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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% Mt = 0.7 GeV5% Mt = 3.5 GeV10% Mt = 7.0 GeV
Uncertainty on light jet scale: Hadronic 1% Mt < 0.7 GeV10% Mt = 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)
tttot = 759±100 pb
Nevt ~ 700/hour
32121 10~ ; ˆ xxxsxs
ggtt
qqtt
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 kinematicallyidentified (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 jetassignment for calibration purposeto avoid bias
– If (limited) b-tagging is available,W jet assignment combinatoricsgreatly 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 eventkinematics
• 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 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
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
1 hard lepton (Pt >20 GeV)
Missing ET (ET >20 GeV)
4 hard jets (PT >40 GeV)
Combined efficiency of requirementsis ~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 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
W l
e-,0
Wouter Verkerke, NIKHEF
‘Standard’ top analysis
• First apply selection cuts
• Assign jets to W, top decays
1 lepton PT > 20 GeV
Missing ET > 20 GeV
4 jets(R=0.4) PT > 40 GeVSelection 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’ Sample175K event = 300 pb-1
‘A7’ Sample145K event = 61 pb-1
Samples for ‘Rome’ study
• Generator: MC@NLO• Includes all LO + NLO m.e.
• Dedicated Generator: AlpGen• Includes all LO W + 4 parton m.e.
HardProcess
Fragmentation,Hadronization &Underlying event
Herwig (Jimmy) [ no pileup ]
ATLAS Full Simulation 10.0.2 (30 min/ev)
ttbar (signal) W+jets (background)
Atlas DetectorSimulation
CPU intensive!
Wouter Verkerke, NIKHEF
Signal-only distributions (Full Simulation)
MW = 78.1±0.8 GeVmtop = 162.7±0.8 GeV
S/B = 1.20 S/B = 0.5
S
B
m(tophad) m(Whad)
TOP CANDIDATE
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 scalecalibration possible fromshift 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(tophad) m(Whad)
TOP CANDIDATE
W CANDIDATE• Plots now include W+jets background
– Background level roughly triples
– Signal still well visible
– Caveat: bkg. cross section quite uncertain
Jet energy scalecalibration possible fromshift in m(W)
L=300 pb-1
(~1 week of running)
Wouter Verkerke, NIKHEF
TOP CANDIDATE
W 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)
B
S
S/B = 0.45
S/B = 1.77
m(Whad)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(tophad) m(tophad)
B
S
S/B = 0.45 S/B = 1.11
SEMI LEPTONIC TOP CANDIDATE
|m(jl)-mt|<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 MW=80.4±10 GeV
160.0 ± 1.0 15.4 ± 1.2 8.3%
+50% 164.1 ± 1.0 17.0 ± 1.5 10%
+100% 165.9 ± 1.4 19.8 ± 2.8 17%
Truth jets 171.1 ± 0.4 7.0 ± 0.2 6.0%
Full simulation 162.7 ± 0.8 15.8 ± 0.8 6.3%
m(top) (GeV) resolution (GeV) (N) stat
Effect ofdetector
simulation
Effect ofincreasingWjets bkg.
Effect ofmW cut
Wouter Verkerke, NIKHEF
Exploiting ttbar as b-jet sample (Full Simulation)
TOP CANDIDATE
W CANDIDATE• 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 AOD b-jet probability Clear enhancementobserved!
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 l 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)#tr
igg
ere
d e
ven
ts /
# e
ven
ts
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, 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 (ttbar) with error < 20% in first running period
– One of the first physics measurements of LHC?
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