Measurement of the W boson helicity in top quark decays at D0
V. M. Abazov,35B. Abbott,75M. Abolins,65B. S. Acharya,28M. Adams,51T. Adams,49M. Agelou,17E. Aguilo,5 S. H. Ahn,30M. Ahsan,59G. D. Alexeev,35G. Alkhazov,39A. Alton,64G. Alverson,63G. A. Alves,2M. Anastasoaie,34
T. Andeen,53S. Anderson,45B. Andrieu,16M. S. Anzelc,53Y. Arnoud,13M. Arov,52A. Askew,49B. A˚ sman,40 A. C. S. Assis Jesus,3O. Atramentov,49C. Autermann,20C. Avila,7C. Ay,23F. Badaud,12A. Baden,61L. Bagby,52 B. Baldin,50D. V. Bandurin,59P. Banerjee,28S. Banerjee,28E. Barberis,63P. Bargassa,80P. Baringer,58C. Barnes,43
J. Barreto,2J. F. Bartlett,50U. Bassler,16D. Bauer,43S. Beale,5A. Bean,58M. Begalli,3M. Begel,71
C. Belanger-Champagne,5L. Bellantoni,50A. Bellavance,67J. A. Benitez,65S. B. Beri,26G. Bernardi,16R. Bernhard,41 L. Berntzon,14I. Bertram,42M. Besanc¸on,17R. Beuselinck,43V. A. Bezzubov,38P. C. Bhat,50V. Bhatnagar,26M. Binder,24
C. Biscarat,42K. M. Black,6I. Blackler,43G. Blazey,52F. Blekman,43S. Blessing,49D. Bloch,18K. Bloom,67 U. Blumenschein,22A. Boehnlein,50O. Boeriu,55T. A. Bolton,59G. Borissov,42K. Bos,33T. Bose,77A. Brandt,78 R. Brock,65G. Brooijmans,70A. Bross,50D. Brown,78N. J. Buchanan,49D. Buchholz,53M. Buehler,81V. Buescher,22
S. Burdin,50S. Burke,45T. H. Burnett,82E. Busato,16C. P. Buszello,43J. M. Butler,62P. Calfayan,24S. Calvet,14 J. Cammin,71S. Caron,33W. Carvalho,3B. C. K. Casey,77N. M. Cason,55H. Castilla-Valdez,32S. Chakrabarti,28 D. Chakraborty,52K. M. Chan,71A. Chandra,48F. Charles,18E. Cheu,45F. Chevallier,13D. K. Cho,62S. Choi,31 B. Choudhary,27L. Christofek,77D. Claes,67B. Cle´ment,18C. Cle´ment,40Y. Coadou,5M. Cooke,80W. E. Cooper,50 D. Coppage,58M. Corcoran,80M.-C. Cousinou,14B. Cox,44S. Cre´pe´-Renaudin,13D. Cutts,77M. C´ wiok,29H. da Motta,2 A. Das,62M. Das,60B. Davies,42G. Davies,43G. A. Davis,53K. De,78P. de Jong,33S. J. de Jong,34E. De La Cruz-Burelo,64 C. De Oliveira Martins,3J. D. Degenhardt,64F. De´liot,17M. Demarteau,50R. Demina,71P. Demine,17D. Denisov,50 S. P. Denisov,38S. Desai,72H. T. Diehl,50M. Diesburg,50M. Doidge,42A. Dominguez,67H. Dong,72L. V. Dudko,37 L. Duflot,15S. R. Dugad,28D. Duggan,49A. Duperrin,14J. Dyer,65A. Dyshkant,52M. Eads,67D. Edmunds,65T. Edwards,44
J. Ellison,48J. Elmsheuser,24V. D. Elvira,50S. Eno,61P. Ermolov,37H. Evans,54A. Evdokimov,36V. N. Evdokimov,38 S. N. Fatakia,62L. Feligioni,62A. V. Ferapontov,59T. Ferbel,71F. Fiedler,24F. Filthaut,34W. Fisher,50H. E. Fisk,50 I. Fleck,22M. Ford,44M. Fortner,52H. Fox,22S. Fu,50S. Fuess,50T. Gadfort,82C. F. Galea,34E. Gallas,50E. Galyaev,55 C. Garcia,71A. Garcia-Bellido,82J. Gardner,58V. Gavrilov,36A. Gay,18P. Gay,12D. Gele´,18R. Gelhaus,48C. E. Gerber,51
