1. top mass in t-tbar  6jets Status Report Physics and Astronomy University of Victoria British Columbia, Canada Top meeting CERN, 21 Feb 2007 Michel Lefebvre Keith Edmonds M.Sc. project Much help from Rolf Seuster

2. Michel Lefebvre, 2007/02/21Status report: top mass in t-tbar -> 6 jets2 Introduction and goals Fully hadronic t-tbar decays  Attempt at measuring the top mass  First see how far one can go without b-tagging only calorimetry  Huge combinatorics and QCD backgrounds

3. Michel Lefebvre, 2007/02/21Status report: top mass in t-tbar -> 6 jets3 Samples Signal sample  5204, athena 11.0.5 AOD 270k events Cone4TowerParticleJets Background samples  QCD multijet, alpgen 2.06, atlfast, athena 12.0.31 home production –generation:p Tmin =25 GeV and R min =0.5 –matching:E Tmin =25 GeV and R jet =0.5 2, 3, 4, 5, 6(inc.) jets

4. Michel Lefebvre, 2007/02/21Status report: top mass in t-tbar -> 6 jets4 Selections Event selection  at least 6 jets in |  | < 3  at least 6 jets satisfying graded p T cuts 85, 65, 50, 40, 40, 40 GeV Hypotheses for each event  only consider the 6 highest p T jets  consider all 90 combinations for t-tbar topology 10 distinct triplet pairs, each triplet with 3 possible W –assumes one cannot distinguish charges and quark types

5. Michel Lefebvre, 2007/02/21Status report: top mass in t-tbar -> 6 jets5 Selections Hypotheses treatment build both W and top 4-vectors from jets Hypotheses selection  M W window cut:40 < M W < 120 GeV   M top window cut:|  M top | < 100 GeV  top p T cut:p T > 100 GeV Perform kinematic fit Hypotheses selection  valid kinematic fitfull accurate cov matrix  kinematic fit  2 cut:  2 < 10

6. Michel Lefebvre, 2007/02/21Status report: top mass in t-tbar -> 6 jets6 P T distribution: signal

7. Michel Lefebvre, 2007/02/21Status report: top mass in t-tbar -> 6 jets7 P T distribution: 6 partons background

8. Michel Lefebvre, 2007/02/21Status report: top mass in t-tbar -> 6 jets8 Kinematic fit: input parameters Experimental jets  initial 4-vectors  we set should not do this when considering merged partons  each jet therefore characterized by the triplet for now assume no b-jet tagging

9. Michel Lefebvre, 2007/02/21Status report: top mass in t-tbar -> 6 jets9 Kinematic fit: simple chi2 consider a simple chi2 fit  assume jet direction exact  constraint both W masses  demand that both tops have the same mass  consider Gaussian for W and top masses easy to improve to likelihood and Breit-Wigner if needed  for a given jet hypothesis consider the chi2 the free parameters are the six fit jet energies

10. Michel Lefebvre, 2007/02/21Status report: top mass in t-tbar -> 6 jets10 Kinematic fit  start each fit with  in the fit use  for use the same values as used in the generation  use the best possible estimate of the jet energy error Here just use

11. Michel Lefebvre, 2007/02/21Status report: top mass in t-tbar -> 6 jets11 Cut flow

12. Michel Lefebvre, 2007/02/21Status report: top mass in t-tbar -> 6 jets12 Jet-parton matching Study events with “true” jet hypothesis Matching criteria  For each parton, look for a matching jet restrict search in a region limited by  R max = 0.2 keep the closest jet in this region  Demand that a jet be matched only once matching efficiencies depends on interparton distances –same top combinations  R(u-b)  = 2.220 ± 0.002  R(ubar-bbar)  = 2.219 ± 0.002  R(u-dbar)  = 2.008 ± 0.002  R(ubar-d)  = 2.010 ± 0.002  R(dbar-b)  = 2.037 ± 0.002  R(d-bbar)  = 2.036 ± 0.002 –other 9 combinations  R   2.40

