Search for Anomalous
Wtb Couplings in Single Top Quark Production
V. M. Abazov,36B. Abbott,75M. Abolins,65B. S. Acharya,29M. Adams,51T. Adams,49E. Aguilo,6M. Ahsan,59 G. D. Alexeev,36G. Alkhazov,40A. Alton,64,*G. Alverson,63G. A. Alves,2M. Anastasoaie,35L. S. Ancu,35T. Andeen,53
B. Andrieu,17M. S. Anzelc,53M. Aoki,50Y. Arnoud,14M. Arov,60M. Arthaud,18A. Askew,49B. A˚ sman,41 A. C. S. Assis Jesus,3O. Atramentov,49C. Avila,8F. Badaud,13L. Bagby,50B. Baldin,50D. V. Bandurin,59P. Banerjee,29
S. Banerjee,29E. Barberis,63A.-F. Barfuss,15P. Bargassa,80P. Baringer,58J. Barreto,2J. F. Bartlett,50U. Bassler,18 D. Bauer,43S. Beale,6A. Bean,58M. Begalli,3M. Begel,73C. Belanger-Champagne,41L. Bellantoni,50A. Bellavance,50
J. A. Benitez,65S. B. Beri,27G. Bernardi,17R. Bernhard,23I. Bertram,42M. Besanc¸on,18R. Beuselinck,43 V. A. Bezzubov,39P. C. Bhat,50V. Bhatnagar,27C. Biscarat,20G. Blazey,52F. Blekman,43S. Blessing,49D. Bloch,19 K. Bloom,67A. Boehnlein,50D. Boline,62T. A. Bolton,59E. E. Boos,38G. Borissov,42T. Bose,77A. Brandt,78R. Brock,65
G. Brooijmans,70A. Bross,50D. Brown,81X. B. Bu,7N. J. Buchanan,49D. Buchholz,53M. Buehler,81V. Buescher,22 V. Bunichev,38S. Burdin,42,†T. H. Burnett,82C. P. Buszello,43J. M. Butler,62P. Calfayan,25S. Calvet,16J. Cammin,71 E. Carrera,49W. Carvalho,3B. C. K. Casey,50H. Castilla-Valdez,33S. Chakrabarti,18D. Chakraborty,52K. M. Chan,55 A. Chandra,48E. Cheu,45F. Chevallier,14D. K. Cho,62S. Choi,32B. Choudhary,28L. Christofek,77T. Christoudias,43 S. Cihangir,50D. Claes,67J. Clutter,58M. Cooke,50W. E. Cooper,50M. Corcoran,80F. Couderc,18M.-C. Cousinou,15 S. Cre´pe´-Renaudin,14V. Cuplov,59D. Cutts,77M. C´ wiok,30H. da Motta,2A. Das,45G. Davies,43K. De,78S. J. de Jong,35
E. De La Cruz-Burelo,64C. De Oliveira Martins,3J. D. Degenhardt,64F. De´liot,18M. Demarteau,50R. Demina,71 D. Denisov,50S. P. Denisov,39S. Desai,50H. T. Diehl,50M. Diesburg,50A. Dominguez,67H. Dong,72T. Dorland,82 A. Dubey,28L. V. Dudko,38L. Duflot,16S. R. Dugad,29D. Duggan,49A. Duperrin,15J. Dyer,65A. Dyshkant,52M. Eads,67
D. Edmunds,65J. Ellison,48V. D. Elvira,50Y. Enari,77S. Eno,61P. Ermolov,38,**H. Evans,54A. Evdokimov,73 V. N. Evdokimov,39A. V. Ferapontov,59T. Ferbel,71F. Fiedler,24F. Filthaut,35W. Fisher,50H. E. Fisk,50M. Fortner,52
H. Fox,42S. Fu,50S. Fuess,50T. Gadfort,70C. F. Galea,35C. Garcia,71A. Garcia-Bellido,82V. Gavrilov,37P. Gay,13 W. Geist,19D. Gele´,19W. Geng,15,65C. E. Gerber,51Y. Gershtein,49D. Gillberg,6G. Ginther,71N. Gollub,41B. Go´mez,8
A. Goussiou,82P. D. Grannis,72H. Greenlee,50Z. D. Greenwood,60E. M. Gregores,4G. Grenier,20Ph. Gris,13 J.-F. Grivaz,16A. Grohsjean,25S. Gru¨nendahl,50M. W. Gru¨newald,30F. Guo,72J. Guo,72G. Gutierrez,50P. Gutierrez,75 A. Haas,70N. J. Hadley,61P. Haefner,25S. Hagopian,49J. Haley,68I. Hall,65R. E. Hall,47L. Han,7K. Harder,44A. Harel,71
J. M. Hauptman,57R. Hauser,65J. Hays,43T. Hebbeker,21D. Hedin,52J. G. Hegeman,34A. P. Heinson,48U. Heintz,62 C. Hensel,22,xK. Herner,72G. Hesketh,63M. D. Hildreth,55R. Hirosky,81J. D. Hobbs,72B. Hoeneisen,12H. Hoeth,26 M. Hohlfeld,22S. Hossain,75P. Houben,34Y. Hu,72Z. Hubacek,10V. Hynek,9I. Iashvili,69R. Illingworth,50A. S. Ito,50
