Measurement of the ttbar production cross section using dilepton events in ppbar collisions

arXiv:1105.5384v2 [hep-ex] 24 Oct 2011

Measurement of the t¯

t

production cross section using dilepton events in p ¯

p

collisions

V.M. Abazov,35 _{B. Abbott,}73 _{B.S. Acharya,}29_{M. Adams,}49 _{T. Adams,}47 _{G.D. Alexeev,}35 _{G. Alkhazov,}39 A. Altona_,61 _{G. Alverson,}60 _{G.A. Alves,}2 _{L.S. Ancu,}34 _{M. Aoki,}48 _{M. Arov,}58 _{A. Askew,}47 _{B. ˚Asman,}41 O. Atramentov,65 _{C. Avila,}8 _{J. BackusMayes,}80 _{F. Badaud,}13 _{L. Bagby,}48 _{B. Baldin,}48 _{D.V. Bandurin,}47 S. Banerjee,29 _{E. Barberis,}60 _{P. Baringer,}56_{J. Barreto,}3 _{J.F. Bartlett,}48 _{U. Bassler,}18 _{V. Bazterra,}49 _{S. Beale,}6

A. Bean,56 _{M. Begalli,}3 _{M. Begel,}71 _{C. Belanger-Champagne,}41 _{L. Bellantoni,}48 _{S.B. Beri,}27 _{G. Bernardi,}17 R. Bernhard,22_{I. Bertram,}42 _{M. Besan¸con,}18_{R. Beuselinck,}43_{V.A. Bezzubov,}38 _{P.C. Bhat,}48 _{V. Bhatnagar,}27

G. Blazey,50S. Blessing,47K. Bloom,64 A. Boehnlein,48 D. Boline,70 E.E. Boos,37 G. Borissov,42 T. Bose,59 A. Brandt,76 _{O. Brandt,}23 _{R. Brock,}62_{G. Brooijmans,}68_{A. Bross,}48 _{D. Brown,}17 _{J. Brown,}17 _{X.B. Bu,}48

M. Buehler,79 _{V. Buescher,}24 _{V. Bunichev,}37 _{S. Burdin}b_,42 _{T.H. Burnett,}80 _{C.P. Buszello,}41 _{B. Calpas,}15 E. Camacho-P´erez,32 _{M.A. Carrasco-Lizarraga,}56_{B.C.K. Casey,}48_{H. Castilla-Valdez,}32_{S. Chakrabarti,}70 D. Chakraborty,50_{K.M. Chan,}54 _{A. Chandra,}78_{G. Chen,}56 _{S. Chevalier-Th´ery,}18_{D.K. Cho,}75 _{S.W. Cho,}31 S. Choi,31 _{B. Choudhary,}28 _{S. Cihangir,}48 _{D. Claes,}64 _{J. Clutter,}56 _{M. Cooke,}48 _{W.E. Cooper,}48_{M. Corcoran,}78

F. Couderc,18 _{M.-C. Cousinou,}15 _{A. Croc,}18 _{D. Cutts,}75 _{A. Das,}45 _{G. Davies,}43 _{K. De,}76 _{S.J. de Jong,}34 E. De La Cruz-Burelo,32 _{F. D´eliot,}18 _{M. Demarteau,}48 _{R. Demina,}69_{D. Denisov,}48 _{S.P. Denisov,}38 _{S. Desai,}48

C. Deterre,18 _{K. DeVaughan,}64 _{H.T. Diehl,}48 _{M. Diesburg,}48 _{A. Dominguez,}64 _{T. Dorland,}80 _{A. Dubey,}28 L.V. Dudko,37 _{D. Duggan,}65_{A. Duperrin,}15 _{S. Dutt,}27 _{A. Dyshkant,}50 _{M. Eads,}64 _{D. Edmunds,}62 _{J. Ellison,}46 V.D. Elvira,48_{Y. Enari,}17_{H. Evans,}52 _{A. Evdokimov,}71_{V.N. Evdokimov,}38 _{G. Facini,}60 _{T. Ferbel,}69_{F. Fiedler,}24

F. Filthaut,34 _{W. Fisher,}62 _{H.E. Fisk,}48 _{M. Fortner,}50 _{H. Fox,}42 _{S. Fuess,}48 _{A. Garcia-Bellido,}69_{V. Gavrilov,}36 P. Gay,13_{W. Geng,}15, 62 _{D. Gerbaudo,}66 _{C.E. Gerber,}49 _{Y. Gershtein,}65 _{G. Ginther,}48, 69 _{G. Golovanov,}35 A. Goussiou,80 _{P.D. Grannis,}70 _{S. Greder,}19 _{H. Greenlee,}48 _{Z.D. Greenwood,}58_{E.M. Gregores,}4_{G. Grenier,}20

Ph. Gris,13 _{J.-F. Grivaz,}16_{A. Grohsjean,}18_{S. Gr¨unendahl,}48 _{M.W. Gr¨unewald,}30_{T. Guillemin,}16 _{F. Guo,}70 G. Gutierrez,48 _{P. Gutierrez,}73 _{A. Haas}c_,68 _{S. Hagopian,}47 _{J. Haley,}60 _{L. Han,}7 _{K. Harder,}44_{A. Harel,}69 J.M. Hauptman,55 _{J. Hays,}43 _{T. Head,}44 _{T. Hebbeker,}21 _{D. Hedin,}50_{H. Hegab,}74 _{A.P. Heinson,}46 _{U. Heintz,}75 C. Hensel,23_{I. Heredia-De La Cruz,}32 _{K. Herner,}61_{G. Hesketh}d_,44_{M.D. Hildreth,}54 _{R. Hirosky,}79 _{T. Hoang,}47

J.D. Hobbs,70 _{B. Hoeneisen,}12 _{M. Hohlfeld,}24 _{Z. Hubacek,}10, 18 _{N. Huske,}17 _{V. Hynek,}10 _{I. Iashvili,}67 R. Illingworth,48_{A.S. Ito,}48 _{S. Jabeen,}75_{M. Jaffr´e,}16_{D. Jamin,}15 _{A. Jayasinghe,}73 _{R. Jesik,}43 _{K. Johns,}45

