b Physics Letters B 626 (2005) 55–64 www.elsevier.com/locate/physlet Measurement of thet t̄ production cross section inpp̄ collisions at √ s = 1.96 TeV in dilepton final states DØ Collaboration V.M. Abazovai, B. Abbottbt, M. Abolinsbk, B.S. Acharyaac, M. Adamsax, T. Adamsav, M. Agelour, J.-L. Agrams, S.H. Ahnae, M. Ahsanbe, G.D. Alexeevai, G. Alkhazovam, A. Alton bj, G. Alversonbi, G.A. Alvesb, M. Anastasoaieah, T. Andeenaz, S. Andersonar, B. Andrieuq, Y. Arnoudn, A. Askewav, B. Åsmanan, A.C.S. Assis Jesusc, O. Atramentovbc, C. Autermannu, C. Avilah, F. Badaudm, A. Badenbg, B. Baldinaw, P.W. Balmag, S. Banerjeeac, E. Barberisbi, P. Bargassabx, P. Baringerbd, C. Barnesap, J. Barretob, J.F. Bartlettaw, U. Basslerq, D. Bauerba, A. Beanbd, S. Beauceronq, M. Begallic, M. Begelbp, A. Bellavancebm, S.B. Beriaa, G. Bernardiq, R. Bernhardaw,1, I. Bertramao, M. Besançonr, R. Beuselinckap, V.A. Bezzuboval, P.C. Bhataw, V. Bhatnagaraa, M. Bindery, C. Biscaratao, K.M. Blackbh, I. Blacklerap, G. Blazeyay, F. Blekmanap, S. Blessingav, D. Blochs, U. Blumenscheinw, A. Boehnleinaw, O. Boeriubb, T.A. Boltonbe, F. Borcherdingaw, G. Borissovao, K. Bosag, T. Bosebo, A. Brandtbv, R. Brockbk, G. Brooijmansbo, A. Brossaw, N.J. Buchananav, D. Buchholzaz, M. Buehlerax, V. Buescherw, S. Burdinaw, S. Burkear, T.H. Burnettbz, E. Busatoq, C.P. Buszelloap, J.M. Butlerbh, J. Camminbp, S. Caronag, W. Carvalhoc, B.C.K. Caseybu, N.M. Casonbb, H. Castilla-Valdezaf, S. Chakrabartiac, D. Chakrabortyay, K.M. Chanbp, A. Chandraac, D. Chapinbu, F. Charless, E. Cheuar, D.K. Chobh, S. Choiau, B. Choudharyab, T. Christianseny, L. Christofekbd, D. Claesbm, B. Cléments, C. Clémentan, Y. Coadoue, M. Cookebx, W.E. Cooperaw, D. Coppagebd, M. Corcoranbx, A. Cotheneto, M.-C. Cousinouo, B. Coxaq, S. Crépé-Renaudinn, D. Cuttsbu, H. da Mottab, M. Dasbf, B. Daviesao, G. Daviesap, G.A. Davisaz, K. Debv, P. de Jongag, S.J. de Jongah, E. De La Cruz-Burelobj, C. De Oliveira Martinsc, S. Deanaq, J.D. Degenhardtbj, F. Déliotr, M. Demarteauaw, R. Deminabp, P. Deminer, D. Denisovaw, S.P. Denisoval, S. Desaibq, H.T. Diehlaw, M. Diesburgaw, M. Doidgeao, H. Dongbq, S. Doulasbi, L.V. Dudkoak, L. Duflotp, S.R. Dugadac, A. Duperrino, J. Dyerbk, A. Dyshkantay, M. Eadsay, D. Edmundsbk, T. Edwardsaq, J. Ellisonau, 0370-2693/$ – see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physletb.2005.08.105 http://www.elsevier.com/locate/physletb http://dx.doi.org/10.1016/j.physletb.2005.08.105 56 DØ Collaboration / Physics Letters B 626 (2005) 55–64 J. Elmsheusery, V.D. Elviraaw, S. Enobg, P. Ermolovak, O.V. Eroshinal, J. Estradaaw, H. Evansbo, A. Evdokimovaj, V.N. Evdokimoval, J. Fastaw, S.N. Fatakiabh, L. Feligionibh, A.V. Ferapontoval, T. Ferbelbp, F. Fiedlery, F. Filthautah, W. Fisherbn, H.E. Fiskaw, I. Fleckw, M. Fortneray, H. Foxw, S. Fuaw, S. Fuessaw, T. Gadfortbz, C.F. Galeaah, E. Gallasaw, E. Galyaevbb, C. Garciabp, A. Garcia-Bellidobz, J. Gardnerbd, V. Gavrilovaj, A. Gays, P. Gaym, D. 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Womersleyaw, D.R. Woodbi, T.R. Wyattaq, Q. Xubj, N. Xuanbb, S. Yacoobaz, R. Yamadaaw, M. Yanbg, T. Yasudaaw, Y.A. Yatsunenkoai, Y. Yenz, K. Yip br, H.D. Yoobu, S.W. Younaz, J. Yubv, A. Yurkewiczbq, A. Zabip, A. Zatserklyaniyay, M. Zdrazilbq, C. Zeitnitzx, D. Zhangaw, X. Zhangbt, T. Zhaobz, Z. Zhaobj, B. Zhoubj, J. Zhubq, M. Zielinski bp, D. Zieminskaba, A. Zieminskiba, R. Zitounbq, V. Zutshiay, E.G. Zverevak a Universidad de Buenos Aires, Buenos Aires, Argentina b LAFEX, Centro Brasileiro de Pesquisas Físicas, Rio de Janeiro, Brazil c Universidade do Estado do Rio de Janeiro, Rio de Janeiro, Brazil d Instituto de Física Teórica, Universidade Estadual Paulista, São Paulo, Brazil 58 DØ Collaboration / Physics Letters B 626 (2005) 55–64 e University of Alberta, Edmonton, Alberta, Simon Fraser University, Burnaby, British