commissioning studies top physics group m. cobal – university of udine atlas week, prague, sep...
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Commissioning StudiesTop Physics Group
M. Cobal – University of Udine
ATLAS Week, Prague, Sep 2003
Under study
In Athens:
Effects of a non-optimized jet energy scale on the Mtop reconstruction (M. Cobal)
First evaluation of (statistical) error on the cross section and on Mtop (M. Cobal and J. Parsons)
Here:
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)
Top measurement during detector/physics commissioning
Gold-plated channel : single lepton
• pT (lep) > 20 GeV• pT
miss > 20 GeV• ≥ 4 jets with pT > 40 GeV• ≥ 2 b-tagged jets
~ 5%S/B ~ 65
Period evts dMtop(stat)
1 year 3x105 0.1 GeV
1 month 7.5x104 0.2 GeV
1 week 1.9x103 0.4 GeV
L = 1033 cm-2s-1
/ (stat)
0.2 %
0.4%
2.5%
To reconstruct Mtop:
Reconstruction of the hadronic part W from jet pair with the closest invariant mass to MW cut on |mjj-mW| < 20 GeV Association of W with a b-tagged-jet
Cut on |mjjb-<mjjb>| < 20 GeV
Kinematic fit
Using remaining l+b-jet, the leptonic part is reconstructed |mlb-<mjjb>| < 35 GeV Kinematic fit to the ttbar hypothesis, with Mtop and MW mass constraints
Systematic error on Mtop (TDR performance, 10 fb-
1)
Initial performance : uncertainty on b-jet scale expected to dominate
b-jet scale uncertainty Mtop
1% 0.7 GeV 5% 3.5 GeV 10% 7 GeV
Cfr: 10% on q-jet scale 3 GeV on Mtop
Results presented at the Athens Workshop
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)
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.
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)
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
jjb
jj
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.
Big 2 events
Reconstructed Mtop
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
Preliminary results with full simulation
TDR top sample(same cuts as fast sim.)
Top mass
W mass
DC1 sample (same cuts as fast sim.)
Top mass
W mass
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
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)
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
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
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
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
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?
……………
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)
(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