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Commissioning ATLAS with top events

Commissioning ATLAS with top events. W. Verkerke. Introduction to physics commissioning. What are we going to do with the first month of data? Many detector-level checks (tracking, calorimetry etc) Try to see large cross section known physics signals

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Commissioning ATLAS with top events

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  1. Commissioning ATLAS with top events W. Verkerke Wouter Verkerke, NIKHEF

  2. Introduction to physics commissioning • What are we going to do with the first month of data? • Many detector-level checks (tracking, calorimetry etc) • Try to see large cross section known physics signals • But to ultimately get to interesting physics, also need to calibrate many higher level reconstruction concepts such as jet energy scales, b-tagging and missing energy • Algorithms benefiting from early data for calibration include • B-tagging • Identify jets originating from b quarks from their topology • Exploit relatively long lifetime of B decays  displaces vertex • Jet energy scale calibration • Relate energy of reconstructed jet to energy of parton • Detector and physics calibration (some fraction of parton energy is undetectable to due production of neutrinos, neutral hadrons etc…). • Dependent of flavor of initial quark  need to measure separately for b jets Wouter Verkerke, NIKHEF

  3. Introduction to physics commissioning • Jet energy scales (cont’d) • Ultimate goal for JE calibration is 1% • At startup calibration will be less known • Important – Illustrated of effect on m(top) measurement • Impacts many measurements, not just m(top) • Need to start data to good use for calibration purposes as quickly as possible • Top physics ideal candidate to do the job • Also candidate for clean physics channel for early cross section measurement Uncertainty on light jet scale:Hadronic 1%  Mt < 0.7 GeV 10%  Mt = 3 GeV Uncertainty On b-jet scale:Hadronic 1%  Mt = 0.7 GeV 5%  Mt = 3.5 GeV 10%  Mt = 7.0 GeV Wouter Verkerke, NIKHEF

  4. Top physics at LHC • Large ttbar production cross section at LHC • Effect of large s at LHC  threshold for ttbar production at lower x • Production gluon dominated at LHC, quark dominated at Tevatron • About 100 times larger than cross section at Tevatron (lumi also much larger) ggtt stt(tot) = 759±100 pb Nevt ~ 700/hour qqtt Wouter Verkerke, NIKHEF

  5. Top physics at topology • Decay products are 2 W bosons and two b quarks • About 99.9% to Wb, ~0.1% decay to Ws and Wd each • For commissioning studies focus on events where one W decays hadronically and the other W decays semi-leptonically • About 30% of total ttbar cross section t t Wouter Verkerke, NIKHEF

  6. What can we learn from ttbar production • Abundant clean source of b jets • 2 out of 4 jets in event are b jets  O(50%) a priori purity(need to be careful with ISR and jet reconstruction) • Remaining 2 jets can be kinematicallyidentified (should form W mass) possibility for further purification t t Wouter Verkerke, NIKHEF

  7. What can we learn from ttbar production • Abundant source of W decays into light jets • Invariant mass of jets should add up to well known W mass • Suitable for light jet energy scale calibration (target prec. 1%) • Caveat: should not use W mass in jetassignment for calibration purposeto avoid bias • If (limited) b-tagging is available,W jet assignment combinatoricsgreatly reduced t t Wouter Verkerke, NIKHEF

  8. What can we learn from ttbar production • Known amount of missing energy • 4-momentum of single neutrino in eachevent can be constrained from eventkinematics • Inputs in calculation: m(top) from Tevatron, b-jet energy scale and lepton energy scale t t Wouter Verkerke, NIKHEF

  9. What can we learn from ttbar production • Two ways to reconstruct the top mass • Initially mostly useful in event selection,as energy scale calibrations must be understood before quality measurementcan be made • Ultimately determine m(top)from kinematic fit to complete event • Needs understanding of bias and resolutionsof all quantities • Not a day 1 topic t t Wouter Verkerke, NIKHEF

