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Top physics during ATLAS commissioning

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... used for Rome analysis New estimate using Alpgen+MLM ... Preliminary results now quoted for 100 pb-1 Plan Finalize Alpgen+MLM matching study ... – PowerPoint PPT presentation

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Title: Top physics during ATLAS commissioning


1
Top physics during ATLAS commissioning
Ivo van VulpenWouter 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 comissioning1)
    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

W bosonCANDIDATE
TOP quark CANDIDATE
Simple (standard) top quark selection
Missing ET gt 20 GeV
Selection efficiency 5
1 lepton PT gt 20 GeV
4 jets(R0.4) PT gt 40 GeV
4
Main results shown in Rome3-jet mass
distributions m(jjj), with and without cut on Mw
Hadronic 3-jet mass
Hadronic 3-jet mass
L300 pb-1 (1 week of running)
m(Whad)
Cut on Mw
Mjjj (GeV)
Mjjj (GeV)
5
Whats new since Rome
6
Whats new since Rome focus on concerns
  • 1) Trigger Effect of electron trigger
    2e15ie25ie60
  • 2) New background estimate from Wjets ?
    Addressing concern about phase space coverage
    A7 sample (Wjets) used for Rome analysis ?
    New estimate using AlpgenMLM 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 4 reconstructed jets with Pt gt 40 GeV
    ? 13.4 Losses
    mainly due to hard analysis cut on jet
    kinematics
  • Electrons At least 1 reconstructed electron
    wth Ptgt 20 GeV ? 62.0 Losses
    mainly due to reconstruction
  • Missing Et gt 20 GeV
    ? 91.8

Electron trigger important for event selection
and cross section measurementNeed
to understand differences between ttbar
and clean Z?ee- or W?e? events
9
Scope of trigger plots
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- (Recmatch)
e- (Recmatch Trigger)
MC truth electron Pt (GeV)
MC truth electron Pt (GeV)
11
Trigger efficiency versus Eta (Pt gt 20 GeV)
Note Events with a reconstructed
electron (Ptgt20 GeV) that
matches the electron from the W decay
(Monte-Carlo truth) Same as
white, but have yes trigger decision
e- (Recmatch)
e- (Recmatch Trigger)
MC truth electron Eta
MC truth electron Eta
12
Background estimate from Wjets
Do you cover the full phase space contributing
to 4 reconstructed jets?
13
What did we have in Rome the A7 sample
  • What is the A7 sample

A7 Alpgen 4 jets W4-partons L.O.
Matrix Element (Herwig) parton shower
W?l?
  • Possible concern about the A7 sample

? Do we cover the full phase space that
contributes to 4 reconstructed jets.
Probably not.? What about W1/2/3-partons hard
gluon(s) from PS ?
14
Towards modeling the full phase space
  • Traditional approach W0jets 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 gt 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 Element
Parton shower
0 ?PT-cut 40 100 GeV
40
15
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
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
    (n0,1,2,3,4,5)
  • 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
  • Generate samples of W n ME partons PS sample
    (n0,1,2,3,4,5)
  • Look at contribution of each sample to W 4 reco
    jets final state

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
  • 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
MLM estimate (127 pb-1)
A7 MLM (unit norm)
A7 estimate (127 pb-1)
20
Summary on Wjets 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), ?(jet), pT(e-), ?(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?m? 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
L200 pb-1
Events / 4.15 GeV
Events / 4.15 GeV
electronmuon estimate for L100 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 lt
Mjj lt 90 GeV
Hadronic 3-jet mass
Hadronic 3-jet mass
L 100 pb-1
L200 pb-1
Events / 4.15 GeV
Events / 4.15 GeV
electronmuon estimate for L100 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
L100 pb-1
Events / 4.15 GeV
Events / 4.15 GeV
electron-only
electron-only
Mjjj (GeV)
Mjjj (GeV)
26
Summary
  • Focused on concerns after Rome
  • New estimate for Wjets background
  • Lower estimate due to mistake in A7 lumi
  • New procedure AlpgenMLM matching 10 higher than
    corrected A7 result
  • First results on impact electron trigger
  • Preliminary results now quoted for 100 pb-1
  • Plan
  • Finalize AlpgenMLM 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
Main loss dueto reconstruc.
Selection criterium
Number of events
ttbar events passing all cuts
Electron trigger important for event selection
and cross section measurementNeed
to understand differences between ttbar
and clean Z?ee- or W?e? events
Pt electron (GeV)
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