S. Jabeen,62M. Jaffre´,16S. Jain,75K. Jakobs,23C. Jarvis,61R. Jesik,43K. Johns,45C. Johnson,70M. Johnson,50 A. Jonckheere,50P. Jonsson,43A. Juste,50E. Kajfasz,15J. M. Kalk,60D. Karmanov,38P. A. Kasper,50I. Katsanos,70 D. Kau,49V. Kaushik,78R. Kehoe,79S. Kermiche,15N. Khalatyan,50A. Khanov,76A. Kharchilava,69Y. M. Kharzheev,36
D. Khatidze,70T. J. Kim,31M. H. Kirby,53M. Kirsch,21B. Klima,50J. M. Kohli,27J.-P. Konrath,23A. V. Kozelov,39 J. Kraus,65T. Kuhl,24A. Kumar,69A. Kupco,11T. Kurcˇa,20V. A. Kuzmin,38J. Kvita,9F. Lacroix,13D. Lam,55 S. Lammers,70G. Landsberg,77P. Lebrun,20W. M. Lee,50A. Leflat,38J. Lellouch,17J. Li,78,**L. Li,48Q. Z. Li,50 S. M. Lietti,5J. K. Lim,31J. G. R. Lima,52D. Lincoln,50J. Linnemann,65V. V. Lipaev,39R. Lipton,50Y. Liu,7Z. Liu,6
A. Lobodenko,40M. Lokajicek,11P. Love,42H. J. Lubatti,82R. Luna,3A. L. Lyon,50A. K. A. Maciel,2D. Mackin,80 R. J. Madaras,46P. Ma¨ttig,26C. Magass,21A. Magerkurth,64P. K. Mal,82H. B. Malbouisson,3S. Malik,67V. L. Malyshev,36 H. S. Mao,50Y. Maravin,59B. Martin,14R. McCarthy,72A. Melnitchouk,66L. Mendoza,8P. G. Mercadante,5M. Merkin,38 K. W. Merritt,50A. Meyer,21J. Meyer,22,xT. Millet,20J. Mitrevski,70R. K. Mommsen,44N. K. Mondal,29R. W. Moore,6
T. Moulik,58G. S. Muanza,20M. Mulhearn,70O. Mundal,22L. Mundim,3E. Nagy,15M. Naimuddin,50M. Narain,77 N. A. Naumann,35H. A. Neal,64J. P. Negret,8P. Neustroev,40H. Nilsen,23H. Nogima,3S. F. Novaes,5T. Nunnemann,25 V. O’Dell,50D. C. O’Neil,6G. Obrant,40C. Ochando,16D. Onoprienko,59N. Oshima,50N. Osman,43J. Osta,55R. Otec,10 G. J. Otero y Garzo´n,50M. Owen,44P. Padley,80M. Pangilinan,77N. Parashar,56S.-J. Park,22,xS. K. Park,31J. Parsons,70 R. Partridge,77N. Parua,54A. Patwa,73G. Pawloski,80B. Penning,23M. Perfilov,38K. Peters,44Y. Peters,26P. Pe´troff,16
M. Petteni,43R. Piegaia,1J. Piper,65M.-A. Pleier,22P. L. M. Podesta-Lerma,33,‡V. M. Podstavkov,50Y. Pogorelov,55 M.-E. Pol,2P. Polozov,37B. G. Pope,65A. V. Popov,39C. Potter,6W. L. Prado da Silva,3H. B. Prosper,49S. Protopopescu,73 J. Qian,64A. Quadt,22,xB. Quinn,66A. Rakitine,42M. S. Rangel,2K. Ranjan,28P. N. Ratoff,42P. Renkel,79S. Reucroft,63
M. Rominsky,75C. Royon,18P. Rubinov,50R. Ruchti,55G. Safronov,37G. Sajot,14A. Sa´nchez-Herna´ndez,33 M. P. Sanders,17B. Sanghi,50G. Savage,50L. Sawyer,60T. Scanlon,43D. Schaile,25R. D. Schamberger,72Y. Scheglov,40
H. Schellman,53T. Schliephake,26S. Schlobohm,82C. Schwanenberger,44A. Schwartzman,68R. Schwienhorst,65 J. Sekaric,49H. Severini,75E. Shabalina,51M. Shamim,59V. Shary,18A. A. Shchukin,39R. K. Shivpuri,28V. Siccardi,19
V. Simak,10V. Sirotenko,50P. Skubic,75P. Slattery,71D. Smirnov,55G. R. Snow,67J. Snow,74S. Snyder,73 S. So¨ldner-Rembold,44L. Sonnenschein,17A. Sopczak,42M. Sosebee,78K. Soustruznik,9B. Spurlock,78J. Stark,14 J. Steele,60V. Stolin,37D. A. Stoyanova,39J. Strandberg,64S. Strandberg,41M. A. Strang,69E. Strauss,72M. Strauss,75 R. Stro¨hmer,25D. Strom,53L. Stutte,50S. Sumowidagdo,49P. Svoisky,55A. Sznajder,3P. Tamburello,45A. Tanasijczuk,1
W. Taylor,6B. Tiller,25F. Tissandier,13M. Titov,18V. V. Tokmenin,36I. Torchiani,23D. Tsybychev,72B. Tuchming,18 C. Tully,68P. M. Tuts,70R. Unalan,65L. Uvarov,40S. Uvarov,40S. Uzunyan,52B. Vachon,6P. J. van den Berg,34 R. Van Kooten,54W. M. van Leeuwen,34N. Varelas,51E. W. Varnes,45I. A. Vasilyev,39M. Vaupel,26P. Verdier,20 L. S. Vertogradov,36M. Verzocchi,50D. Vilanova,18F. Villeneuve-Seguier,43P. Vint,43P. Vokac,10E. Von Toerne,59 M. Voutilainen,68,kR. Wagner,68H. D. Wahl,49L. Wang,61M. H. L. S. Wang,50J. Warchol,55G. Watts,82M. Wayne,55
