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TOP CROSS SECTION MEASUREMENTS AT THE TEVATRON

TOP CROSS SECTION MEASUREMENTS AT THE TEVATRON. SUSANA CABRERA IFIC (CSIC-University of Valencia) on behalf of the CDF & D0 collaborations. XIV International Workshop On Deep Inelastic Scattering Tsukuba, 20-24 APRIL 2006. Top pair production : from TEVATRON to LHC.

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TOP CROSS SECTION MEASUREMENTS AT THE TEVATRON

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  1. TOP CROSS SECTION MEASUREMENTSAT THE TEVATRON SUSANA CABRERA IFIC (CSIC-University of Valencia) on behalf of the CDF & D0 collaborations XIV International Workshop On Deep Inelastic Scattering Tsukuba, 20-24 APRIL 2006

  2. Top pair production : from TEVATRON to LHC At TEVATRON √s=1.96 TeV: Cacciari et al. JHEP 0404:068 (2004) Kidonakis & Vogt PRD 68 114014 (2003) stt (theo)  6.7± 0.8 pb (MTOP=175GeV/c2) σtt(NLO THEO): 12% ACCURACY At LHC √s=14 TeV: 10% qq vs 90% gg R.Bonciani et al. hep-ph/9801375 • Experimental precision in σtt never achieved before • Exhaustive test of the QCD theory • We can find new physics in the top sample. Now at the TEVATRON

  3. OUTLINE & MAIN SIGNATURES • L(e,μ)+JETS CHANNEL: • Kinematics, NN • (CDF, L=760 pb-1) • Secondary vertex b-tag • (CDF, L=695 pb-1) • ALL HADRONIC: • Observed mass spectrum • (DØ, 360 pb-1) • tau+JETS • Missing Et + JETS & Secondary vertex b-tag • (CDF,L=311 pb-1) • DILEPTON CHANNEL: • ee,µµ,eµ • (CDF, L=750 pb-1) • e,µ + track with secondary vertex tag tagging • (DØ, L=370 pb-1) • Dilepton Inclusive (CDF, L=360 pb-1) t →Wb ~ 100% (SM) Main signatures tt  llbb di-lepton 5% e+ bkgrd low tt  lqqbb lepton+jets 30% e+ bkgrd moderate tt  qqqqbb all hadronic 45% bkgrd high

  4. THE DILEPTON CHANNEL. b t e,µ W   W e,µ t b q q Final State from Leading Order Diagram • BACKGROUNDS: • Physics:WW,WZ, Z  tautau • Real MET from neutrinos • High E T jets from extra QCD radiation • MC DRIVEN • Instrumental • Fake leptons in W(→lν)+>=3 jets • DY/Z→ee/μμ with mismeasured MET. • CHALLENGE Determination relies on DATA. SIGNATURE • 2 high P T leptons, PT> 20 GeV • 2 high E T jets from b-quarks • High Missing E T (MET) from neutrinos • ANALYSES STRATEGIES: • ee,µµ,eµ (CDF, L=750 pb-1) • S/B favorable, no btagging needed • TO BE SENSITIVE TO NEW PHYSICS • Looser lepton selection: • e,µ + track (DØ, L=370 pb-1) • Looser event selection: • Dilepton Inclusive (CDF, L=360 pb-1)

  5. Dileptons: ee,μμ,eµ L=750 pb-1 CONTROL REGION NJETS=0,1 SIGNAL REGION N JETS≥2 • Veto Z´sin 76<Mee,μμ<106 • Jet Sig = MET/σ(MET) • Missing ET>25GeV • (away from any jet or lepton) • To enhance S/B: • HT > 200 GeV • ( of ET, leptons, jets & MET) • To reduce fake leptons from W+(≥2jets) • Leptons oppositely charged

  6. Dileptons: ee,μμ,eµ L=750 pb-1 • MAIN SYSTEMATICS • Jet Energy Scale • DY/Z →ee/μμ & fakes background method

  7. e,μ+track vertex btag & eµ L= 370 pb-1 • TO INCREASE ACCEPTANCE • Release lepton ID on second leg • PRICE TO PAY: MORE BACKGROUND • High MET, cut dependent on Meµ,track • Need to use b-tagging • At least 1 b-tagged jet PRE-TAGGED NJETS=1 PRE-TAGGED NJETS>=2

  8. e,μ+track vertex btag & eµ L= 370 pb-1 After b-tagging • MAIN • SYSTEMATIC • Jet Energy Scale

  9. Global high-Pt dilepton analysis L=360 pb-1 WW ttbar W+jets WZ New physics? Missing energy W+g ZZ Ztt DY(ee,mm) Jet multiplicity Higher statistical power with less purity • Fit ALL SM processes in the MET vs NJETS space • In ee,mm: NEX STEP: look for new physics

  10. THE L+JETS CHANNEL q´ b e,µ t W  W q t b q q Final State from Leading Order Diagram • BACKGROUNDS: • Physics: • W+jets (Dominant) • Instrumental: • QCD multijets • -1 jet faking 1 high Pt lepton • -Missing ET from mismeasurements • ANALYSES STRATEGIES: • Event Kinematics S/B ~(1:5) • B-tagging S/B ~(3:1) SIGNATURE • 1 isolated lepton (e,µ) Pt>20 GeV • 2 jets from b-quarks • 2 jets from light quarks • High MET from neutrinos • CHALLENGES: • W+jets: • IF EVENT KINEMATICS: • MC driven: σ(W+jets) NOT precisely known • IF B-TAGGING: • W+HF(b,bb,c) (MC/DATA) • W+LF(MISTAGS) • QCD multijets: • Determination relies on DATA.

