Article
Reference
Measurement of the helicity of W bosons in top-quark decays
CDF Collaboration
CAMPANELLI, Mario (Collab.), et al.
Abstract
We measure the branching fraction of the top quark to longitudinally and right-handed polarized W bosons, F0 and F+, using approximately 200 pb−1 of pp collisions collected by the CDF II detector. We analyze two quantities sensitive to the W helicity: the invariant mass of the charged lepton and the bottom-quark jet in the decay t→Wb→ℓνb (where ℓ=e or μ), and the transverse momentum of the charged lepton. Constrained fits yield F0=0.74+0.22−0.34, and F+
CDF Collaboration, CAMPANELLI, Mario (Collab.), et al . Measurement of the helicity of W bosons in top-quark decays. Physical Review. D , 2006, vol. 73, no. 11, p. 111103
DOI : 10.1103/PhysRevD.73.111103
Available at:
http://archive-ouverte.unige.ch/unige:38350
Disclaimer: layout of this document may differ from the published version.
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Measurement of the helicity of W bosons in top-quark decays
A. Abulencia,23D. Acosta,17J. Adelman,13T. Affolder,10T. Akimoto,53M. G. Albrow,16D. Ambrose,16S. Amerio,42 D. Amidei,33A. Anastassov,50K. Anikeev,16A. Annovi,44J. Antos,1M. Aoki,53G. Apollinari,16J.-F. Arguin,32 T. Arisawa,55A. Artikov,14W. Ashmanskas,16A. Attal,8F. Azfar,41P. Azzi-Bacchetta,42P. Azzurri,44N. Bacchetta,42 H. Bachacou,28W. Badgett,16A. Barbaro-Galtieri,28V. E. Barnes,46B. A. Barnett,24S. Baroiant,7V. Bartsch,30G. Bauer,31
F. Bedeschi,44S. Behari,24S. Belforte,52G. Bellettini,44J. Bellinger,57A. Belloni,31E. Ben-Haim,16D. Benjamin,15 A. Beretvas,16J. Beringer,28T. Berry,29A. Bhatti,48M. Binkley,16D. Bisello,42M. Bishai,16R. E. Blair,2C. Blocker,6
K. Bloom,33B. Blumenfeld,24A. Bocci,48A. Bodek,47V. Boisvert,47G. Bolla,46A. Bolshov,31D. Bortoletto,46 J. Boudreau,45S. Bourov,16A. Boveia,10B. Brau,10C. Bromberg,34E. Brubaker,13J. Budagov,14H. S. Budd,47S. Budd,23
K. Burkett,16G. Busetto,42P. Bussey,20K. L. Byrum,2S. Cabrera,15M. Campanelli,19M. Campbell,33F. Canelli,8 A. Canepa,46D. Carlsmith,57R. Carosi,44S. Carron,15M. Casarsa,52A. Castro,5P. Catastini,44D. Cauz,52 M. Cavalli-Sforza,3A. Cerri,28L. Cerrito,41S. H. Chang,27J. Chapman,33Y. C. Chen,1M. Chertok,7G. Chiarelli,44 G. Chlachidze,14F. Chlebana,16I. Cho,27K. Cho,27D. Chokheli,14J. P. Chou,21P. H. Chu,23S. H. Chuang,57K. Chung,12
W. H. Chung,57Y. S. Chung,47M. Ciljak,44C. I. Ciobanu,23M. A. Ciocci,44A. Clark,19D. Clark,6M. Coca,15 A. Connolly,28M. E. Convery,48J. Conway,7B. Cooper,30K. Copic,33M. Cordelli,18G. Cortiana,42A. Cruz,17J. Cuevas,11 R. Culbertson,16D. Cyr,57S. DaRonco,42S. D’Auria,20M. D’onofrio,19D. Dagenhart,6P. de Barbaro,47S. De Cecco,49 A. Deisher,28G. De Lentdecker,47M. Dell’Orso,44S. Demers,47L. Demortier,48J. Deng,15M. Deninno,5D. De Pedis,49
P. F. Derwent,16C. Dionisi,49J. R. Dittmann,4P. DiTuro,50C. Do¨rr,25A. Dominguez,28S. Donati,44M. Donega,19 P. Dong,8J. Donini,42T. Dorigo,42S. Dube,50K. Ebina,55J. Efron,38J. Ehlers,19R. Erbacher,7D. Errede,23S. Errede,23
R. Eusebi,47H. C. Fang,28S. Farrington,29I. Fedorko,44W. T. Fedorko,13R. G. Feild,58M. Feindt,25J. P. Fernandez,46 R. Field,17G. Flanagan,34L. R. Flores-Castillo,45A. Foland,21S. Forrester,7G. W. Foster,16M. Franklin,21 J. C. Freeman,28Y. Fujii,26I. Furic,13A. Gajjar,29M. Gallinaro,48J. Galyardt,12J. E. Garcia,44M. Garcia Sciveres,28