G. Weber,24M. Weber,50L. Welty-Rieger,54A. Wenger,23,{N. Wermes,22M. Wetstein,61A. White,78D. Wicke,26 G. W. Wilson,58S. J. Wimpenny,48M. Wobisch,60D. R. Wood,63T. R. Wyatt,44Y. Xie,77S. Yacoob,53R. Yamada,50 W.-C. Yang,44T. Yasuda,50Y. A. Yatsunenko,36H. Yin,7K. Yip,73H. D. Yoo,77S. W. Youn,53J. Yu,78C. Zeitnitz,26
S. Zelitch,81T. Zhao,82B. Zhou,64J. Zhu,72M. Zielinski,71D. Zieminska,54A. Zieminski,54,**L. Zivkovic,70 V. Zutshi,52and 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
4Universidade Federal do ABC, Santo Andre´, Brazil
5Instituto de Fı´sica Teo´rica, Universidade Estadual Paulista, Sa˜o Paulo, Brazil 6University of Alberta, Edmonton, Alberta, Canada,
Simon Fraser University, Burnaby, British Columbia, Canada, York University, Toronto, Ontario, Canada,
and McGill University, Montreal, Quebec, Canada
7University of Science and Technology of China, Hefei, People’s Republic of China 8Universidad de los Andes, Bogota´, Colombia
9Center for Particle Physics, Charles University, Prague, Czech Republic 10Czech Technical University, Prague, Czech Republic
11Center for Particle Physics, Institute of Physics, Academy of Sciences of the Czech Republic, Prague, Czech Republic 12
Universidad San Francisco de Quito, Quito, Ecuador
13LPC, Universite´ Blaise Pascal, CNRS/IN2P3, Clermont, France
14LPSC, Universite´ Joseph Fourier Grenoble 1, CNRS/IN2P3, Institut National Polytechnique de Grenoble, Grenoble, France 15CPPM, Aix-Marseille Universite´, CNRS/IN2P3, Marseille, France
16LAL, Universite´ Paris-Sud, IN2P3/CNRS, Orsay, France 17LPNHE, IN2P3/CNRS, Universite´s Paris VI and VII, Paris, France
18CEA, Irfu, SPP, Saclay, France
19IPHC, Universite´ Louis Pasteur, CNRS/IN2P3, Strasbourg, France
20IPNL, Universite´ Lyon 1, CNRS/IN2P3, Villeurbanne, France and Universite´ de Lyon, Lyon, France 21III. Physikalisches Institut A, RWTH Aachen University, Aachen, Germany
22Physikalisches Institut, Universita¨t Bonn, Bonn, Germany 23Physikalisches Institut, Universita¨t Freiburg, Freiburg, 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 30University College Dublin, Dublin, Ireland 31Korea Detector Laboratory, Korea University, Seoul, Korea
32SungKyunKwan University, Suwon, Korea 33CINVESTAV, Mexico City, Mexico
34FOM-Institute NIKHEF and University of Amsterdam/NIKHEF, Amsterdam, The Netherlands 35Radboud University Nijmegen/NIKHEF, Nijmegen, The Netherlands
36Joint Institute for Nuclear Research, Dubna, Russia 37Institute for Theoretical and Experimental Physics, Moscow, Russia
38Moscow State University, Moscow, Russia 39Institute 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
42Lancaster University, Lancaster, United Kingdom 43Imperial College, London, United Kingdom 44University of Manchester, Manchester, United Kingdom
45University of Arizona, Tucson, Arizona 85721, USA
46Lawrence Berkeley National Laboratory and University of California, Berkeley, California 94720, USA 47California State University, Fresno, California 93740, USA
48University of California, Riverside, California 92521, USA 49Florida State University, Tallahassee, Florida 32306, USA 50Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA
