M(bb) mass resolution improvement - PowerPoint PPT Presentation

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M(bb) mass resolution improvement

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  1. M(bb) mass resolution improvement Vadim Kostyukhinon behalf of the Bonn university group

  2. Idea presented 4 months ago Higgs decays into partons, not jets Correct measured jet energy to parton level, not to particle jet level. Use separate corrections for fully hadronic b-jets and leptonic b-jets Check the obtained parton energy on data directly using top events

  3. B-parton energy vs b-jet energy in top quark event ICHEP 2012: mtop= 173.18 ± 0.94GeV (syst+stat, FERMILAB) Calibration idea: constraint fit in semileptonicttbar system. 4 jet energies + Pxmissing,Pymissing(data), 2W+2top masses(constraints); Fit pars: 6 scaling factors + Pzmissing; Ndf=10-7=3  enough for reliable fit. ATLAS paper Eur. Phys. J. C 39 (2005) s63-s90:1% b-jet JES0.7GeV top mass using constraint fit TOP2006(Coimbra) Calibration: b-jet energy scale factors as a function of Pseparately for hadronic,  and electron jets

  4. B-correction in top events KLFitter out of the box for ttbar events. Jets with leptons are removed. KLFitterW selection, MC KLFitter top selection, MC KLFitterPparton/Pjetlight-jet correctionkinematical fit KLFitterPparton/Pjetb-jet correctionkinematical fit B-jet correction is very different from light jet correction! Simple function p0+p1*exp(-p2*p) describes well the jet to parton energy correction for b-jets!

  5. B-correction in top events What about truth Pparton/Pjetratio ? 1) Very similar behaviour to the KLFitter results 2) Also well described by p0+p1*exp(-p2*p) function Caveat: Slightly different coefficients as compared to reco from KLFitter. Not surprising however, KLFitter uses SHIFTED initial parameters (jets, not partons) for kinematical fit. Let’s try the truth Pparton/Pjetcorrection from top quarks for Higgs!

  6. B-correction hadronic jets WH(130), only hadronic jets,parton correction applied: Pcorr ={0.98+0.61*exp(-P*0.031)} * P PT in GeV After Before Works very well  correct Higgs mass is restored, peak width reduced! Remember: correction is taken from top quark.

  7. B-correction muon Additional scale factor 1.057 for  in jet after pT-correction (muon energy is already added to the jet) WH(130), at least one jet has muon, all corrections applied:

  8. B-correction electron Additional scale factor 1.021 for electron in jet after pT-correction WH(130), at least one jet has electron, all corrections applied:

  9. B-jet correction WH(130), all b-jets, all corrections applied: Before: 16.49/118.4=13.9% After: 14.2/130.0=10.9% ~22% improvement in relative peak width (fit) after WH selection cuts. Correct Higgs mass is restored.

  10. B-jet correction on 2012 data (~8pb-1) Uncorrected b-jets Corrected b-jets WH130(50) is shown on top of the background

  11. Conclusions It’s possible to have ~22% more narrow Higgs peak with correct mass using very simple jet to parton b-jet energy correction and separate scaling factors for muon and electron jets. This correction seems universal ( taken from top, works for Higgs). Careful tuning can make the Higgs peak even more narrow (probably?). This correction and scaling factors for e()-jets can be relatively easy checked on data directly using the top quark mass peak. Further work is needed if one wants to obtain the PTparton/PTjet correction function directly from top quark data using kinematical fit. Seems promising, but some systematic issues should be solved. Open question - how much NN approach can improve further… • Warning: • Jets with leptons have on average smaller energy than similar fully hadronic jets due to unmeasureable neutrino(s). Therefore ANY kinematical event selection, using jet energy cut or its derivatives like invariant masses, reduces the relative amount of these jets and the relative fraction of the lost energy in them. • Thus, the correction factors for electron and muon b-jets are not universal and should be calculated after(!) full selection of signal events.