M. Johnson,48 _{D. Johnston,}64 _{A. Jonckheere,}48 _{P. Jonsson,}43 _{J. Joshi,}27 _{A.W. Jung,}48 _{A. Juste,}40 K. Kaadze,57 _{E. Kajfasz,}15 _{D. Karmanov,}37 _{P.A. Kasper,}48_{I. Katsanos,}64 _{R. Kehoe,}77 _{S. Kermiche,}15 N. Khalatyan,48 _{A. Khanov,}74 _{A. Kharchilava,}67 _{Y.N. Kharzheev,}35_{D. Khatidze,}75_{M.H. Kirby,}51_{J.M. Kohli,}27

A.V. Kozelov,38_{J. Kraus,}62 _{S. Kulikov,}38 _{A. Kumar,}67_{A. Kupco,}11 _{T. Kurˇca,}20_{V.A. Kuzmin,}37 _{J. Kvita,}9 S. Lammers,52 _{G. Landsberg,}75 _{P. Lebrun,}20 _{H.S. Lee,}31 _{S.W. Lee,}55 _{W.M. Lee,}48 _{J. Lellouch,}17 _{L. Li,}46

Q.Z. Li,48 _{S.M. Lietti,}5 _{J.K. Lim,}31 _{D. Lincoln,}48 _{J. Linnemann,}62 _{V.V. Lipaev,}38 _{R. Lipton,}48 _{Y. Liu,}7 Z. Liu,6 _{A. Lobodenko,}39_{M. Lokajicek,}11_{R. Lopes de Sa,}70 _{H.J. Lubatti,}80 _{R. Luna-Garcia}e_,32 _{A.L. Lyon,}48 A.K.A. Maciel,2 _{D. Mackin,}78 _{R. Madar,}18 _{R. Maga˜na-Villalba,}32 _{S. Malik,}64 _{V.L. Malyshev,}35 _{Y. Maravin,}57

J. Mart´ınez-Ortega,32R. McCarthy,70 C.L. McGivern,56 M.M. Meijer,34 A. Melnitchouk,63 D. Menezes,50 P.G. Mercadante,4 _{M. Merkin,}37 _{A. Meyer,}21 _{J. Meyer,}23 _{F. Miconi,}19 _{N.K. Mondal,}29 _{G.S. Muanza,}15 M. Mulhearn,79 _{E. Nagy,}15_{M. Naimuddin,}28 _{M. Narain,}75_{R. Nayyar,}28_{H.A. Neal,}61_{J.P. Negret,}8 _{P. Neustroev,}39

S.F. Novaes,5 _{T. Nunnemann,}25 _{G. Obrant,}39 _{J. Orduna,}78_{N. Osman,}15 _{J. Osta,}54 _{G.J. Otero y Garz´on,}1 M. Padilla,46 _{A. Pal,}76 _{N. Parashar,}53 _{V. Parihar,}75_{S.K. Park,}31 _{J. Parsons,}68 _{R. Partridge}c_,75 _{N. Parua,}52

A. Patwa,71 _{B. Penning,}48 _{M. Perfilov,}37 _{K. Peters,}44_{Y. Peters,}44 _{K. Petridis,}44 _{G. Petrillo,}69 _{P. P´etroff,}16 R. Piegaia,1_{J. Piper,}62_{M.-A. Pleier,}71_{P.L.M. Podesta-Lerma}f_,32 _{V.M. Podstavkov,}48_{P. Polozov,}36_{A.V. Popov,}38

M. Prewitt,78_{D. Price,}52_{N. Prokopenko,}38 _{S. Protopopescu,}71_{J. Qian,}61_{A. Quadt,}23_{B. Quinn,}63 _{M.S. Rangel,}2 K. Ranjan,28 _{P.N. Ratoff,}42 _{I. Razumov,}38 _{P. Renkel,}77_{M. Rijssenbeek,}70_{I. Ripp-Baudot,}19 _{F. Rizatdinova,}74

M. Rominsky,48 _{A. Ross,}42_{C. Royon,}18 _{P. Rubinov,}48 _{R. Ruchti,}54_{G. Safronov,}36 _{G. Sajot,}14 _{P. Salcido,}50 A. S´anchez-Hern´andez,32_{M.P. Sanders,}25 _{B. Sanghi,}48 _{A.S. Santos,}5 _{G. Savage,}48_{L. Sawyer,}58_{T. Scanlon,}43 R.D. Schamberger,70 _{Y. Scheglov,}39 _{H. Schellman,}51 _{T. Schliephake,}26_{S. Schlobohm,}80_{C. Schwanenberger,}44 R. Schwienhorst,62 _{J. Sekaric,}56 _{H. Severini,}73 _{E. Shabalina,}23 _{V. Shary,}18 _{A.A. Shchukin,}38 _{R.K. Shivpuri,}28 V. Simak,10_{V. Sirotenko,}48 _{P. Skubic,}73 _{P. Slattery,}69 _{D. Smirnov,}54 _{K.J. Smith,}67 _{G.R. Snow,}64_{J. Snow,}72 S. Snyder,71_{S. S¨oldner-Rembold,}44 _{L. Sonnenschein,}21_{K. Soustruznik,}9 _{J. Stark,}14 _{V. Stolin,}36_{D.A. Stoyanova,}38

M. Strauss,73 _{D. Strom,}49 _{L. Stutte,}48 _{L. Suter,}44 _{P. Svoisky,}73 _{M. Takahashi,}44 _{A. Tanasijczuk,}1 _{W. Taylor,}6 M. Titov,18 _{V.V. Tokmenin,}35_{Y.-T. Tsai,}69 _{D. Tsybychev,}70 _{B. Tuchming,}18 _{C. Tully,}66 _{L. Uvarov,}39 _{S. Uvarov,}39

S. Uzunyan,50 _{R. Van Kooten,}52_{W.M. van Leeuwen,}33_{N. Varelas,}49_{E.W. Varnes,}45_{I.A. Vasilyev,}38 _{P. Verdier,}20 L.S. Vertogradov,35_{M. Verzocchi,}48_{M. Vesterinen,}44_{D. Vilanova,}18 _{P. Vokac,}10_{H.D. Wahl,}47_{M.H.L.S. Wang,}69

J. Warchol,54 _{G. Watts,}80 _{M. Wayne,}54 _{M. Weber}g_,48 _{L. Welty-Rieger,}51 _{A. White,}76 _{D. Wicke,}26 M.R.J. Williams,42 _{G.W. Wilson,}56 _{M. Wobisch,}58 _{D.R. Wood,}60 _{T.R. Wyatt,}44 _{Y. Xie,}48_{C. Xu,}61 _{S. Yacoob,}51

