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Top Pair Production Cross Section with the ATLAS Detector

A. Bangert, N. Ghodbane, T. Goettfert, R. Haertel, A. Macchiolo, R. Nisius, S. Pataraia

Max Planck Institute, Munich

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Introduction

• Measurement of top cross section• Semileptonic channel• Using first ~ 100 pb-1 data from ATLAS in 2008

• Overview• QCD multijets• W+N jets• Selection efficiencies

• Trigger• b-tagging• Summary

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• Estimate background rates:• QCD multijets. • Electroweak W+N jet production.• Top pair production in hadronic and dileptonic channels.

• Estimate εtt• Using Monte Carlo samples.• Include trigger efficiencies.• Consider b-tagging performance.

• Luminosity: • Relative luminosity measured by LUCID.• Absolute calibration from LHC machine parameters.• Relative uncertainty δL~30% expected during initial data-taking.• https://twiki.cern.ch/twiki/pub/Atlas/AtlasTechnicalPaper/Main_jinst_submission.pdf

Pair Production Cross Section

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• Reducible BG• MET is due to

• b→Wc→lνc• Mismeasurement

• Leptons are • Non-prompt• ‘Fake’• Fake Rate R~10-5

(ATLAS TDR)

• R = R(pT, η)

Cuts placed only on jet quantities. (See backup slides.)

ttbar: 5200

W+N jets (W→lν): 8240 – 8251

QCD: 5061 – 5064

Reconstruction: Athena 12.0.6

MET

Electron IsolationLight Jet Fake Rate

A. Doxiadis, M. Kayl, NIKHEF

QCD Multijets

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W+N Jets Background• Most dangerous background to analysis.• Use MC samples to determine selection efficiency εW+Njets.• Nexpected = σW+Njets ∫L dt.

• Theoretical uncertainty on σW+Njets is large.• Uncertainty on background rate will be large.• Effective cuts are therefore needed to reduce background rate.

Systematic Uncertainty on σ(W+)+Njets

CERN-PH-TH/2007-066ttbar: MC@NLO. W+Njets: Alpgen. Analysis cuts: see backup slides.

εtt→evbjjb=25.4 ± 0.001 (stat) % εtt→μvbjjb=28.4 ± 0.001 (stat) %

kT D=0.4

No b-tagging.

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Trigger• e25i and mu20• Plots contain

• ttbar events (sample 5200)• Analysis cuts (see backup slides)• pT(l)>10 GeV

• L1 Trigger, Event Filter (EF)

• Efficiency for e25i to observe an electron in tt→evbjjb is 84%.

• Efficiency for mu20 to observe a muon in tt→μvbjjb is 78%.

A. Macchiolo

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b-tagging• Implement b-tagging to increase S/B.

• Decrease uncertainty on W+N Jets.

• IP3D+SV1 is recommended b-tagger.

• Defining weight cut gives εb-tag, flight, purity.

• Highly sample dependent.

• w>6.25: εb-tag=50%, flight=656, P=0.99.

kT, D=0.4

kT, D=0.7

S. PataraiaSample 5200

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b-tagging performance

• Weights are chosen such that

εb-tag = 50% for all algorithms.

• ‘Purification’:

Removal of true light flavor quarks within ∆R < 0.8 of true heavy flavor quarks or taus. If found, identify and discard reconstructed light flavor jet within ∆R < 0.4 of true light flavor quark.

• IP3D+SV1 is optimized for use on Cone4 jets.

• Good performance for

• Cone4 jets.

• kT, D=0.4 jets.

• Performance is poor for

• Cone7 jets.

• kT, D=0.7 jets.

S. Pataraia

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Summary and Outlook

• Measurement of σtt • in semileptonic

channel • using 100 pb-1 of first

data.

• Background rates• Selection efficiencies• Trigger performance• B-tagging

• Full Dress Rehearsal• LHC comes online…

Invariant Top Mass for t→Wb→jjb

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Backup Slides

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Samples• Semileptonic and dileptonic ttbar events

• MC@NLO and Herwig. • Sample 5200, TID 8037. • no 1mm bug; bug in e isolation.• Filter requires prompt lepton.

• Hadronic ttbar events• MC@NLO and Herwig.• Sample 5204, TID 6015.• 1mm bug.• Filter forbids prompt lepton.

• W+Njets events• Alpgen and Herwig. • Samples 8240-8250, TID 6551-6561.• 1 mm bug.• TruthJetFilter requires 3 jets with pT>30 GeV, |eta|<5.

• QCD multijet events• Alpgen and Herwig.• Samples 5061, 5062, 5063, 5064.• Generated in 11.0.42, reconstructed in 12.0.6.• Filter requires 4 jets with |η|<6.