51University of Illinois at Chicago, Chicago, Illinois 60607, USA 52Northern Illinois University, DeKalb, Illinois 60115, USA
53Northwestern University, Evanston, Illinois 60208, USA 54Indiana University, Bloomington, Indiana 47405, USA 55University of Notre Dame, Notre Dame, Indiana 46556, USA
56Purdue University Calumet, Hammond, Indiana 46323, USA 57
Iowa State University, Ames, Iowa 50011, USA
58University of Kansas, Lawrence, Kansas 66045, USA 59Kansas State University, Manhattan, Kansas 66506, USA 60Louisiana Tech University, Ruston, Louisiana 71272, USA 61University of Maryland, College Park, Maryland 20742, USA
62Boston University, Boston, Massachusetts 02215, USA 63Northeastern University, Boston, Massachusetts 02115, USA
64University of Michigan, Ann Arbor, Michigan 48109, USA 65Michigan State University, East Lansing, Michigan 48824, USA
66University of Mississippi, University, Mississippi 38677, USA 67University of Nebraska, Lincoln, Nebraska 68588, USA 68Princeton University, Princeton, New Jersey 08544, USA 69State University of New York, Buffalo, New York 14260, USA
70Columbia University, New York, New York 10027, USA 71University of Rochester, Rochester, New York 14627, USA 72State University of New York, Stony Brook, New York 11794, USA
73Brookhaven National Laboratory, Upton, New York 11973, USA 74Langston University, Langston, Oklahoma 73050, USA 75University of Oklahoma, Norman, Oklahoma 73019, USA 76Oklahoma State University, Stillwater, Oklahoma 74078, USA
77Brown University, Providence, Rhode Island 02912, USA 78University of Texas, Arlington, Texas 76019, USA 79Southern Methodist University, Dallas, Texas 75275, USA
80
Rice University, Houston, Texas 77005, USA
81University of Virginia, Charlottesville, Virginia 22901, USA 82University of Washington, Seattle, Washington 98195, USA
(Received 10 July 2008; published 25 November 2008)
In0:9 fb1of pp collisions, the D0 Collaboration presented evidence for single top quark production in events with an isolated lepton, missing transverse momentum, and two to four jets. We examine these data to study the Lorentz structure of the Wtb coupling. The standard model predicts a left-handed vector coupling at the Wtb vertex. The most general lowest dimension, CP-conserving Lagrangian admits right-handed vector and or right-right-handed tensor couplings as well. We find that the data prefer the left-handed vector coupling and set upper limits on the anomalous couplings. These are the first direct constraints on a general Wtb interaction and the first direct limits on left- and right-handed tensor couplings.
DOI:10.1103/PhysRevLett.101.221801 PACS numbers: 12.15.Ji, 13.85.Qk, 14.65.Ha
Recently, we presented evidence for single top quark production in pp collisions atpffiffiffis¼ 1:96 TeV [1] based on 0:9 fb1of data collected using the D0 detector [2] at the
Fermilab Tevatron collider. In this Letter, we report an extension of this analysis using the same data set and similar analysis tools to study the consistency of this ex-cess with different hypotheses for the couplings involved in single top quark production. This is the first time such a test has been carried out.