R. Yamada,48 _{W.-C. Yang,}44 _{T. Yasuda,}48 _{Y.A. Yatsunenko,}35 _{Z. Ye,}48 _{H. Yin,}48 _{K. Yip,}71 _{S.W. Youn,}48 J. Yu,76 S. Zelitch,79 T. Zhao,80 B. Zhou,61 J. Zhu,61 M. Zielinski,69 D. Zieminska,52 and L. Zivkovic75

(The D0 Collaboration∗ )

1_{Universidad de Buenos Aires, Buenos Aires, Argentina} 2_{LAFEX, Centro Brasileiro de Pesquisas F´ısicas, Rio de Janeiro, Brazil}

3_{Universidade do Estado do Rio de Janeiro, Rio de Janeiro, Brazil} 4_{Universidade Federal do ABC, Santo Andr´e, Brazil}

5_{Instituto de F´ısica Te´orica, Universidade Estadual Paulista, S˜ao Paulo, Brazil}

6_{Simon Fraser University, Vancouver, British Columbia, and York University, Toronto, Ontario, Canada} 7_{University of Science and Technology of China, Hefei, People’s Republic of China}

8_{Universidad de los Andes, Bogot´a, Colombia} 9_{Charles University, Faculty of Mathematics and Physics,}

Center for Particle Physics, Prague, Czech Republic

10_{Czech Technical University in Prague, Prague, Czech Republic} 11_{Center for Particle Physics, Institute of Physics,}

Academy of Sciences of the Czech Republic, Prague, Czech Republic

12_{Universidad San Francisco de Quito, Quito, Ecuador} 13_{LPC, Universit´e Blaise Pascal, CNRS/IN2P3, Clermont, France}

14_{LPSC, Universit´e Joseph Fourier Grenoble 1, CNRS/IN2P3,}

Institut National Polytechnique de Grenoble, Grenoble, France

15_{CPPM, Aix-Marseille Universit´e, CNRS/IN2P3, Marseille, France} 16_{LAL, Universit´e Paris-Sud, CNRS/IN2P3, Orsay, France} 17_{LPNHE, Universit´es Paris VI and VII, CNRS/IN2P3, Paris, France}

18_{CEA, Irfu, SPP, Saclay, France}

19_{IPHC, Universit´e de Strasbourg, CNRS/IN2P3, Strasbourg, France}

20_{IPNL, Universit´e Lyon 1, CNRS/IN2P3, Villeurbanne, France and Universit´e de Lyon, Lyon, France} 21_{III. Physikalisches Institut A, RWTH Aachen University, Aachen, Germany}

22_{Physikalisches Institut, Universit¨at Freiburg, Freiburg, Germany}

23_{II. Physikalisches Institut, Georg-August-Universit¨at G¨ottingen, G¨ottingen, Germany} 24_{Institut f¨ur Physik, Universit¨at Mainz, Mainz, Germany}

25_{Ludwig-Maximilians-Universit¨at M¨unchen, M¨unchen, Germany} 26_{Fachbereich Physik, Bergische Universit¨at Wuppertal, Wuppertal, Germany}

27_{Panjab University, Chandigarh, India} 28_{Delhi University, Delhi, India}

29_{Tata Institute of Fundamental Research, Mumbai, India} 30_{University College Dublin, Dublin, Ireland}

31_{Korea Detector Laboratory, Korea University, Seoul, Korea} 32_{CINVESTAV, Mexico City, Mexico}

33_{FOM-Institute NIKHEF and University of Amsterdam/NIKHEF, Amsterdam, The Netherlands} 34_{Radboud University Nijmegen/NIKHEF, Nijmegen, The Netherlands}

35_{Joint Institute for Nuclear Research, Dubna, Russia} 36_{Institute for Theoretical and Experimental Physics, Moscow, Russia}

37_{Moscow State University, Moscow, Russia} 38_{Institute for High Energy Physics, Protvino, Russia} 39_{Petersburg Nuclear Physics Institute, St. Petersburg, Russia}

40_{Instituci´o Catalana de Recerca i Estudis Avan¸cats (ICREA) and Institut de F´ısica d’Altes Energies (IFAE), Barcelona, Spain} 41_{Stockholm University, Stockholm and Uppsala University, Uppsala, Sweden}

42_{Lancaster University, Lancaster LA1 4YB, United Kingdom} 43_{Imperial College London, London SW7 2AZ, United Kingdom} 44_{The University of Manchester, Manchester M13 9PL, United Kingdom}

45_{University of Arizona, Tucson, Arizona 85721, USA} 46_{University of California Riverside, Riverside, California 92521, USA}

47_{Florida State University, Tallahassee, Florida 32306, USA} 48_{Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA}

49_{University of Illinois at Chicago, Chicago, Illinois 60607, USA} 50_{Northern Illinois University, DeKalb, Illinois 60115, USA}

51_{Northwestern University, Evanston, Illinois 60208, USA} 52_{Indiana University, Bloomington, Indiana 47405, USA} 53_{Purdue University Calumet, Hammond, Indiana 46323, USA} 54_{University of Notre Dame, Notre Dame, Indiana 46556, USA}

55_{Iowa State University, Ames, Iowa 50011, USA} 56_{University of Kansas, Lawrence, Kansas 66045, USA} 57_{Kansas State University, Manhattan, Kansas 66506, USA} 58_{Louisiana Tech University, Ruston, Louisiana 71272, USA} 59_{Boston University, Boston, Massachusetts 02215, USA} 60_{Northeastern University, Boston, Massachusetts 02115, USA}

61_{University of Michigan, Ann Arbor, Michigan 48109, USA} 62_{Michigan State University, East Lansing, Michigan 48824, USA}

63_{University of Mississippi, University, Mississippi 38677, USA} 64_{University of Nebraska, Lincoln, Nebraska 68588, USA} 65_{Rutgers University, Piscataway, New Jersey 08855, USA} 66_{Princeton University, Princeton, New Jersey 08544, USA} 67_{State University of New York, Buffalo, New York 14260, USA}

68_{Columbia University, New York, New York 10027, USA} 69_{University of Rochester, Rochester, New York 14627, USA} 70_{State University of New York, Stony Brook, New York 11794, USA}

71_{Brookhaven National Laboratory, Upton, New York 11973, USA} 72_{Langston University, Langston, Oklahoma 73050, USA} 73_{University of Oklahoma, Norman, Oklahoma 73019, USA} 74_{Oklahoma State University, Stillwater, Oklahoma 74078, USA}