Y. Gershtein,49D. Gillberg,5G. Ginther,71B. Gmyrek,45N. Gollub,40B. Go´mez,7A. Goussiou,55P. D. Grannis,72 H. Greenlee,50Z. D. Greenwood,60E. M. Gregores,4G. Grenier,19Ph. Gris,12J.-F. Grivaz,15S. Gru¨nendahl,50 M. W. Gru¨newald,29F. Guo,72J. Guo,72G. Gutierrez,50P. Gutierrez,75A. Haas,70N. J. Hadley,61P. Haefner,24 S. Hagopian,49J. Haley,68I. Hall,75R. E. Hall,47L. Han,6K. Hanagaki,50P. Hansson,40K. Harder,59A. Harel,71
R. Harrington,63J. M. Hauptman,57R. Hauser,65J. Hays,53T. Hebbeker,20D. Hedin,52J. G. Hegeman,33 J. M. Heinmiller,51A. P. Heinson,48U. Heintz,62C. Hensel,58K. Herner,72G. Hesketh,63M. D. Hildreth,55R. Hirosky,81 J. D. Hobbs,72B. Hoeneisen,11H. Hoeth,25M. Hohlfeld,15S. J. Hong,30R. Hooper,77P. Houben,33Y. Hu,72Z. Hubacek,9
V. Hynek,8I. Iashvili,69R. Illingworth,50A. S. Ito,50S. Jabeen,62M. Jaffre´,15S. Jain,75K. Jakobs,22C. Jarvis,61 A. Jenkins,43R. Jesik,43K. Johns,45C. Johnson,70M. Johnson,50A. Jonckheere,50P. Jonsson,43A. Juste,50D. Ka¨fer,20 S. Kahn,73E. Kajfasz,14A. M. Kalinin,35J. M. Kalk,60J. R. Kalk,65S. Kappler,20D. Karmanov,37J. Kasper,62P. Kasper,50
I. Katsanos,70D. Kau,49R. Kaur,26R. Kehoe,79S. Kermiche,14N. Khalatyan,62A. Khanov,76A. Kharchilava,69 Y. M. Kharzheev,35D. Khatidze,70H. Kim,78T. J. Kim,30M. H. Kirby,34B. Klima,50J. M. Kohli,26J.-P. Konrath,22 M. Kopal,75V. M. Korablev,38J. Kotcher,73B. Kothari,70A. Koubarovsky,37A. V. Kozelov,38D. Krop,54A. Kryemadhi,81 T. Kuhl,23A. Kumar,69S. Kunori,61A. Kupco,10T. Kurcˇa,19,*J. Kvita,8S. Lammers,70G. Landsberg,77J. Lazoflores,49
A.-C. Le Bihan,18P. Lebrun,19W. M. Lee,52A. Leflat,37F. Lehner,41V. Lesne,12J. Leveque,45P. Lewis,43J. Li,78 Q. Z. Li,50J. G. R. Lima,52D. Lincoln,50J. Linnemann,65V. V. Lipaev,38R. Lipton,50Z. Liu,5L. Lobo,43A. Lobodenko,39
M. Lokajicek,10A. Lounis,18P. Love,42H. J. Lubatti,82M. Lynker,55A. L. Lyon,50A. K. A. Maciel,2R. J. Madaras,46 P. Ma¨ttig,25C. Magass,20A. Magerkurth,64A.-M. Magnan,13N. Makovec,15P. K. Mal,55H. B. Malbouisson,3S. Malik,67 V. L. Malyshev,35H. S. Mao,50Y. Maravin,59M. Martens,50R. McCarthy,72D. Meder,23A. Melnitchouk,66A. Mendes,14
L. Mendoza,7M. Merkin,37K. W. Merritt,50A. Meyer,20J. Meyer,21M. Michaut,17H. Miettinen,80T. Millet,19 J. Mitrevski,70J. Molina,3N. K. Mondal,28J. Monk,44R. W. Moore,5T. Moulik,58G. S. Muanza,15M. Mulders,50 M. Mulhearn,70O. Mundal,22L. Mundim,3Y. D. Mutaf,72E. Nagy,14M. Naimuddin,27M. Narain,62N. A. Naumann,34 H. A. Neal,64J. P. Negret,7P. Neustroev,39C. Noeding,22A. Nomerotski,50S. F. Novaes,4T. Nunnemann,24V. O’Dell,50 D. C. O’Neil,5G. Obrant,39V. Oguri,3N. Oliveira,3D. Onoprienko,59N. Oshima,50R. Otec,9G. J. Otero y Garzo´n,51 M. Owen,44P. Padley,80N. Parashar,56S.-J. Park,71S. K. Park,30J. Parsons,70R. Partridge,77N. Parua,72A. Patwa,73