A. F. Garfinkel,46C. Gay,58H. Gerberich,23E. Gerchtein,12D. Gerdes,33S. Giagu,49P. Giannetti,44A. Gibson,28 K. Gibson,12C. Ginsburg,16K. Giolo,46M. Giordani,52M. Giunta,44G. Giurgiu,12V. Glagolev,14D. Glenzinski,16 M. Gold,36N. Goldschmidt,33J. Goldstein,41G. Gomez,11G. Gomez-Ceballos,11M. Goncharov,51O. Gonza´lez,46 I. Gorelov,36A. T. Goshaw,15Y. Gotra,45K. Goulianos,48A. Gresele,42M. Griffiths,29S. Grinstein,21C. Grosso-Pilcher,13
U. Grundler,23J. Guimaraes da Costa,21C. Haber,28S. R. Hahn,16K. Hahn,43E. Halkiadakis,47A. Hamilton,32 B.-Y. Han,47R. Handler,57F. Happacher,18K. Hara,53M. Hare,54S. Harper,41R. F. Harr,56R. M. Harris,16 K. Hatakeyama,48J. Hauser,8C. Hays,15H. Hayward,29A. Heijboer,43B. Heinemann,29J. Heinrich,43M. Hennecke,25 M. Herndon,57J. Heuser,25D. Hidas,15C. S. Hill,10D. Hirschbuehl,25A. Hocker,16A. Holloway,21S. Hou,1M. Houlden,29 S.-C. Hsu,9B. T. Huffman,41R. E. Hughes,38J. Huston,34K. Ikado,55J. Incandela,10G. Introzzi,44M. Iori,49Y. Ishizawa,53 A. Ivanov,7B. Iyutin,31E. James,16D. Jang,50B. Jayatilaka,33D. Jeans,49H. Jensen,16E. J. Jeon,27M. Jones,46K. K. Joo,27 S. Y. Jun,12T. R. Junk,23T. Kamon,51J. Kang,33M. Karagoz-Unel,37P. E. Karchin,56Y. Kato,40Y. Kemp,25R. Kephart,16
U. Kerzel,25V. Khotilovich,51B. Kilminster,38D. H. Kim,27H. S. Kim,27J. E. Kim,27M. J. Kim,12M. S. Kim,27 S. B. Kim,27S. H. Kim,53Y. K. Kim,13M. Kirby,15L. Kirsch,6S. Klimenko,17M. Klute,31B. Knuteson,31B. R. Ko,15
H. Kobayashi,53K. Kondo,55D. J. Kong,27J. Konigsberg,17K. Kordas,18A. Korytov,17A. V. Kotwal,15A. Kovalev,43 J. Kraus,23I. Kravchenko,31M. Kreps,25A. Kreymer,16J. Kroll,43N. Krumnack,4M. Kruse,15V. Krutelyov,51 S. E. Kuhlmann,2Y. Kusakabe,55S. Kwang,13A. T. Laasanen,46S. Lai,32S. Lami,44S. Lammel,16M. Lancaster,30
R. L. Lander,7K. Lannon,38A. Lath,50G. Latino,44I. Lazzizzera,42C. Lecci,25T. LeCompte,2J. Lee,47J. Lee,27 S. W. Lee,51R. Lefe`vre,3N. Leonardo,31S. Leone,44S. Levy,13J. D. Lewis,16K. Li,58C. Lin,58C. S. Lin,16M. Lindgren,16
E. Lipeles,9T. M. Liss,23A. Lister,19D. O. Litvintsev,16T. Liu,16Y. Liu,19N. S. Lockyer,43A. Loginov,35M. Loreti,42 P. Loverre,49R.-S. Lu,1D. Lucchesi,42P. Lujan,28P. Lukens,16G. Lungu,17L. Lyons,41J. Lys,28R. Lysak,1E. Lytken,46 P. Mack,25D. MacQueen,32R. Madrak,16K. Maeshima,16P. Maksimovic,24G. Manca,29F. Margaroli,5R. Marginean,16 C. Marino,23A. Martin,58M. Martin,24V. Martin,37M. Martı´nez,3T. Maruyama,53H. Matsunaga,53M. E. Mattson,56 R. Mazini,32P. Mazzanti,5K. S. McFarland,47D. McGivern,30P. McIntyre,51P. McNamara,50R. McNulty,29A. Mehta,29 S. Menzemer,31A. Menzione,44P. Merkel,46C. Mesropian,48A. Messina,49M. von der Mey,8T. Miao,16N. Miladinovic,6
J. Miles,31R. Miller,34J. S. Miller,33C. Mills,10M. Milnik,25R. Miquel,28S. Miscetti,18G. Mitselmakher,17 A. Miyamoto,26N. Moggi,5B. Mohr,8R. Moore,16M. Morello,44P. Movilla Fernandez,28J. Mu¨lmensta¨dt,28 A. Mukherjee,16M. Mulhearn,31Th. Muller,25R. Mumford,24P. Murat,16J. Nachtman,16S. Nahn,58I. Nakano,39
A. Napier,54D. Naumov,36V. Necula,17C. Neu,43M. S. Neubauer,9J. Nielsen,28T. Nigmanov,45L. Nodulman,2
O. Norniella,3T. Ogawa,55S. H. Oh,15Y. D. Oh,27T. Okusawa,40R. Oldeman,29R. Orava,22K. Osterberg,22 C. Pagliarone,44E. Palencia,11R. Paoletti,44V. Papadimitriou,16A. Papikonomou,25A. A. Paramonov,13B. Parks,38
S. Pashapour,32J. Patrick,16G. Pauletta,52M. Paulini,12C. Paus,31D. E. Pellett,7A. Penzo,52T. J. Phillips,15 G. Piacentino,44J. Piedra,11K. Pitts,23C. Plager,8L. Pondrom,57G. Pope,45X. Portell,3O. Poukhov,14N. Pounder,41
