1. 1 Top Quark Pair Production at Tevatron and LHC Andrea Bangert, Herbstschule fuer Hochenergiephysik, Maria Laach, September 2007

2. 2 Overview Top pair production Pair production as test of perturbative QCD Top decay Cross section measurements at the Fermilab Tevatron Cross section measurements with the ATLAS detector at the LHC 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: Largest 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: Relies on b-tagging using displaced secondary vertices. Largest uncertainty on ε b-tag, W+Njet, QCD multijet backgrounds. Lepton+jets: Relies on soft lepton b-tag. Main uncertainties are on ε b-tag and mistag rate. MET: Requires missing ET. Selects tau+jets events. Trigger efficiency is dominant systematic uncertainty. Hadronic: Largest uncertainties are on QCD multijet rate and b-tag rate of multijet events.

7. 7 The ATLAS Detector Lead / liquid argon electromagnetic sampling calorimeter. Electron, photon identification and measurements. Hadronic calorimeter. Scintillator-tile barrel calorimeter. Copper / liquid argon hadronic end-cap calorimeter. Tungsten / liquid argon forward calorimeter. Measurements of jet properties. Air-core toroid magnet Instrumented with muon chambers. Muon spectrometer. Measurement of muon momentum.. Inner Detector surrounded by superconducting solenoid magnet. Pixel detector, semiconductor tracker, transition radiation tracker. Momentum and vertex measurements; electron, tau and heavy-flavor identification.

8. 8 Cross Section Measurement with ATLAS LHC starts up in 2008. L = 10 33 cm -2 s -1 ~1 top pair per second Observation of top pair production will be initial landmark for ATLAS. Use ttbar analysis to understand the detector performance. Extract jet energy scale. Determine missing E T and b-tagging performance. Cross section calculation for LHC: m t = 175 GeV, √s = 14 TeV NLO calculation: σ = 803 ± 90 pb NLO + NLL: σ = 833 +52 –39 pb Bonciani, Catani, Mangano, Nason, hep-ph/9801375 A. Shibata

9. 9 Commissioning Analysis Designed to perform first observation of top pair production with ATLAS. L~100 pb -1 Represents ~ 80,000 top pairs. Until data is available, Monte Carlo generated events used to develop analysis. Selection cuts: Designed to select semileptonic ttbar events with e, μ. Exactly one isolated e or μ. p T > 20 GeV |η| < 2.5 At least four jets. First three jets: p T > 40 GeV Fourth jet: p T > 20 GeV |η| < 2.5 missing E T > 20 GeV. No b-tagging is required.

10. 10 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. Cone4

11. 11 Cross Section Studies k T (D=0.4) ~ 10% of sample used as “data” ~ 90% of sample used as model L data = 97 pb-1, N data ~ 45,000 L MC = 970 pb-1, N MC ~ 450,000 Efficiencies for each channel are calculated from Monte Carlo. Number of background events in “data” is determined using information from Monte Carlo. Assume ε data = ε MC. σ·Γ = 246.0 ± 3.5 (stat) pb From Monte Carlo: σ·Γ = 248.5 pb

12. 12 Summary Measurement of σ tt offers test of pQCD. Tevatron: Theoretical calculation, √s = 1.96 TeV: σ = 6.7 ± 0.7 pb CDF experiment: σ = 7.3 ± 0.9 pb LHC: Theoretical calculation, √s = 14 TeV: σ = 833 +52 –39 pb ATLAS analyses currently performed using Monte Carlo generated events. Optimization of event selection and reconstruction, and evaluation of systematic errors is underway. Measurement of σ tt with ATLAS is scheduled for LHC startup in 2008.

13. 13 Backup Slides

14. 14 Tevatron Top Mass

15. 15 Tevatron Cross Section Measurements L = 10 32 cm -2 s -1, √s = 1.96 TeV

16. 16 Atlantis Atlantis is an event display designed for the ATLAS experiment.

17. 17 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 )