The standard model (SM) has been extraordinarily suc-cessful in describing the data taken at the energies of present colliders. However, we know that the electroweak symmetry breaking sector of the SM gives rise to many unanswered questions, making a strong case for new phys-ics beyond the SM. This new physphys-ics can manifest itself in the production of new particles or in corrections to SM processes that change the effective couplings of SM parti-cles. The interactions between quarks and gauge bosons have been measured precisely at the CERN Large Electron Positron collider [3] except for the top quark, which was not kinematically accessible. The large mass of the top quark has prompted speculation that the top quark may play a special role in the mechanism of electroweak sym-metry breaking and thus have nonstandard interactions with weak gauge bosons. We can probe the interactions of top quarks with W bosons via measurements of single top quark production and top quark decays in ttproduction, each yielding complementary information [4].
At the Tevatron, the dominant modes of single top quark production are s-channel and t-channel production. We use the notation ‘‘tb’’ for the sum of the s-channel processes t b and tb and ‘‘tqb’’ for the sum of the t-channel processes tq b andt q b. We assume that single top quark production proceeds exclusively through W boson exchange. Therefore, extensions of the SM in which single top quarks are produced via flavor-changing neutral current interac-tions [5] or the exchange of new massive scalar [6] or vector bosons [7] are not considered here. We further assume thatjVtdj2þ jVtsj2 jVtbj2, i.e., the Wtb vertex dominates top quark production and decay [8]. Finally, we assume that the Wtb vertex is CP-conserving.
The most general, lowest dimension, CP-conserving, Lagrangian for the Wtb vertex is [9]
L ¼ gffiffiffi 2 p W bðfL1PLþ fR1PRÞt ffiffiffig 2 p MW @Wbðf2LPLþ fR2PRÞt þ H:c:; (1)
where MWis the mass of W boson, PL¼ ð1 5Þ=2 is the
left-handed projection operator, and PR¼ ð1 þ 5Þ=2 is
the right-handed projection operator. In the SM the values of the form factors are f1L 1 and fL2 ¼ fR1 ¼ f2R¼ 0. In
this case the predicted cross section for single top quark production is2:9 0:3 pb [10].
The presence of anomalous couplings can change angu-lar distributions and event kinematics as demonstrated by the pT spectrum of the charged lepton from the decay of the top quark in Fig. 1. Such differences can be used to distinguish these couplings [11,12]. The magnitude of the right-handed vector coupling and tensor couplings can be indirectly constrained by the measurement of the b! s branching fraction [13]. Direct constraints on the combi-nation of several couplings can be obtained from the measurement of the W boson helicity in top quark decays [14]. The predicted single top quark production cross sections for the s and t channels combined are 2:7 0:3 pb if fR
1 ¼ 1 and 10:4 1:4 pb if fL2 ¼ 1 or f2R¼ 1,
and the other couplings vanish [11]. In these scenarios the ratio of the s- and t-channel cross section is approximately 1:2 and 6:1, respectively.
Ideally, we would like to set limits on all four couplings fL
1, fL2, fR1, and fR2 simultaneously. This, however, requires
more data than are currently available. We therefore look at two couplings at a time and assume that the other two are negligible. We consider three cases in which we allow the left-handed vector coupling fL
1 and any one of the three
nonstandard couplings to be nonzero. We refer to these as ðL1; L2Þ, ðL1; R1Þ, and ðL1; R2Þ.
We look for events in which the top quark decays to a W boson and a b quark, followed by the decay of the W boson to an electron or a muon and a neutrino. To enhance the signal content of the selected data sample, one or two of the jets are required to be b tagged, i.e., identified as
originat-data (=1) 1 L f bkg 1 L f 2 L f 1 R f 2 R f DØ 0.9 fb-1 [GeV] T Charged Lepton p 0 20 40 60 80 100 120 140 Yield [Events/7.5GeV] 0 10 20 30 40 50 60 70 80
FIG. 1 (color online). Charged lepton pT spectrum from data
and expectation for SM single top production plus background for events with two jets and one b-tagged jet. Superimposed are the distributions from single top quark production with different couplings (all other couplings set to zero) normalized to 10 times the SM single top quark cross section.
ing from b hadrons [15]. For details of the selection criteria and background modeling, see [1].
We model the single top quark signal using the
COMPHEP-SINGLETOP Monte Carlo event generator [16], and the anomalous Wtb couplings are considered in both production and decay in the generated signal samples. The event kinematics for both the s channel and the t channel reproduce distributions from next-to-leading-order calcu-lations [10]. The decay of the top quark and the resulting W boson are carried out in theSINGLETOPgenerator in order to preserve the information about the spin of the particles.
PYTHIA[17] is used to add the underlying event, initial, and final-state radiation and for hadronization. The top quark mass is set to 175 GeV, and the CTEQ6L1 parton distri-bution functions [18] are used.
