summary of commissioning studies top physics group
DESCRIPTION
Summary of Commissioning Studies Top Physics Group. M. Cobal, University of Udine. Top Working Group, CERN October 29 th , 2003. Top Quark Event Yields. NLO Xsect for t-tbar production = 833 pb 8 million t-tbar pairs produced per 10 fb -1 - PowerPoint PPT PresentationTRANSCRIPT
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Summary of Commissioning Studies
Top Physics Group
M. Cobal, University of Udine
Top Working Group, CERNOctober 29th, 2003
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Top Quark Event Yields
• NLO Xsect for t-tbar production = 833 pb8 million t-tbar pairs produced per 10 fb-1
• We reconstruct the top mass in the lepton+jets channel Clean sample (1 isolated lepton, high Etmiss).
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Statistical Error
Period tt events
1 year 8x106
1 month 2x106
1 week 5x105
In the single lepton channel, where we plan to measure m(top) with the best precision:
Period evts Mtop(stat)
1 year 3x105 0.1 GeV
1 month
7.5x104 0.2 GeV
1 week 1.9x103 0.4 GeVL = 1x1033 cm-2s-1
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Top mass precision
One top can be directly reconstructed
Reconstruct t Wb (jj)b
Selection cuts:
1 iso lep, Pt > 20 GeV, || < 2.5, Etmiss > 20At least 4 jets with Pt > 40 GeV and || < 2.5At least 2 b-tagged jets Selection effic. = 5% 126k events, with S/B = 65
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Two methods:
Reconstruction of the hadronic part W from jet pair with the closest invariant mass to m(W) cut on |mjj-mW| < 20 GeV Association of W with a b-tagged-jet
Cut on |mjjb-<mjjb>| < 35 GeV
Kinematic fit
The leptonic part is reconstructed |mlb-<mjjb>| < 35 GeV -30k signal events-14k bkgnd events
Kinematic fit to ttbar, with m(top) and m(W) mass constraintsMain Background is the combinatorial one.
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Systematics for the lepton + jet analyses
At the beginning the jet energy scale will be not known as well as 1%
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Energy scale
From M. Bosman:
- Will start to calibrate calorimeter with weights from MC- Assume:
• EM scale correct to the percent level from the very beginning • fragmentation correctly described in MC• corrections for calorimeter non-compensation and dead material
correct calibration coefficients should be predicted
1) First check fragmentation function with the tracker, then dijet differential cross-section, distribution, check pT balancing across different detectors, etc.
2) Start lo look at in-situ calibration samples: At the very beginning, start with W->jj.
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Taking TDR numbers:
1500 ttbar->bW(l)bW(jj) requiring 4 jets above 40 GeV/day at low L.
In 1 week: 10k W to jj decays In 1 month: 35k W to jj decays
Jets have a pT distribution: ~ 40 to 140 GeV with changing calibration. Consider pT bins of 10 GeV, and bins of 0.3. There are 150 "samples" to consider: After a week, about 70 W per "sample" or a statistical error on m(W) sigma(about 8 GeV with perfect calibration) divided by sqrt(70) This makes ~1% of statistical error
On top there is the systematic errors due to FSR and jet overlap...
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Observed linearity dependence of the top mass shift on the b-jet absolute scale error for the inclusive sample.
Can scale correspondingly: Hadronic Kin fit 1% jet energy uncertainty M(top) = 0.7 0.7 GeV
5% jet energy uncertainty M(top) = 0.7*5 = 3.5 3.5 GeV
10% jet energy uncertainty M(top) = 0.7*10 = 7 7 GeV
b-jet scale
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Here as well linear dependenceIf one performs constrained fit onW-mass, is less important than b-jet scale.
Can scale correspondingly: Hadronic 1% jet energy uncertainty M(top) < 0.7 GeV
10% jet energy uncertainty M(top) = 3 GeV
Light-jet scale
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B-tagging
From S. Rozanov:
Main effects of initial layout:
2 pixel barrel layers rejection of light jets reduced by ~30%. Another important parameter is the efficiency of the pixel chips and modules (not predicted).
Effect of alignment precision:
Precise alignment of ID could be reached only after a FEW MONTHS work. (studies undergoing) Impact of misalignment much higher than effect of 2 or 3 layers. Can also compromise a jet energy calibration based on W from tt at startup: could be difficult to select W’s over background.
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Estimates for initial (t-tbar) measurement
• Initial lum = 1x1033 cm-2 s-1 t-tbar production rate = 0.85 Hz
~ 500k t-tbar events produced per week
• With same analysis and detector performance as in Physics TDR, predict:– Selection of 8000 single lepton plus jets events, S/B =
65
– In ± 35 GeV window around m(top), would have:• 1900 signal events• 900 bkgnd events (dominated by “wrong
combinations” from t-tbar events)
stat error on (t-tbar) 2% after 1 week
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• What happens with degraded initial detector performance?
– eg. Consider case where b-tagging is not available in early running:
– Drop b-tagging requirement: signal effic. increases from 5% to 20%, but bkgnd increases faster
– For one week, would select 32000 signal events, but with S/B = 6
– Biggest problem comes from large increase in combinatorial bkgnd when trying to reconstruct t Wb (jj)b with b-tagging
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W jj t Wb (jj)b
– Fit of m(jjb) spectrum provides Xsect measurement with stat. error 7%
– Even with no b-tagging, can measure (t-tbar) to < 10% with two days of integrated luminosity at 1x1033
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Results presented
An initial uncertainty of 5% on the b-jet energy scale, gives a top massuncertainty of 3.5 for the mass reconstuction.If we go to 10% , the uncertainty on the top mass is of ~7 GeV
An initial uncertainty of 10% on the light jet energy scale, gives a top mass uncertainty of 3 GeV for the mass reconstuction. Kinematic fit less sensitive to light jet energy scale. But can have very large combinatorial background in case of b-tagging not working
After 1 week of data taking we should be able to measure the cross-section with a 2% statistical error
Even without b-tagging, with two days of data taking, can measure at < 10% (stat. error)
In Athens:
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In Prague:
First evaluation of Mtop, assuming no b-tagging at the startup (V. Kostiouchine)
Investigation of differences found in the combinatorial backgnd between TDR and DC1 (V. Kostiouchine)
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Mtop reconstruction in ATLAS at startup
Work done by V. Kostioukhine
Assumptions:
• No jet energy calibration, no b-tagging.• Uniform calorimeter response • Good lepton identification.
