Top physics during ATLAS commissioning

Top physics during ATLAS commissioning

Ivo van VulpenWouter Verkerke

Structure of the talk:

Reminder you of the goals of the study and main results presented in Rome Overview new results since Rome

Ivo van Vulpen (NIKHEF) ATLAS physics week (October 2005) Slide 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

1 lepton PT > 20 GeV

Missing ET > 20 GeV

4 jets(R=0.4) PT > 40 GeV

Selection efficiency = ~5 %

TOP quark CANDIDATE

W bosonCANDIDATE

Simple (standard) top quark selection:


Main results shown in Rome:

3-jet mass distributions m(jjj), with and without cut on Mw

Hadronic 3-jet mass

m(Whad)L=300 pb-1

(~1 week of running)

Cut on Mw

Hadronic 3-jet mass

Mjjj (GeV) Mjjj (GeV)


What’s new since Rome


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-1

More realistic estimate for integrated luminosity during LHC commissioning run


Trigger Performance

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


Impact various selection criteria on ttbar selection efficiency

Electron trigger important for event selection and cross section measurementNeed to understand differences between ttbar and clean Ze+e- or Weν events

• 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 %

Fraction of events passing cuts


Scope of trigger plots

Data Reconstruction AnalysisTrigger

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

Investigate trigger performance:

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


Trigger efficiency versus Pt (no pt-cut)

83.9 %

MC truth electron Pt (GeV)

Remaining questions: What object triggered the events with low-Pt e-‘s ? Why do we lose electrons Pt = 100 GeV in barrel ?

e- (Rec+match)e- (Rec+match + Trigger)

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

MC truth electron Pt (GeV)


Trigger efficiency versus Eta (Pt > 20 GeV)

MC truth electron Eta

e- (Rec+match) e- (Rec+match + Trigger)

MC truth electron Eta

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


Background estimate from W+jets

“Do you cover the full phase space contributing to 4 reconstructed jets?”


Cross section presented on wiki was wrong by factor ~2 Background goes down!

• ‘Good’ news: A7 cross section wrong on wiki:

What did we have in Rome: the A7 sample

• What is the A7 sample

W l

A7 = ‘Alpgen+ 4 jets’:

= W+4-partons L.O. Matrix Element + (Herwig) parton shower

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 ?

• Possible concern about the A7 sample

Wlν


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 element for 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

Matrix ElementParton shower

0 PT-cut 40 100 GeV

40


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

W+1jetsW+0jets

W+2/3/4jets

(Plot taken from presentation by M. Mangano)

W

PT W-boson = net PT radiation


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

Sample (# of ME partons)

#R

eco jets


#Even

ts

• # Alpgen ME partons versus # reconstructed jets

• Constribution of ME parton samples in selected events (4 reconstr. jets)


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


#R

eco jets


#Even

ts

• # Alpgen ME partons versus # reconstructed jets

• Constribution of parton samples in ttbar sample (4 reconstr. jets)

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


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

A7 estimate (127 pb-1) MLM estimate (127 pb-1) A7 & MLM (unit norm)

Amount of background increases by ~10% Shape consistent

W + 0 ME part.W + 1 ME part. W + 2 ME part. W + 3 ME part. W + 4 ME part. W ≥ 5 ME part.


More plots on W+ n ME MLM shape vs A7

MLM: PT of W-boson

PT of leading jet

A7 estimate (127 pb-1) MLM estimate (127 pb-1) A7 & MLM (unit norm)

pT, distributions of all jets and the electron consistent between A7 and MLM

W + 0 ME part.W + 1 ME part. W + 2 ME part. W + 3 ME part. W + 4 ME part. W ≥ 5 ME part.


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 Wν decays in study (need to be generated)


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 ?“


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.

Mjjj mass (GeV) Mjjj mass (GeV)

100 pb-1200 pb-1

Hadronic 3-jet mass


electron-only

L =100 pb-1

electron+muon estimate for L=100 pb-1

L=200 pb-1

Hadronic 3-jet mass

Even

ts /

4.1

5 G

eV

Even

ts /

4.1

5 G

eV


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


electron-only

L =100 pb-1

electron+muon estimate for L=100 pb-1

L=200 pb-1

Hadronic 3-jet mass

Even

ts /

4.1

5 G

eV

Even

ts /

4.1

5 G

eV Hadronic 3-jet mass


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. “


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


electron-only

L =100 pb-1

electron-only

L=100 pb-1

Hadronic 3-jet mass

Even

ts /

4.1

5 G

eV

Even

ts /

4.1

5 G

eV Hadronic 3-jet mass

Minimum Jet PT = 40 GeV Minimum Jet PT = 30 GeV


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


Backup slidesBackup slides


Impact various selection criteria on ttbar selection efficiency

ttbar events passing all cuts

100 %

Pt of 4th jet (GeV)

Pt electron (GeV)

Jet Pt-cut

Electron Pt-cut

Main loss due to kinem. cuts (also # jets)

Main loss dueto reconstruc.

Nu

mb

er

of

even

ts

Selection criterium

Electron (62.0%)

Et-miss (91.8%)

Jets (13.4%)

Electron trigger important for event selection and cross section measurementNeed to understand differences between ttbar

and clean Ze+e- or Weν events

Nu

mb

er

of

jets

Nu

mb

er

of

even

ts

Documents

Top physics during ATLAS commissioning