Background contributions from Wþ jets and tt produc-tion are simulated using the ALPGEN leading-order Monte Carlo event generator [19] interfaced to PYTHIA. A parton-jet matching algorithm [20] is used to avoid double counting. The response of the D0 detector to the Monte Carlo events is simulated using GEANT [21]. Simulated events are processed through the same recon-struction software used for data, and efficiencies and res-olutions are corrected to match the performance of the reconstruction for data. The tt background is normalized using the theoretical cross section [22]. The multijet back-ground, which arises from events in which a jet is mis-identified as an isolated electron or muon, is modeled using events from data containing lepton candidates failing the isolation requirements and that otherwise resemble the signal events.
To increase the search sensitivity, we divide our data into 12 independent analysis channels based on the lepton (e or ), jet multiplicity (2, 3, or 4), and number of b tagged jets (1 or 2). The Wþ jets background is normalized such that the number of events predicted by the simulation agrees with the number of events observed in each analysis chan-nel before b tagging is applied.
After all cuts, we select 1398 b tagged leptonþ jets events, which we expect to contain 62 13 single top quark events, 348 80 tt events, 849 222 W þ jets
events, and 202 48 multijet events. A detailed break-down of the sample composition in each channel is given in Ref. [1]. Within each channel, the signal efficiency of the complete selection does not depend strongly on the as-sumed Wtb coupling. The selection efficiencies for signal with different Wtb couplings vary between ð1:07 0:15Þ% and ð1:52 0:16Þ% for tb events with 1 b tag, between ð0:86 0:13Þ% and ð1:14 0:14Þ% for tqb events with 1 b tag, betweenð0:40 0:08Þ% and ð0:60 0:10Þ% for tb events with 2 b tags, and between ð0:07 0:01Þ% and ð0:10 0:02Þ% for tqb events with 2 b tags.
Systematic uncertainties in the signal and background models are described in detail in Ref. [1]. The dominant contributions to the uncertainties in the background esti-mate come from: the normalization of the tt background (18%), which includes the top quark mass uncertainty; the normalization of the Wþ jets and multijets backgrounds to data (17%–27%), which includes the uncertainty in the fraction of events with heavy flavor production; and the b-tagging efficiencies (12%–17% for double-tagged events). The uncertainties from the jet energy scale correc-tions (1%–20%) and the b tagging probabilities affect both the shape and normalization of the simulated distributions. All other components contribute at the level of a few percent.
We use boosted decision trees [23,24] to discriminate between the single top quark signal and background. For training, we divide our data into only four independent analysis channels defined by lepton flavor and b tag multi-plicity. Each channel contains events with 2, 3, or 4 jets. For each of the three coupling scenarios, the signal samples consist of a sample of events generated with left-handed vector coupling set to one, i.e., with SM coupling, and a sample of events generated with the nonstandard coupling set to one and all other couplings set to zero. The back-ground sample consists of events from all backback-ground sources in proportions according to the background model described above.
We use 50 variables in the training: the 49 variables that were used in Ref. [1] plus the lepton pT which helps
distinguish the signals with different couplings, as can be
data (=1) 1 L f bkg 1 L f 2 L f -1 DØ 0.9 fb (a)
Decision Tree Output 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Yield [Events/0.1] 0 20 40 60 80 100 data (=1) 1 L f bkg 1 L f 1 R f DØ 0.9 fb-1 (b)
Decision Tree Output 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Yield [Events/0.1] 0 20 40 60 80 100 120 data (=1) 1 L f bkg 1 L f 2 R f DØ 0.9 fb-1 (c)
Decision Tree Output 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Yield [Events/0.1] 0 20 40 60 80 100 120 140 160
FIG. 2 (color online). Boosted decision tree output distributions for data and sum of the SM signal and backgrounds for events with two jets and one b-tagged jet for (a) theðL1; L2Þ scenario, (b) the ðL1; R1Þ scenario, and (c) the ðL1; R2Þ scenario. Superimposed are the distributions for the single top quark signals with different couplings normalized to 5 times the SM single top quark cross section.
seen in Fig. 1. The variables describe individual object kinematics, global event kinematics, and angular correla-tions. The boosted decision trees produce a continuous output distribution ranging from zero to one, with back-ground tending closer to zero and signal tending closer to one. Figure2shows representative output distributions for the data and the sum of the SM signal and backgrounds for the electron channel with two jets and one b-tagged jet in each of the three anomalous coupling scenarios.
