The top quark pair production cross-section ${\sigma}_{t\bar{t}}$ has been
measured in $p\bar{p}$ collisions at center of mass energies of 1.96 TeV using
Tevatron Run 2 data. In the begining of Run 2 both CDF and D\O\
$\sigma_{t\bar{t}}$ measurements in the {\it dilepton} channel
$t\bar{t}{\to}WbW\bar{b}{\to}\bar{\ell}{\nu}_{\ell}b{\ell}^{'}\
bar{\nu}_{{\ell}^{'}}\bar{b}$ and in the {\it lepton plus jets} channel
$t\bar{t}{\to}WbW\bar{b}{\to}q\bar{q}^{'}b{\ell}\bar{\nu}_{\ell
}\bar{b}+\bar{\ell}{\nu}_{\ell}bq\bar{q}^{'}\bar{b}$ agree with the NLO
(Next-to-Leading-Order) theoretical predictions. The presence of a top signal
in Tevatron data has been reestablished.Comment: 4 pages, 3 figure

Cabrera, S.

English

arXiv

The top quark pair production cross-section {sigma}{sub t{bar t}} has been measured in p{bar p} collisions at center of mass energies of 1.96 TeV using Tevatron Run 2 data. In the beginning of Run 2 both CDF and D0 {sigma}{sub t{bar t}} measurements in the dilepton channel t{bar t} {yields} WbW{bar b} {yields} {bar {ell}}{nu}{sub {ell}}b{ell}{prime} {bar {nu}}{sub {ell}{prime}}{bar b} and in the lepton plus jets channel t{bar t} {yields} WbW{bar b} {yields} q{bar q}{prime} b{ell}{bar {nu}}{sub {ell}}{bar b} + {bar {ell}}{nu}{sub {ell}}bq{bar q}{prime} {bar b} agree with the NLO (Next-to-Leading-Order) theoretical predictions. The presence of a top signal in Tevatron data has been reestablished