  10. How to identify ttbar events • Commissioning study  Want to restrict ourselves to basic (robust) quantities • Apply some simple cuts • Hard pT cuts really clean upsample (ISR). • Possible becauseof high production rate    Combined efficiency of requirementsis ~5%  still have ~10 evts/hour 4 hard jets (PT >40 GeV) 1 hard lepton (Pt >20 GeV) Missing ET(ET >20 GeV)  Wouter Verkerke, NIKHEF

  11. Can this be done? • Selecting ttbar with b-tagging expected to be easy: S/B is O(100) • But we would like to start without b-tagging • Major worry: background. Can we see a signal? • Does the idea hold with increasingly realistic detector simulation? • Short history of study • Freiburg 2004: Initial Fast Simulations studies by M. Cobal and S. Bentvelsen demonstrate viability of idea • Rome 2005: Repeat studies with Full simulation (I. van Vulpen & W. Verkerke) • Oct 2005 Physics week: Improve background estimates, add effects of trigger efficiency today Wouter Verkerke, NIKHEF

  12. W+4jets (largest bkg) Problematic if 3 jets line up m(t) and W + remaining jet also line up to m(t) Cannot be simulated reliablyby Pythia or Herwig. Requires dedicated event generator AlpGen Ultimately get rate from data Z+4 jets rate and MC (Z+4j)/(W+4J) ratio Vast majority of events can be rejected exploiting jet kinematics. QCD multi-jet events Problematic if one jets goes down beampipe (thus giving ETmiss) and one jets mimics electron Cross section large and not well unknown, but mostly killed by lepton ID and ETmiss cuts. Rely on good lepton ID and ETmiss to suppress e-,p0 W  l n Backgrounds that you worry about Wouter Verkerke, NIKHEF

  13. ‘Standard’ top analysis • First apply selection cuts • Assign jets to W, top decays Missing ET > 20 GeV 1 lepton PT > 20 GeV Selection efficiency = 5.3% 4 jets(R=0.4) PT > 40 GeV W CANDIDATE TOP CANDIDATE 1 Hadronic top: Three jets with highest vector-sum pT as the decay products of the top 2 W boson: Two jets in hadronic top with highest momentum in reconstructed jjj C.M. frame. Wouter Verkerke, NIKHEF

  14. Samples for ‘Rome’ study ttbar (signal) W+jets (background) • Generator: MC@NLO • Includes all LO + NLO m.e. • Dedicated Generator: AlpGen • Includes all LO W + 4 parton m.e. Hard Process CPU intensive! Fragmentation, Hadronization & Underlying event Herwig (Jimmy) [ no pileup ] ATLAS Full Simulation 10.0.2 (30 min/ev) Atlas DetectorSimulation ‘T1’ Sample 175K event = 300 pb-1 ‘A7’ Sample 145K event = 61 pb-1 Wouter Verkerke, NIKHEF

  15. W CANDIDATE TOP CANDIDATE Signal-only distributions (Full Simulation) • Clear top, W mass peaks visible • Background due to mis-assignment of jets • Easier to get top assignment right than to get W assignment right • Masses shifted somewhat low • Effect of (imperfect) energy calibration m(tophad) m(Whad) MW = 78.1±0.8 GeV mtop = 162.7±0.8 GeV L=300 pb-1 (~1 week of running) Jet energy scalecalibration possible fromshift in m(W) S B S/B = 0.5 S/B = 1.20 Wouter Verkerke, NIKHEF

  16. W CANDIDATE TOP CANDIDATE Signal + Wjets background (Full Simulation) • Plots now include W+jets background • Background level roughly triples • Signal still well visible • Caveat: bkg. cross section quite uncertain m(tophad) m(Whad) Jet energy scalecalibration possible fromshift in m(W) S L=300 pb-1 (~1 week of running) B S/B = 0.27 S/B = 0.45 Wouter Verkerke, NIKHEF