75_{Brown University, Providence, Rhode Island 02912, USA} 76_{University of Texas, Arlington, Texas 76019, USA} 77_{Southern Methodist University, Dallas, Texas 75275, USA}

78_{Rice University, Houston, Texas 77005, USA} 79_{University of Virginia, Charlottesville, Virginia 22901, USA}

80_{University of Washington, Seattle, Washington 98195, USA}

(Dated: May 26th, 2011)

We present a measurement of the t¯t production cross section σt¯tin p¯p collisions at√s = 1.96 TeV

using 5.4 fb−1_{of integrated luminosity collected with the D0 detector. We consider final states with}

at least two jets and two leptons (ee, eµ, µµ), and events with one jet for the the eµ final state as well. The measured cross section is σt¯t=7.36+0.90_−0.79(stat + syst) pb. This result combined with

the cross section measurement in the lepton + jets final state yields σt¯t=7.56+0.63_−0.56(stat + syst) pb,

which agrees with the standard model expectation. The relative precision of 8% of this measurement is comparable to the precision of the latest theoretical calculations.

PACS numbers: 14.65.Ha, 13.85.Qk

I. INTRODUCTION

The precise measurement of the top quark pair (t¯t) pro-duction cross section (σt¯t) and its comparison with the theoretical predictions provide important tests of pertur-bative quantum choromodynamics (QCD). At present, the most precise predictions of σt¯tare given by approx-imate next to next-to-leading order (NNLO) calcula-tions [1–3], that set a goal for the experimental preci-sion of the σt¯t measurement of (6 – 9)%. Furthermore, because σt¯tdepends on the top quark mass (mt), it can

∗_{with visitors from} a_{Augustana College, Sioux Falls, SD, USA,} b_{The University of Liverpool, Liverpool, UK,}c_{SLAC, Menlo Park,} CA, USA, d_{University College London, London, UK,} e_Centro de Investigacion en Computacion - IPN, Mexico City, Mexico, f_{ECFM, Universidad Autonoma de Sinaloa, Culiac´an, Mexico, and} g_{Universit¨at Bern, Bern, Switzerland.}

be used to measure mt [4, 5]. Comparing the standard model (SM) prediction with the measured σt¯t value al-lows testing for the presence of physics beyond the SM, for instance, scenarios in which the top quark would de-cay into a charged Higgs boson and a b quark [5].

In this Letter we present an updated measurement of σt¯tin p¯p collisions at√s = 1.96 TeV in the dilepton (ℓℓ′, ℓ = e, µ) channel. Within the SM, top quarks decay al-most 100% of the time into a W boson and a b quark. We consider the leptonic decays of both W bosons from top quark decay into eνe, µνµ, or τ ντ (throughout this letter, e, µ, τ refer to both charge conjugate states: e±_{, µ}± _or τ±_{), where only leptonic decays of the τ are considered.} This measurement complements the σt¯tmeasurements in the lepton+jets (ℓj) channel, in which one of the W bosons decays hadronically into a q ¯q′

pair and the other W boson decays leptonically [6, 7], as well as mea-surements in the all-hadronic channel, in which both W bosons decay hadronically [8].

The measurement is based on data collected with the D0 detector during Run II of the Fermilab Tevatron Collider that correspond to an integrated luminosity of 5.4 ± 0.3 fb−1_{. This result supersedes our previous} mea-surement [9], which used a dataset five times smaller than the one considered here. The CDF Collaboration has measured σt¯t in the ℓℓ′ channel using 2.8 fb−1 of inte-grated luminosity [10]. The ATLAS and CMS Collabo-rations recently published their first σt¯tmeasurements in pp collisions at√s = 7 TeV [11, 12].

The D0 detector is described in detail in [13]. The region of the D0 detector closest to the interaction re-gion contains a tracking system consisting of a silicon microstrip tracker and a central fiber tracker, both lo-cated inside a superconducting solenoid magnet which generates a magnetic field of 2 T. Hits in these two de-tectors are used to reconstruct tracks from charged parti-cles in the pseudorapidity region |η| < 3 [30]. Surround-ing the two trackSurround-ing subdetectors are liquid-argon ura-nium calorimeters, segmented into electromagnetic and hadronic sections. The central section of the calorime-ter (CC) covers pseudorapidities |η| < 1.1, and the two end calorimeters (EC) extend coverage to |η| ≈ 4.2 with all three housed in separate cryostats. The muon system surrounds the calorimeter and consists of three layers of tracking detectors and scintillator trigger counters cov-ering |η| < 2. A toroidal iron magnet with a field of 1.8 T is located outside the innermost layer of the muon detector. The luminosity is calculated from the rate of inelastic p¯p collisions measured with plastic scintillator arrays located in front of the EC cryostats [14].

The D0 trigger is based on a three-level pipeline sys-tem. The first level is implemented in custom-designed hardware. The second level uses high-level processors to combine information from the different sub-detectors to construct simple physics objects. The software-based third level uses full event information obtained with a simplified reconstruction algorithm.

II. OBJECT IDENTIFICATION

The t¯t dilepton final state contains two leptons (elec-trons, muons or an electron and a muon), at least two jets, and significant missing transverse momentum (/pT) from escaping neutrinos.

Electrons are identified as energy clusters with radius R =p(∆η)2_{+ (∆φ)}2_{< 0.2 in the calorimeter (φ is the} azimuthal angle) which are consistent in their longitudi-nal and transverse profiles with those of an electromag-netic shower. More than 90% of the energy of the electron candidate must be deposited in the electromagnetic part of the calorimeter, and less than 20% of its energy may be deposited in an annulus of 0.2 < R < 0.4 around its direction. This cluster has to be matched to an inner detector track. We consider electrons in the CC with |η| < 1.1 and in the EC with 1.5 < |η| < 2.5. In addi-tion, we make a requirement on the electron likelihood

discriminant, based on tracking and calorimeter infor-mation, at a cut value chosen to have a high selection efficiency (near 85%) for electrons, and good rejection (near 90%) for jets misidentified as electrons. Electrons fulfilling all these criteria are called ”tight electrons”.

