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 Estimate background rates: QCD multijets. Electroweak W+N jet production. Top pair production in hadronic and dileptonic channels. Estimate ε tt 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. Pair Production Cross Section

4 Reducible BG MET is due to b→Wc→lνc Mismeasurement Leptons are Non-prompt ‘Fake’ Fake Rate R~10 -5 (ATLAS TDR) R = R(p T, η) Cuts placed only on jet quantities. (See backup slides.) ttbar: 5200 W+N jets (W→lν): 8240 – 8251 QCD: 5061 – 5064 Reconstruction: Athena MET Electron Isolation Light Jet Fake Rate A. Doxiadis, M. Kayl, NIKHEF QCD Multijets

5 W+N Jets Background Most dangerous background to analysis. Use MC samples to determine selection efficiency ε W+Njets. N expected = σ W+Njets ∫L dt. Theoretical uncertainty on σ W+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/ ttbar: W+Njets: Alpgen. Analysis cuts: see backup slides. ε tt→evbjjb =25.4 ± (stat) % ε tt→μvbjjb =28.4 ± (stat) % k T D=0.4 No b-tagging.

6 Trigger e25i and mu20 Plots contain ttbar events (sample 5200) Analysis cuts (see backup slides) p T (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

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 ε b-tag, f light, purity. Highly sample dependent. w>6.25: ε b-tag =50%, f light =656, P=0.99. k T, D=0.4 k T, D=0.7 S. Pataraia Sample 5200

8 b-tagging performance Weights are chosen such that ε b-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. k T, D=0.4 jets. Performance is poor for Cone7 jets. k T, D=0.7 jets. S. Pataraia

9 Summary and Outlook Measurement of σ tt 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 and Herwig. Sample 5200, TID no 1mm bug; bug in e isolation. Filter requires prompt lepton. Hadronic ttbar events and Herwig. Sample 5204, TID mm bug. Filter forbids prompt lepton. W+Njets events Alpgen and Herwig. Samples , TID mm bug. TruthJetFilter requires 3 jets with p T >30 GeV, |eta|<5. QCD multijet events Alpgen and Herwig. Samples 5061, 5062, 5063, Generated in , reconstructed in Filter requires 4 jets with |η|<6. Lead jet must have p T (j 1 )>80 GeV. 3 subsequent jets must have p T >40 GeV.

12 Selection Cuts Inclusive k T 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 p T (j 1 )>80 GeV. Three subsequent jets must have p T (j)>40 GeV. Commissioning Analysis Cuts Exactly one isolated, high-p T electron or muon. E ∆R=0.20 < 6 GeV. p T (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 p T (j) > 40 GeV. Fourth jet has p T (j 4 ) > 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

14 Uncertainty on σ W+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/ Systematic Variation at TevatronSystematic Variation at LHC Proton-antiproton collisions, 1.96 TeV, W ±, Cone7 jets, E T (j)>10 GeV, |η|<2 Proton-proton collisions, 14 TeV, only W +, Cone4 jets, E T (j)>20 GeV, |η|<4.5

15 ProcessCross section [pb] MLM match rate ε filter σ*f [pb] L [pb -1 ]Weight (100 pb -1 ) Initial events Final events Efficiency [%] stat. uncertainty Weighted final events ttbartt→(evb)(jjb) 854N/A tt→(μvb)(jjb) tt→(τvb)(jjb) tt→(lvb)(lvb) tt→(jjb)(jjb) W→ev2 partons partons partons partons W→μvW→μv2 partons partons partons partons W→τvW→τv2 partons partons partons partons unknown

16 Trigger L1 trigger 4x4 matrix of calorimeter towers. 2x2 central core is “Region of Interest” 12 surrounding towers measure isolation L1_em25i requires E T >18 GeV in ROI in EM calorimeter E T <2 GeV in ROI in hadron calorimeter Isolation: E T <3 GeV in EM calorimeter E T <2 GeV in hadron calorimeter Event Filter requires E T >18 GeV ε trigger = 99% for electrons with p T >25 GeV. L1_mu20 requires E T >17.5 GeV. Sample 5200, true tt→evbjjb events semileptonic selection cuts, p T (e) > 10 GeV