Top Pair Production Cross Section with the ATLAS Detector

1. 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

2. 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

3. Pair Production Cross Section • 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

4. QCD Multijets • 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) Reconstruction: Athena 12.0.6 MET Light Jet Fake Rate Electron Isolation ttbar: 5200 W+N jets (W→lν): 8240 – 8251 QCD: 5061 – 5064 A. Doxiadis, M. Kayl, NIKHEF Cuts placed only on jet quantities. (See backup slides.)

5. 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 εtt→evbjjb=25.4 ± 0.001 (stat) % εtt→μvbjjb=28.4 ± 0.001 (stat) % kT D=0.4 No b-tagging. ttbar: MC@NLO.W+Njets: Alpgen.Analysis cuts: see backup slides. CERN-PH-TH/2007-066

6. 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 is84%. • Efficiency for mu20 to observe a muon in tt→μvbjjb is78%. A. Macchiolo

7. 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 Sample 5200 S. Pataraia

8. 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

9. 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

10. Backup Slides

11. 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.

12. 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.

13. Lepton Fake Rate A. Doxiadis, M. Kayl, Nikhef, ATL-COM-PHYS-2008-04

14. 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

16. 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