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Update on tt-bar signal and background simulation

This update provides a summary of the last meeting, including information on MC@NLO simulation, running in ATLAS, comparison to LO generators, and issues with AlpGen and Pythia. It also discusses the top mass reconstruction, changes in the Pt(tt-bar) system in Pythia, and the W+jets background.

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Update on tt-bar signal and background simulation

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  1. Update on tt-bar signal and background simulation Stan Bentvelsen

  2. Resume of last meeting • MC@NLO: • Matching NLO calculations of QCD process with parton shower MC simulation • Fully exclusive events generated • Hard emissions treated as in NLO • Soft emissions handled by MC shower (Herwig) • No ‘double counting’ between these two • Running in ATLAS: • Create event file using ‘runNLO’ program (extern) • Contains kinematic of hard NLO process • Interface to Herwig via McAtNLO_i

  3. 13.5% 86.5% Resume: Weights ‘standard’ tt production process -1706 • Weights: ±w • ‘unweighted’ events, up to a sign!(practically weight ±1) • Efficient event generation possible • NLO distributions (without MC showering) are non-physical tt production cross section MCatNLO: 842 pb HERWIG: 458 pb PYTHIA: 490 pb (nb: no consistent pdf’s!)

  4. Resume: Comparison to LO generators • Pt(tt system) • Herwig & MCatNLO agree at low Pt, • At large Pt MCatNLO ‘harder’ • PYTHIA completely off Same distribution on linear scale All distributions normalised to 1

  5. AlpGen ‘hard multiparton’ generator • Many hard processes – with extra n-jets (‘light jets’) • E.g.: tt+n-jets, W+n-jets • Exact (LO) matrix element • Alpgen generates file with hard scattering • To be fed into Herwig/Pythia shower MC’s • Problems: (AlpGen v1.3 & Herwig_i-00-01-18) • Compiler optimization problems on Linux gcc 3.2 • Works fine under gcc 2.96; subtle problem! • Solution: -Compiler optimization flag change to –O (was –O2) -Use f90 version of the generator • Interface in Herwig not comply to Alpgen V1.3 • On list t.b.d. for next release (do not know actual status) • Private version working

  6. Alpgen: tt+1jet • Inputs • Mtop=175 • 1 extra light jet • Jet: Pt>10, ||<2.5, R>0.4 • Initial grid 3 * 200000 • Events: 40.106 • Produced 60 samples • Production • Un-weighting to single lepton (e,,) decay • Effective : 293 pb • 1.9 106 events generated (8 10-4 efficiency) • 18.1% (351000) events pass first selection • ETmiss>20 GeV, lepton (e, ) Pt>20 and >=4 jets Pt>40

  7. tt-system alpgen affected by extra gluon Previously problems, now solved! Histograms normalized to unity Extra jet: Pt-min = 10 GeV |η| < 2.5 R>0.4 AlpGen tt+1-jet production Alpgen looks ok!

  8. Top mass reconstruction • Simple kinematic reconstruction • First selection: • Event ETmiss>20 GeV • 1 lepton (e,) with pT>20 GeV • At least 4 jets (cone size R=0.4) with ||<2.5 and pT>40 GeV • Reconstructed W: • |W(reco)-W(true)|<20 GeV • 1 b-tagged jet: Opening angle (b,W) < (b,l) • 2 b-tagged jets: Combination with maximum resulting Pt for top • ‘Commissioning’ (i.e. no b-tag):Exactly 4 jets

  9. Reconstructed top mass Changes wrt Herwig (selection wrt previous)

  10. Pt tt-bar system in Pythia • Suggestion by Sjöstrand: • Increase ISR phase space for Pythia generator • Set process scale Q2=s • MSTP(32) = 10 • Raise maximum scale of initial shower to s • MSTP(68) = 2 (default: maximum scale upto Q2) • Do not use cone restrictions from ISR to top quarks • MSTP(67) = 0 • Events generated; results not yet shown to author. Equiv. upto s?

