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Top Quark Pair Production at Tevatron and LHC

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Top decay. Cross section measurements at the Fermilab Tevatron ... Momentum and vertex measurements; electron, tau and heavy-flavor identification. 8 ... – PowerPoint PPT presentation

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Title: Top Quark Pair Production at Tevatron and LHC


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
scale µ µR µF
Parton Density Functions
  • Partonic cross section sij
  • 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.

Measurement of s serves as experimental test of
pQCD.
4
Test of Perturbative QCD
vs 1.96 TeV
5
Top Decay
  • Top lifetime is tt10-24 s
  • No top hadrons or bound states.
  • G(t?Wb) 100
  • G(W ?l?)1/3, G(W?qq)2/3
  • Top events identified by decay products
  • tt ? Wb Wb ? lvb lvb
  • dileptonic
  • Low background rates
  • G 10.3
  • tt ? Wb Wb ? lvb jjb
  • leptonjets
  • Manageable background
  • G 43.5
  • tt ? Wb Wb ? jjb jjb
  • hadronic or all jets
  • High multijet background rates
  • G 46.2

6
Tevatron Measurements
Kidonakis Vogt s 6.8 0.6 pb Cacciari et
al s 6.7 0.7 pb
CDF, mt 170 GeV s 7.7 0.9 pb CDF, mt 175
GeV s 7.3 0.9 pb
CDF Cross Section
  • Dilepton Largest uncertainty on estimate of
    Zjet, ?jet backgrounds.
  • Leptonjets NN exploits kinematics and topology
    to distinguish ttbar from Wjet, QCD multijet
    backgrounds.
  • Leptonjets Relies on b-tagging using displaced
    secondary vertices. Largest uncertainty on
    eb-tag, WNjet, QCD multijet backgrounds.
  • Leptonjets Relies on soft lepton b-tag. Main
    uncertainties are on eb-tag and mistag rate.
  • MET Requires missing ET. Selects taujets
    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
  • Inner Detector surrounded by superconducting
    solenoid magnet.
  • Pixel detector, semiconductor tracker,
    transition radiation tracker.
  • Momentum and vertex measurements electron, tau
    and heavy-flavor identification.
  • 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.

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, vs 14 TeV
  • NLO calculation s 803 90 pb
  • NLO NLL s 833 5239 pb
  • Bonciani, Catani, Mangano, Nason, hep-ph/9801375

A. Shibata
9
Commissioning Analysis
  • Designed to perform first observation of top pair
    production with ATLAS.
  • L100 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 gt 20 GeV
  • ? lt 2.5
  • At least four jets.
  • First three jets pT gt 40 GeV
  • Fourth jet pT gt 20 GeV
  • ? lt 2.5
  • missing ET gt 20 GeV.
  • No b-tagging is required.

10
Top Quark and W Boson Masses
  • 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.
  • 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.

Cone4
11
Cross Section Studies
  • 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 edata eMC.

sG 246.0 3.5 (stat) pb From Monte Carlo sG
248.5 pb
12
Summary
  • Measurement of stt offers test of pQCD.
  • Tevatron
  • Theoretical calculation, vs 1.96 TeV s 6.7
    0.7 pb
  • CDF experiment s 7.3 0.9 pb
  • LHC
  • Theoretical calculation, vs 14 TeV s 833
    5239 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 stt 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, vs 1.96 TeV
16
Atlantis
  • Atlantis is an event display designed for the
    ATLAS experiment.

17
Statistical Error on e and s
  • Error on efficiency de v(e (1- e) / Ni)
  • dNe vNe, dNµ vNµ
  • dse dNe / Ldata ee
  • dsµ dNµ / Ldata eµ
  • ds v(dse2 dsµ2)
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