G. Pawloski,80P. M. Perea,48E. Perez,17K. Peters,44P. Pe´troff,15M. Petteni,43R. Piegaia,1J. Piper,65M.-A. Pleier,21 P. L. M. Podesta-Lerma,32V. M. Podstavkov,50Y. Pogorelov,55M.-E. Pol,2A. Pomposˇ,75B. G. Pope,65A. V. Popov,38 C. Potter,5W. L. Prado da Silva,3H. B. Prosper,49S. Protopopescu,73J. Qian,64A. Quadt,21B. Quinn,66M. S. Rangel,2 K. J. Rani,28K. Ranjan,27P. N. Ratoff,42P. Renkel,79S. Reucroft,63M. Rijssenbeek,72I. Ripp-Baudot,18F. Rizatdinova,76 S. Robinson,43R. F. Rodrigues,3C. Royon,17P. Rubinov,50R. Ruchti,55V. I. Rud,37G. Sajot,13A. Sa´nchez-Herna´ndez,32 M. P. Sanders,61A. Santoro,3G. Savage,50L. Sawyer,60T. Scanlon,43D. Schaile,24R. D. Schamberger,72Y. Scheglov,39
H. Schellman,53P. Schieferdecker,24C. Schmitt,25C. Schwanenberger,44A. Schwartzman,68R. Schwienhorst,65 J. Sekaric,49S. Sengupta,49H. Severini,75E. Shabalina,51M. Shamim,59V. Shary,17A. A. Shchukin,38W. D. Shephard,55
R. K. Shivpuri,27D. Shpakov,50V. Siccardi,18R. A. Sidwell,59V. Simak,9V. Sirotenko,50P. Skubic,75P. Slattery,71 R. P. Smith,50G. R. Snow,67J. Snow,74S. Snyder,73S. So¨ldner-Rembold,44X. Song,52L. Sonnenschein,16A. Sopczak,42 M. Sosebee,78K. Soustruznik,8M. Souza,2B. Spurlock,78J. Stark,13J. Steele,60V. Stolin,36A. Stone,51D. A. Stoyanova,38 J. Strandberg,64S. Strandberg,40M. A. Strang,69M. Strauss,75R. Stro¨hmer,24D. Strom,53M. Strovink,46L. Stutte,50
S. Sumowidagdo,49P. Svoisky,55A. Sznajder,3M. Talby,14P. Tamburello,45W. Taylor,5P. Telford,44J. Temple,45 B. Tiller,24M. Titov,22V. V. Tokmenin,35M. Tomoto,50T. Toole,61I. Torchiani,22S. Towers,42T. Trefzger,23 S. Trincaz-Duvoid,16D. Tsybychev,72B. Tuchming,17C. Tully,68A. S. Turcot,44P. M. Tuts,70R. Unalan,65L. Uvarov,39
S. Uvarov,39S. Uzunyan,52B. Vachon,5P. J. van den Berg,33R. Van Kooten,54W. M. van Leeuwen,33N. Varelas,51 E. W. Varnes,45A. Vartapetian,78I. A. Vasilyev,38M. Vaupel,25P. Verdier,19L. S. Vertogradov,35M. Verzocchi,50 F. Villeneuve-Seguier,43P. Vint,43J.-R. Vlimant,16E. Von Toerne,59M. Voutilainen,67,†M. Vreeswijk,33H. D. Wahl,49 L. Wang,61M. H. L. S. Wang,50J. Warchol,55G. Watts,82M. Wayne,55G. Weber,23M. Weber,50H. Weerts,65N. Wermes,21 M. Wetstein,61A. White,78D. Wicke,25G. W. Wilson,58S. J. Wimpenny,48M. Wobisch,50J. Womersley,50D. R. Wood,63 T. R. Wyatt,44Y. Xie,77N. Xuan,55S. Yacoob,53R. Yamada,50M. Yan,61T. Yasuda,50Y. A. Yatsunenko,35K. Yip,73 H. D. Yoo,77S. W. Youn,53C. Yu,13J. Yu,78A. Yurkewicz,72A. Zatserklyaniy,52C. Zeitnitz,25D. Zhang,50T. Zhao,82
B. Zhou,64J. Zhu,72M. Zielinski,71D. Zieminska,54A. Zieminski,54V. Zutshi,52and E. G. Zverev3
(D0 Collaboration)
1Universidad de Buenos Aires, Buenos Aires, Argentina 2LAFEX, Centro Brasileiro de Pesquisas Fı´sicas, Rio de Janeiro, Brazil
3Universidade do Estado do Rio de Janeiro, Rio de Janeiro, Brazil 4Instituto de Fı´sica Teo´rica, Universidade Estadual Paulista, Sa˜o Paulo, Brazil
5University of Alberta, Edmonton, Alberta, Canada, Simon Fraser University, Burnaby, British Columbia, Canada,
York University, Toronto, Ontario, Canada, and McGill University, Montreal, Quebec, Canada
