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Top Quark Pair Production at Tevatron and LHC. Andrea Bangert, Herbstschule fuer Hochenergiephysik, Maria Laach, September 2007. Overview. Top pair production Pair production as test of perturbative QCD Top decay Cross section measurements at the Fermilab Tevatron - PowerPoint PPT Presentation
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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)