Columbia, York University, Toronto, Ontario, and McGill University, Montreal, Quebec, Canada f Institute of High Energy Physics, Beijing, People’s Republic of China g University of Science and Technology of China, Hefei, People’s Republic of China h Universidad de los Andes, Bogotá, Colombia i Center for Particle Physics, Charles University, Prague, Czech Republic j Czech Technical University, Prague, Czech Republic k Center for Particle Physics, Institute of Physics, Academy of Sciences of the Czech Republic, Prague, Czech Republic l Universidad San Francisco de Quito, Quito, Ecuador m Laboratoire de Physique Corpusculaire, IN2P3-CNRS, Université Blaise Pascal, Clermont-Ferrand, France n Laboratoire de Physique Subatomique et de Cosmologie, IN2P3-CNRS, Universite de Grenoble 1, Grenoble, France o CPPM, IN2P3-CNRS, Université de la Méditerranée, Marseille, France p IN2P3-CNRS, Laboratoire de l’Accélérateur Linéaire, Orsay, France q LPNHE, IN2P3-CNRS, Universités Paris VI and VII, Paris, France r DAPNIA/Service de Physique des Particules, CEA, Saclay, France s IReS, IN2P3-CNRS, Université Louis Pasteur, Strasbourg, and Université de Haute Alsace, Mulhouse, France t Institut de Physique Nucléaire de Lyon, IN2P3-CNRS, Université Claude Bernard, Villeurbanne, France u III Physikalisches Institut A, RWTH Aachen, Aachen, Germany v Physikalisches Institut, Universität Bonn, Bonn, Germany w Physikalisches Institut, Universität Freiburg, Freiburg, Germany x Institut für Physik, Universität Mainz, Mainz, Germany y Ludwig-Maximilians-Universität München, München, Germany z Fachbereich Physik, University of Wuppertal, Wuppertal, Germany aa Panjab University, Chandigarh, India ab Delhi University, Delhi, India ac Tata Institute of Fundamental Research, Mumbai, India ad University College Dublin, Dublin, Ireland ae Korea Detector Laboratory, Korea University, Seoul, Republic of Korea af CINVESTAV, Mexico City, Mexico ag FOM-Institute NIKHEF and University of Amsterdam/NIKHEF, Amsterdam, The Netherlands ah Radboud University Nijmegen/NIKHEF, Nijmegen, The Netherlands ai Joint Institute for Nuclear Research, Dubna, Russia aj Institute for Theoretical and Experimental Physics, Moscow, Russia ak Moscow State University, Moscow, Russia al Institute for High Energy Physics, Protvino, Russia am Petersburg Nuclear Physics Institute, St. Petersburg, Russia an Lund University, Lund, Royal Institute of Technology and Stockholm University, Stockholm, and Uppsala University, Uppsala, Sweden ao Lancaster University, Lancaster, United Kingdom ap Imperial College, London, United Kingdom aq University of Manchester, Manchester, United Kingdom ar University of Arizona, Tucson, AZ 85721, USA as Lawrence Berkeley National Laboratory and University of California, Berkeley, CA 94720, USA at California State University, Fresno, CA 93740, USA au University of California, Riverside, CA 92521, USA av Florida State University, Tallahassee, FL 32306, USA aw Fermi National Accelerator Laboratory, Batavia, IL 60510, USA ax University of Illinois at Chicago, Chicago, IL 60607, USA ay Northern Illinois University, DeKalb, IL 60115, USA az Northwestern University, Evanston, IL 60208, USA ba Indiana University, Bloomington, IN 47405, USA bb University of Notre Dame, Notre Dame, IN 46556, USA bc Iowa State University, Ames, IA 50011, USA bd University of Kansas, Lawrence, KA 66045, USA be Kansas State University, Manhattan, KA 66506, USA bf Louisiana Tech University, Ruston, LA 71272, USA bg University of Maryland, College Park, MD 20742, USA bh Boston University, Boston, MA 02215, USA DØ Collaboration / Physics Letters B 626 (2005) 55–64 59 f bi Northeastern University, Boston, MA 02115, USA bj University of Michigan, Ann Arbor, MI 48109, USA bk Michigan State University, East Lansing, MI 48824, USA bl University of