We use Bayesian statistics [25] to compare the output distribution of the decision trees from data to the expecta-tions for single top quark production, taking all systematic uncertainties and their correlations into account. For any pair of values of the two couplings that are considered nonzero, we compute the expected output distribution by superimposing the distributions from the two signal samples with the nonstandard coupling and from the back-ground samples in the appropriate proportions. In the case of theðL1; L2Þ scenario, the two amplitudes interfere, and
we use a superposition of three signal samples: one with left-handed vector couplings, one with the left-handed tensor coupling only set to one, and one with both cou-plings set to one to take into account the effect of the interference. We then compute a likelihood as a product over all bins and channels. Here we use 12 channels de-fined by lepton flavor, b tag multiplicity, and jet multi-plicity (2, 3, or 4). We assume Poisson distributions for the observed counts and flat non-negative prior probabilities for the signal cross sections. The prior for the combined signal acceptance and background yields is modeled with a multivariate Gaussian describing the effect of systematic uncertainties, including correlations.
A two-dimensional posterior probability density is com-puted as a function ofjfL
1j2 andjfXj2, where fX is any of
the other three nonstandard couplings, in each channel. These probability distributions are shown in Fig. 3. We quote the values of the couplings that maximize the two-dimensional likelihood as our measurements. In all three scenarios we measure zero for the right-handed vector and left- and right-handed tensor couplings. We compute 95% C.L. upper limits on these couplings by integrating out the left-handed vector coupling to get a one-dimensional pos-terior probability density. The measured values are given in Table I. The data favor the left-handed vector hypothesis over the alternative hypotheses.
In summary, we have studied the excess observed in 0:9 fb1 of D0 data in the search for single top quark
production. We attribute this excess to single top quark production and study its consistency with different hypoth-eses for the structure of the Wtb coupling and find that the data prefer the left-handed vector coupling over the alter-native hypotheses studied. These are the first direct con-straints on a general Wtb interaction and the first direct limits on left- and right-handed tensor couplings.
We thank the staffs at Fermilab and collaborating insti-tutions and acknowledge support from the DOE and NSF (USA); CEA and CNRS/IN2P3 (France); FASI, Rosatom, and RFBR (Russia); CNPq, FAPERJ, FAPESP, and FUNDUNESP (Brazil); DAE and DST (India); Colciencias (Colombia); CONACyT (Mexico); KRF and KOSEF (Korea); CONICET and UBACyT (Argentina);
max posterior 68% CL contour 90% CL contour 95% CL contour -1 DØ 0.9 fb (a) 0 0.5 1 1.5 2 2.5 3 3.5 4 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 -1 DØ 0.9 fb (b) 0 0.5 1 1.5 2 2.5 3 3.5 4 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 -1 DØ 0.9 fb (c) 0 0.5 1 1.5 2 2.5 3 3.5 4 0 0.5 1 1.5 2 2.5 3 3.5 4 -1 DØ 0.9 fb (d) 0 0.5 1 1.5 2 2.5 3 3.5 4 0 0.5 1 1.5 2 2.5 3 3.5 4 -1 DØ 0.9 fb (e) 2 | 1 L |f 0 0.5 1 1.5 2 2.5 3 3.5 4 2| 2 R |f 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 -1 DØ 0.9 fb (f) 2 | 1 L |f 0 0.5 1 1.5 2 2.5 3 3.5 4 2| 2 R |f 2 | 1 L |f 2| 1 R |f 2 | 1 L |f 2| 1 R |f 2 | 1 L |f 2| 2 L |f 2 | 1 L |f 2| 2 L |f 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
FIG. 3 (color online). Plots of the two-dimensional posterior probability density for the anomalous couplings. The plots on the left show the expectation for left-handed vector couplings, and the plots on the right show the observed posterior from our data. The upper row (a),(b) shows the plots for theðL1; L2Þ scenario, the middle row (c),(d) for theðL1; R1Þ scenario, and the bottom row (e),(f ) for theðL1; R2Þ scenario.
TABLE I. Measured values of the total cross section for single top production and one-dimensional limits on Wtb couplings in the three scenarios.
Scenario Cross section Coupling (L1, L2) 4:4þ2:32:5pb jfL 1j2¼ 1:4þ0:60:5 jfL 2j2<0:5 at 95% C.L. (L1, R1) 5:2þ2:63:5pb jfL 1j2¼ 1:8þ1:01:3 jfR 1j2<2:5 at 95% C.L. (L1, R2) 4:5þ2:22:2pb jfL 1j2¼ 1:4þ0:90:8 jfR 2j2<0:3 at 95% C.L.
