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

Wouter Verkerke, NIKHEF 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 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 b-jet scale: Hadronic 1%   M t = 0.7 GeV 5%   M t = 3.5 GeV 10%   M t = 7.0 GeV Uncertainty on light jet scale: Hadronic 1%   M t < 0.7 GeV 10%   M t = 3 GeV

Wouter Verkerke, NIKHEF 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)  tt tot = 759±100 pb N evt ~ 700/hour gg  tt qq  tt

Wouter Verkerke, NIKHEF 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 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 kinematically identified (should form W mass)  possibility for further purification t t

Wouter Verkerke, NIKHEF 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 jet assignment for calibration purpose to avoid bias –If (limited) b-tagging is available, W jet assignment combinatorics greatly reduced t t

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

Wouter Verkerke, NIKHEF 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 measurement can be made –Ultimately determine m(top) from kinematic fit to complete event Needs understanding of bias and resolutions of all quantities Not a day 1 topic t t

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

Wouter Verkerke, NIKHEF 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 Backgrounds that you worry about 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 reliably by 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 W  l e-,0e-,0

Wouter Verkerke, NIKHEF ‘Standard’ top analysis First apply selection cuts Assign jets to W, top decays 1 lepton P T > 20 GeV Missing E T > 20 GeV 4 jets(R=0.4) P T > 40 GeV Selection efficiency = 5.3% 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. W CANDIDATE

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

Wouter Verkerke, NIKHEF Signal-only distributions (Full Simulation) M W = 78.1±0.8 GeV m top = 162.7±0.8 GeV S/B = 1.20 S/B = 0.5 S B m(top had )m(W had ) TOP CANDIDAT E W CANDIDATE 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 Jet energy scale calibration possible from shift in m(W) L=300 pb -1 (~1 week of running)

Wouter Verkerke, NIKHEF Signal + Wjets background (Full Simulation) S/B = 0.45 S/B = 0.27 S B m(top had )m(W had ) TOP CANDIDAT E W CANDIDATE Plots now include W+jets background –Background level roughly triples –Signal still well visible –Caveat: bkg. cross section quite uncertain Jet energy scale calibration possible from shift in m(W) L=300 pb -1 (~1 week of running)

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

Wouter Verkerke, NIKHEF Signal + Wjets background (Full Simulation) TOP CANDIDATE Can also clean up sample by with requirement on m(jl) [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 m(top had ) B S S/B = 0.45 S/B = 1.11 SEMI LEPTONIC TOP CANDIDATE |m(jl)-m t |<30 GeV L=300 pb -1 (~1 week of running)

Wouter Verkerke, NIKHEF Effect if increasing realism Evolution of m(top) resolution, yield with improving realism Hadronic M W = 80.4±10 GeV ± ± % +50%164.1 ± ± 1.510% +100%165.9 ± ± 2.817% Truth jets171.1 ± ± % Full simulation162.7 ± ± % m(top) (GeV) resolution (GeV) (N) stat Effect of detector simulation Effect of increasing Wjets bkg. Effect of m W cut

Wouter Verkerke, NIKHEF Exploiting ttbar as b-jet sample (Full Simulation) TOP CANDIDAT E W CANDIDATE Simple demonstration use of ttbar sample to provide b enriched jet sample –Cut on m(W had ) and m(top had ) masses –Look at b-jet prob for 4 th 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 enhancement observed!

Wouter Verkerke, NIKHEF 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  l W + 4 partons (32 pb * ) W + 3 partons (80 pb * ) W + 5 partons (15 pb * ) parton is reconstructed as 2 jets 2 parton reconstructed as single jets * These are the cross sections with the analysis cuts on lepton and jet pT applied at the truth level

Wouter Verkerke, NIKHEF 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 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) Nominal analysis cut Electron pT (GeV) #triggered events / # events 73.5%

Wouter Verkerke, NIKHEF 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, E T -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 (ttbar) with error < 20% in first running period –One of the first physics measurements of LHC?