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Semileptonic Boosted Tops

Semileptonic Boosted Tops. Brock Tweedie Johns Hopkins University 10 July 09. K. Rehermann & B.T., To Appear. The Problem. b-jet. l. ~ m t / p t. n. Isolation probability ( D R bl > 0.4). Our Philosophy. Try to use these nonisolated leptons Avoid using MET for discrimination

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Semileptonic Boosted Tops

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  1. Semileptonic Boosted Tops Brock Tweedie Johns Hopkins University 10 July 09 K. Rehermann & B.T., To Appear

  2. The Problem b-jet l ~ mt / pt n Isolation probability (DRbl > 0.4)

  3. Our Philosophy • Try to use these nonisolated leptons • Avoid using MET for discrimination • Do not b-tag

  4. Leptons Inside of Jets • Physics backgrounds • Heavy flavor (prompt and radiative) • p+ decays in flight • Instrumental backgrounds • p0C “e” • p+C “m” Save for real experimentalists!

  5. Event Generation • PYTHIA and HERWIG ttbar continuum and generic dijet • Includes prompt heavy flavor, light meson decays-in-flight in LHC-like detector volume • Basic requirement: muon with Pt > 30 GeV • ~4% pass rate for dijet…per-jet probability ~2% I will exclusively use PYTHIA plots / #s. HERWIG is practically identical.

  6. m+jets Event Reconstruction • Set aside leading muon • Put remaining particles into perfect 0.1x0.1 calorimeter • Cluster with C/A • Set R according to Ht in hemisphere opposite the muon • Jet Pt > 50 GeV • Leading jet == hadronic top • Jet closest to muon == b-jet from semilep top

  7. Semi-Leptonic Tops vs Light Jets b b • m + jet + MET • hard m and MET • mT(m+MET) ~ mW • mass = mt n n b n m b m p m K • m + jet + MET + JUNK • Soft/collinear singularities • Splittings more common late in the shower (more gluons!)

  8. Mini-Isolation DR ~ mt / Ptt DR ~ ? DR ~ mb / Ptb DR ~ mb / Ptb m n m n B B B W t

  9. Mini-Isolation • Many options for cone definition: • R ~ 1/Ptb ~ 1/Ptm • R ~ 1/Ptt • R = fixed # • … • They all perform comparably • R ~ 1/Ptm convolves additional discriminating power from muon Pt distributions

  10. Mini-Isolation Isolation cone DR = (15 GeV) / Ptm top light jet W+jets Demand >90% isolated

  11. (Thaler & Wang) xm = 1 - mb2/mbm2 top light jet W+jets Demand xm > 0.5

  12. Mini-Isolation After xm Cut top light jet W+jets

  13. xm After Mini-Isolation Cut top light jet W+jets

  14. Efficiencies of Leptonic Cuts (Pt ~ 1 TeV)

  15. Efficiencies of Leptonic Cuts (Pt ~ 2 TeV)

  16. Semi-Leptonic Tops vs W-strahlung b q’ • m + jet + MET • hard m and MET • mT(m+MET) ~ mW • mass = mt n n q b n m W m m • m + jet + MET • hard m and MET • mT(m+MET) ~ mW • mW < mass < sqrt(s-hat)

  17. Ideal Top Mass Distributions top light jet W+jets

  18. DRbm • Wjj MadGraph 2 C 4 top light jet W+jets

  19. Ideal-Mass vs DRbm Discrimination

  20. Efficiencies of Leptonic Cuts (Pt ~ 1 TeV)

  21. Efficiencies of Leptonic Cuts (Pt ~ 2 TeV)

  22. Backgrounds with Top-Mass Cut Now use MET for global even reco. Define hn == hm. Hadronic top-mass cut efficiencies: et ~ 85% / eq/g ~ 25%

  23. Backgrounds with Top-Tag

  24. Resonance Efficiencies (incorporates m+jets BR) top-mass cut top-tag

  25. Summary • Assuming this all works in the detector, light QCD can be made negligible practically “for free” • In principle, rejection factors at the ~50,000 level • Allows for a comfortable margin of theory error • W-strahlung is still non-negligible • O(1) rejection “for free” by exploiting geometry

  26. Summary • Still various additional discriminators after incorporating MET • mT(top), m(top) • Internal angular variables • Possibilities for improvements in high-Pt t-tagging and b-tagging

  27. Summary • We will be seeing how these perform in full CMS simulation in the coming months

  28. Extras

  29. Resonances

  30. Discovery Reach Estimates S/sqrt(B) > 5 & S > 15 G = 0% G = 15%

  31. Subjet Rates * old PYTHIA results

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