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Fermilab FERMILAB-Conf-03/139-E  June 2003TOP QUARK PRODUCTION CROSS-SECTION AT THE TEVATRON RUN 2S.CABRERA, for the CDF and D collaborations.Department of Physics, Box 90305, Duke University, Durham, NC 27708-0305, USAThe top quark pair production cross-section tt has been measured in pp collisions at centerof mass energies of 1.96 TeV using Tevatron Run 2 data. In the begining of Run 2 both CDFand D tt measurements in the dilepton channel tt!WbWb!̀`b`0`0b and in the leptonplus jets channel tt!WbWb!qq0b``b+̀`bqq0b agree with the NLO (Next-to-Leading-Order)theoretical predictions. The presence of a top signal in Tevatron data has been reestablished.1 IntroductionTo date, all direct measurements of top quark production have been performed by the CDF andD experiments at the Fermilab Tevatron collider in pp collisions. At the Tevatron top quarksare produced predominatly in pairs through the QCD proceses qq!tt and gg!tt. Top quarkscan also be produced singly via the electroweak vertex Wtb with about half the cross section,but with nal states diÆcult to extract from background.In Run 1, at center of mass energiesps = 1:8 TeV, the top pair production cross-sectionwas expected to be 5:19+0:52 0:68 pb at mtop = 175 GeV=c2 a with a 90% (20%) contribution fromqq!tt (gg!tt). The precision of the measured cross-sections by the Tevatron from about 100pb 1 of data in Run 1 was approximately 25% b. The ratio of cross-sections at 1.96 TeV (Run1) and 1.8 TeV (Run 2) is 1.2950.015, with an expected Run 2 cross-section of 6:70+0:71 0:88 pbwith 85% (15%) contribution from qq!tt (gg!tt) 2. In Run 2a, with the increased center ofmass energy and the expected integrated luminosity of 2 fb 1 we should measure tt to betterthan 7% precision and observe single top production for the rst time with a 20% precision onthe cross-section measurement 5.aResults BCMN 1 updated in 2 taking into account the most recent determinations of systematic uncertaintiesin the extraction of the PDFs.bThe tt from all channels combined measurement by CDF3 and D 4 was 6:5+1:7 :14 pb for mtop =176:16:6 GeV=c2 and 5:9 1:7 pb for mtop = 172:16:8 GeV=c2 respectively.Within the SM the top quark decays almost exclusively into Wb. The tt dilepton channel,where both W's decay leptonically to e or , has the smallest BR: 5%. In the so called \leptonplus jets" one W decays leptonically and the other hadronically giving a higher BR: 30%.The Tevatron has delivered about 170 pb 1 in Run 2a up until January 2003. The detectorupgrades have been extensive. CDF has expanded the silicon coverage and installed a new driftchamber. D has a new inner tracking (silicon and ber trackers) with a new 2T superconductingsolenoid. CDF has extended the electron identication to rapidity regions jj > 1 with a newplug calorimeter and the coverage of the muon systems.2 tt measurements in the dilepton channelThe background processes that mimic the top dilepton signature are Drell-Yan (Z=!e+e ; ),Z! , WW/WZ and processes with a real lepton and a jet or a track faking a second lepton.Dilepton selection starts with 2 high-Pt (Pt > 20 GeV=c) e or  oppositely charged. CDFrequires both leptons to be well isolated from nearby calorimeter activity greatly reducing thefake lepton, Wbb and bb backgrounds. The dilepton invariant mass, Mee or M, is required tobe outside the interval 76   106 GeV=c2 to reject Z!`+` X events. D discriminates tt fromZ's in this interval by demanding larger 6ET c than in the region outside this interval.A large 6ET is required as a signature of the two W decay neutrinos. All backgrounds withreal 6ET contribution due to the presence of neutrinos are reduced. In addition CDF requiresj6ET j > 50 GeV if ( 6ET ; ` or j) < 200 to eliminate instrumental contributions to the 6ET dueto mismeasured energies of lepton or jets d (see Figure 2). (GeV)TE0 20 40 60 80 100 120 140,closest l,jet)TE (φ∆00.511.522.533.5tMC tData eeµµData µData e2, After all cuts≥jets NTE vs φ∆CDF Run II Preliminary - Number of jets in W+jets1 2 3 4Number of tagged events0510152025303540BackgroundBackground errorstBackground + t errorstBkgnd + t)-1Data (57.5 pbCDF II preliminary≥Number of tagged eventsFigure 1: On the left side, the 5 CDF tt dilepton candidates found in 72 pb 1 in the plane ( 6ET ; nearest ` or j)versus 6ET in comparison with MC Herwig tt. On the right side, number of events in the W + jets sample with atleast one b-tag: the 3 and 4 jet bins are used to extract tt.Two high energy jets are demanded as expected by the fragmentation of the top decay bquarks. The backgrounds with softer jets originating from QCD radiation are reduced. Finally,to enhance the signal-to-background ratio, large HT is requirede. The results from CDF andD are summarized in Tables 1 and 2.3 tt measurements in the lepton+jets channelThe CDF event selection required one e or  with Pt > 20 GeV=c, 6ET > 20 GeV and at least 3high Et jets. Cosmic rays, electron conversions, Drell-Yan and tt dilepton events are removed.c 6ET is the missing energy transverse to the beam direction.d( 6ET ; ` or j) is the azimutal separation between the vector 6ET and the nearest lepton or jet.eHT is the scalar sum of the transverse energy of the leptons, jets and neutrinos in the event.Table 1: Run 2 CDF results in the tt dilepton channel for a data sample of 72 pb 1Source ee  e ``All Backgrounds 0.1030.056 0.0930.054 0.1000.037 0.300.12Expected tt!