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Top physics during ATLAS commissioning

Top physics during ATLAS commissioning. Ivo van Vulpen Wouter Verkerke. Structure of the talk:  Reminder you of the goals of the study and main results presented in Rome  Overview new results since Rome.

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Top physics during ATLAS commissioning

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  1. Top physics during ATLAS commissioning Ivo van VulpenWouter Verkerke

  2. Structure of the talk:  Reminder you of the goals of the study and main results presented in Rome  Overview new results since Rome

  3. Goals for top physics during comissioning:1) Can we see the top peak in the LHC commissioning run ?With 300 pb-1 Without b-tagging 2) Can we help commission the ATLAS detector using these events ? Calibrate light jet energy scale Calibrate missing ET Obtain enriched b-jet sample Cross section W bosonCANDIDATE TOP quark CANDIDATE Simple (standard) top quark selection: Missing ET > 20 GeV Selection efficiency = ~5 % 1 lepton PT > 20 GeV 4 jets(R=0.4) PT > 40 GeV

  4. Main results shown in Rome:3-jet mass distributions m(jjj), with and without cut on Mw Hadronic 3-jet mass Hadronic 3-jet mass L=300 pb-1 (~1 week of running) m(Whad) Cut on Mw Mjjj (GeV) Mjjj (GeV)

  5. What’s new since Rome

  6. What’s new since Rome: focus on concerns 1) Trigger Effect of electron trigger: 2e15i+e25i+e60 2) New background estimate from W+jets Addressing concern about phase space coverage A7 sample (W+jets) used for Rome analysis New estimate using Alpgen+MLM matching 3) 100 pb-1More realistic estimate for integrated luminosity during LHC commissioning run

  7. Trigger Performance “How much ‘good’ electron events do we lose by including the trigger ?”

  8. Impact various selection criteria on ttbar selection efficiency Fraction of events passing cuts Jets: 4 reconstructed jets with Pt > 40 GeV  13.4% Losses mainly due to hard analysis cut on jet kinematics Electrons At least 1 reconstructed electron wth Pt> 20 GeV  62.0% Losses mainly due to reconstruction Missing Et > 20 GeV  91.8 % Electron trigger important for event selection and cross section measurementNeed to understand differences between ttbar and clean Ze+e- or Weν events

  9. Trigger  Investigate trigger performance: step 1: Require reconstructed good e- (with/without Pt cut) step 2: Require e- to point back to MC truth e- from W decay step 3: Look at trigger decision Scope of trigger plots Data Reconstruction Analysis “How much ‘good’ electron events do we lose by including the trigger ?”

  10. Remaining questions: What object triggered the events with low-Pt e-‘s ? Why do we lose electrons Pt = 100 GeV in barrel ? Trigger efficiency versus Pt (no pt-cut) Note: Events with a reconstructed electron (no Pt-cut) that matches the electron from the W decay (Monte-Carlo truth) Same as white, but have ‘yes’ trigger decision 83.9 % e- (Rec+match) e- (Rec+match + Trigger) MC truth electron Pt (GeV) MC truth electron Pt (GeV)

  11. Trigger efficiency versus Eta (Pt > 20 GeV) Note: Events with a reconstructed electron (Pt>20 GeV) that matches the electron from the W decay (Monte-Carlo truth) Same as white, but have ‘yes’ trigger decision e- (Rec+match) e- (Rec+match + Trigger) MC truth electron Eta MC truth electron Eta

  12. Background estimate from W+jets “Do you cover the full phase space contributing to 4 reconstructed jets?”

  13. W  l n • ‘Good’ news: A7 cross section wrong on wiki: Cross section presented on wiki was wrong by factor ~2 Background goes down! What did we have in Rome: the A7 sample • What is the A7 sample A7 = ‘Alpgen+ 4 jets’:= W+4-partons L.O. Matrix Element + (Herwig) parton shower Wlν • Possible concern about the A7 sample  Do we cover the full phase space that contributes to 4 reconstructed jets. Probably not. What about W+1/2/3-partons + hard gluon(s) from PS ?

  14. Towards modeling the full phase space • ‘Traditional’ approach : W+0jets Matrix Elements(ME) + Parton Shower (PS): • Would covers full phase space, but … • Extremely inefficient for high-Pt jet sample • Parton shower does not correctly describe hard gluon emission • remember: we require 4 jets with Pt > 40 GeV • Idea for improvement: • Use parton shower for low-Pt radiation • Use matrix elementfor high-Pt radiation • Practical translation: • Generate separate samples of W + 0,1,2,3,4,5 ME partons • add arton shower to each sample • Cannot simply add samples because of double counting from hard parton showers • Solution: Alpgen + MLM matching (M. Mangano)In a nutshell: kill events with too high PT-gluons in PS • After matching can add W + n ME partons samples Parton shower Matrix Element 0 PT-cut 40 100 GeV 40

  15. W+2/3/4jets W+0jets W+1jets Does MLM matching work ? • Look at PT distribution of W-boson at Tevatron • Region of high W-boson transverse momentum described by matrix element computation • Sum of MLM-matched W + n ME parton samples describes CDF data well (Plot taken from presentation by M. Mangano) W PT W-boson = net PT radiation