Mississippi, University, MS 38677, USA bm University of Nebraska, Lincoln, NE 68588, USA bn Princeton University, Princeton, NJ 08544, USA bo Columbia University, New York, NY 10027, USA bp University of Rochester, Rochester, NY 14627, USA bq State University of New York, Stony Brook, NY 11794, USA br Brookhaven National Laboratory, Upton, NY 11973, USA bs Langston University, Langston, OK 73050, USA bt University of Oklahoma, Norman, OK 73019, USA bu Brown University, Providence, RI 02912, USA bv University of Texas, Arlington, TX 76019, USA bw Southern Methodist University, Dallas, TX 75275, USA bx Rice University, Houston, TX 77005, USA by University of Virginia, Charlottesville, VA 22901, USA bz University of Washington, Seattle, WA 98195, USA Received 26 May 2005; received in revised form 25 July 2005; accepted 27 August 2005 Available online 15 September 2005 Editor: L. Rolandi Abstract We present a measurement of the top quark pair (t t̄) production cross section inpp̄ collisions at √ s = 1.96 TeV using events with two charged leptons in the final state. This analysis utilizes an integrated luminosity of 224–243 pb−1 collected with the DØ detector at the Fermilab Tevatron Collider. We observe 13 events in thee+e−, eµ andµ+µ− channels with an expected background of 3.2 ± 0.7 events. For a top quark mass of 175 GeV, we measure at t̄ production cross section o σt t̄ = 8.6+3.2 −2.7(stat) ± 1.1(syst) ± 0.6(lumi) pb, consistent with the standard model prediction.  2005 Elsevier B.V. All rights reserved. PACS: 13.85.Lg; 13.85.Qk; 14.65.Ha rk f fla- ged a- ry d he of b- e me y. ing - a - The top quark was discovered[1] in 1995 at the Fermilab Tevatron Collider inpp̄ collisions at √ s = 1.8 TeV. Its observation completed the third qua weak isospin doublet suggested by the absence o vor changing neutral current interactions[2] and mea- surement of theb quark weak isospin[3]. By virtue of its large mass (mt = 178.0 ± 4.3 GeV[4]), the top quark could decay into exotic particles, e.g., a char Higgs boson[5]. Such decays would lead to a me suredt t̄ production cross section (σtt̄ ) apparently de- pendent on thet t̄ final state. It is therefore necessa to precisely measureσtt̄ in all decay channels an compare it with the standard model prediction. T E-mail address: clement@fnal.gov(C. Clément). 1 Visitor from University of Zurich, Zurich, Switzerland. increased luminosity and higher collision energy√ s = 1.96 TeV at the Run II of Tevatron permit su stantially more accurate measurement ofσtt̄ in all final states. In the SU(2) × U(1) electroweak model with on Higgs doublet[6], each top quark of at t̄ pair is ex- pected to decay approximately 99.8% of the ti to a W boson and ab quark [7]. Dilepton final states arise when bothW bosons decay leptonicall These occur along with two energetic jets result from the hadronization of theb quarks and missing transverse energy (/ET ) from the high transverse mo mentum (pT ) neutrinos. In this Letter, we present measurement ofσtt̄ with 224–243 pb−1 of pp̄ col- lider data at √ s = 1.96 TeV collected with the up graded DØ detector[8]. We consider thee+e−, eµ mailto:clement@fnal.gov 60 DØ Collaboration / Physics Letters B 626 (2005) 55–64 ay % - er u- yo- nd lla- by a- nt tion umi- ere are o tio ith ine l, r two en- the cays ents o- ise - as c- tion to- in a en- ith rged e- se- king t” uce e- de- the ng side 2% e und uon to in- ia ius n- tion of . It ted ron d to the jets d n - rithm andµ+µ− final states. The electrons and muons m originate either directly from aW boson or indi- rectly from aW → τν decay. The correspondingt t̄ branching fractions(B) are 1.58%, 3.16%, and 1.57 [7] for the e+e−, eµ, and µ+µ− channels, respec tively. The DØ detector has a silicon microstrip track and a central fiber tracker located within a 2 T s perconducting solenoidal magnet[8]. The