1

Top Quark Pair Production at Tevatron and LHC

Andrea Bangert, Herbstschule fuer Hochenergiephysik, Maria Laach, September 2007

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

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

4

Test of Perturbative QCD

√s = 1.96 TeV

5

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%

6

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.

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

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

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.

• 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.

10

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

11

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

12

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.

13

Backup Slides

14

Tevatron Top Mass

15

Tevatron Cross Section Measurements

L = 1032cm-2s-1, √s = 1.96 TeV

16

Atlantis

Atlantis is an event display designed for the ATLAS experiment.

17

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)