A muon is identified as a segment in at least one layer of the muon system in the full acceptance of the muon system that is matched to a track in the central tracking system. Reconstructed muons must satisfy two isolation criteria. First, the transverse energy deposited in an an-nulus around the muon 0.1 < R < 0.4 (ETµ,iso) has to be less than 15% of the transverse momentum of the muon (pµT). Second, the sum of the transverse momenta of the tracks in a cone of radius R < 0.5 around the muon track in the central tracking system (pµ,iso_T ) has to be less than 15% of pµ_T. Muons that fulfill these isolation criteria are referred to as ”tight isolated muons”.

Monte Carlo (MC) generated events are processed through a geant3 [15] based simulation of the D0 de-tector and the same reconstruction programs used for the data. To simulate the effects from additional p¯p interactions, zero bias events with no trigger require-ments selected randomly in collider data are overlayed on the fully simulated MC events. Residual differences between data and MC simulation in the electron and muon pT resolutions and identification efficiencies are corrected. These corrections are derived from a sam-ple of Z/γ⋆_{→ ℓℓ events in data and MC, applying tight} requirements on one of the two leptons for selecting the events and using the other one to measure the efficiencies and resolutions.

Jets are identified with a cone algorithm with radius R < 0.5 [16] in the range |η| < 2.5. A jet energy scale correction (JES) is determined by calibrating the energy deposited in the jet cone using transverse momentum bal-ance in γ+jet and dijet events. If a muon overlaps with the jet cone, the momentum of that muon is added to the jet pT , assuming that the muon originates from a semileptonic decay of a hadron belonging to the jet.

We require that the jets be matched to at least two tracks originating from the vertex of the primary p¯p in-teraction (PV). Jets in MC are corrected for the residual differences between data and MC in the energy resolu-tion and JES. These correcresolu-tion factors are measured by comparing data and MC in (Z/γ⋆→ ee)+jets events.

We use a neural-network (NN) tagging algorithm [17] to identify jets from b quarks. The algorithm combines information from the impact parameters of the tracks and variables that characterize the presence and properties of secondary vertices within the jet in a single discriminant. In order to use this information for b tagging, the jet is required to be matched to a jet built from tracks. Jets fulfilling this requirement are called taggable jets. The NN discriminant has a value close to one for the b quark jets and close to zero for the light quark and gluon jets.

The /pT is reconstructed from the energy deposited in the calorimeter cells. Corrections for lepton and jet pT are propagated into the /pT. The missing transverse

mo-mentum significance (σ/pT) is defined in each event as a likelihood discriminant constructed using the ratio of /pT to its uncertainty.

More details about object identification can be found in [18].

III. EVENT SELECTION AND BACKGROUND

ESTIMATION

The main sources of background in the ℓℓ′

channel come from Drell-Yan and Z boson production (Z/γ⋆_→ ℓℓ), diboson production (WW, WZ, ZZ ), and instrumen-tal background. The instrumeninstrumen-tal background mainly arises from multijet and (W → ℓν)+jets events in which one or two jets are misidentified as electrons and/or muons originating from the semileptonic decay of a heavy flavor hadron.

For this analysis we consider events that passed at least one of a set of single lepton triggers for the ee and µµ channels. For the eµ channel, we consider events selected by a mixture of single and multilepton triggers and lep-ton+jet triggers. Efficiencies for single lepton triggers have been measured with Z/γ⋆_{→ ℓℓ data. These} effi-ciencies are found to be around 99% for the ee channel and 80% for µµ. For the eµ channel the trigger efficiency is close to 100%.

In order to separate t¯t signal events from background, the following selection is applied:

• We require at least one PV in the beam interaction region with |z| < 60 cm, where z is the coordinate along the beam axis, and z = 0 in the center of the detector. At least three tracks must be associated with this PV.

• We require at least two isolated leptons with pT > 15 GeV, originating from the same PV, i.e., the dif-ference between the z coordinates of the two lepton tracks should be less than 2 cm, where the z coordi-nate is calculated at the point of the track’s closest approach to the beam.

• We select the two highest pT leptons with opposite charges.

• In the eµ final state, we require the distance be-tween the electron and the muon directions to be R(e, µ) > 0.3 to reduce the background from bremsstrahlung.

• In the eµ channel, we consider events with at least one jet with pT > 20 GeV. In the ee and µµ channels, we require at least two jets with pT > 20 GeV.

• To further improve the signal purity of the selected sample, we apply additional selection criteria based on global event properties. In the eµ channel with exactly one jet, we require HT > 105 GeV, where

HT is the scalar sum of the transverse momenta of the leading lepton and the two leading jets. In the eµ final state with two jets, we require HT > 110 GeV. In the ee final state, we require σ/pT> 5, while in the µµ channel we require /pT> 40 GeV and σ/pT> 5.

In order to estimate the signal efficiency and the back-ground contamination, we use the MC simulation for all contributions but for the instrumental background, the latter being derived from data. The t¯t and Z/γ⋆ _events are generated with the tree level matrix element gener-ator alpgen [19] interfaced with the pythia [20] gen-erator for parton showering and hadronization. Dibo-son events are generated with pythia. All simulated samples are generated using the CTEQ6L1 parton dis-tribution functions (PDFs) [21]. The Z/γ⋆ _{samples are} normalized to the NNLO cross section computed with the fewz program [22]. We separately simulate Z/γ⋆ with heavy flavor (HF) quarks, Z/γ⋆_{+b¯b (or Z/γ}⋆_+c¯c), using alpgen and enhance the corresponding leading or-der cross sections by a factor estimated with the mcfm program [23]. The diboson samples are normalized to the next-to-leading order cross section calculated with mcfm_{. Uncertainties in these normalization factors are} taken into account as systematic uncertainties. In addi-tion, we apply a correction to the Z/γ⋆_{+jets simulation} based on data to address the imperfect modeling of the Z boson pT in the MC [24].

