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 We reconstruct the top mass in the lepton+jets channel Clean sample (1 isolated lepton, high Etmiss).

Statistical Error Periodtt events 1 year8x month2x week5x10 5 In the single lepton channel, where we plan to measure m(top) with the best precision: Periodevts  M top (stat) 1 year3x GeV 1 month7.5x GeV 1 week1.9x GeV L = 1x10 33 cm -2 s -1

Top mass precision One top can be directly reconstructed Reconstruct t  Wb  (jj)b Selection cuts: 1 iso lep, Pt > 20 GeV, |  | 20 At least 4 jets with Pt > 40 GeV and |  | < 2.5 At least 2 b-tagged jets Selection effic. = 5%  126k events, with S/B = 65

Two methods:  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 - | < 35 GeV  Kinematic fit The leptonic part is reconstructed |m l b - | < 35 GeV -30k signal events -14k bkgnd events Kinematic fit to ttbar, with m(top) and m(W) mass constraints Main Background is the combinatorial one.

Systematics for the lepton + jet analyses At the beginning the jet energy scale will be not known as well as 1%

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.

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...

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) = GeV 5% jet energy uncertainty   M(top) = 0.7*5 = GeV 10% jet energy uncertainty   M(top) = 0.7*10 = 7 7 GeV b-jet scale

Here as well linear dependence If one performs constrained fit on W-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

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.

Estimates for initial  (t-tbar) measurement Initial lum = 1x10 33 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

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 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

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 1x10 33

Results presented  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) In Athens:

In Prague:  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)

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 jjj 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

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 1x10 33

Conclusions  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)  Additional studies (e.g. di-lepton) undergoing