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

Wouter Verkerke, NIKHEF

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

– 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


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


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


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


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



• 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



• 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



• 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


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


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


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


‘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


‘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!


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)


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



TOP CANDIDATE

W CANDIDATE


• 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




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



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


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!


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


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.


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%


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?

Documents

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