Commissioning Studies Top 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 M top reconstruction (M. Cobal)  First evaluation of (statistical) error on the cross section and on M top (M. Cobal and J. Parsons) Here:  First evaluation of M top, 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 p T (lep) > 20 GeV p T miss > 20 GeV ≥ 4 jets with p T > 40 GeV ≥ 2 b-tagged jets  ~ 5% S/B ~ 65 PeriodevtsdM top (stat) 1 year3x GeV 1 month7.5x GeV 1 week1.9x GeV L = cm -2 s -1  /  (stat) 0.2 % 0.4% 2.5%

To reconstruct M top :  Reconstruction of the hadronic part W from jet pair with the closest invariant mass to M W cut on |m jj -m W | < 20 GeV Association of W with a b-tagged-jet Cut on |m jjb - | < 20 GeV  Kinematic fit Using remaining l+b-jet, the leptonic part is reconstructed |m l b - | < 35 GeV Kinematic fit to the ttbar hypothesis, with M top and M W mass constraints

Systematic error on M top (TDR performance, 10 fb -1 ) Initial performance : uncertainty on b-jet scale expected to dominate b-jet scale uncertainty  M top 1% 0.7 GeV 5% 3.5 GeV 10% 7 GeV Cfr: 10% on q-jet scale  3 GeV on M top

Results presented at the Athens Workshop  An initial uncertainty of 5% on the b-jet energy scale, gives a top mass uncertainty 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)

M top 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 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: M W 1 =M W 2 and M t 1 = M t 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 M top

Signal selection:  ( 4jets exactly+  2 cut) ~40% (~200k evt) W+jets selection:  with the same cuts ~9% (~8k evt)  2 signal  2 W+jets 3-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 M top 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 M jjb <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 P t >20 GeV, P t miss >20 GeV, at least 4 jets with P t >40GeV, 2 b-jets (parton level). 2 non-b jets with min|M jet-jet – M W | taken as W decay products. b jet is selected so that P t 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 jj massjjb 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 jj mass top

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 !! top jj 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 p T 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 week What about B systematics ? M (jj) W  jj difference of distributions for events in the top peak and for events in the side-bands Feedback on detector performance: -- m (top) wrong  jet scale ? -- golden-plated sample to commission b-tag