6University of Science and Technology of China, Hefei, People’s Republic of China 7Universidad de los Andes, Bogota´, Colombia
8Center for Particle Physics, Charles University, Prague, Czech Republic 9Czech Technical University, Prague, Czech Republic
10Center for Particle Physics, Institute of Physics, Academy of Sciences of the Czech Republic, Prague, Czech Republic 11Universidad San Francisco de Quito, Quito, Ecuador
12Laboratoire de Physique Corpusculaire, IN2P3-CNRS, Universite´ Blaise Pascal, Clermont-Ferrand, France 13Laboratoire de Physique Subatomique et de Cosmologie, IN2P3-CNRS, Universite de Grenoble 1, Grenoble, France
14CPPM, IN2P3-CNRS, Universite´ de la Me´diterrane´e, Marseille, France 15IN2P3-CNRS, Laboratoire de l’Acce´le´rateur Line´aire, Orsay, France
16LPNHE, IN2P3-CNRS, Universite´s Paris VI and VII, Paris, France 17DAPNIA/Service de Physique des Particules, CEA, Saclay, France
18IPHC, IN2P3-CNRS, Universite´ Louis Pasteur, Strasbourg, France, and Universite´ de Haute Alsace, Mulhouse, France 19Institut de Physique Nucle´aire de Lyon, IN2P3-CNRS, Universite´ Claude Bernard, Villeurbanne, France
20III. Physikalisches Institut A, RWTH Aachen, Aachen, Germany 21Physikalisches Institut, Universita¨t Bonn, Bonn, Germany 22Physikalisches Institut, Universita¨t Freiburg, Freiburg, Germany
23Institut fu¨r Physik, Universita¨t Mainz, Mainz, Germany 24Ludwig-Maximilians-Universita¨t Mu¨nchen, Mu¨nchen, Germany 25Fachbereich Physik, University of Wuppertal, Wuppertal, Germany
26Panjab University, Chandigarh, India 27Delhi University, Delhi, India
28Tata Institute of Fundamental Research, Mumbai, India 29University College Dublin, Dublin, Ireland
30Korea Detector Laboratory, Korea University, Seoul, Korea 31SungKyunKwan University, Suwon, Korea
32CINVESTAV, Mexico City, Mexico
33FOM-Institute NIKHEF and University of Amsterdam/NIKHEF, Amsterdam, The Netherlands 34Radboud University Nijmegen/NIKHEF, Nijmegen, The Netherlands
35Joint Institute for Nuclear Research, Dubna, Russia 36Institute for Theoretical and Experimental Physics, Moscow, Russia
37Moscow State University, Moscow, Russia 38Institute for High Energy Physics, Protvino, Russia 39Petersburg Nuclear Physics Institute, St. Petersburg, Russia
40Lund University, Lund, Sweden, Royal Institute of Technology and Stockholm University, Stockholm, Sweden,
and Uppsala University, Uppsala, Sweden
41Physik Institut der Universita¨t Zu¨rich, Zu¨rich, Switzerland 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 57Iowa 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
80Rice University, Houston, Texas 77005, USA 81University of Virginia, Charlottesville, Virginia 22901, USA
82University of Washington, Seattle, Washington 98195, USA (Received 26 September 2006; published 5 February 2007)
*On leave from IEP SAS Kosice, Slovakia.