• Lead jet must have pT(j1)>80 GeV. • 3 subsequent jets must have pT>40 GeV.

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Selection Cuts

• Inclusive kT algorithm, E scheme, D=0.4. • Hard Jet Cuts

• No cuts are applied to lepton quantities.• Require at least four jets:

• |η| < 5.0.• Lead jet must have pT(j1)>80 GeV.• Three subsequent jets must have pT(j)>40 GeV.

• Commissioning Analysis Cuts• Exactly one isolated, high-pT electron or muon.

• E∆R=0.20 < 6 GeV.• pT(l) > 20 GeV, |η| < 2.5.• Muons are reconstructed by Staco. • Electron candidates must:

• Be reconstructed by eGamma.• Fulfill (isEM==0).

• At least four jets. • |η| < 2.5• First three jets have pT(j) > 40 GeV.• Fourth jet has pT(j4) > 20 GeV.• Jets are removed if ∆R(j,e)<0.4.

• Missing Transverse Energy: MET > 20 GeV.

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Lepton Fake Rate

A. Doxiadis, M. Kayl, Nikhef, ATL-COM-PHYS-2008-04

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Uncertainty on σW+Njets

• “Comparative study of various algorithms for the merging of parton showers and matrix elements in hadronic collisions”

• J. Alwall, S. Hőche, F. Krauss, N. Lavesson, L. Lőnnblad, F. Maltoni, M. Mangano, M. Moretti, C. Papadopoulos, F. Piccinini, S. Schumann, M. Treccani, J. Winter, M. Worek.

• CERN-PH-TH/2007-066

Systematic Variation at Tevatron Systematic Variation at LHC

Proton-antiproton collisions, 1.96 TeV, W±,

Cone7 jets, ET(j)>10 GeV, |η|<2

Proton-proton collisions, 14 TeV, only W+,

Cone4 jets, ET(j)>20 GeV, |η|<4.5

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Process Cross section [pb]

MLM match rate

εfilter σ*f [pb]

L [pb-1] Weight(100 pb-1)

Initial events

Final

events

Efficiency [%]

stat. uncertainty

Weighted

final events

ttbar tt→(evb)(jjb)

854 N/A 0.54 461 770.7 0.130

94304 23937 25.38 0.14 3112

tt→(μvb)(jjb) 94489 26980 28.55 0.15 3507

tt→(τvb)(jjb) 95292 6718 7.05 0.08 873

tt→(lvb)(lvb) 70324 5313 7.56 0.10 691

tt→(jjb)(jjb) 802 0.46 369 178.2 0.561 65742 1100 1.67 0.05 617

W→ev 2 partons 2032 0.402 0.262 214 102.6 0.975 21950 15 0.068 0.018 15

3 partons 771 0.301 0.534 124 90.8 1.102 11250 113 1.00 0.09 125

4 partons 273 0.252 0.780 53.7 111.8 0.894 6000 429 7.15 0.33 384

5 partons 91 0.264 0.929 22.3 221.8 0.451 4950 774 15.64 0.52 349

W→μv 2 partons 2032 0.402 0.020 16.3 198.9 0.503 3250 46 1.42 0.21 23

3 partons 771 0.304 0.276 64.7 178.5 0.560 11550 268 2.32 0.14 150

4 partons 273 0.229 0.576 36.0 88.9 1.125 3200 362 11.31 0.56 407

5 partons 91 0.264 0.84 20.2 235.4 0.425 4750 1003 21.12 0.59 426

W→τv 2 partons 2032 0.415 0.104 87.7 234.9 0.426 20600 6 0.029 0.012 3

3 partons 771 0.303 0.373 87.1 149.2 0.670 13000 38 0.29 0.05 25

4 partons 273 0.245 0.686 45.9 108.9 0.918 5000 101 2.02 0.20 93

5 partons 91 0.264 0.866 20.8 0 0 0 0 unknown unknown unknown

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Trigger

• L1 trigger• 4x4 matrix of calorimeter towers.• 2x2 central core is “Region of Interest”• 12 surrounding towers measure isolation

• L1_em25i requires• ET>18 GeV in ROI in EM calorimeter • ET<2 GeV in ROI in hadron calorimeter• Isolation:

• ET<3 GeV in EM calorimeter• ET<2 GeV in hadron calorimeter

• Event Filter requires ET>18 GeV• εtrigger = 99% for electrons with pT>25 GeV.• L1_mu20 requires ET>17.5 GeV.

Sample 5200, true tt→evbjjb events

semileptonic selection cuts, pT(e) > 10 GeV