The instrumental background is estimated directly from data. In the ee and eµ channels we determine the contribution of events with jets misidentified as trons using the signal data sample but without the elec-tron likelihood discriminant requirement. We extract the number of events with jets misidentified as electrons, nf, and the number of events with real electrons, ne, by maxi-mizing the function of the electron likelihood distribution

L = N Y i=1 [neS(xi) + nfB(xi)] e−(ne+nf) N ! , (1)

where N is the number of selected events, xi is the elec-tron likelihood discriminant value in the event i, and S(xi) and B(xi) are the signal and background proba-bility density functions (pdfs). The signal pdf is mea-sured in Z/γ⋆_{→ ee data events. The background pdf} is measured in eµ events with the same selection as the analysis sample but inverting the opposite sign lepton re-quirement (i.e., requiring leptons of the same sign) with-out any topological requirement but using muon with reversed isolation requirements: ETµ,iso/p

µ

T > 0.2 and pµ,iso_T /pµ_T > 0.2. The total number of events with a jet misidentified as an electron is given by nf scaled for the integral of B(x) over the region satisfying the likelihood requirement. The estimation is performed separately in the CC and EC. We find that the contribution of in-strumental background to the ee channel is negligible. The limited size of the sample used to measure the back-ground pdf is the main contribution to the systematic

uncertainty on the instrumental background. Together with a possible pT dependence of the signal and back-ground pdfs, these sources typically lead to systematic uncertainties of about 50%.

We also determine the number of events with an iso-lated muon arising from jets in the eµ and µµ channels. This number is estimated as nµ_f = Nloosefµ, where Nloose is the number of events in the same sign sample with loose isolation criteria on the muon: E_Tµ,iso/pµ_T < 0.5 and pµ,iso_T /pµ_T < 0.5, and fµ is the misidentification rate for isolated muons. In the µµ final state, we apply these loose isolation criteria only to one randomly cho-sen muon. In the eµ channel, the number of events with jets misidentified as electrons in the same sign sample is subtracted from Nloose. The misidentification rate, fµ, is determined in a dimuon sample with at least one jet. In this sample we require one muon to be close to the jet (R(µ, jet) < 0.5) with reversed isolation criteria E_Tµ,iso/pµ_T > 0.15 and pµ,iso_T /pµ_T > 0.15. The other muon defined as the probe, should pass the loose isolation crite-ria E_Tµ,iso/pµ_T < 0.5 and pµ,iso_T /pµ_T < 0.5. We compute fµ as the ratio of the number of events in which the probe muon passes the tight isolation criteria to the total num-ber of events in this same sign sample. The systematic uncertainty on the fµ determination is about 10% and results mainly from the statistical uncertainty due to the limited size of the sample used for the muon misidenti-fication rate calculation and the potential dependence of the misidentification rate on pT and /pT.

The number of predicted background events as well as the expected number of signal events in the four chan-nels are shown in Table I. The t¯t events have two b quark jets in the final state, but most of the background events have jets produced by light quarks or gluons. In order to achieve a better separation between signal and background when measuring the cross section, we use the distribution of the smallest of the two b-tagging NN discriminants of the two leading jets. If a jet does not satisfy the requirements to enter the NN computation (non-taggable jet), a value of -1 is assigned to it. These NN discriminant distributions for the different channels are shown in Fig. 1

We measure the t¯t cross section σt¯tby simultaneously fitting the NN distributions in the four channels and max-imizing the likelihood function

L =Y i

Y j

P [nij, µij(σt¯t)] , (2)

where i runs over the channels and j over the bins of the NN distribution, and P [n, µ(σt¯t)] is the Poisson proba-bility function to observe n events when µ(σt¯t) events are expected.

IV. RESULTS AND UNCERTAINTIES

The main systematic uncertainties for the measure-ment of the t¯t cross section are described in the following.

A 6.1% uncertainty [14] directly affects the cross section measurement because of the luminosity uncertainty but also the expected numbers of Z/γ⋆ _{and diboson} back-ground events. Uncertainties in lepton identification ef-ficiencies are determined by evaluating possible sources of bias in the data driven method used for the efficiency measurements and the possible impact of data/MC dif-ferences in Z/γ⋆_{→ ℓℓ events. Uncertainties in the lepton} energy resolution are determined by comparing the width of the Z boson invariant mass distributions in data and MC.

The uncertainty in the relative JES between data and MC for light quark jets has been evaluated by shifting the jets in MC by their corresponding JES uncertainty. The uncertainty on the difference between the light and b quark JES (1.8%) is estimated by propagating the differ-ence in the single pion response between data and MC to the MC JES for b quark jets. Jet energy resolution uncer-tainties are estimated by comparing the resolutions mea-sured in Z/γ⋆_{+jets events in data and in MC. The} un-certainty on the jet identification efficiency is estimated by comparing the efficiencies measured in dijet events for data and MC. The b quark identification uncertainties in-clude uncertainties in the probability of tagging a b quark jet, the probability of tagging a light quark jet or gluon, and the probability for a jet not to be taggable [17].

To estimate the uncertainty in the trigger efficiency, we use events selected with the same criteria as the t¯t signal but without jet requirements. In all four channels this selection is dominated by Z/γ⋆_{events. We compute} the ratio of the expected and observed number of events for two cases: when both leptons are allowed to fire the trigger or when only one lepton is allowed to fire the trigger. The difference in these ratios is used to estimate the uncertainty on the trigger efficiency.

Several uncertainties on the signal modeling are con-sidered. The effects of higher order corrections and the hadronization modeling are estimated as the dif-ference in signal efficiencies using the default alpgen+ pythia_{simulation and using events generated with the} mc@nlo_{[25] + herwig [26] simulation. The uncertainty} coming from color reconnection is evaluated by compar-ing the t¯t efficiency using pythia v6.4 tune Apro and pythia_{v6.4 tune ACRpro [27]. The uncertainty on} ini-tial (ISR) and final (FSR) state radiation is evaluated by varying the ISR/FSR parameters in pythia up to 20% and evaluating the change in the signal efficiency. The uncertainty due to PDFs is estimated by reweighting the signal efficiency to the CTEQ6.1M PDFs [21] and look-ing at the efficiency variation for each eigenvector set that define the CTEQ6.1M uncertainty range. The un-certainty due to the simulation of b quark fragmentation is assigned to be the difference between tuning the pa-rameters of the b quark fragmentation function to LEP or SLD data [28].

The uncertainty in the background normalization in-cludes the theoretical uncertainties in the cross section and the uncertainty due to the correction for the Z boson

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pT modeling. We also take into account an uncertainty due to the limited statistics of the signal and background templates of the NN discriminant. For the following sys-tematic uncertainties, we take into account effects that change the shape of the differential distribution of the b-tagging NN output discriminant: jet energy scale, jet resolution, jet identification, and b quark identification uncertainties.