†Visitor from Helsinki Institute of Physics, Helsinki, Finland. ‡Visitor from Lewis University, Romeoville, IL, USA
We present a measurement of the fraction fof right-handed W bosons produced in top quark decays, based on a candidate sample of tt events in the ‘ jets and dilepton decay channels corresponding to an integrated luminosity of 370 pb 1collected by the D0 detector at the Fermilab Tevatron p pCollider at
s
p
1:96 TeV. We reconstruct the decay angle for each lepton. By comparing the cosdistribution from the data with that for the expected background and signal for various values of f(where we assume that the fraction of longitudinally-polarized W bosons has the standard model value of 0.70), we find
f 0:056 0:080stat 0:057syst (f< 0:23 at 95% C.L.), consistent with the standard model prediction of f 3:6 104.
DOI:10.1103/PhysRevD.75.031102 PACS numbers: 14.70.Fm, 12.15.Ji, 14.65.Ha
The top quark is by far the heaviest of the known fermions and is the only one that has a Yukawa coupling of order unity to the Higgs boson in the standard model (SM). We search for evidence of new physics in t ! Wb decay by measuring the helicity of the W boson. In the standard model, the top quark decays via the V A charged current interaction, almost always to a W boson and a b quark. A different form for the t ! Wb coupling would alter the fractions of W bosons produced in each of the three possible polarization states. For any linear com-bination of V and A currents at the t ! Wb vertex, the fraction f0 of longitudinally-polarized W bosons is 0:697 0:012 [1] at the world average top quark mass
mtof 172:5 2:3 GeV [2].
In this analysis, we fix f0 at 0.70 and measure the positive helicity fraction f. In the standard model, fis
predicted at next-to-leading order to be 3:6 104[3]. A measurement of f that differs significantly from this
value would be an unambiguous indication of new physics. For example, an fvalue of 0.30 would indicate a purely
V Acharged current interaction.
Measurements of the b ! s decay rate have indirectly limited the V A contribution in top quark decays to less than a few percent [4]. Direct measurements of the V A contribution are still necessary because the limit from
b ! sassumes that the electroweak penguin contribution
is dominant. Direct measurements of the longitudinal frac-tion found f0 0:91 0:39 [5], f0 0:56 0:31 [6], and f0 0:740:22
0:34 [7]. Direct measurements of f have
set limits of f< 0:18 [8], f< 0:27 [7], and f< 0:25
[9] at the 95% C.L. The analysis presented in this article improves upon that reported in Ref. [9] by using a larger data set, including the dilepton decay channel of the tt pair, and employing enhanced analysis techniques.
The angular distribution of the down-type decay prod-ucts of the W boson (charged lepton or d, s quark) in the rest frame of the W boson can be described by introducing the decay angle of the down-type particle with respect to the top quark direction. The dependence of the distribution of cos on f,
!c / 21 c2
f0 1 c2f 1 c2f;
(1) where f, f0, and f must sum to one and c cos,
forms the basis for our measurement. We proceed by
selecting a data sample enriched in tt events, reconstruct-ing the four vectors of the two top quarks and their decay products, and then calculating cos. This distribution in cos is compared with templates for different fvalues,
suitably corrected for background and reconstruction ef-fects, using a binned maximum likelihood method. In the
‘ jets channel, the kinematic reconstruction is done with a fit that constrains the W boson mass to its measured value and the top quark mass to 175 GeV, while in the dilepton channel, the kinematics are solved algebraically with the top quark mass fixed to 172.5 GeV.
The D0 detector [10] comprises three main systems: the central-tracking system, the calorimeters, and the muon system. The central-tracking system is located within a 2 T solenoidal magnet. The next layer of detection involves three liquid-argon/uranium calorimeters: a central section covering pseudorapidities [11] jj & 1, and two end calo-rimeters extending coverage to jj 4, all housed in separate cryostats. The muon system is located outside the calorimetry, and consists of a layer of tracking detec-tors and scintillation trigger counters before 1.8 T toroids, followed by two similar layers after the toroids.