  11. Various pythia options Pythia0: All 3 options set Pythia1: MSTP(32) = 10 Pythia2: MSTP(68) = 2 Pythia3: MSTP(67) = 0 Pythia ‘overshoot’ hard Pt spectrum by opening phase space Cone restriction little effect by itself Pt tt-bar system in Pythia • None of the Pythia options describe the hard Pt spectrum as in Herwig or MC@NLO (n.b: NLO ME calculations coincide at high Pt with MC@NLO)

  12. Pt tt-bar system in Pythia Changes wrt Standard pythia (selection wrt previous) ~20% variation

  13. Top mass with pythia • Large differences in Pt spectrum for various Pythia settings • Upto 20% difference in final selection efficiencies • Effect on resulting top mass less dramatic • Does these settings have consequences for other processes in Pythia? • Need to get opinion of Sjöstrand • No final conclusion on this yet

  14. W+jets background • Most important background: W+n jets • Leptonic decay of W, and n=4 extra jets • In Pythia only relevant process: qq’W (+q(g) ) • No ‘hard’ matrix element for 4 extra jets • I.e.: 3 or 4 extra jets need to be generated by • Fragmentation • Decays • Detector response • Reconstruction • MC@NLO has NLO qq’ W+X • No ‘hard’ matrix element for 4 extra jets • Generated 350k events, only 1 event passed first selection • lepton (e, ) Pt>20 and >=4 jets Pt>40 • Alpgen does have ‘hard’ matrix element for 4 extra jets Very unlikely and no reliable rate nor distributions

  15. Due to small generation efficiencies in Alpgen: Use local NIKHEF LCG grid Currently 30% of total LCG grid This will change soon Total 240 CPU’s Mix of PIII: 0.8, 1.2, 2.0 and 2.6 GHz machines NIKHEF data processing facility AlpGen jobs running!

  16. NIKHEF data processing facility • For alpgen event generation (+Atlfast): • Many tries to debug ‘job submission’ • Taking advantage of ‘empty farm’ • Total submitted jobs: 2303 • Total GHzHrs (equivalent hours on 1 GHz machine): 15469 (!) Large fraction of ‘playing around’ as well…

  17. Alpgen: W+4jets • Main use of background production • Inputs • W+4 extra light jets • Jet: Pt>10, ||<3.0, R>0.3 • No lepton cuts • Initial grid: 200000*3 • Events: 150·106 • Jobs: 198 • Production: • Un-weighting to W lepton (e,,) decay • Effective : 4390 pb • 108401 events generated (3.6 10-6 efficiency) • 2.57% (2784) events pass first selection • ETmiss>20 GeV, lepton (e, ) Pt>20 and >=4 jets Pt>40

  18. Alpgen: W+4jets (2) • Main use of background production • Inputs • W+4 extra light jets • Jet: Pt>10, ||<2.5, R>0.4 • No lepton cuts • Initial grid: 200000*3 • Events: 150·106 • Jobs: 98 • Lower maximum weight by factor 10 (?? Can I do this??) • Un-weighting to W lepton (e,,) decay • Effective : 2430 pb • 380740 events generated (2.6 10-5 efficiency) • 3.41% (13002) events pass first selection • ETmiss>20 GeV, lepton (e, ) Pt>20 and >=4 jets Pt>40

  19. Alpgen: W+4jets (3) • Main use of background production • Inputs • W+4 extra light jets • Jets: Pt>10, ||<2.5, R>0.4 • Lepton: Pt>30, ||<3.0, Etmiss>30. • Initial grid: 200000*3 • Events: 200·106 • Jobs: 100 • Lower maximum weight by factor 10 (?? Can I do this??) • Un-weighting to W lepton (e,,) decay • Effective : 106 pb • 39810 events generated (2 10-6 efficiency) • 25.8% (10264) events pass first selection • ETmiss>20 GeV, lepton (e, ) Pt>20 and >=4 jets Pt>40 Chosen too large for fair comparison with other data sets Data set still included in next plots just for comparison

  20. AlpGen W+4jet comparisons All histograms normalized to unity

  21. Reconstructed top mass • Normalized according to same luminosity • Large difference of Alpgen3 due to hard lepton, Pt>30 • Difference Alpgen1 and Alpgen2 only from  and R Some more work needed to check these statements. E.g. make harder cuts on parton level data sets 1 and 2 to see if coincides exactly with data set 3

  22. Luminosity: 10 pb-1 MC@NLO signal Alpgen1 sample Luminosity: 150 pb-1 MC@NLO signal Alpgen2 sample Top signal + background

  23. Top signal + background • Requiring 1 b-tag, 150 pb-1: • No mis-tag rate included…

  24. Request for DC2 (proposal) • Use for commissioning studies • Initial detector layout • Signal tt-bar events: 1 fb-1 (10%) • 2 event generators • MC@NLO 830k events (eff slightly less due to w<0) • Pythia 830k events (i.e. NLO normalisation!) • Full decay modes W’s • Background events: 200 pb-1 • Alpgen, according to Alpgen1 sample • Alpgen W+4jet 600k events • Only leptonic decays of W (e,)

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