Maximizing the likelihood function in Eq. 2 and using the above systematic uncertainties, we measure the cross section assuming a top quark mass mt= 172.5 GeV and find

σt¯t= 8.05+0.50−0.48 (stat) +1.05

−0.97 (syst) pb. (3) In order to reduce the influence of systematic uncer-tainties on the cross section measurement, in the

fol-lowing we use nuisance parameters [29] to constrain the overall uncertainty using the data NN output distribu-tion itself. Using this technique, the likelihood (Eq. 2) is modified, L =Y i Y j P [nij, µij(σt¯t, νk)] Y k G(νk; 0, SD), (4) where G(νk; 0, SD) denotes the Gaussian probability den-sity with mean at zero and width corresponding to one standard deviation (SD) of the considered systematic un-certainty. Correlations of systematic uncertainties be-tween channels and bebe-tween the different samples are naturally taken into account by assigning the same nui-sance parameter to the correlated systematic uncertain-ties. In Eq. 4, the free parameters of the fit are νk and

TABLE I: Numbers of expected and observed events assuming the SM t¯t cross section for a top quark mass of mt= 172.5 GeV

(7.45 pb). Expected numbers of events are shown with their systematic uncertainties. The uncertainty on the ratio between observed and expected numbers of events takes into account the statistical uncertainty in the observed number of events (Nobs)

and the systematic uncertainty in the expected number of events (Nexp).

Channel Z → ℓℓ Diboson Instrumental

background t¯t → ℓ¯ℓb¯bν ¯ν Nexp Nobs ObservedExpected

ee+2jet 12.6 ± 2.0 3.0 ± 0.4 - 45.6 ± 5.3 61.1 ± 7.1 74 1.21 ± 0.20

µµ+2jet 67.3 ± 9.7 5.1 ± 0.7 7.6 ± 1.2 59.8 ± 6.6 139.8 ± 15.7 144 1.03 ± 0.14 eµ+2jet 30.3 ± 4.2 8.6 ± 1.2 22.7 ± 8.6 191.5 ± 18.8 253.1 ± 24.3 281 1.11 ± 0.13 eµ+1jet 40.9 ± 4.8 20.7 ± 2.4 25.3 ± 10.5 52.1 ± 9.4 139.0 ± 16.5 150 1.08 ± 0.16

σt¯t.

As can be seen from Eq. 3, the systematic uncertain-ties are the limiting uncertainuncertain-ties in the precision of the t¯t cross section measurement. Varying the systematic uncertainties and constraining them with data can there-fore improve the measurement. Using nuisance parame-ters we find an overall improvement of the uncertainty of 20% and reach a relative precision of 11% in the t¯t cross section:

σt¯t= 7.36+0.90−0.79(stat + syst) pb.

The uncertainties on the t¯t cross section are summa-rized in Table II. For each category of systematic un-certainty listed in Table II, the corresponding nuisance parameters are set to their fitted value, and shifted by the uncertainty on the fit. In the columns “+σ” and “−σ,” the positive and negative systematic uncertainties on the measured cross section for each category are listed.

We combine this measurement with the cross section measurement in the non-overlapping ℓj channel [6] us-ing the same nuisance parameter approach and takus-ing correlations between common systematics uncertainties into account. In the ℓj channel, the events are sepa-rated into events with three or at least four jets, of which zero, one, or at least two jets are b-tagged. In events that have three or four jets but no b-tagged jets or events with three jets and one b-tagged jet, we use a topologi-cal discriminant to improve the separation of signal and background. In [6], the separation into these channels and application of topological methods is referred to as the combined method. For this combination, we do not simultaneously fit the heavy flavor fraction for W +jet processes (W +HF) in the ℓj channel as was done in [6], making it unnecessary to use ℓj events with only two jets. With this change compared to [6], the measured ℓj t¯t cross section is

σt¯t= 7.90+0.78−0.69(stat + syst) pb.

The combination of the measurements in the dilepton and lepton + jet final states is done by maximizing the product of the likelihood function for the ℓℓ′_{and ℓj} chan-nels, which yields

σt¯t= 7.56+0.63−0.56(stat + syst) pb

for mt= 172.5 GeV. This combination has a relative pre-cision of 8% and represents an improvement of about 12% relative to the ℓj cross section measurement alone. The uncertainties for this combined measurement are summa-rized in Table II.

Due to acceptance effects, the t¯t efficiency depends on the assumed mtin the MC. We extract the t¯t cross section using simulated t¯t events with different values of mt. The resulting cross sections can be fitted with the following functional form: σt¯t(mt) = 1 m4 t [a + b(mt− 170 GeV) + c(mt− 170 GeV)2+ d(mt− 170 GeV)3] , (5) with a = 6.5178 × 109 _GeV4_{, b = 7.884 × 10}7 _GeV3_, c = 9.3069 × 105 GeV2, and d = −2.42 × 103 GeV and where σt¯t and mt are in pb and GeV, respectively. The relative uncertainty on the cross section for differ-ent mass points is the same as the one obtained for mt = 172.5 GeV. Figure 2 shows this parameterization for the measurement as a function of top quark mass to-gether with approximate NNLO computations [1–3].

V. CONCLUSION

In this Letter we presented an updated measurement of the t¯t production cross section in the dilepton final state using 5.4 fb−1 of integrated luminosity. This cross sec-tion measurement yields σt¯t=7.36+0.90−0.79(stat + syst) pb and has a relative precision of +12%−11%. It is currently the most precise measurement of the t¯t cross section in the dilepton channel. Combining this measurement with our result in the lepton + jets channel [6] yields 7.56+0.63−0.56(stat + syst) pb which corresponds to a relative precision of 8%. This measurement is in good agreement with the SM prediction.

We thank the staffs at Fermilab and collaborating institutions, 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 (In-dia); Colciencias (Colombia); CONACyT (Mexico); KRF and KOSEF (Korea); CONICET and UBACyT (Ar-gentina); FOM (The Netherlands); STFC and the Royal

TABLE II: Breakdown of uncertainties on the t¯t cross sections in the ℓℓ′_{channel and for the combined ℓℓ}′_{and ℓj measurement}

using the nuisance parameter technique. The ±σ give the impact on the measured cross section when the nuisance parameters describing the considered category are shifted by ±1 SD from their fitted mean. See text for further details.