  16. Applying MLM to estimate W + 4 reco jet background • Generate samples of W + n ME partons + PS sample (n=0,1,2,3,4,5) • Look at contribution of each sample to W + 4 reco jets final state # Alpgen ME partons versus # reconstructed jets Constribution of ME parton samples in selected events (4 reconstr. jets) #Events #Reco jets Sample (# of ME partons) Sample (# of ME partons)

  17. Applying MLM to estimate W + 4 reco jet background Background dominated by W + 4 ME parton sample But other samples also contribute due to small differences in jet definition in MLM matching and reconstruction, effects of detector simulation etc… Does not affect validity of procedure but strong mismatch will increase number of significantly contributing samples • Generate samples of W + n ME partons + PS sample (n=0,1,2,3,4,5) • Look at contribution of each sample to W + 4 reco jets final state # Alpgen ME partons versus # reconstructed jets Constribution of parton samples in ttbar sample (4 reconstr. jets) #Events #Reco jets Sample (# of ME partons) Sample (# of ME partons)

  18. Result: W + 4 reco jet background from MLM matching • Bottom line for W + 4 jets background in 3-jet invariant mass m(jjj)Add all W + n ME partons samples and normalize sum to 127 pb-1(luminosity of A7 sample) • Including full phase space adds ~10% background w.r.t A7 samples MLM estimate (127 pb-1) A7 & MLM (unit norm) A7 estimate (127 pb-1) W + 0 ME part.W + 1 ME part.W + 2 ME part.W + 3 ME part.W + 4 ME part.W ≥ 5 ME part. Amount of background increases by ~10% Shape consistent

  19. More plots on W+ n ME MLM shape vs A7 W + 0 ME part.W + 1 ME part.W + 2 ME part.W + 3 ME part.W + 4 ME part.W ≥ 5 ME part. MLM: PT of W-boson pT, h distributions of all jets and the electron consistent between A7 and MLM PT of leading jet MLM estimate (127 pb-1) A7 & MLM (unit norm) A7 estimate (127 pb-1)

  20. Summary on W+jets background • Evaluated background on full phase space by including W + 0,1,2,3,4,5 ME partons + PS using MLM technique - Background level increases by ~10% w.r.t. A7 sample - M(jjj), pT(jet), η(jet), pT(e-), η(e-) shapes all consistent between A7 and MLM sample • To do: study effect of varying MLM matching parameters • Can e.g. vary PT threshold between PS and ME • Check that result is not strongly dependent on choice of matching parameters • Include Wmν decays in study (need to be generated)

  21. Results for 100 pb-1 “What are the results of the study when using a more conservative estimate for the luminosity collected during the commissioning run ?“

  22. Results for 100 pb-1 (no cut on reconstructed W mass) Note 1: Background ~factor 2 lower due to initial mistake in A7 lumi Note 2: Error bars now reflect statistical error of 100 pb-1 instead of statistical error of MC sample as was done for Rome plots. Hadronic 3-jet mass Hadronic 3-jet mass 100 pb-1 200 pb-1 L =100 pb-1 L=200 pb-1 Events / 4.15 GeV Events / 4.15 GeV electron+muon estimate for L=100 pb-1 electron-only Mjjj mass (GeV) Mjjj mass (GeV) Mjjj (GeV) Mjjj (GeV)

  23. Results for 100 pb-1 (with cut on reconstructed W mass) Distribution of 3-jet invariant mass after a cut on the mass of the reconstructed W-boson: 70 < Mjj < 90 GeV Hadronic 3-jet mass Hadronic 3-jet mass L =100 pb-1 L=200 pb-1 Events / 4.15 GeV Events / 4.15 GeV electron+muon estimate for L=100 pb-1 electron-only Mjjj (GeV) Mjjj (GeV)

  24. Relax cut on minimum PT requirement for jets “Top peak close to rising edge of background distribution when using a minimum jet PT-cut at Pt = 40 GeV. “

  25. Relaxed cut on minimum PT requirement for jets • Top peak on rising edge background distribution: Try relaxing cut on minimum jet-PT In Note: investigate stability and effects from changed selection criteria Minimum Jet PT = 40 GeV Minimum Jet PT = 30 GeV Hadronic 3-jet mass Hadronic 3-jet mass L =100 pb-1 L=100 pb-1 Events / 4.15 GeV Events / 4.15 GeV electron-only electron-only Mjjj (GeV) Mjjj (GeV)

  26. Summary • Focused on concerns after Rome • New estimate for W+jets background • Lower estimate due to mistake in A7 lumi • New procedure Alpgen+MLM matching 10% higher than corrected A7 result • First results on impact electron trigger • Preliminary results now quoted for 100 pb-1 • Plan • Finalize Alpgen+MLM matching study • Evaluate some outstanding issues (b-tag, calibrations, etc.) • Write note

  27. Backup slides

  28. Impact various selection criteria on ttbar selection efficiency Jet Pt-cut 100 % Main loss due to kinem. cuts (also # jets) Number of jets Number of events Electron (62.0%) Et-miss (91.8%) Jets (13.4%) Pt of 4th jet (GeV) Electron Pt-cut Selection criterium Main loss dueto reconstruc. Number of events ttbar events passing all cuts Electron trigger important for event selection and cross section measurementNeed to understand differences between ttbar and clean Ze+e- or Weν events Pt electron (GeV)

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