surrounding liquid-argon/uranium calorimeter has a central cr stat covering pseudo-rapidities|η| up to 1.1,2 and two end cryostats extending coverage to|η| ≈ 4 [9]. A muon system[10] resides beyond the calorimetry, a consists of a layer of tracking detectors and scinti tion trigger counters before 1.8 T toroids, followed two similar layers after the toroids. Luminosity is me sured using plastic scintillator arrays located in fro of the end cryostats. The trigger and data acquisi systems are designed to accommodate the high l nosities of Run II. The data used in this analysis w collected by requiring two leptons (e or µ) in the hard- ware trigger and one or two leptons in the softw triggers[8]. To extract thet t̄ signal, we select events with tw high-pT isolated leptons, large/ET , and at least two jets. We further improve the signal to background ra by selecting events with kinematics compatible w t t̄ events. To derive the cross section we determ the overall efficiencyε (including trigger, geometrica and event selection efficiencies) fort t̄ and the numbe of expected background events. We distinguish categories of backgrounds: “physics” and “instrum tal”. Physics backgrounds are processes in which charged leptons arise from electroweak boson de and the/ET originates from highpT neutrinos. This signature arises inZ/γ ∗ → τ+τ− where theτ leptons decay leptonically, andWW/WZ (diboson) produc- tion. Instrumental backgrounds are defined as ev in which (a) a jet or a lepton within a jet fakes the is lated lepton signature, or (b) the/ET originates from misreconstructed jet or lepton energies or from no in the calorimeter. 2 Rapidity y and pseudo-rapidityη are defined as func tions of the polar angleθ and parameterβ as y(θ,β) ≡ 1 2 ln [(1+ β cosθ)/(1− β cosθ)]; η(θ) ≡ y(θ,1), whereβ is the ra- tio of a particle’s momentum to its energy. The electrons used in the analysis are defined clusters of calorimeter cells for which (a) the fra tion of energy deposited in the electromagnetic sec of the calorimeter has to be at least 90% of the tal cluster energy, (b) the energy is concentrated narrow cone and isolated from further calorimeter ergy, (c) the shape of the shower is compatible w that of an electron, (d) the electron matches a cha track in the tracking system. In order to further r move backgrounds we use (e) a discriminant that lects prompt isolated electrons based on the trac system and calorimeter information[11]. Electrons which fulfill criteria (a) to (e) are referred to as “tigh electrons. For background calculations we introd “loose” electrons for which only (a) and (b) are r quired. The muons considered in the analysis are fined as tracks reconstructed in the three layers of muon system, with a matching track in the tracki system. The energy deposited in the calorimeter in a hollow cone around the muon must be less than 1 of the muonpT . To further remove background, th sum of the charged track momenta in a cone aro the muon track has to be smaller than 12% of the m pT . Muons that fulfill all these criteria are referred as “tight” muons. For background calculations, we troduce “loose” muons for which the isolation criter are relaxed. Jets are reconstructed with a fixed cone of rad R = 0.53 and must be confirmed by the indepe dent calorimeter trigger readout. Jet energy calibra is applied to the jets[13]. The /ET is equal in mag- nitude and opposite in direction to the vector sum all significant calorimeter cell transverse energies is corrected for the transverse momenta of all isola muons, as well as for the corrections to the elect and jet energies. Event selections for each channel are optimize minimize the expected statistical uncertainty on cross section. We select events with at least two with p j T > 20 GeV and|y| < 2.5 (see footnote2) and two leptons withp T > 15 GeV. Muons are