t ¯ tb ¯ t ¯ tH ( b ¯ Introduction b b ) analysis Exotics searches Summary Discussion Heavy flavour modelling in top-related analyses at ATLAS Andrea Knue Heavy Flavour Production at the LHC Durham, 20-22nd April 2016 1/ 28 – Heavy Flavour Modelling and Top-Quarks – Andrea Knue
t ¯ tb ¯ t ¯ tH ( b ¯ Introduction b b ) analysis Exotics searches Summary Discussion Outline Motivation t ¯ t + b (¯ b ) measurement SM search: t ¯ tH ( H → b ¯ b ) Exotics searches Summary Discussion 2/ 28 – Heavy Flavour Modelling and Top-Quarks – Andrea Knue
t ¯ tb ¯ t ¯ tH ( b ¯ Introduction b b ) analysis Exotics searches Summary Discussion Motivation top-quark events often produced with additional jets @ the LHC ֒ → want to test pQCD prediction ֒ → important, often irreducible background for SM and exotics will show today three examples: tb ¯ 1.) SM measurement ( t ¯ b ) tH ( b ¯ 2.) SM search ( t ¯ b )) 3.) Exotics searches (four tops, Ht + X , H + ) 3/ 28 – Heavy Flavour Modelling and Top-Quarks – Andrea Knue
t ¯ tb ¯ t ¯ tH ( b ¯ Introduction b b ) analysis Exotics searches Summary Discussion Outline Motivation t ¯ t + b (¯ b ) measurement SM search: t ¯ tH ( H → b ¯ b ) Exotics searches Summary Discussion 4/ 28 – Heavy Flavour Modelling and Top-Quarks – Andrea Knue
t ¯ tb ¯ t ¯ tH ( b ¯ Introduction b b ) analysis Exotics searches Summary Discussion tb ¯ t ¯ b production @ 8 TeV Eur. Phys. J. C (2016) 76:11 want to understand and test QCD best predictions from fixed order calc. @ NLO but: large uncertainties on predictions ֒ → we need experimental measurements! measurement @ 8TeV: cross-section in fiducial phase space t + b ¯ b and ratio of σ ( t ¯ t + b ¯ b ) measure t ¯ t + b , t ¯ σ ( t ¯ t + jj ) 5/ 28 – Heavy Flavour Modelling and Top-Quarks – Andrea Knue
t ¯ tb ¯ t ¯ tH ( b ¯ Introduction b b ) analysis Exotics searches Summary Discussion Analysis strategy Fiducial t ¯ t + b cross-section ➔ e µ & lepton+jets channel ➔ two (one) leptons, ≥ 3 (5) jets, ≥ 3 b -tagged jets ➔ use b -tagging discriminant to extract t ¯ t + b / c / l t + b ¯ Fiducial t ¯ b cross-section ➔ dilepton channel: cut- and fit-based ➔ two leptons, ≥ 4 b -tagged jets t + b ¯ ➔ use b -tagging discr. to extract t ¯ b , t ¯ t + bX , t ¯ t + cX , t ¯ t + lX t + b ¯ σ ( t ¯ b ) /σ ( t ¯ t + jj ) ratio ➔ dilepton channel, at least 4 particle jets ➔ allows to cancel some uncertainties 6/ 28 – Heavy Flavour Modelling and Top-Quarks – Andrea Knue
t ¯ tb ¯ t ¯ tH ( b ¯ Introduction b b ) analysis Exotics searches Summary Discussion Pre-/Postfit plots for lepton+jets Extract cross-sections: ➔ profile likelihood fit of templates to data ➔ allows to strongly constrain the uncertainties (right plot) ➔ insignificant uncertainties are excluded from the fit 7/ 28 – Heavy Flavour Modelling and Top-Quarks – Andrea Knue
t ¯ tb ¯ t ¯ tH ( b ¯ Introduction b b ) analysis Exotics searches Summary Discussion Impact of systematic uncertainties Impact on final result ➔ dilepton: stat. and syst. of similar size ➔ lepton+jets: limited by systematics ➔ systematics dominated by tagging and modelling uncertainties ➔ impact on µ : PS and scale variations 8/ 28 – Heavy Flavour Modelling and Top-Quarks – Andrea Knue
t ¯ tb ¯ t ¯ tH ( b ¯ Introduction b b ) analysis Exotics searches Summary Discussion Comparison of generator setups Sample Generator Shower m b [GeV] Comments b -quarks in ME, NLO+PS t + b ¯ t ¯ aMC@NLO Pythia8 4.8 BDDP scale b t + b ¯ t ¯ aMC@NLO Pythia8 4.8 H T / 4 scale b t + b ¯ t ¯ Powhel Pythia8 0.0 H T / 2 scale b t + b ¯ t ¯ b @ LO+PS t ¯ t +(0-3) p. Madgraph5 Pythia6 4.8 b -quarks only in PS enhanced g → b ¯ t ¯ t (a) Pythia8 Pythia8 4.8 b suppr. g → b ¯ t ¯ t (b) Pythia8 Pythia8 4.8 b enhanced g → b ¯ t ¯ t (c) Pythia8 Pythia8 4.8 b t ¯ t Powheg Pythia6 0.0 Options for g → b ¯ b splitting a) wgtq3: mass dep. terms in splitting kernel, b) wgtq5: no mass dep. terms, sgtq=1: small g → b ¯ b rate c) wgtq6: mass dep. terms, sgtq=0.25: high g → b ¯ b rate 9/ 28 – Heavy Flavour Modelling and Top-Quarks – Andrea Knue