13. Michel Lefebvre, 2007/02/21Status report: top mass in t-tbar -> 6 jets13 Jet-parton matching Matching efficiencies u76.1% ± 0.1%ubar76.2%± 0.1% dbar70.2% ± 0.1%d70.2%± 0.1% b79.1% ± 0.1%bbar79.2%± 0.1% Jet-parton distances   R < 0.2 is a reasonable criteria

14. Michel Lefebvre, 2007/02/21Status report: top mass in t-tbar -> 6 jets14 Need to characterize kinematics uncertainties  required for the kinematic fit Consider 1-to-1 matched jet-parton pairs  study the quantities  here focus on a few aspects Jet kinematics uncertainties statistics in the E,  plane

15. Michel Lefebvre, 2007/02/21Status report: top mass in t-tbar -> 6 jets15 Jet kinematics: energy energy bias energy resolution

16. Michel Lefebvre, 2007/02/21Status report: top mass in t-tbar -> 6 jets16 Jet kinematics: energy bias ?

17. Michel Lefebvre, 2007/02/21Status report: top mass in t-tbar -> 6 jets17 Jet kinematics: resolution

18. Michel Lefebvre, 2007/02/21Status report: top mass in t-tbar -> 6 jets18 Signal truth kinematic fit results 2 ×1104 entries 2 ×1038 entries

19. Michel Lefebvre, 2007/02/21Status report: top mass in t-tbar -> 6 jets19 Signal truth kinematic fit results

20. Michel Lefebvre, 2007/02/21Status report: top mass in t-tbar -> 6 jets20 Cut flow: signal

21. Michel Lefebvre, 2007/02/21Status report: top mass in t-tbar -> 6 jets21 Selections: signal top mass all hypotheses valid fit hypotheses 1-to-1 matched jet-parton pairs (overlaid, not stacked) with combinatorics  recall that 10% of all hypotheses combinatorics have correct top triplets

22. Michel Lefebvre, 2007/02/21Status report: top mass in t-tbar -> 6 jets22 Selections: signal top mass final hypotheses (after chi2 cut)valid fit hypotheses chi2 cut 1-to-1 matched jet-parton pairs (overlaid, not stacked) with combinatorics Need to revisit combinatorics strategy perhaps keep only one (best?) combination per event?

23. Michel Lefebvre, 2007/02/21Status report: top mass in t-tbar -> 6 jets23 Selections: signal W mass all hypotheses valid fit hypotheses 1-to-1 matched jet-parton pairs (overlaid, not stacked) with combinatorics  recall that 10% of all hypotheses combinatorics has correct top triplets

24. Michel Lefebvre, 2007/02/21Status report: top mass in t-tbar -> 6 jets24 Cut flow: all samples

25. Michel Lefebvre, 2007/02/21Status report: top mass in t-tbar -> 6 jets25 Selections: top mass all hypotheses valid fit hypotheses signal 1-to-1 matched jet-parton pairs (overlaid, not stacked) signal with combinatorics QCD 6 partons QCD 5 partons QCD 4 partons

26. Michel Lefebvre, 2007/02/21Status report: top mass in t-tbar -> 6 jets26 Selections: top mass valid fit hypotheses signal 1-to-1 matched jet-parton pairs (overlaid, not stacked) signal with combinatorics QCD 6 partons QCD 5 partons QCD 4 partons chi2 cut final hypotheses (after chi2 cut) Need larger background samples!!

27. Michel Lefebvre, 2007/02/21Status report: top mass in t-tbar -> 6 jets27 Selections: W mass all hypotheses valid fit hypotheses signal 1-to-1 matched jet-parton pairs (overlaid, not stacked) signal with combinatorics QCD 6 partons QCD 5 partons QCD 4 partons

28. Michel Lefebvre, 2007/02/21Status report: top mass in t-tbar -> 6 jets28 Possible way forward... need b-tagging explore existing background samples  SUSY samples? revisit jet-parton hypotheses strategy study energy biases  can they be extracted from the study of semileptonic decays?  how do they affect the kinematic fits? improve kinematic fit  better energy resolution parameterization obtain this in situ?  allow jets direction to vary  move to likelihood with Breit-Wigner tackle b-jet energy scale  obtain this in situ?