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Top Quark Pair Production at Tevatron and LHC

Top Quark Pair Production at Tevatron and LHC. Andrea Bangert, Young Scientist Workshop, 23.07.2007, Ringberg Castle. Overview. Top pair production Pair production as test of perturbative QCD Top decay Cross section measurements at the Tevatron

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Top Quark Pair Production at Tevatron and LHC

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  1. Top Quark Pair Production at Tevatron and LHC Andrea Bangert, Young Scientist Workshop, 23.07.2007, Ringberg Castle

  2. Overview • Top pair production • Pair production as test of perturbative QCD • Top decay • Cross section measurements at the Tevatron • Cross section measurements with the ATLAS detector • Conclusions

  3. Top Production scale μ = μR = μF Parton Density Functions • Partonic cross section σij • Short-distance hard scattering. • Calculated to NLO in perturbative QCD. • Parton density functions f(x,μ2) • Non-perturbative but universal. • Determined from fits to experimental data. Measurement of σ serves as experimental test of pQCD.

  4. Test of Perturbative QCD √s = 1.96 TeV

  5. Top Decay • Top lifetime is τt~10-24 s • No top hadrons or bound states. • Γ(t→Wb) ~ 100% • Γ(W →lν)=1/3, Γ(W→qq’)=2/3 • Top events identified by decay products: • tt → Wb Wb → lvb lvb • “dileptonic” • Low background rates • Γ = 10.3% • tt → Wb Wb → lvb jjb • “lepton+jets” • Manageable background • Γ = 43.5% • tt → Wb Wb → jjb jjb • “hadronic” or “all jets” • High multijet background rates • Γ = 46.2%

  6. Tevatron Measurements Kidonakis + Vogt: σ = 6.8 ± 0.6 pb Cacciari et al:σ = 6.7 ± 0.7 pb CDF,mt = 170 GeV: σ = 7.7 ± 0.9 pb CDF, mt = 175 GeV: σ = 7.3 ± 0.9 pb CDF Cross Section • Dilepton: Uncertainty on estimate of Z+jet, γ+jet backgrounds. • Lepton+jets: NN exploits kinematics and topology to distinguish ttbar from W+jet, QCD multijet backgrounds. • Lepton+jets: b-tagging using displaced secondary vertices. Uncertainty on εb-tag, W+Njet, QCD multijet backgrounds. • Lepton+jets: soft lepton b-tag. Uncertainty on εb-tag and mistag rate. • MET: Require missing ET. Selects tau+jets events. Trigger efficiency is dominant systematic uncertainty. • Hadronic: Uncertainty on QCD multijet rate, b-tag rate of multijet events.

  7. Cross Section Measurement with ATLAS • LHC starts up in 2008. • L = 1033cm-2s-1 • ~1 top pair per second • Use ttbar analysis to understand the detector performance. • Extract jet energy scale. • Determine missing ET and b-tagging performance. The ATLAS Detector • NLO calculation: σ = 803 ± 90 pb • NLO + NLL: σ = 833 +52–39 pb • Bonciani, Catani, Mangano, Nason, hep-ph/9801375 A. Shibata

  8. kT (D=0.4) Commissioning Analysis • Designed to perform first observation of top pair production with ATLAS. • L~100 pb-1 • 80000 top pairs. • Selection of semileptonic ttbar events: • one e or μ, 4 jets, missing ET. • Reconstruction: Take trijet combination with highest pT to represent t→Wb→jjb. • σ·Γ = 246.0 ± 3.5 (stat) pb • From Monte Carlo:σ·Γ = 248.5 pb

  9. Top Quark and W Boson Masses • mt = 163.4 ± 1.6 (stat) GeV • Generated top mass is 175 GeV. • mW = 78.90 ± 0.5 GeV. • Generated W mass is 80.4 GeV. • Trijet combination with maximal pT represents t→Wb→jjb. • Dijet combination with maximal pT represents W→jj. • Fit mass distribution using Gaussian and polynomial; mean is fitted mass.

  10. Summary • Measurement of σtt offers test of pQCD. • Theoretical calculation, √s = 1.96 TeV: σ = 6.7 ± 0.7 pb • CDF experiment: σ = 7.3 ± 0.9 pb • Theoretical calculation, √s = 14 TeV: σ = 833 +52–39 pb • ATLAS analyses currently performed using Monte Carlo generated events. • Optimization of event selection, evaluation of systematic errors is underway. • Measurement of σtt with ATLAS is scheduled for LHC startup in 2008.

  11. Backup Slides

  12. Tevatron Measurements L = 1032cm-2s-1, √s = 1.96 TeV

  13. Atlantis Atlantis is an event display designed for the ATLAS experiment.

  14. Comissioning Analysis Selection Cuts • MET > 20 GeV. • Exactly one e or μ with: • pT > 20 GeV • |η| < 2.5 • E(∆R<0.2)<6 GeV • In order to avoid the crack in the LAr calorimeter, exclude electrons with 1.35<|ηe|<1.57. • 3 jets with pT(j)>40 GeV. • 1 additional jet with pT(j4)>20 GeV. • |mjj - mW| < 10 GeV.

  15. The Commissioning Analysis • Designed to perform first observation of top pair production with ATLAS. • L~100 pb-1 • 80000 top pairs. • Selection of semileptonic ttbar events: • one e or μ • 4 jets • missing ET. • Reconstruction: Take trijet combination with highest pT to represent t→Wb→jjb. • Discard event if no dijet combination W→jj has mjj~mW. Nikhef, Udine/ICTP, A.Shibata CSC sample #5200, event generator MC@NLO

  16. Statistical Error on ε and σ • Error on efficiency: δε= √(ε (1- ε) / Ni) • δNe = √Ne, δNμ = √Nμ • δσe = δNe / Ldata εe • δσμ= δNμ/ Ldata εμ • δσ = √(δσe2 + δσμ2)

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