t ¯ tb ¯ t ¯ tH ( b ¯ Introduction b b ) analysis Exotics searches Summary Discussion Results for fiducial cross-sections QCD only measurements (subtract t ¯ tH and t ¯ tV ) measurement at 8 TeV still limited by statistics uncertainty on t ¯ t + b : 25% (lepton+jets) and 32% (dilepton) t + b ¯ uncertainty on t ¯ b : 35% (cut-based) and 36% (fit-based) most generators: good agreement with data → extreme option with highest rate of g → b ¯ ֒ b : strongly disfavoured 10/ 28 – Heavy Flavour Modelling and Top-Quarks – Andrea Knue
t ¯ tb ¯ t ¯ tH ( b ¯ Introduction b b ) analysis Exotics searches Summary Discussion Results ratio: Comparison for diff. g → b ¯ b rates → enhanced g → b ¯ b rate (mass. dep. terms) g → b ¯ → reduced b rate (no mass. dep.) → enhanced g → b ¯ b rate (mass. dep. terms) ➔ results sensitive to modelling of gluon splitting in PS ➔ model with highest g → b ¯ b rate overestimates the cross section by factor of 2! 11/ 28 – Heavy Flavour Modelling and Top-Quarks – Andrea Knue
t ¯ tb ¯ t ¯ tH ( b ¯ Introduction b b ) analysis Exotics searches Summary Discussion Outline Motivation t ¯ t + b (¯ b ) measurement SM search: t ¯ tH ( H → b ¯ b ) Exotics searches Summary Discussion 12/ 28 – Heavy Flavour Modelling and Top-Quarks – Andrea Knue
t ¯ tb ¯ t ¯ tH ( b ¯ Introduction b b ) analysis Exotics searches Summary Discussion Motivation: Why search for t ¯ tH ? ➔ after discovery of the Higgs Boson: ֒ → what are its properties? ֒ → is it really the SM particle? ➔ important: directly measure the top-Higgs Yukawa coupling Y t � ` ➔ top quark heaviest fermion: � W � g t ֒ → Y t largest: ≈ 1 b b t H t b g t b 0 q + W ➔ any deviation would be sign for BSM processes q ➔ but: challenging search 13/ 28 – Heavy Flavour Modelling and Top-Quarks – Andrea Knue
t ¯ tb ¯ t ¯ tH ( b ¯ Introduction b b ) analysis Exotics searches Summary Discussion tH ( H → b ¯ Why t ¯ b ) ? ¯ t Challenges: largest BR for Higgs decay, but: g b g tb ¯ irreducible bkg from t ¯ b ¯ large uncertainties on t ¯ b t +HF g ¯ t t g How to cope with irreducible bkg? t ➔ exploit as much info as possible ➔ use a NN to get best possible S/B separation ¯ b ➔ use signal-depleted regions to constrain bkg and unc g ➔ combined nuisance parameter fit to all regions b → analysis here: 8 TeV, 20.3 fb − 1 ֒ Eur. Phys. J. C (2015) 75:349 14/ 28 – Heavy Flavour Modelling and Top-Quarks – Andrea Knue
t ¯ tb ¯ t ¯ tH ( b ¯ Introduction b b ) analysis Exotics searches Summary Discussion Lepton+jets channel Eur. Phys. J. C (2015) 75:349 Event selection ➔ 1 isolated lepton (25 GeV) ➔ at least 4 jets, 2 b -tagged jets ➔ MVA based b -tagging ➔ no cut on MET/ m T , W ➔ ttbar modelling with Powheg Discriminating variables signal-depleted regions: H had T signal-enriched regions: MVA output ֒ → input to MVA are 10 variables per region ➔ note: if for g → b ¯ b the b’s are within the same jet, events can migrate between different jet/tag categories 15/ 28 – Heavy Flavour Modelling and Top-Quarks – Andrea Knue
t ¯ tb ¯ t ¯ tH ( b ¯ Introduction b b ) analysis Exotics searches Summary Discussion How to define different t ¯ t +HF processes? take antiKt (R=0.4) particle level jets ( p T > 15 GeV, | η | < 2.5) use truth matching to hadrons with ∆ R < 0 . 4, p T > 5 GeV now check additional jets in the event: → if add. jet contains one B/D hadron: t ¯ ֒ t + b → if add. jet contains more than one B/D hadron: t ¯ ֒ t + B → if event not categ. as t ¯ ֒ t + b / B and particle jet contains C-hadron: t ¯ t + c ¯ c 16/ 28 – Heavy Flavour Modelling and Top-Quarks – Andrea Knue
t ¯ tb ¯ t ¯ tH ( b ¯ Introduction b b ) analysis Exotics searches Summary Discussion HF categories for different setups ➔ Sherpa does not include t + b ¯ t ¯ b production via MPI and FSR Samples used for analysis: t + b ¯ ➔ Powheg+Pythia6: t ¯ b only from PS ➔ Madgraph+Pythia6: t ¯ t +(0–3) partons (MLM matching) ➔ Sherpa+OpenLoops: (4FS, massive b -quarks) t + b ¯ t + b ¯ → expected to model t ¯ b better, reweight Powheg to Sherpa for t ¯ ֒ b 17/ 28 – Heavy Flavour Modelling and Top-Quarks – Andrea Knue
t ¯ tb ¯ t ¯ tH ( b ¯ Introduction b b ) analysis Exotics searches Summary Discussion Sensitivity to variations in Sherpa Other variations ➔ impact of MSTW larger than NNPDF ➔ impact of shower recoil visible Scale variations ➔ large impact from µ R variation ➔ large impact from functional form of µ R 18/ 28 – Heavy Flavour Modelling and Top-Quarks – Andrea Knue
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