This measurement uses a data sample recorded with the D0 experiment and corresponds to an integrated luminosity of about 370 pb1of p pcollisions atps 1:96 TeV. The data sample consists of tt candidate events from the
‘ jets decay channel tt ! WWb b ! ‘qq0b b and the dilepton channel tt ! WWb b ! ‘‘00b b, where
‘and ‘0are electrons or muons. The ‘ jets final state is characterized by one charged lepton, at least four jets (two of which are b jets), and significant missing transverse energy (E6 T). The dilepton final state is characterized by two charged leptons of opposite sign, at least two jets, and significant E6 T.
We simulate tt signal events with mt 172:5 GeV for
different values of fwith theALPGENMonte Carlo (MC)
program [12] for the parton-level process (leading order) andPYTHIA[13] for gluon radiation and subsequent hadro-nization. As the interference term between V A and
V A is suppressed by the small mass of the b quark
and is therefore negligible [14], samples with f 0:00
and f 0:30 are used to create cos templates for any
fvalue by a linear interpolation of the templates. The MC
samples used to model background events with real leptons are also generated usingALPGENandPYTHIA.
The ‘ jets event selection [15] requires an isolated lepton (e or ) with transverse momentum pT > 20 GeV,
no other lepton with pT > 15 GeV in the event, E6 T >
20 GeV, and at least four jets. Electrons are required to have jj < 1:1 and are identified by their energy deposition and isolation in the calorimeter, their transverse and lon-gitudinal shower shapes, and information from the tracking system. Also, a discriminant combining the above infor-mation must be consistent with the expectation for a high-pT isolated electron [15]. Muons are identified using information from the muon and tracking systems, and must satisfy isolation requirements based on the energies of calorimeter clusters and the momenta of tracks around the muon. They are required to have jj < 2:0 and to be isolated from jets. Jets are reconstructed using the Run II midpoint cone algorithm with cone radius 0.5 [16], and are required to have rapidity jyj < 2:5 and pT> 20 GeV.
Backgrounds in the ‘ jets channel arise predomi-nantly from W jets production and multijet production where one of the jets is misidentified as a lepton and spurious E6 T appears due to mismeasurement of the trans-verse energy in the event. We determine the number of multijet background events Nmj from the data, using the technique described in Ref. [15]. We calculate Nmjfor each bin in the cosdistribution from the data sample to obtain the multijet cos templates.
To discriminate between tt pair production and back-ground, a discriminantD with values in the range 0 to 1 is calculated using input variables which exploit differences in kinematics and jet flavor. The kinematic variables con-sidered are: HT (defined as the scalar sum of the jet pT
values), the minimum dijet mass of the jet pairs mjj min, the
2from the kinematic fit, the difference in azimuthal angle between the lepton and E6 T directions, and aplanarity
A and sphericity S [17] (calculated from the four leading jets and the lepton). Only the four leading jets in pT are considered in computing these variables.
We utilize the fact that background jets arise mostly from light quarks or gluons while two of the jets in tt events arise from b quarks by considering the impact parameters with respect to the primary vertex of all tracks within the jet cone. Based on these values, we calculate the probability PPV for each jet to originate from the primary vertex. We then average the two smallest PPV values to form a continuous variable hPPVi that tends to be small for
ttevents and large for backgrounds. Including PPV as a
continuous variable in the discriminant results in similar background discrimination but better efficiency than ap-plying a simple cut on PPV.
The discriminant is built separately for the e jets and
jets channels, using the method described in Refs. [15,18]. Background events tend to haveD values near 0, while tt events tend to have values near 1. We consider all possible combinations of the above variables for use in the discriminant, and all possible requirements
on the D value, and choose the variables and D criterion that give the smallest expected uncertainty on f. In the
e jets channel,S, HT, hPPVi, and 2are used, andD is
required to be >0:65. In the jets channel, A, HT,
mjj min, hPPVi, 2, and are used, andD is required to
be >0:80. In both channels there is no measurable depen-dence on the value of fof the efficiency for tt events to
satistfy theD requirement.
