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

P 2 Resume of last meeting  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

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

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

P 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 ) 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

P 6 Alpgen: tt+1jet Inputs M top =175 1 extra light jet Jet: P t >10, |  | 0.4 Initial grid 3 * Events: Produced 60 samples Production  Un-weighting to single lepton (e, ,  ) decay  Effective  : 293 pb events generated ( efficiency) 18.1% (351000) events pass first selection  E T miss >20 GeV, lepton (e,  ) P t >20 and >=4 jets P t >40

P 7 AlpGen tt+1-jet production 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 looks ok!

P 8 Top mass reconstruction Simple kinematic reconstruction First selection:  Event E T miss >20 GeV  1 lepton (e,  ) with p T >20 GeV  At least 4 jets (cone size  R=0.4) with |  | 40 GeV Reconstructed W:  |W(reco)-W(true)|<20 GeV 1. 1 b-tagged jet: Opening angle  (b,W) <  (b,l) 2. 2 b-tagged jets: Combination with maximum resulting Pt for top 3. ‘Commissioning’ (i.e. no b-tag): Exactly 4 jets

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

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

P 11 Pt tt-bar system in Pythia 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 None of the Pythia options describe the hard Pt spectrum as in Herwig or (n.b: NLO ME calculations coincide at high Pt with

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

P 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

P 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  has NLO qq’  W+X No ‘hard’ matrix element for 4 extra jets Generated 350k events, only 1 event passed first selection  lepton (e,  ) P t >20 and >=4 jets P t >40  Alpgen does have ‘hard’ matrix element for 4 extra jets Very unlikely and no reliable rate nor distributions

P 15 NIKHEF data processing facility 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 AlpGen jobs running!

P 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): (!) Large fraction of ‘playing around’ as well…

P 17 Alpgen: W+4jets Main use of background production Inputs W+4 extra light jets Jet: P t >10, |  | 0.3 No lepton cuts Initial grid: *3 Events: 150·10 6 Jobs: 198 Production:  Un-weighting to W lepton (e, ,  ) decay  Effective  : 4390 pb events generated ( efficiency) 2.57% (2784) events pass first selection  E T miss >20 GeV, lepton (e,  ) P t >20 and >=4 jets P t >40

P 18 Alpgen: W+4jets (2) Main use of background production Inputs W+4 extra light jets Jet: P t >10, |  | 0.4 No lepton cuts Initial grid: *3 Events: 150·10 6 Jobs: 98 Lower maximum weight by factor 10 (?? Can I do this??)  Un-weighting to W lepton (e, ,  ) decay  Effective  : 2430 pb events generated ( efficiency) 3.41% (13002) events pass first selection  E T miss >20 GeV, lepton (e,  ) P t >20 and >=4 jets P t >40

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

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

P 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

P 22 Top signal + background Luminosity: 10 pb -1  signal  Alpgen1 sample Luminosity: 150 pb -1  signal  Alpgen2 sample

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

P 24 Request for DC2 (proposal) Use for commissioning studies  Initial detector layout Signal tt-bar events: 1 fb -1 (10%)  2 event generators events (eff slightly less due to w<0) Pythia830k events (i.e. NLO normalisation!)  Full decay modes W’s Background events: 200 pb -1  Alpgen, according to Alpgen1 sample Alpgen W+4jet600k events  Only leptonic decays of W (e,  )