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Top Mass in t-tbar: Status Report from CERN

This report provides an update on the progress of measuring the top mass in fully hadronic t-tbar decays without b-tagging, using only calorimetry. It includes information on the event selection, kinematic fit, jet-parton matching, and jet kinematics uncertainties.

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Top Mass in t-tbar: Status Report from CERN

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

  2. 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 Status report: top mass in t-tbar -> 6 jets

  3. 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: pTmin=25 GeV and Rmin=0.5 • matching: ETmin=25 GeV and Rjet=0.5 • 2, 3, 4, 5, 6(inc.) jets Status report: top mass in t-tbar -> 6 jets

  4. Selections • Event selection • at least 6 jets in || < 3 • at least 6 jets satisfying graded pT cuts • 85, 65, 50, 40, 40, 40 GeV • Hypotheses for each event • only consider the 6 highest pT 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 Status report: top mass in t-tbar -> 6 jets

  5. Selections • Hypotheses treatment build both W and top 4-vectors from jets • Hypotheses selection • MW window cut: 40 < MW < 120 GeV • Mtop window cut: |Mtop| < 100 GeV • top pT cut: pT > 100 GeV • Perform kinematic fit • Hypotheses selection • valid kinematic fit full accurate cov matrix • kinematic fit 2 cut: 2 < 10 Status report: top mass in t-tbar -> 6 jets

  6. PT distribution: signal Status report: top mass in t-tbar -> 6 jets

  7. PT distribution: 6 partons background Status report: top mass in t-tbar -> 6 jets

  8. 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 Status report: top mass in t-tbar -> 6 jets

  9. 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 Status report: top mass in t-tbar -> 6 jets

  10. 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 Status report: top mass in t-tbar -> 6 jets

  11. Cut flow Status report: top mass in t-tbar -> 6 jets

  12. 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 Rmax = 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 Status report: top mass in t-tbar -> 6 jets

  13. Jet-parton matching • Matching efficiencies • u 76.1% ± 0.1% ubar 76.2% ± 0.1% • dbar 70.2% ± 0.1% d 70.2% ± 0.1% • b 79.1% ± 0.1% bbar 79.2% ± 0.1% • Jet-parton distances • R < 0.2 is a reasonable criteria Status report: top mass in t-tbar -> 6 jets

  14. Jet kinematics uncertainties • 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 statistics in the E, plane Status report: top mass in t-tbar -> 6 jets

  15. Jet kinematics: energy energy bias energy resolution Status report: top mass in t-tbar -> 6 jets

  16. Jet kinematics: energy bias ? Status report: top mass in t-tbar -> 6 jets

  17. Jet kinematics: resolution Status report: top mass in t-tbar -> 6 jets

  18. Signal truth kinematic fit results 2 ×1104 entries 2 ×1038 entries Status report: top mass in t-tbar -> 6 jets

  19. Signal truth kinematic fit results Status report: top mass in t-tbar -> 6 jets

  20. Cut flow: signal Status report: top mass in t-tbar -> 6 jets

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

  22. Selections: signal top mass valid fit hypotheses final hypotheses (after chi2 cut) 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? Status report: top mass in t-tbar -> 6 jets

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

  24. Cut flow: all samples Status report: top mass in t-tbar -> 6 jets

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

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

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

  28. 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? Status report: top mass in t-tbar -> 6 jets

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