F. Prakoshyn,14A. Pronko,16J. Proudfoot,2F. Ptohos,18G. Punzi,44J. Pursley,24J. Rademacker,41A. Rahaman,45 A. Rakitin,31S. Rappoccio,21F. Ratnikov,50B. Reisert,16V. Rekovic,36N. van Remortel,22P. Renton,41M. Rescigno,49 S. Richter,25F. Rimondi,5K. Rinnert,25L. Ristori,44W. J. Robertson,15A. Robson,20T. Rodrigo,11E. Rogers,23S. Rolli,54 R. Roser,16M. Rossi,52R. Rossin,17C. Rott,46A. Ruiz,11J. Russ,12V. Rusu,13D. Ryan,54H. Saarikko,22S. Sabik,32 A. Safonov,7W. K. Sakumoto,47G. Salamanna,49O. Salto,3D. Saltzberg,8C. Sanchez,3L. Santi,52S. Sarkar,49K. Sato,53
P. Savard,32A. Savoy-Navarro,16T. Scheidle,25P. Schlabach,16E. E. Schmidt,16M. P. Schmidt,58M. Schmitt,37 T. Schwarz,33L. Scodellaro,11A. L. Scott,10A. Scribano,44F. Scuri,44A. Sedov,46S. Seidel,36Y. Seiya,40A. Semenov,14
F. Semeria,5L. Sexton-Kennedy,16I. Sfiligoi,18M. D. Shapiro,28T. Shears,29P. F. Shepard,45D. Sherman,21 M. Shimojima,53M. Shochet,13Y. Shon,57I. Shreyber,35A. Sidoti,44A. Sill,16P. Sinervo,32A. Sisakyan,14J. Sjolin,41 A. Skiba,25A. J. Slaughter,16K. Sliwa,54D. Smirnov,36J. R. Smith,7F. D. Snider,16R. Snihur,32M. Soderberg,33A. Soha,7
S. Somalwar,50V. Sorin,34J. Spalding,16F. Spinella,44P. Squillacioti,44M. Stanitzki,58A. Staveris-Polykalas,44 R. St. Denis,20B. Stelzer,8O. Stelzer-Chilton,32D. Stentz,37J. Strologas,36D. Stuart,10J. S. Suh,27A. Sukhanov,17
K. Sumorok,31H. Sun,54T. Suzuki,53A. Taffard,23R. Tafirout,32R. Takashima,39Y. Takeuchi,53K. Takikawa,53 M. Tanaka,2R. Tanaka,39M. Tecchio,33P. K. Teng,1K. Terashi,48S. Tether,31J. Thom,16A. S. Thompson,20 E. Thomson,43P. Tipton,47V. Tiwari,12S. Tkaczyk,16D. Toback,51K. Tollefson,34T. Tomura,53D. Tonelli,44 M. To¨nnesmann,34S. Torre,44D. Torretta,16S. Tourneur,16W. Trischuk,32R. Tsuchiya,55S. Tsuno,39N. Turini,44 F. Ukegawa,53T. Unverhau,20S. Uozumi,53D. Usynin,43L. Vacavant,28A. Vaiciulis,47S. Vallecorsa,19A. Varganov,33
E. Vataga,36G. Velev,16G. Veramendi,23V. Veszpremi,46T. Vickey,23R. Vidal,16I. Vila,11R. Vilar,11I. Vollrath,32 I. Volobouev,28F. Wu¨rthwein,9P. Wagner,51R. G. Wagner,2R. L. Wagner,16W. Wagner,25R. Wallny,8T. Walter,25 Z. Wan,50M. J. Wang,1S. M. Wang,17A. Warburton,32B. Ward,20S. Waschke,20D. Waters,30T. Watts,50M. Weber,28
W. C. Wester III,16B. Whitehouse,54D. Whiteson,43A. B. Wicklund,2E. Wicklund,16H. H. Williams,43P. Wilson,16 B. L. Winer,38P. Wittich,43S. Wolbers,16C. Wolfe,13S. Worm,50T. Wright,33X. Wu,19S. M. Wynne,29A. Yagil,16 K. Yamamoto,40J. Yamaoka,50Y. Yamashita.,39C. Yang,58U. K. Yang,13W. M. Yao,28G. P. Yeh,16J. Yoh,16K. Yorita,13
T. Yoshida,40I. Yu,27S. S. Yu,43J. C. Yun,16L. Zanello,49A. Zanetti,52I. Zaw,21F. Zetti,44X. Zhang,23 J. Zhou,50and S. Zucchelli5
(CDF Collaboration)
1Institute of Physics, Academia Sinica, Taipei, Taiwan 11529, Republic of China
2Argonne National Laboratory, Argonne, Illinois 60439, USA
3Institut de Fisica d’Altes Energies, Universitat Autonoma de Barcelona, E-08193, Bellaterra (Barcelona), Spain
4Baylor University, Waco, Texas 76798, USA
5Istituto Nazionale di Fisica Nucleare, University of Bologna, I-40127 Bologna, Italy
6Brandeis University, Waltham, Massachusetts 02254, USA
7University of California, Davis, Davis, California 95616, USA
8University of California, Los Angeles, Los Angeles, California 90024, USA
9University of California, San Diego, La Jolla, California 92093, USA
10University of California, Santa Barbara, Santa Barbara, California 93106, USA
11Instituto de Fisica de Cantabria, CSIC-University of Cantabria, 39005 Santander, Spain
12Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA
13Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637, USA
14Joint Institute for Nuclear Research, RU-141980 Dubna, Russia
15Duke University, Durham, North Carolina 27708, USA
16Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA
17University of Florida, Gainesville, Florida 32611, USA
18Laboratori Nazionali di Frascati, Istituto Nazionale di Fisica Nucleare, I-00044 Frascati, Italy
19University of Geneva, CH-1211 Geneva 4, Switzerland
20Glasgow University, Glasgow G12 8QQ, United Kingdom
21Harvard University, Cambridge, Massachusetts 02138, USA
22Division of High Energy Physics, Department of Physics, University of Helsinki and Helsinki Institute of Physics, FIN-00014, Helsinki, Finland
A. ABULENCIAet al. PHYSICAL REVIEW D73,111103(R) (2006)
111103-2
23University of Illinois, Urbana, Illinois 61801, USA
24The Johns Hopkins University, Baltimore, Maryland 21218, USA
25Institut fu¨r Experimentelle Kernphysik, Universita¨t Karlsruhe, 76128 Karlsruhe, Germany
26High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305, Japan
27Center for High Energy Physics: Kyungpook National University, Taegu 702-701, Korea;
Seoul National University, Seoul 151-742, Korea;
and SungKyunKwan University, Suwon 440-746, Korea
28Ernest Orlando Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
29University of Liverpool, Liverpool L69 7ZE, United Kingdom
30University College London, London WC1E 6BT, United Kingdom
31Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
32Institute of Particle Physics: McGill University, Montre´al, Canada H3A 2T8 and University of Toronto, Toronto, Canada M5S 1A7
33University of Michigan, Ann Arbor, Michigan 48109, USA
34Michigan State University, East Lansing, Michigan 48824, USA
35Institution for Theoretical and Experimental Physics, ITEP, Moscow 117259, Russia
36University of New Mexico, Albuquerque, New Mexico 87131, USA
37Northwestern University, Evanston, Illinois 60208, USA
38The Ohio State University, Columbus, Ohio 43210, USA
39Okayama University, Okayama 700-8530, Japan
40Osaka City University, Osaka 588, Japan
41University of Oxford, Oxford OX1 3RH, United Kingdom
42University of Padova, Istituto Nazionale di Fisica Nucleare, Sezione di Padova-Trento, I-35131 Padova, Italy
43University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
44Istituto Nazionale di Fisica Nucleare Pisa, Universities of Pisa, Siena, Italy and Scuola Normale Superiore, I-56127 Pisa, Italy
45University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA
46Purdue University, West Lafayette, Indiana 47907, USA
47University of Rochester, Rochester, New York 14627, USA
48The Rockefeller University, New York, New York 10021, USA
49Istituto Nazionale di Fisica Nucleare, Sezione di Roma 1, University of Rome ‘‘La Sapienza,’’ I-00185 Roma, Italy
50Rutgers University, Piscataway, New Jersey 08855, USA
51Texas A&M University, College Station, Texas 77843, USA
52Istituto Nazionale di Fisica Nucleare, University of Trieste/Udine, Italy
53University of Tsukuba, Tsukuba, Ibaraki 305, Japan
54Tufts University, Medford, Massachusetts 02155, USA
55Waseda University, Tokyo 169, Japan
56Wayne State University, Detroit, Michigan 48201, USA
57University of Wisconsin, Madison, Wisconsin 53706, USA
58Yale University, New Haven, Connecticut 06520, USA (Received 8 November 2005; published 7 June 2006)
