34FOM-Institute NIKHEF and University of Amsterdam/NIKHEF, Amsterdam, The Netherlands 35
Radboud University Nijmegen/NIKHEF, Nijmegen, The Netherlands
36Joint Institute for Nuclear Research, Dubna, Russia 37
Institute for Theoretical and Experimental Physics, Moscow, Russia
38Moscow State University, Moscow, Russia 39
Institute for High Energy Physics, Protvino, Russia
40Petersburg Nuclear Physics Institute, St. Petersburg, Russia
41Lund University, Lund, Sweden, Royal Institute of Technology and Stockholm University, Stockholm, Sweden,
and Uppsala University, Uppsala, Sweden
42Physik Institut der Universita¨t Zu¨rich, Zu¨rich, Switzerland 43
Lancaster University, Lancaster, United Kingdom
44Imperial College, London, United Kingdom 45
University of Manchester, Manchester, United Kingdom
46University of Arizona, Tucson, Arizona 85721, USA 47
Lawrence Berkeley National Laboratory and University of California, Berkeley, California 94720, USA
48California State University, Fresno, California 93740, USA 49University of California, Riverside, California 92521, USA 50
Florida State University, Tallahassee, Florida 32306, USA
51Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA 52
University of Illinois at Chicago, Chicago, Illinois 60607, USA
53Northern Illinois University, DeKalb, Illinois 60115, USA 54
Northwestern University, Evanston, Illinois 60208, USA
55Indiana University, Bloomington, Indiana 47405, USA 56University of Notre Dame, Notre Dame, Indiana 46556, USA
57Purdue University Calumet, Hammond, Indiana 46323, USA 58Iowa State University, Ames, Iowa 50011, USA 59
University of Kansas, Lawrence, Kansas 66045, USA
60Kansas State University, Manhattan, Kansas 66506, USA 61
Louisiana Tech University, Ruston, Louisiana 71272, USA
62University of Maryland, College Park, Maryland 20742, USA 63Boston University, Boston, Massachusetts 02215, USA 64Northeastern University, Boston, Massachusetts 02115, USA
65University of Michigan, Ann Arbor, Michigan 48109, USA 66
Michigan State University, East Lansing, Michigan 48824, USA
67University of Mississippi, University, Mississippi 38677, USA 68
University of Nebraska, Lincoln, Nebraska 68588, USA
69Princeton University, Princeton, New Jersey 08544, USA 70
State University of New York, Buffalo, New York 14260, USA
71Columbia University, New York, New York 10027, USA 72University of Rochester, Rochester, New York 14627, USA 73State University of New York, Stony Brook, New York 11794, USA
74Brookhaven National Laboratory, Upton, New York 11973, USA 75
Langston University, Langston, Oklahoma 73050, USA
76University of Oklahoma, Norman, Oklahoma 73019, USA 77
Oklahoma State University, Stillwater, Oklahoma 74078, USA
78Brown University, Providence, Rhode Island 02912, USA 79
University of Texas, Arlington, Texas 76019, USA
80Southern Methodist University, Dallas, Texas 75275, USA 81Rice University, Houston, Texas 77005, USA 82University of Virginia, Charlottesville, Virginia 22901, USA
(Received 18 August 2006; published 22 January 2007)
We present the first experimental discrimination between the2e=3and4e=3top quark electric charge scenarios, using top quark pairs (tt) produced inppcollisions at
s p
1:96 TeVby the Fermilab Tevatron Collider. We use370 pb1of data collected by the D0 experiment and select events with at least one high
transverse momentum electron or muon, high transverse energy imbalance, and four or more jets. We discriminate betweenb- andb-quark jets by using the charge and momenta of tracks within the jet cones. The data are consistent with the expected electric charge,jqj 2e=3. We exclude, at the 92% C.L., that the sample is solely due to the production of exotic quark pairsQQ withjqj 4e=3. We place an upper limit on the fraction ofQQ pairs <0:80at the 90% C.L.
DOI:10.1103/PhysRevLett.98.041801 PACS numbers: 14.65.Ha, 13.40.f, 13.85.Rm, 14.80.j
The heavy particle discovered by the CDF and D0 Collaborations at the Fermilab Tevatron proton-antiproton collider in 1995 [1] is widely recognized to be the top quark. Currently measured properties of the particle are consistent with standard model (SM) expectations for the top quark. However, many of the properties of the particle are still poorly known. In particular, its electric charge, a fundamental quantity characterizing a particle, has not yet been determined.
