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 1

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 2

Top Production scale μ = μR = μF Parton Density Functions • Partonic cross section σij • Short-distance • Calculated hard scattering. to NLO in perturbative QCD. • Parton density functions f(x, μ 2) • Non-perturbative but universal. • Determined from fits to experimental data. Measurement of σ serves as experimental test of p. QCD. 3

Test of Perturbative QCD √s = 1. 96 Te. V 4

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 “dileptonic” • Low background rates • Γ = 10. 3% tt → Wb Wb → lvb jjb “lepton+jets” • Manageable background • Γ = 43. 5% tt → Wb Wb → jjb “hadronic” or “all jets” • High multijet background rates • Γ = 46. 2% 5

Tevatron Measurements Kidonakis + Vogt: σ = 6. 8 ± 0. 6 pb Cacciari et al: σ = 6. 7 ± 0. 7 pb CDF Cross Section CDF, mt = 170 Ge. V: σ = 7. 7 ± 0. 9 pb CDF, mt = 175 Ge. V: σ = 7. 3 ± 0. 9 pb • 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 6 -tag rate of multijet events.

Cross Section Measurement with ATLAS • LHC starts up in 2008. • L = 1033 cm-2 s-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 7 A. Shibata

Commissioning Analysis • Designed to perform first observation of top pair production with ATLAS. • • • Selection of semileptonic ttbar events: • • L~100 pb-1 80000 top pairs. one e or μ, 4 jets, missing ET. Reconstruction: Take trijet combination with highest p. T to represent t→Wb→jjb. k. T (D=0. 4) • σ·Γ = 246. 0 ± 3. 5 (stat) pb • From Monte Carlo: σ·Γ = 248. 5 pb 8

Top Quark and W Boson Masses • mt = 163. 4 ± 1. 6 (stat) Ge. V • Generated top mass is 175 Ge. V. • m. W = 78. 90 ± 0. 5 Ge. V. • Generated W mass is 80. 4 Ge. V. • Trijet combination with maximal p. T represents t→Wb→jjb. • Dijet combination with maximal p. T represents W→jj. • Fit mass distribution using Gaussian and polynomial; mean is fitted mass. 9

Summary • • • Measurement of σtt offers test of p. QCD. Theoretical calculation, √s = 1. 96 Te. V: σ = 6. 7 ± 0. 7 pb CDF experiment: σ = 7. 3 ± 0. 9 pb Theoretical calculation, √s = 14 Te. V: 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. 10

Backup Slides 11

Tevatron Measurements L = 1032 cm-2 s-1, √s = 1. 96 Te. V 12

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

Comissioning Analysis Selection Cuts • MET > 20 Ge. V. • Exactly one e or μ with: • p. T > 20 Ge. V • |η| < 2. 5 • E(∆R<0. 2)<6 Ge. V • In order to avoid the crack in the LAr calorimeter, exclude electrons with 1. 35<|ηe|<1. 57. • 3 jets with p. T(j)>40 Ge. V. • 1 additional jet with p. T(j 4)>20 Ge. V. • |mjj - m. W| < 10 Ge. V. 14

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 p. T to represent t→Wb→jjb. • Discard event if no dijet combination W→jj has mjj~m. W. Nikhef, Udine/ICTP, A. Shibata CSC sample #5200, event generator MC@NLO 15

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 = √(δσe 2 + δσμ 2) 16