We measure the branching fraction of the top quark to longitudinally and right-handed polarizedW bosons,F0andF, using approximately200 pb1ofpp collisions collected by the CDF II detector. We analyze two quantities sensitive to theWhelicity: the invariant mass of the charged lepton and the bottom- quark jet in the decayt!Wb!‘b(where‘eor), and the transverse momentum of the charged lepton. Constrained fits yield F00:740:220:34, and F<0:27 at the 95% confidence level. These measurements are consistent with the standard model predictions.
DOI:10.1103/PhysRevD.73.111103 PACS numbers: 14.65.Ha, 12.15.Ji, 13.88.+e
The top quark is the most massive known elementary fermion, with mt175 GeV=c2 [1,2]. At the Fermilab Tevatron proton-antiproton collider, with a center-of- mass energy of
ps
1:96 TeV, most top quarks are pair-produced via the strong interaction [3,4]. However, the decayt!Wbproceeds entirely via the weak interac- tion. Given theVAstructure of the weak interaction in the standard model (SM), in the limit of a massless bottom quark the top quark can decay to either a left-handed or
longitudinally polarizedWboson [5] and a bottom quark.
The fraction F0 of longitudinally polarized W bosons is enhanced due to the large coupling of the top quark to the Higgs field responsible for electroweak symmetry break- ing. The leading-order SM prediction is [6]
F0 t!W0b
t!W0b t!Wb m2t
2M2Wm2t; (1)
whereW0andWindicate longitudinally and transversely polarizedW’s, respectively, andMW 80:4 GeV=c2is the W boson mass [7]. Formt175 GeV=c2,F0 0:70. A deviation from this prediction could indicate non-SM physics such as large CP-violation in top-quark decays [8], as could a value for the right-handed fraction F
different from its SM value of104 [9].
We use two observables inttcandidate events to mea- sure the W helicity. The first is the decay angle of the charged lepton in the W decay frame, measured with respect to the top-quark direction, and the second is the transverse momentumpT of the charged lepton. Leptons from longitudinally polarized W boson decays have a symmetric angular distribution / 1cos2 , while left-handed W decays have an asymmetric distribution/ 1cos 2. We can approximatecos by relating it to the invariant mass of the system composed of thebquark and the charged leptonMlb:
cos p‘pbE‘Eb
jp‘jjpbj ’ 2M2lb
m2tM2W1; (2)
a variable that depends only on lab-frame momenta. The second observable, the charged leptonpT, exploits the fact that charged leptons from left-handedW decays are pref- erentially emitted in the backward direction with respect to theWdirection of motion, leading to a softerpT in the lab frame, while the leptons from right-handedW’s are pref- erentially emitted forward and thus have a harder pT spectrum. LongitudinalW decays represent an intermedi- ate case. Figure 1 shows the predictedcos and leptonpT distributions formt175 GeV=c2, after the event selec- tion and reconstruction described below.
A measurement ofF0 has been previously reported by the CDF Collaboration [10] using 100 pb1 of data from the 1992 –1996 Tevatron collider run (Run I). Using thepTtechnique, a value of0:910:37stat 0:13syst was obtained. Using the same data set, CDF has also placed a limit on the right-handed helicity fraction ofF<0:18 at the 95% confidence level (C.L.) with the cos tech- nique [11]. The D0 Collaboration has used 125 pb1 of Run I data to obtain F0 0:560:31[12], and has re- cently reportedF 0:000:13stat 0:07syst[13].