ℓℓ′ _ℓℓ′_+ℓj

Source +σ [pb] _{−σ [pb]} +σ [pb] _{−σ [pb]}

Statistical +0.50 _−0.48 +0.20 _−0.20

Muon identification +0.11 _−0.11 +0.07 _−0.06

Electron identification and smearing +0.24 _−0.23 +0.13 _−0.13

Signal modeling +0.34 _−0.33 +0.16 _−0.06

Triggers +0.19 _−0.19 +0.05 _−0.05

Jet energy scale +0.13 _−0.12 +0.04 _−0.04

Jet reconstruction and identification +0.21 _−0.20 +0.12 _−0.09

b-tagging +0.06 _−0.06 +0.16 _−0.14 Background normalization +0.29 _−0.27 +0.11 _−0.10 W +HF fraction - - +0.12 _−0.04 Instrumental background +0.18 _−0.17 +0.05 _−0.04 Luminosity +0.57 _−0.51 +0.48 _−0.43 Other +0.10 _−0.10 +0.06 _−0.06 Template statistics +0.08 _−0.08 +0.04 _−0.04

Society (United Kingdom); MSMT and GACR (Czech Republic); CRC Program and NSERC (Canada); BMBF

and DFG (Germany); SFI (Ireland); The Swedish Re-search Council (Sweden); and CAS and CNSF (China).

[1] V. Ahrens, A. Ferroglia, M. Neubert, B. D. Pecjak, and L. L. Yang, J. High Energy Phys. 09, 097 (2010); V. Ahrens, A. Ferroglia, M. Neubert, B. D. Pecjak, and L. L. Yang, Nucl. Phys. Proc. Suppl. 205–206, 48 (2010). [2] S. Moch and P. Uwer, Phys. Rev. D 78, 034003 (2008); U. Langenfeld, S. Moch, and P. Uwer, Phys. Rev. D 80, 054009 (2009).

[3] N. Kidonakis and R. Vogt, Phys. Rev. D 68, 114014 (2003); N. Kidonakis, Phys. Rev. D 82, 114030 (2010). [4] V. M. Abazov et al. (D0 Collaboration), Phys. Lett. B

679, 177 (2009).

[5] V. M. Abazov et al. (D0 Collaboration), Phys. Rev. D 80, 071102 (2009).

[6] V. M. Abazov et al. (D0 Collaboration), Phys. Rev. D 84_{, 012008 (2011).}

[7] V. M. Abazov et al. (D0 Collaboration), Phys. Rev. Lett. 100, 192004 (2008).

[8] V. M. Abazov et al. (D0 Collaboration), Phys. Rev. D 76, 072007 (2007); V. M. Abazov et al. (D0 Collaboration), Phys. Rev. D 82, 032002 (2010).

[9] V. M. Abazov et al. (D0 Collaboration), Phys. Lett. B 626_{, 55 (2005).}

[10] T. Aaltonen et al. (CDF Collaboration), Phys. Rev. D 82, 052002 (2010).

[11] G. Aad et al. (ATLAS Collaboration), Eur. Phys. J. C 71, 1577 (2011).

[12] V. Khachatryan et al. (CMS Collaboration), Phys. Lett. B 695, 424 (2011).

[13] V. M. Abazov et al. (D0 Collaboration), Nucl. In-strum. Methods Phys. Res. A 565, 463 (2006).

[14] T. Andeen et al., FERMILAB-TM-2365 (2007).

[15] R. Brun, F. Carminati, CERN Program Library Long Writeup W5013, 1993 (unpublished).

[16] G. C. Blazey et al., in Proceedings of the Workshop: “QCD and Weak Boson Physics in Run II,” edited by U. Baur, R. K. Ellis, and D. Zeppenfeld (Fermilab, Batavia, IL, 2000) p. 47, arXiv:hep-ex/0005012; see Sec. 3.5 for details.

[17] V. M. Abazov et al. (D0 Collaboration) Nucl. In-strum. Methods Phys. Res. A 620, 490 (2010).

[18] V. M. Abazov et al. (D0 Collaboration), Phys. Rev. D 76_{, 052006 (2007).}

[19] M. L. Mangano, M. Moretti, F. Piccinini, R. Pittau, and A. D. Polosa, J. High Energy Phys. 07, 001 (2003). [20] T. Sj¨ostrand, S. Mrenna, and P. Z. Skands, J. High

En-ergy Phys. 05, 026 (2006).

[21] P. M. Nadolsky et al., Phys. Rev. D 78, 013004 (2008). [22] R. Gavin, Y. Li, F. Petriello, and S. Quackenbush,

arXiv:1011.3540 [hep-ph].

[23] R. K. Ellis, Nucl. Phys. Proc. Suppl. 160, 170 (2006). [24] V. M. Abazov et al. (D0 Collaboration), Phys. Lett. B

693_{, 522 (2010).}

[25] S. Frixione and B. R. Webber, J. High Energy Phys. 0206, 029 (2002), S. Frixione, P. Nason, B.R. Webber, J. High Energy Phys. 0308, 007 (2003).

[26] G. Corcella, I. G. Knowles, G. Marchesini, S. Moretti, K. Odagiri, P. Richardson, M. H. Seymour, B. R. Web-ber, J. High Energy Phys. 01, 010 (2001).

[27] A. Buckley, H. Hoeth, H. Lacker, H. Schulz and J. E. von Seggern, Eur. Phys. J. C 65, 331 (2010);

P. Z. Skands and D. Wicke, Eur. Phys. J. C 52, 133 (2007).

[28] Y. Peters, K. Hamacher and D. Wicke, FERMILAB-TM-2425-E.

[29] P. Sinervo, In Proceedings of PHYSTAT 2003: “Statisti-cal Problems in Particle Physics, Astrophysics, and

Cos-Top quark mass (GeV)

(pb)

σ

Top quark mass (GeV)

150 160 170 180 190

(pb)

σ

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FIG. 2: Dependence of the experimental and theoretical [1–3] t¯t cross sections on the top quark mass. The colored dashed lines represent the uncertainties for all three theoretical cal-culations from the choice of the PDF and the renormaliza-tion and factorizarenormaliza-tion scales (added quadratically). The data point shows the combined ℓℓ′ _{and ℓj cross section}

measure-ment for mt= 172.5 GeV, the black curve is experimental t¯t

cross section as a function of mt, and the gray band

corre-sponds to the total experimental uncertainty.

mology”, edited by L. Lyons, R. Mount, and R. Reitmeyer (SLAC, Menlo Park, CA, 2003), p. 122, SLAC-R-703. [30] The pseudorapidity is defined as η = − ln[tan(θ/2)],

where θ is the polar angle with respect to the proton beam.