accepte in the region|η| < 2.0, while electrons must be withi |η| < 1.1 or 1.5< |η| < 2.5. The two leptons are re 3 Jets are defined using the iterative seed-based cone algo with R = √ ( φ)2 + ( η)2 = 0.5 (whereφ is the azimuthal an- gle), including mid-points as described in Section 3.5 (p. 47) of[12]. DØ Collaboration / Physics Letters B 626 (2005) 55–64 61 e c- on ec- ion o c- t he ise e ro- on. - nta ri- ived ) us- - r o- ors ts to de- cy, om P- the ss the es- r ck- am- the the lec- ons - ave al ke ly- e in ure ht ke e- ress ding a. ght h a ted quired to be of opposite signs in thee+e− andµ+µ− channels. A cut on /ET is crucial to reduce the otherwis large Z/γ ∗ background. This background is parti ularly severe in thee+e− and µ+µ− channels. Due to different resolutions in electron energies and mu momenta, the optimization leads to different sel tions in the three channels. In theeµ channel, we require/ET > 25 GeV and φ(/ET ,µ) > 0.25, where φ(/ET ,µ) is the azimuthal angle between the/ET and the muon. The latter gives additional reject againstZ/γ ∗ → ττ background in events with tw jets. In thee+e− channel, we veto events with diele tron invariant mass 80� Mee � 100 GeV and require /ET > 35 GeV (/ET > 40 GeV) for Mee > 100 GeV (Mee < 80 GeV). In theµ+µ− channel, we accep events with/ET > 35 GeV. This cut is tightened at low and high values of φ(/ET ,µ1) whereµ1 denotes the leadingpT muon. Events with φ(/ET ,µ1) > 175◦ are removed. The final selection in theeµ channel requiresH T = p 1 T + ∑ (p j T ) > 140 GeV, wherep 1 T denotes thepT of the leading lepton. This cut effectively rejects t largest backgrounds for this final state which ar from Z/γ ∗ → τ+τ− and diboson production. Th e+e− analysis uses a cut on sphericityS = 3(ε1 + ε2)/2 > 0.15, whereε1 and ε2 are the two leading eigenvalues of the normalized momentum tensor[14]. This requirement rejects events in which jets are p duced in a planar geometry through gluon radiati The final selection applied in theµ+µ− channel fur- ther rejects theZ/γ ∗ → µ+µ− background. We com pute for eachµ+µ− event theχ2 of a fit to theZ → µ+µ− hypothesis given the measured muon mome and known resolutions. Selecting events withχ2 > 2 is more effective than selecting on the dimuon inva ant mass for this channel. Signal acceptances and efficiencies are der from a combination of Monte Carlo simulation (MC and data. Top quark pair production is simulated ing ALPGEN[15] with mt = 175 GeV. PYTHIA[16] is used for fragmentation and decay.B hadron and τ lepton decays are modeled via EVTGEN[17] and TAUOLA [18], respectively. A full detector simula tion using GEANT[19] is performed. Lepton trigge and identification efficiencies as well as lepton m mentum resolutions are derived fromZ/γ ∗ → + − ( = e,µ) data. These per-lepton normalization fact and momentum smearings are applied to MC even ensure the simulated samples provide an accurate scription of the data. The jet reconstruction efficien jet energy resolution and/ET resolution in the MC are adjusted to their measured values in data. To calculate the expected number of events fr physics backgrounds, we useZ/γ ∗ → τ+τ− and di- boson MC samples generated with PYTHIA and AL GEN, respectively. TheZ/γ ∗ → τ+τ− contribution is normalized to the cross section measured by DØ[20]. For the diboson processes, diboson+ 2 jets events are generated at leading order (LO) and are scaled by ratio of the next-to-leading order to LO inclusive cro sections derived for diboson inclusive production[21]. Instrumental backgrounds are determined from data. Fake electrons can arise from jets comprised sentially of a leadingπ0/η and an overlapping o conversion-produced track. We estimate this ba ground by calculating the fractionfe of loose elec- trons which appear as tight electrons in a control s ple dominated by fake electrons. In