1 Top Quark Pair Production at Tevatron and LHC Andrea Bangert, Herbstschule fuer Hochenergiephysik,...
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Transcript of 1 Top Quark Pair Production at Tevatron and LHC Andrea Bangert, Herbstschule fuer Hochenergiephysik,...
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Top Quark Pair Production at Tevatron and LHC
Andrea Bangert, Herbstschule fuer Hochenergiephysik, Maria Laach, September 2007
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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
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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: 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 εb-tag, W+Njet, QCD multijet backgrounds.• Lepton+jets: Relies on soft lepton b-tag. Main uncertainties are on εb-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.
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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.
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Cross Section Measurement with ATLAS• LHC starts up in 2008.• L = 1033cm-2s-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 ET and b-tagging performance.
• Cross section calculation for LHC: • mt = 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
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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.
• pT > 20 GeV• |η| < 2.5
• At least four jets.• First three jets: pT > 40 GeV• Fourth jet: pT > 20 GeV• |η| < 2.5
• missing ET > 20 GeV.• No b-tagging is required.
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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.
Cone4
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Cross Section Studies
kT (D=0.4)
• ~ 10% of sample used as “data”
• ~ 90% of sample used as model
• Ldata = 97 pb-1, Ndata ~ 45,000
• LMC = 970 pb-1, NMC ~ 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 εdata = εMC.
σ·Γ = 246.0 ± 3.5 (stat) pbFrom Monte Carlo: σ·Γ = 248.5 pb
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Summary
• Measurement of σtt 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 σtt with ATLAS is scheduled for LHC startup in 2008.
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Backup Slides
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Tevatron Top Mass
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Tevatron Cross Section Measurements
L = 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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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
2)