1. Top physics during ATLAS commissioning
Niks
Ivo van Vulpen Wouter Verkerke

2. Structure of the talk:
 Reminder you of the goals of the study and main results presented in Rome  Overview new results since Rome

3. Goals for top physics during comissioning: 1) Can we see the top peak in the LHC commissioning run ? With 300 pb 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
W boson CANDIDATE
TOP quark CANDIDATE
Simple (standard) top quark selection:
Missing ET > 20 GeV
Selection efficiency = ~5 %
1 lepton PT > 20 GeV
4 jets(R=0.4) PT > 40 GeV

4. Main results shown in Rome: 3-jet mass distributions m(jjj), with and without cut on Mw
Hadronic 3-jet mass
Hadronic 3-jet mass
L=300 pb-1
(~1 week of running)
m(Whad)
Cut on Mw
Mjjj (GeV)
Mjjj (GeV)

5. What’s new since Rome

6. 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

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

8. Impact various selection criteria on ttbar selection efficiency
Fraction of events passing cuts
Jets: 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  % Losses mainly due to reconstruction
Missing Et > 20 GeV  %
Electron trigger important for event selection and cross section measurement Need to understand differences between ttbar and clean Ze+e- or Weν events

9. Scope of trigger plots Trigger
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:
Data
Reconstruction
Analysis
“How much ‘good’ electron events do we lose by including the trigger ?”

10. Trigger efficiency versus Pt (no pt-cut)
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
83.9 %
e- (Rec+match)
e- (Rec+match + Trigger)
Remaining questions:  What object triggered the events with low-Pt e-‘s ?  Why do we lose electrons Pt = 100 GeV in barrel ?
MC truth electron Pt (GeV)
MC truth electron Pt (GeV)

11. Trigger efficiency versus Eta (Pt > 20 GeV)
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
e- (Rec+match)
e- (Rec+match Trigger)
MC truth electron Eta
MC truth electron Eta

12. Background estimate from W+jets
“Do you cover the full phase space contributing to 4 reconstructed jets?”

13. What did we have in Rome: the A7 sample
W  l n
What is the A7 sample
A7 = ‘Alpgen+ 4 jets’: = W+4-partons L.O. Matrix Element (Herwig) parton shower
Wlν
Possible concern about the A7 sample
 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 ?
Cross section presented on wiki was wrong by factor ~2 Background goes down!
‘Good’ news: A7 cross section wrong on wiki:

14. 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
Parton shower
Matrix Element
0 PT-cut GeV 40

15. Look at PT distribution of W-boson at Tevatron
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
(Plot taken from presentation by M. Mangano)
W+1jets
W+0jets
W+2/3/4jets W
PT W-boson = net PT radiation

16. 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
# Alpgen ME partons versus # reconstructed jets
Constribution of ME parton samples in selected events (4 reconstr. jets)
#Events
#Reco jets
Sample (# of ME partons)
Sample (# of ME partons)

17. Applying MLM to estimate W + 4 reco jet background
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
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
# Alpgen ME partons versus # reconstructed jets
Constribution of parton samples in ttbar sample (4 reconstr. jets)
#Events
#Reco jets
Sample (# of ME partons)
Sample (# of ME partons)

18. 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)
W + 0 ME part. W + 1 ME part. W + 2 ME part. W + 3 ME part. W + 4 ME part. W ≥ 5 ME part.
Amount of background increases by ~10%
Shape consistent

19. More plots on W+ n ME MLM shape vs A7
W + 0 ME part. W + 1 ME part. W + 2 ME part. W + 3 ME part. W + 4 ME part. W ≥ 5 ME part.
MLM: PT of W-boson
pT, h distributions of all jets and the electron consistent between A7 and MLM
PT of leading jet
A7 estimate (127 pb-1)
MLM estimate (127 pb-1)
A7 & MLM (unit norm)

20. 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 Wmν decays in study (need to be generated)

21. 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 ?“

22. 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.
Hadronic 3-jet mass
Hadronic 3-jet mass
100 pb-1
200 pb-1
L =100 pb-1
L=200 pb-1
Events / 4.15 GeV
Events / 4.15 GeV
electron+muon estimate for L=100 pb-1
electron-only
Mjjj mass (GeV)
Mjjj mass (GeV)
Mjjj (GeV)
Mjjj (GeV)

23. 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
Hadronic 3-jet mass
Hadronic 3-jet mass
L =100 pb-1
L=200 pb-1
Events / 4.15 GeV
Events / 4.15 GeV
electron+muon estimate for L=100 pb-1
electron-only
Mjjj (GeV)
Mjjj (GeV)

24. 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. “

25. 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
Minimum Jet PT = 40 GeV
Minimum Jet PT = 30 GeV
Hadronic 3-jet mass
Hadronic 3-jet mass
L =100 pb-1
L=100 pb-1
Events / 4.15 GeV
Events / 4.15 GeV
electron-only
electron-only
Mjjj (GeV)
Mjjj (GeV)

26. Focused on concerns after Rome
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

27. Backup slides

28. Impact various selection criteria on ttbar selection efficiency
Jet Pt-cut
100 %
Main loss due to kinem. cuts (also # jets)
Number of jets
Number of events
Electron (62.0%)
Et-miss (91.8%)
Jets (13.4%)
Pt of 4th jet (GeV)
Electron Pt-cut
Selection criterium
Main loss due to reconstruc.
Number of events
ttbar events passing all cuts
Electron trigger important for event selection and cross section measurement Need to understand differences between ttbar and clean Ze+e- or Weν events
Pt electron (GeV)