To date, it is possible to interpret the discovered particle as either a charge2e=3or4e=3quark. In the published top quark analyses of the CDF and D0 Collaborations [2], there is a twofold ambiguity in pairing thebquarks and the W bosons in the reaction pp !tt!WWbb, and
equivalently, in the electric charge assignment of the mea-sured particle. In addition to the SM assignment, t! Wb, ‘‘t’’!Wbis also conceivable, in which case ‘‘t’’ would actually be an exotic quark Q with charge q 4e=3 (charge-conjugate processes are implied). It is possible to fit Z!‘‘ and Z!bb data assuming a
top quark mass of mt270 GeV and a right-handed b quark that mixes with the isospin1=2component of an exotic doublet of charge 1e=3 and 4e=3 quarks, Q1; Q4R [3]. In this scenario, the 4e=3 charge quark is the particle discovered at the Tevatron, and the top quark, with mass of 270 GeV, would have so far escaped detection.
In this Letter, we report the first experimental discrimi-nation between the2e=3and 4e=3 charge scenarios. We also consider the case where the analyzed sample contains an admixture of SM top quarks and exotic quarks and place an upper limit on the exotic quark fraction. Our search strategy assumes each quark decays 100% of the time to a W boson and abquark. We use the lepton-plus-jets chan-nel which arises when one W boson decays leptonically and one decays hadronically. The charged leptons (e=) originate from a direct W decay or fromW !!e=. We require that the final state have at least twob-quark jets. The data used in this Letter were collected by the D0 experiment from June 2002 through August 2004 and correspond to an integrated luminosity of370 pb1.
The D0 detector includes a tracking system, calorime-ters, and a muon spectrometer [4]. The tracking system is
made up of a silicon microstrip tracker (SMT) and a central fiber tracker, located inside a 2 T superconducting sole-noid. The SMT, with a typical strip pitch of 50–80m, allows a precise determination of the primary interaction vertex (PV) and an accurate determination of the impact parameter of a track relative to the PV [5]. The tracker design provides efficient charged-particle measurements in the pseudorapidity region jj<3 [6]. The calorimeter consists of a barrel section covering jj<1:1, and two end caps extending to jj 4:2. The muon spectrometer encapsulates the calorimeter up tojj 2:0and consists of three layers of drift chambers and two or three layers of scintillators [7]. A 1.8 T iron toroidal magnet is located outside the innermost layer of the muon detector.
We select data samples in the electron and muon chan-nels by requiring an electron with transverse momentum pT>20 GeVandjj<1:1, or a muon withpT>20 GeV
andjj<2:0. The leptons are required to be isolated from other particles using calorimeter and tracking information. More details on the lepton identification and trigger re-quirements are given in Ref. [8].Wboson candidate events are then selected in both channels by requiring missing transverse energy, 6ET, in excess of 20 GeV due to the neutrino. To remove multijet background, 6ET is required to be noncollinear with the lepton direction in the trans-verse plane. Jets are defined using a cone algorithm [9] with radiusR0:5[10]. These events must be accom-panied by four or more jets withpT>15 GeVand rapidity jyj<2:5. After all the above selection requirements are applied, we have a total of 231 (277) events in the muon (electron) channel.
with two SVT-tagged jets, the largest (second largest) background is Wbb (single top quark [13]) production with a contribution of 5% (1%) to the number of selected events.
The top or antitop quark whoseWboson decays leptoni-cally (hadronileptoni-cally) is referred to as the leptonic (hadronic) top and the associated b-quark is denoted b‘ (bh). To compute the top quark charge we need to (i) decide which of the two SVT-tagged jets areb‘andbhand (ii) determine ifb‘ and bh are b- or b quarks. The detected final state partons in thettcandidate events comprise theb‘ andbh quarks, two quarks from the hadronically decaying W boson, and one muon or one electron. The four highest pT jets can be assigned to the set of final state quarks according to many permutations and there are at least two ways to assign the SVT-tagged jets tob‘ andbh. For each permutation, the measured four vectors of the jets and lepton are fitted to the tt event hypothesis, taking into account the experimental resolutions and constraining the mass of twoW bosons to its measured value and the top quark mass to 175 GeV. We decide which of the SVT-tagged jets areb‘andbhby selecting the permutation with the highest probability of arising from attevent. Studies on simulatedttshow that this gives the correct assignment in about 84% of the events.
We measure the absolute value of the top quark charge on each side of the event, given byQ1 jq‘qb‘jon the
leptonic side andQ2 j q‘qbhjon the hadronic side.
