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Jet Combination Algorithms for Minimizing Higgs Mass Resolution

P. Grannis Feb. 14, 2001. Jet Combination Algorithms for Minimizing Higgs Mass Resolution. There are 2 key ingredients for the Higgs search sensitivity: 1) High efficiency / purity b tagging 2) Best multi-jet mass resolution (H → bb system) PYTHIA study: pp → WH → (qq’) (bb)

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Jet Combination Algorithms for Minimizing Higgs Mass Resolution

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  1. P. Grannis Feb. 14, 2001 Jet Combination Algorithms for Minimizing Higgs Mass Resolution • There are 2 key ingredients for the Higgs search sensitivity: • 1) High efficiency / purity b tagging • 2) Best multi-jet mass resolution (H → bb system) • PYTHIA study:pp → WH → (qq’) (bb) • M(H) = 120 GeV • with ISR/FSR/fragmentation • smear particle energies dE/E = 0.5 • Cone algorithm jets (typically R=0.5)

  2. b-tagging: dR(b quark – jet ) < 0.1 AND dp(b quark– jet /p < 0.15 for either initial or final PYTHIA b quark. 56% efficient per jet Selection/”trigger” : at least 4 jets (2 b-tagged) with ET > 15 GeV and |h| < 2.5 Denote non-b-tagged jets as “q-jets” at least 2 di-q jet combinations within 12 GeV of obs. W mass jets within |h|<2.5 all reconstructed jets # di-q within 2 s

  3. b-jet matching distributions

  4. Mass for pair of jets matched to q’s from W smeared no smearing Central gaussian s =4.32 GeV Central gaussian s =7.35 GeV

  5. Effect of FSR at partonic level: Combine b and q partons if Mbq < 10 GeV 2 original b-tagged partons Mass of the b and bbar tagged jets (only) smeared No smearing s =6.59 GeV large low side tail

  6. Seek algorithm for adding FSR q jets to the b-jets to improve dMH . Base it on cuts on observed b-jet/q-jet mass: smeared No smearing

  7. If Mbq < Mcut for any q jet, add q jet momentum to b system. For Mcut = 25 GeV, s = 6.67 GeV, but distribution is more symmetric than for bb jets only. (But still non-Gaussian.) For Mcut = 45 GeV, get high side tail from association with non-FSR jets. Mcut = 25 GeV Mcut = 35 GeV Mcut = 45 GeV

  8. Amalgamating any q-jet is wrong – should not add a jet from W → qq. If exempt q-jet from the best W → qq combination from amalgamation: s = 6.62 GeV Mcut = 25 Mcut = 35 Mcut = 45 GeV Variations: exempt any q-jet in a reasonable W → qqno better exempt non W → qq for each W comb, and weightno better R = 0.3 cone jetsno better choose W → qq with best W H pT balanceno better rescale jet energies by factor to give minimum |pTW– pTH| worse Note: resolution for subsample with exactly 2 b-jets and 2 q-jets in W is somewhat better: s = 6.41 GeV (lose 30% of sample)

  9. Conclusions: • Algorithm for amalgamating jets can symmetrize Mbb distribution and remove some of low mass tail (useful since bknd falls steeply with Mbb • Resolution of Gaussian core is not improved by FSR amalgamation • Further studies for reducing dMbb should aim at improving dEjet/Ejet resolution: Use of kT, energy flow or clustering algorithms (could choose FSR jets to amalgamate as above and recluster energy in vicinity of these to add to b system?) Jet by jet energy scale corrections using multivariate predictors for true E based on e.g. presence of/momentum of leptons, subclustering, track multiplicity, secondary vertex missing pT, etc. (these multivariate energy corrections worked rather well for optimizing the momentum of B hadrons in OPAL. Used H-matrix predictor of B momentum using lepton/track momenta)

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