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

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

4. 4 Test of Perturbative QCD √s = 1.96 TeV

5. 5 Top Decay Top lifetime is τ t ~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. 6 Tevatron Measurements CDF Cross Section CDF, m t = 170 GeV: σ = 7.7 ± 0.9 pb CDF, m t = 175 GeV: σ = 7.3 ± 0.9 pb Kidonakis + Vogt: σ = 6.8 ± 0.6 pb Cacciari et al: σ = 6.7 ± 0.7 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 ε b-tag, W+Njet, QCD multijet backgrounds. Lepton+jets: soft lepton b-tag. Uncertainty on ε b-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. 7 Cross Section Measurement with ATLAS LHC starts up in 2008. L = 10 33 cm -2 s -1 ~1 top pair per second Use ttbar analysis to understand the detector performance. Extract jet energy scale. Determine missing E T and b-tagging performance. NLO calculation: σ = 803 ± 90 pb NLO + NLL: σ = 833 +52 –39 pb Bonciani, Catani, Mangano, Nason, hep-ph/9801375 The ATLAS Detector A. Shibata

8. 8 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 E T. 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

9. 9 Top Quark and W Boson Masses 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. m t = 163.4 ± 1.6 (stat) GeV Generated top mass is 175 GeV. m W = 78.90 ± 0.5 GeV. Generated W mass is 80.4 GeV.

10. 10 Summary Measurement of σ tt 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 σ tt with ATLAS is scheduled for LHC startup in 2008.

11. 11 Backup Slides

12. 12 Tevatron Measurements L = 10 32 cm -2 s -1, √s = 1.96 TeV

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

14. 14 Comissioning Analysis Selection Cuts MET > 20 GeV. Exactly one e or μ with: p T > 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<|η e |<1.57. 3 jets with p T (j)>40 GeV. 1 additional jet with p T (j 4 )>20 GeV. |m jj - m W | < 10 GeV.

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

16. 16 Statistical Error on ε and σ Error on efficiency: δ ε = √(ε (1- ε) / N i ) δN e = √N e, δN μ = √N μ δσ e = δN e / L data ε e δσ μ = δN μ / L data ε μ δσ = √(δσ e 2 + δσ μ 2 )