Here we report measurements ofF0 andFthat combine thecos andpT techniques.
The CDF II detector [14] consists of a charged-particle tracking system in a magnetic field of 1.4 T, segmented electromagnetic and hadronic calorimeters, and muon de- tectors. A silicon microstrip detector provides tracking over the radial range 1.5– 28 cm and is used to detect displaced secondary vertices. The fiducial region of the silicon detector covers the pseudorapidity range jj<2, while the central tracking system and muon chambers provide coverage forjj<1[15]. For electron identifica- tion we use the calorimeter regionjj<1, while for jet identification we usejj<2:5. A three-level trigger sys-
tem selects events with electron (muon) candidates with ETpT>18 GeV (18 GeV=c), which form the data set for this analysis.
In the decay process tt!WbWb, events can be classified based on the observed number of isolated charged leptons with large transverse momentum, where a lepton signifies an electron or muon of either charge;
typically these leptons come from the decay W !‘.
Transverse momentum for electrons from W decay is best measured at CDF using the transverse energy ET deposited in the calorimeter, while for muons the trans- verse momentumpT is measured by the tracking system.
We will use the symbol pT to denote the appropriate calorimeter- or tracking-based quantity. The 193 pb1
‘‘dilepton’’ sample [16] consists of events with two oppo- sitely charged lepton candidates, each with pT>
20 GeV=c. Events in this sample are required to have missing transverse energy 6ET>25 GeV, and two or more jets with pseudorapidity jj<2:5 and transverse energy ET >15 GeV. The scalar sum of the transverse energy of the jets, leptons, and6ET, is required to be greater than 200 GeV. We observe 13 events in this sample, with a predicted total background from WW pairs, Z!, the Drell-Yan process, and hadrons misidentified as leptons (‘‘fakes’’) of2:70:7events. The162 pb1 ‘‘lepton plus jets’’ sample [17] consists of events with a single isolated lepton candidate withpT>20 GeV=c,6ET>20 GeV, and three or more jets with jj<2 and ET>15 GeV. To reduce backgrounds, we require that one or more jets have a displaced secondary-vertex tag, indicating that it
θ * cos
-1 -0.5 0 0.5 1 1.5 2
Arbitrary Units
Longitudinal Left-handed Right-handed
[GeV/c]
Lepton p
T0 20 40 60 80 100 120 140 160 180 200
Arbitrary Units
Longitudinal Left-handed Right-handed
FIG. 1. Distributions of reconstructed cos (upper plot) and lepton pT (lower) for top-quark decays to left-handed, right- handed, and longitudinally polarizedWbosons.
A. ABULENCIAet al. PHYSICAL REVIEW D73,111103(R) (2006)
111103-4
is consistent with the decay of a long-livedbhadron. Fifty- seven events pass the selection cuts, of which approxi- mately 2=3 are tt events. The largest remaining back- grounds come fromW plus jets events containing bottom or charm jets, QCD multijet events, andWplus light-quark events misidentified asb’s.
ThepTanalysis [18] uses both samples, while thecos analysis [19] uses the lepton plus jets sample only. In addition to the selection requirements described above, events selected for the cos analysis are required to have a fourth jet with ET>8 GeV and jj<2. Thirty- seven events pass this cut. The presence of four jets allows the event to be kinematically reconstructed as attevent [1]
with the top mass constrained to 175 GeV=c2, and to associate the appropriate jet to the lepton in Eq. (2). We find that 31 of the 37 events pass a cut on the fit quality, with an estimated background of6:90:9events.
To create reconstructed cos templates for tt signal events, we use theMADEVENT[20] Monte Carlo program.
Hadronization and fragmentation are carried out using
PYTHIA[21]. Events for thepTanalysis are generated using
HERWIG[22]. In both cases, we fix the helicity in the top rest frame of one W boson, while the other W takes on values according to the SM prediction. The events are then passed through the CDF simulation and reconstruction algorithms. The lepton from the W whose helicity was fixed is used to create the templates; we find that the helicity of the otherW has a negligible effect on the PT distribution of this lepton. For the lepton plus jets sample, all backgrounds except QCD are modeled with Monte Carlo simulations. We model the QCD background using lepton plus jets events where the primary lepton is nonisolated. For the dilepton sample all but the fake back- ground is modeled with Monte Carlo. We model the latter background using lepton plus jet events containing jets that could be misidentified as a charged lepton.