We then perform a binned Poisson maximum likelihood fit to compare the observed distribution of events in D to the sum of the distributions expected from tt, W jets, and multijet events. Nmj is constrained to the expected value within the known uncertainty. The likelihood is then maxi-mized with respect to the numbers of tt, W jets, and multijet events, which are multiplied by the appropriate efficiency for theD selection to determine the composition of the sample used for measuring cos.
In the dilepton channel, backgrounds arise from pro-cesses such as WW jets or Z jets. These propro-cesses are either rare or require false E6 T from mismeasurement of jet and lepton energy, allowing a good signal to background ratio to be attained using only kinematic selection criteria. The selection is detailed in Ref. [19]. Events are required to have two leptons with opposite charge and pT> 15 GeV
and two or more jets with pT> 20 GeV and jyj < 2:5.
Additional criteria are applied in the ee and channels to suppress Z ! ‘‘, and in the e channel the sum of the two leading jet pT’s and the leading lepton pT must be greater than 122 GeV. We place a more stringent require-ment on electron identification than is used in Ref. [19].
TableIlists the composition of each sample as well as the number of observed events in the data. We observe a disparity between the number of tt events in the e jets channel and jets channel, which is unexpected since the selection efficiencies for the two channels are similar. The statistical significance of the discrepancy in the event distribution is slightly above 2. The disparity appears to be a feature of the data sample used in this analysis, as it occurs regardless of the choice of variables used to define D. Further, it has no direct impact on this analysis, which relies only upon the distribution of events in cos.
TABLE I. Number of events observed in each tt decay chan-nel, the background level as determined by a fit to the D distribution in the ‘ jets channels and the expectation from the background production rate and selection efficiency in the dilepton channels, and the expected signal yield assuming stan-dard model tt production with a top quark mass of 175 GeV.
Observed Background Expected tt
e jets 51 5:3 0:9 32.9
jets 19 3:3 0:4 26.4
e 15 2:2 0:6 8.9
ee 4 0:8 0:2 3.3
1 0:4 0:1 2.4
The top quark and W boson four-momenta in the se-lected ‘ jets events are reconstructed using a kinematic fit which is subject to these constraints: two jets must form the invariant mass of the W boson, the lepton and the E6 T together with the neutrino pz component must form the invariant mass of the W boson, and the masses of the two reconstructed top quarks must be 175 GeV. Among the 12 possible jet combinations, the solution with the minimal 2
from the kinematic fit is chosen; MC studies show this yields the correct solution in about 60% of all cases. The cosdistribution obtained in the ‘ jets data after the full selection and compared to standard and V A model expectations is shown in Fig.1(a).
Dilepton events are rarer than ‘ jets events, but have the advantage that coscan be calculated for each lepton, thus providing two measurements per event. The presence of two neutrinos in the dilepton final state makes the system kinematically underconstrained. However, if a top quark mass is assumed, the kinematics can be solved algebraically with a four-fold ambiguity in addition to the two-fold ambiguity in pairing jets with leptons. For each lepton, we calculate the value of cosresulting from each solution with each of the two leading jets associated with the lepton. To account for detector resolution we perform a Monte Carlo integration over the space of parton kinematics consistent with the measured quantities by repeating the above procedure 100 times, fluctuating the
jet and lepton energies within their resolutions for each iteration. The primary benefit of this integration is that 20% of tt events do not have a kinematic solution when the measured quantities are used, but almost all of these do have a solution in the allowed parton kinematic space and therefore can be retained for the analysis. The average of these values is taken as the cosfor that lepton. The cos distribution obtained in dilepton data is shown in Fig.1(b).
We compute the binned Poisson likelihood Lf for the
data to be consistent with the sum of signal and back-ground templates at each of seven chosen fvalues. The
background normalization is constrained to be consistent within errors with the expected value by a Gaussian term in the likelihood. A parabola is fit to the lnLf points to
determine the likelihood as a function of f.