  17. W CANDIDATE TOP CANDIDATE Signal + Wjets background (Full Simulation) • Now also exploit correlation between m(tophad) and m(Whad) • Show m(tophad) only for events with |m(jj)-m(W)|<10 GeV m(tophad) m(tophad) L=300 pb-1 (~1 week of running) m(Whad) S S/B = 1.77 B S/B = 0.45 Wouter Verkerke, NIKHEF

  18. Signal + Wjets background (Full Simulation) • Can also clean up sample by with requirement on m(jln) [semi-leptonic top] • NB: There are two m(top) solutions for each candidate due to ambiguity in reconstruction of pZ of neutrino • Also clean signal quite a bit • m(W) cut not applied here TOP CANDIDATE SEMI LEPTONIC TOP CANDIDATE m(tophad) m(tophad) L=300 pb-1 (~1 week of running) S |m(jln)-mt|<30 GeV B S/B = 1.11 S/B = 0.45 Wouter Verkerke, NIKHEF

  19. Effect if increasing realism • Evolution of m(top) resolution, yield with improving realism m(top) (GeV) resolution (GeV) s(N) stat Effect ofdetectorsimulation Effect ofincreasingWjets bkg. Effect ofmW cut Wouter Verkerke, NIKHEF

  20. W CANDIDATE TOP CANDIDATE Exploiting ttbar as b-jet sample (Full Simulation) • Simple demonstration use of ttbar sample to provide b enriched jet sample • Cut on m(Whad) and m(tophad) masses • Look at b-jet prob for 4th jet (must be b-jet if all assignments are correct) W+jets (background) ‘random jet’, no b enhancement expected ttbar (signal) ‘always b jet if all jet assignment are OK’ b enrichment expected and observed AOD b-jet probability Clear enhancementobserved! AOD b-jet probability Wouter Verkerke, NIKHEF

  21. Moving beyond Rome – Improving the analysis • We know that we underestimate the level of background • Only generating W + 4 partons now, but W + 3,5 partons may also result in W + 4 jet final state due to splitting/merging W + 3 partons (80 pb*) W + 4 partons (32 pb*) W + 5 partons (15 pb*) W  l n W  l n W  l n 2 parton reconstructed as single jets parton is reconstructed as 2 jets * These are the cross sections with the analysis cuts on lepton and jet pT applied at the truth level Wouter Verkerke, NIKHEF

  22. Moving beyond Rome – Improving the analysis • Improving the W + 4 jets background estimate • Need to simulate W + 3,5 parton matrix elements as well • But not trivial to combine samples: additional parton showering in Herwig/Jimmy leads to double counting if samples are naively added • But new tool available in AlpGen v2.03: MLM matching prescription. • Explicit elimination of double counting by reconstructing jets in event generator and killing of ‘spillover’ events. • Work in progress • Expected for upcoming Oct Physics week • To set upper bound: naïve combination of W + 3,4,5 parton events would roughly double W+jets background. Wouter Verkerke, NIKHEF

  23. Moving beyond Rome – effect of trigger • Look at Electron Trigger efficiency • Event triggered on hard electron • Triggering through 2E15i, E25i, E60 channels • Preliminary trigger efficiency as function of lepton pT • Efficiency = fraction of events passing all present analysis cuts that are triggered • Analysis cuts on electron include requirements on isem flag and etcone40 • Includes effects of ‘untriggerable’ events due to cracks etc… • In cooperation with M. Wielers (work in progress) 73.5% #triggered events / # events Nominal analysis cut Wouter Verkerke, NIKHEF Electron pT (GeV)

  24. Summary • Can reconstruct top and W signal after ~ one week of data taking without using b tagging • Can progressively clean up signal with use of b-tag, ET-miss, event topology • Many useful spinoffs • Hadronic W sample  light quark jet energy scale calibration • Kinematically identified b jets useful for b-tag calibration • Continue to improve realism of study and quality of analysis • Important improvement in W+jets estimate underway • Incorporate and estimate trigger efficiency to few (%) • Also continue to improve jet assignment algorithms • Expect estimate of s(ttbar) with error < 20% in first running period • One of the first physics measurements of LHC? Wouter Verkerke, NIKHEF

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