top quark pair production at lhc and tevatron.pdf

7/28/2019 Top Quark Pair Production at LHC and Tevatron.pdf

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Top Quark Pair Production atTevatron 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 theFermilab Tevatron

Cross section measurements with theATLAS detector at the LHC

Conclusions


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Top Production

Partonic cross section ij

Short-distance hard scattering.

Calculated to NLO in perturbative QCD.

Parton density functions f(x,2)

Non-perturbative but universal.

Determined from fits to experimental data.

Parton Density Functions

Measurement of serves as experimental test of pQCD.

scale = R = F


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Test of Perturbative QCD

s = 1.96 TeV


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Top Decay

Top lifetime is t~10-24 s No top hadrons or bound states.

(tWb) ~ 100%

(W l)=1/3, (Wqq)=2/3 Top events identified by decay

products:

tt Wb Wb lvb lvb dileptonic

Low background rates

= 10.3% tt Wb Wb lvb jjb lepton+jets

Manageable background = 43.5%

tt Wb Wb jjb jjb hadronic or all jets

High multijet background rates = 46.2%


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Tevatron Measurements

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

Kidonakis + Vogt: = 6.8 0.6 pbCacciari et al: = 6.7 0.7 pb

Dilepton: Largest uncertainty on estimate ofZ+jet, +jet backgrounds. Lepton+jets: NN exploits kinematics andtopology to distinguish ttbar from W+jet, QCD

multijet backgrounds. Lepton+jets: Relies on b-tagging usingdisplaced secondary vertices. Largest

uncertainty on b-tag, W+Njet, QCD multijetbackgrounds.

Lepton+jets: Relies on soft lepton b-tag. Mainuncertainties are on b-tag and mistag rate. MET: Requires missing ET. Selects tau+jetsevents. 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 argonelectromagnetic samplingcalorimeter.

Electron, photon identificationand measurements.

Hadronic calorimeter. Scintillator-tile barrel calorimeter. Copper / liquid argon hadronic

end-cap calorimeter.

Tungsten / liquid argon forwardcalorimeter.

Measurements of jet properties. Air-core toroid magnet

Instrumented with muonchambers.

Muon spectrometer.

Measurement of muonmomentum.

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: = 803 90 pb NLO + NLL: = 833 +5239 pb Bonciani, Catani, Mangano, Nason, hep-ph/9801375

A. Shibata


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Commissioning Analysis

Designed to perform firstobservation of top pairproduction with ATLAS. L~100 pb-1

Represents ~ 80,000 top pairs.

Until data is available, MonteCarlo generated events used todevelop analysis.

Selection cuts:

Designed to select semileptonicttbar events with e, .

Exactly one isolated e or. pT > 20 GeV

|| < 2.5 At least four jets.

First three jets: pT > 40 GeV

Fourth jet: pT > 20 GeV

|| < 2.5

missing ET > 20 GeV. No b-tagging is required.

= 0.53no cuts, = 0.71one lepton, = 0.82four jets, = 0.82missing Et,0

20000

40000

60000

80000

100000

120000

140000

Cut Flow

= 17.55%Semileptonic ttbar with e,

= 25.13%,Semileptonic ttbar with


= 7.73%Dileptonic ttbar,


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Top Quark and W Boson Masses

Mass [GeV]0 50 100 150 200 250 300 350 400

Events

/4.0

GeV

0

100

200

300

400

500

600

-1Mass of Best Hadronic W Candidate for L = 97 pb





= 0.10%Hadronic ttbar,

= 8.63%W+N Jets Background,

= 13.25%MCatNLO leptonic ttbar,

2 / DoF = 1.21= 4.72,= 78.90,Signal + Background,

Combinatorial + Physics Background

Mass [GeV]0 100 200 300 400 500 600 700 800

E

vents/4.0

GeV

0

50

100

150

200

250

300

350

-1Top Quark Mass for L = 97 pb





= 0.10%Hadronic ttbar,

= 8.63%W+N Jets Background,




Trijet combination with maximal pTrepresents tWbjjb.Dijet combination with maximal pTrepresents Wjj.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

Mass [GeV]0 100 200 300 400 500 600 700 800

Events/4.0

GeV

0

2040

60

80

100

120

140

160

180

200

220

-1Top Quark Mass for L = 97 pb








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 fromMonte Carlo.

Assume data = MC.

= 246.0 3.5 (stat) pbFrom Monte Carlo: = 248.5 pb


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Summary

Measurement oftt offers test of pQCD. Tevatron: Theoretical calculation, s = 1.96 TeV: = 6.7 0.7 pb CDF experiment: = 7.3 0.9 pb

LHC: Theoretical calculation, s = 14 TeV: = 833 +5239 pb ATLAS analyses currently performed using Monte Carlo

generated events.

Optimization of event selection and reconstruction, andevaluation of systematic errors is underway.

Measurement oftt with ATLAS is scheduled for LHCstartup in 2008.


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Backup Slides


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Tevatron Top Mass

)2

Top Quark Mass (GeV/c150 160 170 180 190 200

Tevatron 1.8170.9(Run I/Run II, March 2007)

/dof = 9.2/102

Lepton+Jets: D0 2.7170.5)

-1( 900 pb

Lepton+Jets: CDF 2.5170.9)

-1( 940 pb

Dilepton: D0 8.0172.5)

-1

(1000 pb

Dilepton: CDF 5.6164.5)

-1(1030 pb

All-Jets: CDF

4.3171.1)-1( 943 pb

Best Tevatron Run II (preliminary, March 2007)


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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 and

Error on efficiency: = ( (1- ) / Ni) Ne = Ne, N = N

e = Ne / Ldata

e = N / L

data

= (e2

+ 2

)

top quark pair production at lhc and tevatron.pdf

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