thee+e− channel the control sample consists of events that satisfied trigger and have two loose electrons. In theeµ chan- nel the events in the control sample must satisfy trigger and have one tight muon and one loose e tron. Contributions from processes with real electr (W → eν andZ/γ ∗ → e+e−) are suppressed by re quiring /ET < 10 GeV in bothe+e− andeµ channels and|Mee − MZ| > 15 GeV in thee+e− channel only. We also veto events in which both loose electrons h a matching track. We observe thatfe measured in the e+e− andeµ control samples agree within statistic errors. The predicted number of events with a fa electron in the final sample is obtained by multip ing the number ofe+e− (eµ) events with one loos electron and one tight electron (muon) byfe. An isolated muon can be mimicked by a muon a jet when the jet is not reconstructed. We meas the fractionfµ of loose muons that satisfy the tig muon criteria in a control sample dominated by fa muons. In theµ+µ− channel the control sample is d fined as events that have two loose muons. To supp physics processes with real isolated muons the lea pT muon is required to fail the tight muon criteri This cuts efficientlyZ/γ ∗ → µ+µ− events but also W → µν events where a second-leading muon mi arise from a muon in a jet. The number of events wit fake muon contributing to the final sample is estima 62 DØ Collaboration / Physics Letters B 626 (2005) 55–64 on ep- sing al ing at nt by p- of e- u- le od sim . ved ted ep- nd the ti- , en- of that ved via- s. ach of ien- he uc- el t in - re- in- mi- by counting the number of events with one tight mu and a loose muon and multiplying it byfµ. In theeµ channel the contribution from events where both l tons are fake leptons is already accounted for by u fe. The remaining contribution from events with a re electron and a fake muon, is determined by combin fe and a fake ratefµ obtained on a control sample th satisfies theeµ trigger. The processesZ/γ ∗ → + − ( = e,µ), while lacking highpT neutrinos, might have a significa amount of measured/ET due to limited/ET resolution. In thee+e− channel, this background is estimated measuring a/ET misreconstruction rate on data and a plying it to the simulation. We observe that the/ET spectrum ine+e− events with 80� Mee � 100 GeV agrees well with the/ET spectrum observed inγ + 2 jets candidate events. We obtain the/ET misrecon- struction rate in data as the ratio of the number ofγ +2 jets events passing the/ET selection divided by the number failing the selection. The/ET misreconstruc- tion rate is also consistent withZ/γ ∗ → e+e− +2 jets simulation. This rate is multiplied by the number events that fail the/ET selections but pass all other s lections. In theµ+µ− channel, the expected contrib tion ofZ/γ ∗ → µ+µ− background in the final samp is derived from events simulated with ALPGEN. Go agreement is observed between the data and the ulation in the variables/ET and φ(/ET ,µ1). This al- lows us to obtain the probability for aZ/γ ∗ → µ+µ− event to pass the/ET selection from the simulation The sample is normalized to the number of obser Z/γ ∗ → µ+µ− events in the data with 70� Mµµ � 110 GeV before the/ET selection. The number of observed events and estima physics and instrumental backgrounds in the dil ton + 2 jets sample, the integrated luminosities a the ε × B for the t t̄ signal are given inTable 1for each channel. We observe 5, 8 and 0 events in e+e−, eµ andµ+µ− channels, respectively. We es mate the probability to observe�5, �8, and exactly 0 events in thee+e−, eµ, andµ+µ− channels as 22% 43%, and 5%, respectively, using the measuredσtt̄ and taking into account systematic uncertainties. By g erating pseudo-experiments we estimate that 20% the possible outcomes have lower likelihoods than of our observation. The significance of the obser t t̄ signal over the background