Status and Progress of PhD Research

1. Status and Progress of PhD Research Andrea Bangert MPI, 13.02.2007

2. Introduction Title of thesis: “Measurement of the Top-Antitop Production Cross Section with the ATLAS Detector at the LHC” Current status: Analysis of Monte Carlo samples in preparation for analysis of data.

3. Analysis of Monte Carlo Samples • Hard matrix elements are generated by MC@NLO, AcerMC, Alpgen. (ASCII file.) • Parton shower and hadronization of events performed by Herwig or Pythia. (Pool file.) • Detector simulation is performed, events are reconstructed in Athena. (ESD → AOD). • AOD → ROOT Ntuple • Analysis is performed upon Ntuple, histograms are filled in ROOT.

4. 2006 • Used MC@NLO to generate 1500 ttbar events for each top mass. • Herwig performed parton shower and hadronization. • No detector simulation or event reconstruction was performed. • June 2006 • Learned techniques of event generation. • June 2006 • Attended CERN School of Particle Physics. • September 2006 • Began analysis on Ntuples. • November 2006 • Attended two C++ programming courses at CERN.

5. Current Status of Analysis • Samples • Selection Cuts • Fit to Top Mass, W boson Mass • Agreement between independent Monte Carlo samples • Estimate of pair production cross section

6. Samples • Semileptonic and dileptonic ttbar events, • csc11.005200, σ=461 pb, L=1072 pb-1 • NLO event generator MC@NLO / Herwig, CTEQ6M PDFs • Athena 11.0.42 • Hadronic ttbar events, • csc11.005204, σ=369 pb, L=105 pb-1 • NLO event generator MC@NLO / Herwig, CTEQ6M PDFs • Athena 11.0.42 • Semileptonic and dileptonic ttbar events, • csc11.005205, σ=460 pb, L=643 pb-1 • LO event generator AcerMC / Pythia, CTEQ6M PDFs • Athena 11.0.42 • Inclusive W+N partons background, • rome.003017, σ=1200 pb, L=136 pb-1 • LO event generator Alpgen, CTEQ5M PDFs • Generated with Athena 10.0.2 • Reconstructed Athena 11.0.42

7. Selection Cuts • Consider events in the semileptonic ttbar channel where the lepton is an electron or a muon. • Γ = 29.9% • Exactly one isolated, high-pT electron or muon: • For e “isolated” means E∆R=0.45<7 GeV • For μ “isolated” means E∆R=0.20<1 GeV • pT(l)>20 GeV, |η|<2. • Require (isEM==0) for electrons • At least four jets: • Jets are reconstructed using Cone4 algorithm. • |η|<2.5 • pT(j1)>50 GeV, pT(j2)>40GeV, • pT(j3)>30 GeV, pT(j4)>20GeV • Missing ET>20 GeV • No b-tagging is required.

8. Fit to Top Mass, W Mass • mtfit = 163.7 ± 1.5 GeV • mtPDG = 174.2 ± 3.3 GeV mWfit = 83.05 ± 1.14 GeV mWPDG = 80.40 ± 0.03 GeV

9. Agreement between independent Monte Carlo samples • Used AcerMC leptonic ttbar sample as “data”. • Weighted all other samples to “data” luminosity: • wsample = Ldata / Lsample • Rough agreement between “data” and Monte Carlo in normalization and shape of distributions. • Efficiencies do not agree: • εAcerMC = 0.0649 ± 0.0005 • εMC@NLO = 0.0803 ± 0.0004 • For MC@NLO sample: • εe = 18.7%, εμ = 5.60% • ετ = 1.98%, εll = 4.97% • Identification of event channel is not available for AcerMC sample.

10. Estimate of ttbar Cross Section • MC@NLO ttbar sample was used as Monte Carlo. • AcerMC ttbar sample was used as “data”. • LMC = 1072 pb-1, Ldata = 643 pb-1 • Number of initial and final events in each decay channel was known from Monte Carlo. • εdata = εMC was assumed. • For semileptonic ttbar events with electron or muon: • σdata = (Nedata/LdataεeMC) + (Nμdata/LdataεμMC) • σdata = 200 ± 2 (stat) pb • σtheory = 232 pb

11. Summary and Goals • Cross section estimate is σdata = 200 ± 2 (stat) pb • σtheory = 232 pb • Poor cross section estimate reflects difference between εdata and εMC. • εdata / εMC ~σdata / σtheory ~ 0.8 • Need to understand differences between AcerMC and MC@NLO samples. • Perform cut flow analysis. • Switch from Cone4 to kT jet reconstruction algorithm. • Use new csc sample for W+N partons background when available. • Obtain QCD multijet background sample. • Partially reconstruct leptonic W and leptonic top.

12. Goals for 2007 • Presentation at DPG in March: • “Study of Top-Antitop Production with the ATLAS Detector at the LHC” • Presentation to top working group at CERN during “Trigger and Physics Week” in March. • Attend tutorial on event generators in April. • Contribute to CSC note on top cross section. Deadline is May. • Perform local alignment validation using CSC data; write up results.

13. Backup Slides

14. Cross Section Estimate • Number of initial events in each channel was known from Monte Carlo. • Number of final events in each channel after selection cuts was obtained from Monte Carlo. • Efficiency was computed for each channel: • εMC = NfMC/ NiMC • Fraction of events in each channel was calculated using Monte Carlo: • FeMC = NfMC(e) / NfMC • Fraction of final events per decay channel and efficiency obtained from Monte Carlo were assumed to be valid for “data”. • Nfdata(e) = FeMCNfdata, Nfdata(μ) = FμMCNfdata

15. Information from Monte Carlo • MC@NLO leptonic ttbar sample.

16. Statistical Error on Cross Section • Nfdata(e) = FeMC Nfdata = 12049 = Ne • Nfdata(μ) = FμMC Nfdata = 3606 = Nμ • δNe = √Ne, δNμ = √Nμ • δσe = δNe / Ldataεe = 0.91 pb • δσμ= δNμ/Ldataεμ = 1.67 pb • δσ = √(δσe2 + δσμ2) = 1.9 pb

17. isEM: Quality Control for Electrons • The isEM flag is designed to identify electrons and reject jets. • Represents result of combinations of cuts imposed on quantities reconstructed within: • Electromagnetic and hadronic calorimeters: • Very little hadronic leakage (1st bit). • Energy deposit in electromagnetic calorimeter is narrow in width (2nd bit). • Energy deposit in electromagnetic calorimeter has one narrow maximum, no substructure (3rd bit). • Inner detector: • At least nine precision hits from pixel detector and semiconductor tracker; small transverse impact parameter. • η and φ of track are extrapolated to calorimeter cluster; extrapolated and measured values are required to match. • Energy measured in electromagnetic calorimeter is required to match momentum measured in inner detector. • Requiring isEM==0 demands that each electron candidate pass all of above cuts before being accepted.

18. Cross Section as function of mt Scale dependence of ttbar production rate at fixed order. “Top Quark Physics”, hep-ph/0003033