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Summary of Commissioning Studies Top Physics Group

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

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Summary of Commissioning Studies Top Physics Group

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  1. Summary of Commissioning StudiesTop Physics Group M. Cobal, University of Udine Top Working Group, CERN October 29th, 2003

  2. 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).

  3. Statistical Error In the single lepton channel, where we plan to measure m(top) with the best precision: L = 1x1033 cm-2s-1

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

  5. 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 • |mlnb-<mjjb>| < 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.

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

  7. 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 • Firstcheck fragmentation function with the tracker, thendijet • differential cross-section, h distribution, check pT balancing across • different detectors, etc. • Start lo look at in-situ calibration samples: At the very beginning, start • with W->jj.

  8. Taking TDR numbers: 1500 ttbar->bW(ln)bW(jj) requiring 4 jets above 40 GeV/day at low L. In 1 week: 10k W to jj decays In1 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 h 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...

  9. b-jet scale Observed linearity dependence of the top mass shift on the b-jet absolute scale error for the inclusive sample. Can scale correspondingly: HadronicKin fit 1% jet energy uncertainty  dM(top) = 0.7 0.7 GeV 5% jet energy uncertainty  dM(top) = 0.7*5 = 3.5 3.5 GeV 10% jet energy uncertainty  dM(top) = 0.7*10 = 7 7 GeV

  10. Light-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  dM(top) < 0.7 GeV 10% jet energy uncertainty  dM(top) = 3 GeV

  11. B-tagging From S. Rozanov: Main effects of initial layout: 2 pixel barrel layersrejection 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.

  12. 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

  13. 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

  14. 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

  15. Results presented In Athens: • 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 s at < 10% (stat. error)

  16. 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)

  17. 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.

  18. 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)

  19. Signal selection without b-tag Lepton+4jets exactly (DR=0.4) e: signal ~76% with respect to 4jet W+jets ~83% with respect to 4jets Select among them 2 jets with maximal jjj jj Select the 3-jet combination with maximal

  20. A kinematical constraint fit can be used for a further selection: MW1=MW2 and Mt1=Mt2.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 lessthan a fixed value. Having 3 jets from t-quark decay,there are 3 possible jet assignments for W(jj)b.

  21. Reconstructed Mtop Big 2 events

  22. W+jets selection: e with the same cuts ~9% (~8k evt) Signal selection:e ( 4jets exactly+2 cut) ~40% (~200k evt) 3-jet massW+jets 2 signal 2 W+jets

  23. Preliminary results with full simulation TDR top sample (same cuts as fast sim.) W mass Top mass

  24. W mass DC1 sample (same cuts as fast sim.) Top mass

  25. Conclusions on Mtop • Attsignal can be selected without b-tagging and precise jet energy calibration • Signal / backgnd ratio is ~20 in this case (~70 in the region Mjjb<200 GeV) . Here only W+jetsevents are considered as background. • Such a clean sample could be also used for jet energy calibration. • Results confirmed by full simulation

  26. 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)

  27. 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 Ptjet-jet-b -> max jj mass jjb mass top t-quark peak after application of constraint fit

  28. DC1 +jets sample DC1 sample with application of “TDR-like” generation level cuts DC1 sample Same selection jjb mass jj mass jj mass jjb mass top top t-quark peak after application of constraint fit

  29. DC1 e+jets sample DC1 sample with application of “TDR-like” generation level cuts DC1 sample Selection: the same jjb mass jj mass jj mass jjb mass top top t-quark peak after application of constraint fit

  30. DC1 summary e,+jets sample DC1 sample with application of “TDR-like” generation level cuts DC1 sample Same selection jjb mass jjb mass jj mass jj mass top top t-quark peak after application of constraint fit  agreement with TDR !!

  31. Next Steps • More detailed MC study: W + jets background. • Study of background level dependence on b-tagging e. • 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? ……………

  32. 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)

  33. t  bjj W  jj W  jj M (jj) M (bjj) M (jj)  (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 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 ? 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

  34. 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

  35. 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 s at < 10% (stat. error) • Additional studies (e.g. di-lepton) undergoing

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