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TDR signal+backgrounds estimation
In case of no b-tag:
tt signal: ~500k evt ( 4 times reduction due to b-tag)W+jets: ~85k evt (50 times reduction due to b-
tag)
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Signal selection without b-tag
Lepton+4jets exactly (R=0.4): signal ~76% with respect to
4jet W+jets ~83% with respect to
4jets
Select the 3-jet combination with maximal
Select among them 2 jets with maximal
3
1iiPP
2
1iiPP
jjj
jj
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Having 3 jets from t-quark decay,there are 3 possible jet assignments for W(jj)b.
• A kinematical constraint fit can be used for a further selection: MW
1=MW
2 and Mt
1=Mt
2.
An approximate calibration is obtained with the W peak
• Select the combination with lowest 2 out of the 3 available. Event is accepted is this minimal 2 is less than a fixed value.
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Big 2 events
Reconstructed Mtop
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Signal selection: ( 4jets exactly+2 cut) ~40% (~200k evt)
W+jets selection: with the same cuts ~9% (~8k evt)
2 signal 2 W+jets3-jet mass W+jets
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Preliminary results with full simulation
TDR top sample(same cuts as fast sim.)
Top mass
W mass
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DC1 sample (same cuts as fast sim.)
Top mass
W mass
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Conclusions on Mtop
1. A tt signal can be selected without b-tagging and precise jet energy calibration
2. Signal / backgnd ratio is ~20 in this case (~70 in the region Mjjb<200 GeV) . Here only W+jets events are considered as background.
3. Such a clean sample could be also used for jet energy calibration.
4. Results confirmed by full simulation
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Combinatorial background in DC1 data
Work done by V. Kostioukhine
• Increase of the combinatorial background in DC1 samples with respect to the TDR ones
• Vadim checked better and.....
W(TDR) W (DC1)
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TDR +jets sample
Selection: 1 lep with Pt>20 GeV, Pt miss >20 GeV, at least 4 jets with
Pt>40GeV, 2 b-jets (parton level). 2 non-b jets with min|Mjet-jet – MW|
taken as W decay products. b jet is selected so that Pt jet-jet-b -> max
t-quark peak after application of constraint fit
jj mass jjb mass
top
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DC1 +jets sample
Same selection
DC1 sample
t-quark peak after application of constraint fit
DC1 sample with application of“TDR-like” generation level cuts
jj mass jjb mass
top top
jj mass jjb mass
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DC1 e+jets sample Selection: the same
DC1 sample
t-quark peak after application of constraint fit
DC1 sample with application of“TDR-like” generation level cuts
jj mass jjb mass jjb massjj mass
toptop
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DC1 summary e,+jets sample
Same selection DC1 sample with application of“TDR-like” generation level cuts
DC1 sample
t-quark peak after application of constraint fit agreement with TDR !!
toptop
jj mass jj massjjb mass jjb mass
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Next Steps
More detailed MC study: W + jets background.
Study of background level dependence on b-tagging .
Measure the cross-section and top mass assuming different efficiency for the b-tagging (and no b-tagging at all) and looking at various channels. What is the minimal b-tagging needed?
……………
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First look at data in 2007
Study of high pT isolated electrons and muons
Select a “standard” top sample, and a “golden” top sample with tighter cuts.
Try to reconstruct the two top masses (in single lepton events, one top decays hadronically, the other one leptonically)
Take top events: try a first measurement of the cross section, and of the mass in various channels (as a cross check, since systematic errors are different)
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(tt) : initial measurement dominated by L and detector uncertainties 10-20%?
In addition, very pessimistic scenario considered : b-tag not yet available S increases by ~ 4 S/B decreases from 65 to 6 large combinatorial background
W jj t bjj
M (jj) M (bjj)
Still a top peak is visible Statistical error from fit: from 2.5% (perfect b-tag) to 7% (no b-tag) for ~ one weekWhat about B systematics ?
M (jj)
W jj
difference of distributionsfor events in the top peak andfor events in the side-bands
Feedback on detector performance:-- m (top) wrong jet scale ? -- golden-plated sample to commission b-tag
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W jj t Wb (jj)b
– Fit of m(jjb) spectrum provides Xsect measurement with stat. error 7%
– Even with no b-tagging, can measure (t-tbar) to < 10% with two days of integrated luminosity at 1x1033
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Conclusions
An initial uncertainty of 5% on the b-jet energy scale, gives a top massuncertainty of 3.5 for the mass reconstuction.If we go to 10% , the uncertainty on the top mass is of 7 GeV
An initial uncertainty of 10% on the light jet energy scale, gives a top mass uncertainty of 3 GeV for the mass reconstuction. Kinematic fit less sensitive to light jet energy scale. But can have very large combinatorial background in case of b-tagging not working
After 1 week of data taking we should be able to measure the cross-section with a 2% statistical error
Even without b-tagging, with two days of data taking, can measure at < 10% (stat. error)
Additional studies (e.g. di-lepton) undergoing