``b`0`0b 0.470.05 0.590.07 1.440.16 2.50.3Data 1 1 3 5Table 2: Run 2 D results in the tt dilepton channelSource ee  eL pb 1 48.2 42 33All Backgrounds 1.000.48 0.60.30 0.070.01Expected tt!``b`0`0b 0.250.02 0.30.04 0.500.01Data 4 2 1To increase the signal-to-background ratio, CDF uses the Silicon Vertex Detector to identifythe b-quark displaced vertices. A jet is b-tagged if it contains a secondary vertex with at leasttwo charged tracks andLxyxy> 3 f . The eÆciency for identifying at least one of the b quarksfrom tt decays is about 45%, which is measured using tt MC and corrected with a data to MCscale factor. The mistags from light quarks and gluon jets are evaluated using the negativerate of Lxy extracted from inclusive jet data and applied to W+jets data. The W/Z+heavyavour: g!bb; cc background is evaluated from W+jets data, the b tag rate and the Run 1avour composition in W+jets events. The non-W background is evaluated from W+jets dataassuming it is at in the plane of lepton calorimeter isolation versus 6ET , and extrapolated fromthe low isolation and small 6ET (non-W) region to the high isolation and large 6ET (W dominated)region. Small contributions from diboson WW/WZ, Drell-Yan and single top production areevaluated from MC (see results in Table 3).Table 3: Run 2 CDF results in the tt lepton plus jets channel with displaced vertex taggingSource W+1jet W+2jets W+3jets W4jetsBackground 33.85.0 16.42.4 2.880.05 0.870.2SM Background plus tt 34.05.0 18.652.4 7.351.4 7.622.0Data before tagging 4913 768 99 26Data (1b-tag) 31 26 7 8The D topological analysis does not use b-tagging. First, a data sample enriched with Wevents is preselected by demanding a loose e or  with Pt > 20 GeV=c, 6ET > 20 GeV and a SoftMuon Tag veto. Then, the QCD background is evaluated from data for each jet multiplicity.In the e-channel this background is due to 0 and  QCD compton in jets faking e's and inthe -channel is due to real 's from heavy avour decays. The W + 4 jets background isestimated using the Berends scaling law. Finally, the topological cuts are applied to furtherreduce background: at least 4 jets, and large values of Ht and A g (see results in Table 4).The D Soft Muon Tag analysis has same preselection as the topological analysis. Thetopological requirements on Ht and A are milder and at least 3 high-Et jets are required.fLxy is the distance in the transverse plane to the beam direction between the secondary vertex and theprimary vertex. xy is the resolution in the determination of Lxy.gThe Aplanarity A measures the relative activity perpendicular to the plane of maximum activity.Background is reduced by demanding one low momentum  in a jet coming from the semileptonicb decay (see results in Table 5).Table 4: Run 2 D results in the tt lepton plus jets topologic analysisNW NQCD All BG Exp Signal Nobs L(pb 1)e+jets 1.30.5 1.40.4 2.70.6 1.8 4 49.5+jets 2.10.9 0.60.4 2.71.1 2.4 4 40Table 5: Run 2 D results in the tt lepton plus jets Soft Muon Tag analysisAll BG Exp Signal Nobs L(pb 1)e+jets 0.20.1 0.5 2 49.5+jets 0.70.4 0.8 0 404 Summary and conclusionsThe tt production cross-section in pp collisions atps = 1:96 TeV has been determined from thenumber of observed top candidates in a given channel, the estimated background, the integratedluminosity and the tt acceptance for a top mass 175 GeV=c2 h: tt =Nobs NbkgARL. All results arein agreement with the NLO prediction: 6:70+0:71 0:88 pb. Attributing the excess of events over theexpected backgrounds to tt production in the decay channels considered, we obtain the followingrst Run 2 results: CDF dilepton channels: tt = 13:2  5:9(stat) 1:5(sys) 0:8(lum) pb. CDF lepton plus jets channels: tt = 5:3  1:9(stat)  0:8(sys) 0:3(lum) pb. D dilepton channels: tt = 29:9+21:0 15:7(stat)+14:1 6:1 (sys) 3:0(lum) pb. D lepton plus jets channels: tt = 5:8+4:3 3:4(stat)+4:1 2:6(sys) 0:6(lum) pb. D all combined channels: tt = 8:5+4:5 3:6(stat)+6:3 3:5(sys) 0:8(lum) pb.AcknowledgmentsThanks to all the participants in the CDF and D top physics groups. Their extraordinaryamount of eort made the Winter 2003 measurements reported in this proceedings a reality.Thanks to P. Azzi-Bachetta, J. Konigsberg, E. Barberis and C. Gerber to motivate and coordi-nate all this eort.References1. R. Bonciani et al, Nucl. Phys. B 529, 424 (1998),[arXiv:hep-ph/9801375].2. M. Cacciari et al, CERN-TH-2003-054, [arXiv:hep-ph/0303085].3. T. Aolder et al.,Phys. Rev. D 64, 0320022 (2001).4. V.M. Abazov et al.,Phys. Rev. D 67, 012004 (2003).5. M. Kruse for the CDF collaboration. FERMILAB-CONF-03/050-E.6. R. Zitoun, these proceedings.hFor the latest results on top mass see 6.

Top quark production cross-section at the Tevatron Run 2

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