The data are fit separately to thecos andpTtemplates using likelihood functions that include a Gaussian con- straint on the background, as well as corrections for trigger and event selection cuts that have helicity-dependent biases, such as those on the leptonpT. Because the statis- tical power of the sample is insufficient to fitF andF0 simultaneously, we constrain F to zero when fitting for F0; when fitting forFwe constrainF0to 0.70. (If thetWb interaction vertex is expanded to include aVAterm,F0 is unaffected [6].) We requirePFi1, resulting in a one- parameter fit. The results of the fits to the various subsam- ples are shown in Table I. The reconstructedcos distri- bution from the data and the best-fit templates are shown in Fig. 2. The observedcos distribution extends somewhat beyond the physical range 1cos 1 because the world-average top andWmasses are used in Eq. (2), rather than the true event-by-event reconstructed masses, whose much larger uncertainties would unnecessarily smear the cos distribution obtained from theMlbapproximation. In
the dilepton sample, the best-fit value of F0 falls at 0:540:350:25, outside the physical range. In this case, the observed distribution of leptonpT is softer than any com- ponent of signal or background in our model. A measured central value of0:54or less is expected 0.5% of the time for a trueF0 of 0.7; however the dilepton result is consis- tent with the lepton plus jets result at the2level when the uncertainties on both measurements are properly taken into account. Moreover a previous analysis of the kinematics of these dilepton data [23] has found them to be consistent with the SM at the 1.0%– 4.5% level, and measurements of thettcross section [16] and top mass [24] in our dilepton samples are also consistent with the SM. We therefore carry out oura prioridecision to perform a combined pT fit to the two samples. The lepton pT distribution for the two samples and the results of the fit are shown in Fig. 3.
TABLE I. Summary of results for thecos ,pT, and combined measurements of F0 and F. N is the number of events or leptons used in the measurement. Where two uncertainties are given the first is statistical and the second is systematic.
Uncertainties on the combined measurements are the total sta- tistical and systematic uncertainty. In obtaining the limits, the likelihood function is integrated over the physical region [0,1]
only.
Analysis N F0 F
cos 31 0:990:290:350:19 0:230:160:08 pT (dilepton) 26 0:540:350:250:16 0:470:100:09 pT (lepjets) 57 0:950:350:420:17 0:110:210:190:10 pT (combined) 83 0:310:370:230:17 0:180:140:120:12 Combined . . . 0:740:220:34 0:000:200:19 95% C.L. limit . . . <0:95,>0:18 <0:27
θ* cos
-1 -0.5 0 0.5 1 1.5 2
Events
0 1 2 3 4 5 6
Data Best Fit Longitudinal
×) Left-handed (25 Background
FIG. 2. The reconstructedcos distribution for the lepton plus jets sample, overlaid with signal and background templates according to their best-fit values. The left-handed template has been scaled up by a factor of 25.
The dominant systematic uncertainties in thecos and pTanalyses arise from uncertainties in the top-quark mass, the background shape and normalization, the effects of initial- and final-state radiation (ISR/FSR), and the parton distribution functions (PDFs). We determine these uncer- tainties by performing Monte Carlo experiments in which the systematic parameter in question is varied by1and the resulting simulated data are fit to the default templates.
We compare the meanF0orFreturned by the likelihood fit with the default (unfluctuated) value. The results are summarized in Table II. The sum in quadrature of all sources of systematic uncertainty leads to a final result of F0 0:990:290:35stat: 0:19syst: for the cos analysis andF00:310:370:23stat: 0:17syst:for thepTanalysis.
We combine the results of the cos and pT analyses taking into account both the statistical and systematic correlations between the two techniques. Statistical corre- lations arise because the two analyses share the subset of the lepton plus jets sample that passes the fit quality cut on the top mass reconstruction. Common sources of system- atic uncertainty include the top mass uncertainty and back- ground normalizations. The correlation coefficients of 0:6 are determined via Monte Carlo experiments. The
combined result is F0 0:740:220:34 (stat:syst:). In addi- tion, we findF 0:000:200:19(stat:syst:) andF<0:27 at the 95% C.L. These results are consistent with the SM predictions ofF00:70,F0.
We thank Tim Stelzer and Fabio Maltoni for their help with the MADEVENT calculations. We thank the Fermilab staff and the technical staffs of the participating institutions for their vital contributions. This work was supported by the U.S. Department of Energy and National Science Foundation; the Italian Istituto Nazionale di Fisica Nucleare; the Ministry of Education, Culture, Sports, Science and Technology of Japan; the Natural Sciences and Engineering Research Council of Canada; the National Science Council of the Republic of China; the Swiss National Science Foundation; the A. P. Sloan Foundation;
the Bundesministerium fu¨r Bildung und Forschung, Germany; the Korean Science and Engineering Foundation and the Korean Research Foundation; the Particle Physics and Astronomy Research Council and the Royal Society, UK; the Russian Foundation for Basic Research; the Comisio´n Interministerial de Ciencia y Tecnologı´a, Spain; in part by the European Community’s Human Potential Programme under Contract No. HPRN- CT-2002-00292; and the Academy of Finland.