Systematic uncertainties are evaluated in ensemble tests by varying the parameters (see Table II) which can affect the shapes of the cos distributions or the relative con-tribution from signal and background sources. Ensembles are formed by drawing events from a model with the parameter under study varied. These are compared to the standard costemplates in a maximum likelihood fit. The average shift in the resulting f value is taken as the
systematic uncertainty and is shown in Table II. The total systematic uncertainty is then taken into account in the likelihood by convoluting the latter with a Gaussian with a width that corresponds to the total systematic uncertainty. The dominant uncertainties arise from the uncertainties on the top quark mass and on the jet energy scale (JES). The mass of the top quark is varied by 2:3 GeV and the JES by 1 around their nominal values.
The statistical uncertainty on the cos templates is taken as a systematic uncertainty estimated by fluctuating the templates according to their statistical uncertainty, and noting the RMS of the resulting distribution when fitting to the data.
The effect of gluon radiation in the modeling of tt events is studied with an alternate MC sample that includes tt events generated with an additional hard parton by ALPGEN. These events are mixed with the standard tt events
TABLE II. Systematic uncertainties on f for the two chan-nels and for their combination.
Source ‘ jets Dilepton Combined
Jet energy scale 0.038 0.039 0.038
Top quark mass 0.019 0.028 0.021
Template statistics 0.037 0.024 0.028
ttmodel 0.006 0.018 0.009
Background model 0.007 0.007 0.005
Heavy flavor fraction 0.018 0.015
Calibration 0.018 0.010 0.016 Total 0.063 0.059 0.057 * θ cos -1 -0.5 0 0.5 1 Events 0 5 10 15 20 25 -1 DØ data, 370 pb l+jets (V-A) → t t l+jets (V+A) → t t background (a) * θ cos -1 -0.5 0 0.5 1 Events 0 2 4 6 8 10 12 14 -1 DØ data, 370 pb ll (V-A) → t t ll (V+A) → t t background (b)
FIG. 1. cos distribution observed in (a) ‘ jets and (b) dilepton events. The standard model prediction is shown as the solid line, while a model with a pure V A interaction would result in the distribution given by the dashed line.
according to the ratio of the leading order cross sections for these two processes. Effects of the chosen factorization scale Q in the generation of the W jets events are eval-uated using a sample generated with a different choice of
Q. The systematic uncertainty on the jet flavor composition in the W jets background is derived using alternate MC samples in which the fraction of b and c jets are varied by 20% about the nominal value [20]. The difference found between the input fvalue and the reconstructed fvalue
in ensemble tests is taken as the systematic uncertainty on the calibration of the analysis.
The systematic uncertainties are conservatively assumed to be fully correlated except for those due to template statistics and the calibration of the individual analyses, which are completely uncorrelated, and the MC model systematic uncertainties, which are partially correlated. Assuming a fixed value of 0.70 for f0, we find
f 0:109 0:094stat 0:063syst (2)
using ‘ jets events, and
f 0:089 0:154stat 0:059syst (3)
using dilepton events. Combination of these results yields
f 0:056 0:080stat 0:057syst: (4)
We also calculate a Bayesian confidence interval (using a flat prior distribution which is nonzero only in the physi-cally allowed region of f 0:0–0:3) which yields
f< 0:23 at 95%C:L: (5)
Expressed as a measurement of fVA, the fractional V A component in the t ! Wb coupling, the combined result is equivalent to:
fVA 0:187 0:267stat 0:190syst (6)
or
fVA< 0:77 at 95%C:L: (7)
As seen in Fig.1(a), there is a deficit of ‘ jets data events in the central region of cos. We estimate the significance of this effect by performing a likelihood ratio test to evaluate the goodness-of-fit for the best-fit model and find that the probability of obtaining a worse fit is 1.3%. We also evaluate the goodness-of-fit for the standard model hypothesis and find a fit probability of 0.8% (statis-tical). Thus we conclude that the discrepancy is not a statistically significant indication of non-SM physics. We have studied the subset of our MC ensemble tests in which the mock data has a lower fit probability than the collider data does and find that our sensitivity to the value of fin
this subset is the same as in the entire set of ensembles. With a larger dataset, we plan to determine whether this discrepancy persists, and to repeat the analysis with both
fand f0 unconstrained.
In summary, we have measured the fraction of right-handed W bosons in tt decays in the ‘ jets and dilepton channels, and find f 0:056 0:080stat 0:057syst.
This is the most precise measurement of fto date and is
consistent with the standard model prediction of f
3:6 104[3].
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); 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.
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