The charge of the lepton is indicated byq‘, andqb‘andqbh
are the charges of the SVT-tagged jets on the leptonic and hadronic side of the event. The charges qb
‘ and qbh are
determined by combining thepT and charge of the tracks contained within a cone of R0:5 around the
SVT-tagged jet axis. Based on an optimization using simulated ttevents generated withALPGEN[14] andGEANT[15] for a full D0 detector simulation, we define an estimator for jet charge qjet Piqip0T:i6=
P
ip0T:i6 where the subscript i
runs over all tracks with pT>0:5 GeV and within 0.1 cm of the PV in the direction parallel to the beam axis. To determine the expected distributions for the top quark chargesQ1 andQ2, it is crucial to determine the expected
distributions forqjetin the case of ab-quark or ab-quark
jet. In5%of thettevents, one of the SVT-tagged jets is actually ac-quark jet arising fromW !cs(or its charge conjugate). Therefore, we also need to determine the ex-pected distribution for qjet in the case ofc- and c-quark
jets.
We derive the expected distributions of jet charge from dijet collider data, enhanced in heavy flavor (bandc). We select events with exactly two jets, both SVT-tagged, with pT>15 GeVandjyj<2:5. The method requires that the two jets are of charge-conjugate flavors. To ensure this, we enhancebbandccproduced by flavor creation [16–18], by requiring the azimuthal distance between the jets to be larger than 3.0 and one jet (designated asj1) to contain a
muon with pT>4 GeV. We refer to this sample as the ‘‘tight dijet sample,’’ to j1 as the ‘‘tag jet,’’ and to the
second jetj2as the ‘‘probe jet.’’
The fraction of cc events in the tight dijet sample is estimated using the distribution of the muon transverse momentum with respect to the tag jet axis (prel
T ). We fit theprel
T distribution with a sum of twoprelT templates, one for b-quark jets (including both prompt and cascade de-cays) and one for semimuonic decays inside c-quark jets. This leads to a fractionxcofccevents of121%in the tight
dijet sample and since the light-flavor tagging efficiency is 15 times lower, we also conclude that the fraction of lighter flavor jets is negligible. The muon inside the tag jet comes either (i) from a directBmeson decay, (ii) aB!D meson cascade decay, (iii) an oscillated neutral Bmeson, or (iv) a direct Dmeson decay. We find that further con-tribution from indirect Dmeson decay can be neglected. Charge-flipping processes (ii) and (iii) lead to a muon of opposite charge to that of the quark initiating the tag jet and therefore of same sign as the quark initiating the probe jet. We find, with PYTHIA [19] simulated events andEVTGEN [20] for heavy flavor decays, that charge-flipping processes are x 301% of the bb events in the tight dijet sample. This fraction is experimentally confirmed by studying charge correlation between muons in back-to-back muon-tagged dijet events.
We denote the charge distributions for the probe jet when the muon on the tag side is positive or negative as P and P. Similarly we define Pf to be the charge distribution when the jet is of flavorfb,b, c,c. Given
the fractions ofccevents and of charge-flipping processes we can write
P0:69Pb0:30Pb0:01Pc
P 0:30Pb0:69Pb0:01Pc:
(1)
P and P are distributions observed in data and are admixtures of the quark charge distributions. Eqs. (1) are not sufficient to extract the four probability density func-tions (PDFs)Pf. Therefore we define a ‘‘loose dijet sam-ple,’’ wherej1is not required to be SVT tagged. Using the same techniques as for the tight dijet sample, we find that xc 192% and the same fraction of charge-flipping processes as for the tight dijet sample. We refer to P0
(P0
) as the observed PDFs forqjeton the probe jet in the
loose dijet sample, when the tag muon is positive (nega-tive). Thus we can write
P0
0:567Pb0:243Pb0:19Pc;
P0
0:243Pb0:567Pb0:19Pc:
(2)
We solve Eqs. (1) and (2) to obtain thePfforb-,b-, c-, and
c-quark jets.
tracking efficiency is rapidity dependent. Therefore we must account for the different jetpTandyspectra between the probe jets of the dijet samples and theb-quark jets in preselected tt events. The Pf’s obtained above are cor-rected by weighting the data events to thepTandyspectra of SVT-tagged jets in tt events. Figure 1(a) shows the resultingPbandPb.
We derive the expected distributions forQ1 andQ2 by
applying the assignment procedure between the SVT-tagged jets and the bh, b‘ quarks on simulated ttevents using our calculated Pf’s. The true flavor f of the SVT-tagged jets is determined from the simulation information. The values of qbh and qb‘ are obtained by randomly sampling the distribution of Pf for the corresponding flavors. About 1% of ttcandidate events contain a SVT-tagged light-flavor jet. In this case the PDF forqjetis taken
from simulation. In the case of ajqj 4e=3exotic quark, the expected distributions of exotic quark charge are de-rived by computing Q1 j q‘qb‘j andQ2 jq‘
qb
hj, following the same procedure as for the SM top
quark. The uncertainty on the mass of the top quark [21] is propagated as a systematic uncertainty.
