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Top Quark Pair Production at Tevatron and LHC

Andrea Bangert, Young Scientist Workshop, 23.07.2007, Ringberg Castle

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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

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Top Production

• 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.

Parton Density Functions

Measurement of σ serves as experimental test of pQCD.

scale μ = μR = μF

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Test of Perturbative QCD

√s = 1.96 TeV

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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%

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Tevatron Measurements

CDF Cross SectionCDF, mt = 170 GeV: σ = 7.7 ± 0.9 pbCDF, mt = 175 GeV: σ = 7.3 ± 0.9 pb

Kidonakis + Vogt: σ = 6.8 ± 0.6 pbCacciari 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 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.

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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.

• 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

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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.

kT (D=0.4)

• σ·Γ = 246.0 ± 3.5 (stat) pb• From Monte Carlo: σ·Γ = 248.5 pb

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Top Quark and W Boson Masses

•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.

• 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.

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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.

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Backup Slides

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Tevatron MeasurementsL = 1032cm-2s-1, √s = 1.96 TeV

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Atlantis

Atlantis is an event display designed for the ATLAS experiment.

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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.

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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

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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 + δσμ

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