  11. l+jets with kinematics & ANN • Backgrounds: • W+(>=n jets) (MAIN) • MC (ALPGEN+HERWIG) driven • QCD multijets (3.7%) • DATA driven • “l+jets” Event Selection: • To reduce QCD multijet: • If MET<30 GeV: 0.5<ΔΦ(MET,leading jet) <2.5 • METHOD: • 7 KINEMATIC & TOPOLOGICAL VARIABLES • HT, Aplanarity, min(Mjj), min(ΔRjj) • ηMAX, , Sum(Pz)/Sum(E T), E T(2nd-j)+ET(3rd-j) • ANN (Artificial Neuronal Network) • Maximize discrimination ttbar against W+jets • Take correlations into account

  12. l+jets with kinematics: 760 pb-1 Binned Likelihood Fit • 4% QCD • 80% W+jets • 15% ttbar • Main • systematics: • 8.3% Jet Energy scale • 10.2% W+jets Q2 scale CDF Preliminary (760 pb-1)

  13. L+jets secondary vertex tag L=695 pb -1 • EVENT SELECTION: • >=1 b-tag • HT >200 GeV CONTROL REGION SIGNAL REGION 2 b tags 156 158 53.0+-6.3 17.2+-1.9

  14. L+jets secondary vertex tag L=695 pb -1 s(tt) =  8.2 ± 0.6 (stat) ± 1.0 (syst) ±0.5 (lumi) pb SYSTEMATIC ERROR DOMINATES

  15. MET+jets secondary vertex tag L=311 pb -1 • EVENT SELECTION: • Multijet trigger: • 4 high ET jets • High SumEt. • Veto high PT e or µ • ≥ 1 btag tau+jets l+jets:µ,e not identified CHALLENGE Very small S/B ! 1st)CONTROL REGION N JETS=3 Measure probability to get +btag from QCD multijets and fake MET. 2nd) SIGNAL REGION NJETS≥ 4 Apply mistag probability to sample before btagging. • 3rd) Optimize S/B with KINEMATIC cuts • Met/ √SumEt ≥ 4 GeV ½ • minΔΦ(Met,jets) ≥ 0.4 rad S/B:1/5 BEFORE BTAGGING

  16. MET+jets secondary vertex tag L=311 pb -1 After btagging : >=1btag • MAIN SYSTEMATICS: • 8.2% Generator • 10% Background predicion S/B=1.14 N expected (ttbar)=56.5 (MTOP=178 GeV/c2)

  17. ALL HADRONIC: OBSERVED MASS SPECTRUM with secondary vertex tagging • EVENT SELECTION: • 6 JETS: 2 b-tagged jetsET>45 GeV • 2 non b-tagged jets ET>20 GeV • 2 jets ET>15 GeV 2-jet mass: Mjj with 2 non-btg jets • BACKGROUND METHOD: • SHAPE: from pretagged multijet data • random jets as b-jets • Kinematic correlations: Pt-bjet, dRbb • RATE: Mjj < 65 GeV shape normalized to DATA 3-jet mass: Mbjj (1btg jet, 2 non-btg jets) TIGHT Aplanarity>0.05 Centrality>0.7 Sphericity>0.5 ΔR bb>1 LOOSE No kinematic cuts MEDIUM Aplanarity>0.05 Centrality>0.6 Sphericity>0.2 ΔRbb>1

  18. ALL HADRONIC: OBSERVED MASS SPECTRUM & secondary vertex tagging MAIN SYSTEMATICS: 25% Bkg Model. 15% Jet Energy Scale. 18% b-tagging efficiency σ(tt) = 12.1  ± 4.9 (stat) ± 4.6 (syst) pb

  19. CDF & DØ SUMMARY WEIGHT 11% 32% 50% 2% 6% -2% BEST SINGLE σMEASURED CDF COMBINED σtt=7.3±0.9 pb 15% improvement w.r.t best single σmeasured

  20. σttbar vs √s & MTOP

  21. CONCLUSIONS • Importance of s tt measurements • Check of perturbative QCD • Starting point to measure top quark properties: mass, charge, W helicity…..window for NEW PHYSICS. • Background for Higgs and other searches. • Current CDF precision from combined result ~12% , reach the current accuracy of the NLO QCD calculations CDF PRELIMINARY (MTOP=175 GeV/c2) • Combining 6 measurements with data samples up to 760 pb-1 the systematic uncertainty dominates over the statistical. • New challenge with 1 fb-1 (combining CDF & D0 )is to reduce the systematic uncertainties (Jet Energy Scale and b-tagging)

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