The expected distributions ofQ1 andQ2 for the
back-ground are obtained by (i) performing the assignment procedure between SVT-tagged jets and thebh,b‘ quarks
onWbbsimulated events, and (ii) using the true jet flavors f to sample the correspondingPf’s. The resulting distri-butions ofQ1andQ2for the background are added to the
top charge distributions in the SM and exotic cases. We denote PSM (Pex) the PDFs for Q1 and Q2 including the background contributions in the SM (exotic) case.
For 16 of the 21 selected lepton-plus-jet events, the kinematic fit converges and we can assign the SVT-tagged jets to the b‘ andbh quarks, thus providing 32 measure-ments of the top quark charge. Figure 1(b)shows the 32 observed values of Q1 andQ2 overlaid with the SM and
exotic charge distributions.
To discriminate between the SM and the exotic hypoth-eses, we form the ratio of the likelihood of the observed set of chargesqiarising from a SM top quark to the likelihood for the set of qi arising from the exotic scenario,
Q
iPSMqi=QiPexqi. The subscript i runs over all
32 available measurements. The value of the ratio is de-termined in data and compared with the expected distribu-tions forin the SM and exotic scenarios. We find that the observed set of charges agrees well with those of a SM top quark. The probability of our observation is 7.8% in the case where the selected sample contains only exotic quarks with charge jqj 4e=3, including systematic uncertain-ties. Thus, we exclude at the 92.2% C.L. that the selected data set is solely composed of an exotic quark with
TABLE I. Expected and observed confidence levels as function of the cumulated systematic uncertainties.
Systematic Observed Expected
Statistical uncertainty only 95.8 95.3
Fraction ofccevents 95.8 95.2
Charge-flipping processes 95.7 95.2
Weighting with respect topTandyspectra 94.4 94.1
Fraction of flavor creation 93.7 93.4
Statistical error onPf 93.3 93.1
Jet energy calibrationa 92.4 91.8
Top quark mass 92.2 91.2
aReference [22].
Jet charge [e]
-1 -0.5 0 0.5 1
(a.u.)f
P
0 0.02 0.04 0.06 0.08 0.1 0.12
b
P
b
P
(a)
-1
, 370pb D0
Top quark charge [e]
0 0.5 1 1.5 2
Number of events 0 2 4 6 8 10 12 14
Data |q| = 2e/3 |q| = 4e/3
-1
, 370pb
D0 (b)
Fraction of exotic quarks
-0.5 0 0.5 1 1.5
-ln(L)
106 107 108 109 110
Stat. only Stat. + syst. Physical region
-1
, 370pb
D0 (c)
jqj 4e=3. The corresponding expected C.L. is 91.2%. TableIsummarizes the dominant systematic uncertainties and their cumulative effect on the C.L.
It is not excluded that the data contain a mixture of two heavy quarks, one with jqj 2e=3 and one with jqj
4e=3. We perform an unbinned maximum likelihood fit to the observed set ofqiin data to determine the fractionof exotic quark pairs. The likelihood of the observed set ofqi can be expressed as a function ofby
L; q Y
Ndata
i1
1PSMqi Pexqi: (3)
Figure 1(c) shows lnL as function of . We fit 0:130:66stat 0:11syst, consistent with the SM. Using a Bayesian prior equal to one in the physically allowed region01and zero otherwise, we obtain
0 <0:52 at the 68% C.L. and 0 <0:80 at the 90% C.L.
In summary, we present the first experimental discrimi-nation between the 2e=3 and 4e=3 top quark electric charge scenarios. The observed top quark charge is con-sistent with the SM prediction. The hypothesis that only an exotic quark with chargejqj 4e=3is produced has been excluded at the 92% C.L. We also place an upper limit of 0.80 at the 90% C.L. on the fraction of exotic quark pairs in the double tagged lepton-plus-jets sample.
We thank the staffs at Fermilab and collaborating insti-tutions, and acknowledge support from the DOE and NSF (U.S.A.); CEA and CNRS/IN2P3 (France); FASI, Rosatom and RFBR (Russia); CAPES, CNPq, FAPERJ, FAPESP, and FUNDUNESP (Brazil); DAE and DST (India); Colciencias (Colombia); CONACyT (Mexico); KRF and KOSEF (Korea); CONICET and UBACyT (Argentina); FOM (The Netherlands); PPARC (United Kingdom); MSMT (Czech Republic); CRC Program, CFI, NSERC, and WestGrid Project (Canada); BMBF and DFG (Germany); SFI (Ireland); The Swedish Research Council (Sweden); Research Corporation; Alexander von Humboldt Foundation; and the Marie Curie Program.
*On leave from IEP SAS Kosice, Slovakia.
†Visitor from Helsinki Institute of Physics, Helsinki,
Finland.
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