is 3.8 standard de tions. - Table 1 Expected signal (assumingmt = 175 GeV andσt t̄ = 7 pb) and background event yields fore+e−, eµ, andµ+µ− channels. In- strumental backgrounds include/ET and fake lepton background Total uncertainties are given Channel e+e− eµ µ+µ− Integrated luminosity (pb−1) 243 228 224 Physics backgrounds 0.3± 0.1 0.7± 0.2 0.2± 0.1 Instrumental backgrounds 0.7± 0.1 0.2± 0.1 1.1+0.4 −0.3 Total background 0.9± 0.1 0.9± 0.2 1.4± 0.4 ε × B (10−3) 1.1+0.1 −0.2 3.2+0.4 −0.3 1.0± 0.1 Expected signal 1.9+0.2 −0.3 5.1+0.6 −0.5 1.6± 0.2 Total prediction 2.8± 0.3 6.1+0.6 −0.5 2.9± 0.6 Observed 5 8 0 Table 2 Summary of systematic uncertainties onσt t̄ Source σtt̄ (pb) Jet energy calibration +0.8–0.7 Jet identification +0.3–0.6 Muon identification +0.5–0.4 Electron identification ±0.3 Trigger +0.3–0.2 Other +0.2–0.3 Total ±1.1 To compute the cross section, we calculate in e channel the probability to observe the number events seen in the data as a function ofσtt̄ given the number of background events and the signal effic cies. The combined cross section is the value ofσtt̄ that maximizes the product of the likelihoods in t three channels. The resulting top quark pair prod tion cross section at √ s = 1.96 TeV in dilepton final states is σtt̄ = 8.6+3.2 −2.7(stat) ± 1.1(syst) ± 0.6(lumi) pb for mt = 175 GeV, within errors of the standard mod theoretical prediction of 6.77 ± 0.42 pb [22] and in agreement with the recent result in Ref.[23]. We find σtt̄ also consistent with measurements carried ou different final states[11,24]. The total systematic un certainty is obtained by varying the background p diction and signal efficiencies within their uncerta ties and taking into account correlations. The do nant systematic uncertainties are given inTable 2. In DØ Collaboration / Physics Letters B 626 (2005) 55–64 63 Fig. 1. Predicted and observed (a) number of events with 0, 1 and 2 or more jets with all other selections applied, (b)/ET and (c) leading lepton pT in dilepton events after all selections. TheZ/γ ∗ contribution includese+e−, τ+τ− → eµ, andµ+µ− final states. Thet t̄ prediction is shown forσt t̄ = 7 pb. d to s nce nts ns ing nd ll. re - se- DF b- on- gen- ding o- stri- well pair - ith ing OE e); Pq, E T ed , BF po- the 95) 95) . ley, ell, addition, a 6.5% systematic uncertainty is assigne the luminosity measurement[25]. The top quark mas affects the signal efficiency, resulting in a depende of σtt̄ on mt given bydσtt̄ /dmt = −0.08 pb/GeV for mt in the range 160 GeV to 190 GeV. Fig. 1(a) shows that the observed number of eve with 0, 1, and 2 or more jets, with all other selectio applied, is consistent with the prediction (assum σtt̄ = 7 pb). Fig. 1(b) shows that the observed a predicted/ET spectra after all selections agree we Other kinematic distributions in dilepton events a also well described by the sum oft t̄ signal and back ground contributions at various steps of the event lection. The leading leptonpT spectrum in thet t̄ dilep- ton final states has recently been studied by the C Collaboration[26] and a mild excess has been o served at low transverse momenta. This is not c firmed by our data, as shown inFig. 1(c). To test agreement between data and the prediction, we erate pseudo-experiments from the predicted lea leptonpT spectrum and use our measuredσtt̄ to nor- malize thet t̄ signal. We find that 31% of the pseud experiments are less consistent with the parent di bution than the data. We conclude that data agree with the prediction. In summary, we have measured the top quark production cross section at √ s = 1.96 TeV in e+e−, eµ andµ+µ− final states to beσtt̄ = 8.6+3.2 −2.7(stat) ± 1.1(syst) ± 0.6(lumi) pb for mt = 175 GeV, in agree ment with the standard model prediction and w measurements in other final states. 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