[1] T. Affolderet al.(CDF Collaboration), Phys. Rev. D63, 032003 (2001).
[2] V. Abazovet al.(D0 Collaboration), Nature (London)429, 638 (2004).
[3] M. Cacciari et al., J. High Energy Phys. 04 (2004) 068.
[4] N. Kidonakis and R. Vogt, Phys. Rev. D 68, 114014 (2003).
[5] Charge-conjugation symmetry implies that the t quark decays to either a longitudinally- or right-handed- polarized W. Throughout the remainder of this Letter, a ‘‘left-handedW’’ refers to either a left-handedWor a right-handedW.
[6] G. L. Kane, G. A. Ladinsky, and C.-P. Yuan, Phys. Rev. D 45, 124 (1992).
TABLE II. Summary of systematic uncertainties for the mea- surements ofF0 andF.
Systematic Source pTMethod cos Method
F0 F F0 F
Top Mass 0.11 0.09 0.08 0.04
Bkg. Modeling 0.10 0.06 0.13 0.05
ISR/FSR 0.04 0.03 0.03 0.02
PDF 0.03 0.03 0.04 0.01
MC Statistics 0.01 <0:01 0.01 0.01
Acceptance Correction 0.02 0.01 <0:005 <0:005 Trigger Correction 0.02 0.02 . . . . Jet Energy Scale . . . 0.09 0.04
MC Modeling . . . 0.04 0.02
b-tagging . . . 0.01 <0:005
Total 0.17 0.12 0.19 0.08
[GeV/c]
Lepton pT
0 20 40 60 80 100 120 140 160 180 200
Entries/20 GeV/c
0 5 10 15 20 25 30 35 40 45 50
Data Best Fit Longitudinal Left-handed Background
FIG. 3. Distribution of lepton pT for the lepton plus jets and dilepton samples, overlaid with the total signal and background templates according to their best-fit values.
A. ABULENCIAet al. PHYSICAL REVIEW D73,111103(R) (2006)
111103-6
[7] S. Eidelman et al. (Particle Data Group), Phys. Lett. B 592, 1 (2004).
[8] J. Cao, R. J. Oakes, F. Wang, and J. M. Yang, Phys. Rev. D 68, 054019 (2003); E. Malkawi and C.-P. Yuan, Phys. Rev.
D50, R4462 (1994).
[9] M. Fischeret al., Phys. Rev. D63, 031501(R) (2001).
[10] T. Affolderet al.(CDF Collaboration), Phys. Rev. Lett.84, 216 (2000).
[11] D. Acosta et al. (CDF Collaboration), Phys. Rev. D 71, 031101 (2005).
[12] V. M. Abazovet al.(D0 Collaboration), Phys. Lett. B617, 1 (2005).
[13] V. M. Abazovet al.(D0 Collaboration), Phys. Rev. D72, 011104 (2005).
[14] D. Acostaet al.(CDF II Collaboration), Phys. Rev. D71, 032001 (2005).
[15] In the CDF geometry,is the polar angle with respect to the proton beam axis, andis the azimuthal angle. The pseudorapidity is lntan=2. The transverse mo- mentumpTis the component of the momentum projected onto the plane perpendicular to the beam axis. The trans- verse energy ET of a shower or calorimeter tower is
Esin, where E is the energy deposited. 6ET P
iETini, where ni is the unit vector in the azimuthal plane that points from the beam line to theith calorimeter tower.
[16] D. Acostaet al.(CDF II Collaboration), Phys. Rev. Lett.
93, 142001 (2004).
[17] D. Acostaet al.(CDF II Collaboration), Phys. Rev. D71, 052003 (2005).
[18] N. Goldschmidt, Ph.D. thesis, University of Michigan [FERMILAB Report No. 2005-57, 2005 (unpublished)].
[19] T. Vickey, Ph.D. thesis, University of Illinois [FERMILAB Report No. 2004-49, 2004 (unpublished)].
[20] F. Maltoni and T. Stelzer, J. High Energy Phys. 02 (2003) 27.
[21] T. Sjostrand et al., Comput. Phys. Commun. 135, 238 (2001).
[22] G. Corcellaet al., J. High Energy Phys. 01 (2001) 10.
[23] D. Acostaet al.(CDF II Collaboration), Phys. Rev. Lett.
95, 022001 (2005).
[24] A. Abulencia et al., FERMILAB Report No. 05-551-E, 2005 [Phys. Rev. Lett. (to be published)].