FOM (The Netherlands); STFC (United Kingdom); MSMT and GACR (Czech Republic); CRC Program, CFI, NSERC, and WestGrid Project (Canada); BMBF and DFG (Germany); SFI (Ireland); The Swedish Research Council (Sweden); CAS and CNSF (China); and the Alexander von Humboldt Foundation (Germany).
*Visitor from Augustana College, Sioux Falls, SD, USA.
†
Visitor from The University of Liverpool, Liverpool, United Kingdom.
‡Visitor from ECFM, Universidad Autonoma de Sinaloa,
Culiaca´n, Mexico.
xVisitor from II. Physikalisches Institut,
Georg-August-University, Go¨ttingen, Germany.
kVisitor from Helsinki Institute of Physics, Helsinki,
Finland.
{Visitor from Universita¨t Zu¨rich, Zu¨rich, Switzerland.
**Deceased.
[1] V. M. Abazov et al. (D0 Collaboration), Phys. Rev. Lett. 98, 181802 (2007); Phys. Rev. D 78, 012005 (2008). [2] V. M. Abazov et al. (D0 Collaboration), Nucl. Instrum.
Methods Phys. Res., Sect. A 565, 463 (2006).
[3] The LEP Collaborations and the LEP Electroweak Working Group, Report No. LEPEWWG/2007-01. [4] For example, T. Tait and C.-P. Yuan, Phys. Rev. D 63,
014018 (2000).
[5] V. M. Abazov et al. (D0 Collaboration), Phys. Rev. Lett. 99, 191802 (2007).
[6] V. M. Abazov et al. (D0 Collaboration), arXiv:0807.0859 [Phys. Rev. Lett. (to be published)].
[7] V. M. Abazov et al. (D0 Collaboration), Phys. Rev. Lett. 100, 211803 (2008).
[8] T. Affolder et al. (CDF Collaboration), Phys. Rev. Lett. 86, 3233 (2001); D. Acosta et al. (CDF Collaboration), Phys. Rev. Lett. 95, 102002 (2005); V. M. Abazov et al. (D0 Collaboration), Phys. Lett. B 639, 616 (2006).
[9] G. L. Kane, G. A. Ladinsky, and C.-P. Yuan, Phys. Rev. D 45, 124 (1992).
[10] Z. Sullivan, Phys. Rev. D 70, 114012 (2004).
[11] E. Boos, L. Dudko, and T. Ohl, Eur. Phys. J. C 11, 473 (1999).
[12] D. O. Carlson, E. Malkawi, and C.-P. Yuan, Phys. Lett. B 337, 145 (1994); E. Malkawi and C.-P. Yuan, Phys. Rev. D 50, 4462 (1994); A. P. Heinson, A. S. Belyaev, and E. Boos, Phys. Rev. D 56, 3114 (1997).
[13] F. Larios, M. A. Perez, and C.-P. Yuan, Phys. Lett. B 457, 334 (1999); G. Burdman, M. C. Gonzales-Garcia, and S. F. Novaes, Phys. Rev. D 61, 114016 (2000), and references therein.
[14] V. M. Abazov et al. (D0 Collaboration), Phys. Rev. Lett. 100, 062004 (2008).
[15] T. Scanlon, Ph.D. thesis, Imperial College, University of London [Fermilab Report No. FERMILAB-THESIS-2006-43].
[16] E. Boos et al., Phys. At. Nucl. 69, 1317 (2006); E. Boos et al. (CompHEP Collaboration), Nucl. Instrum. Methods Phys. Res., Sect. A 534, 250 (2004).
[17] T. Sjo¨strand et al., arXiv:hep-ph/0308153; we used
PYTHIAversion 6.323.
[18] J. Pumplin et al., J. High Energy Phys. 07 (2002) 012; D. Stump et al., J. High Energy Phys. 10 (2003) 046. [19] M. L. Mangano et al., J. High Energy Phys. 07 (2003) 001;
we usedALPGENversion 2.05.
[20] M. L. Mangano et al., J. High Energy Phys. 01 (2007) 013. [21] R. Brun and F. Carminati, CERN Program Library Long
Writeup Report No. W5013, 1993 (unpublished). [22] N. Kidonakis and R. Vogt, Phys. Rev. D 68, 114014
(2003).
[23] L. Breiman et al., Classification and Regression Trees (Wadsworth, Stamford, 1984).
[24] Y. Freund and R. E. Schapire, in Machine Learning: Proceedings of the Thirteenth International Conference, edited by L. Saitta (Morgan Kaufmann, San Fransisco, 1996), p. 148.
[25] I. Bertram et al., Fermilab Report No. FERMILAB-TM-2104, 2000.