Search for the Higgs boson in the t th production mode using the ATLAS detector

Transcription

1 Search for the Higgs boson in the t th production mode using the ATLAS detector Rustem Ospanov (University of Pennsylvania) on behalf of the ATLAS collaboration Large Hadron Collider Physics Conference, June 2nd, / 18

2 Introduction Higgs boson with m 125 GeV discovered via γγ, ZZ, WW decays Higgs boson Yukawa couplings probe flavor structure of the SM Higgs Yukawa couplings accessible at LHC: τ, µ, b and top quark Strong evidence for H ττ: 4.1σ for ATLAS and 3.2σ for CMS Figure by Hitoshi Murayama Evidently the Higgs system knows something that we do not know: the difference between the generations - Martinus Veltman. 2 / 18

3 Higgs-top quark coupling t th coupling Drives SM properties at high energy scale Indirect measurement via gluon fusion production Direct measurement via t th production t th production: σ(t th) 130 fb for m H = 125 GeV at s = 8 TeV 2600 events with 20.3 fb 1 Main background is t t + jets with σ 250 pb Decay modes: B(H b b) = 8 - dominant mode B(H γγ) = clean resonance signature B(H WW, ZZ, ττ) multi-lepton final states 3 / 18

4 t th t t + b b analysis strategy Tag tt decay by 1-lepton and 2-lepton Categorize by number jets and b-jets Increase sensitivity with different S/B Constrain systematic uncertainty by fitting regions with low S/B t th 6 jets 4 b-jets 1-lepton: t th 6 jets 4 b-jets 1 electron or muon 4 jets 2 b-jets 2-lepton: t th 4 jets 4 b-jets 2 electron or muon 2 jets 2 b-jets t th signal > 90% 4 / 18

5 t th t t + b b analysis strategy Tag tt decay by 1-lepton and 2-lepton Categorize by number jets and b-jets Increase sensitivity with different S/B Constrain systematic uncertainty by fitting regions with low S/B 1-lepton: t th 6 jets 4 b-jets 1 electron or muon 4 jets 2-lepton: 2 b-jets t th 4 jets 4 b-jets 2 electron or muon 2 jets 2 b-jets 1-lepton ATLAS Preliminary Simulation s L dt = 20.3 fb m H = 125 GeV B S / B S / B S / = 8 TeV, j, 2 b S/B < 0.1% j, 2 b S/B = 0.1% j, 2 b S/B = 0.2% B S / B S / B S / j, 3 b S/B = 0.2% j, 3 b S/B = 0.4% j, 3 b S/B = 0.9% S/ B for control regions S/ B for signal regions B S / B S / B S / Single lepton j, 4 b S/B = 1.3% 5 j, 4 b S/B = 2.3% 6 j, 4 b S/B = 3.8% 5 / 18

6 t th t t + b b analysis strategy Tag tt decay by 1-lepton and 2-lepton Categorize by number jets and b-jets Increase sensitivity with different S/B Constrain systematic uncertainty by fitting regions with low S/B 1-lepton: t th 6 jets 4 b-jets 1 electron or muon 4 jets 2-lepton: 2 b-jets t th 4 jets 4 b-jets 2 electron or muon 2 jets 2 b-jets 2-lepton ATLAS Preliminary Simulation s L dt = 20.3 fb m H = 125 GeV B S / B S / B S / = 8 TeV, j, 2 b S/B < 0.1% j, 2 b S/B = 0.1% j, 2 b S/B = 0.3% B S / B S / j, 3 b S/B = 0.7% j, 3 b S/B = 1.8% S/ B for control regions S/ B for signal regions B S / Dilepton j, 4 b S/B = 6.8% / 18

7 t th t t + b b: backgrounds t t + jets Dominant background +5% 6% uncertainty Modeled with Powheg+Pythia Difference between Powheg and Madgraph is taken as uncertainty and pt t are re-weighted to unfolded 7 TeV data p tt T tt + b b same final state as signal Other backgrounds: t tw /Z - Madgraph+Pythia W/Z+jets - Alpgen+Pythia Diboson - Alpgen+Herwig QCD multi-jet - data-driven 1-lepton background composition 4 j, 2 b 4 j, 3 b 4 j, 4 b ATLAS Preliminary Simulation = 125 GeV m H s = 8 TeV 5 j, 2 b 5 j, 3 b tt+light 5 j, 4 b tt+cc tt+bb tt+v W+jets Z+jets Diboson Single top Multijet 6 j, 2 b 6 j, 3 b 6 j, 4 b Single lepton 7 / 18

8 t th t t + b b: backgrounds t t + jets Dominant background +5% 6% uncertainty Modeled with Powheg+Pythia Difference between Powheg and Madgraph is taken as uncertainty and pt t are re-weighted to unfolded 7 TeV data p tt T tt + b b same final state as signal Other backgrounds: t tw /Z - Madgraph+Pythia W/Z+jets - Alpgen+Pythia Diboson - Alpgen+Herwig QCD multi-jet - data-driven 1-lepton yields - post-fit 5 jets, 6 jets, 6 jets, 4 b-tags 3 b-tags 4 b-tags t th (125) 11 ± 1 ± 9 69 ± 3 ± ± 2 ± 23 t t+ light 78 ± ± ± 11 t t + c c 45 ± ± ± 19 t t + b b 149 ± ± ± 40 t t + V 3.3 ± ± ± 2.7 non-t t 23.2 ± ± ± 2.2 Total 309 ± ± ± 27 Data / 18

9 t th t t + b b: multi-variate discriminant inputs Event shape variables: event centrality,... Object pair properties: m min R b b,... Object kinematics: p jet5 T,... Event kinematics: HT had, N pt >40 GeV,... Modeling of variables and correlations checked in signal depleted regions 1-lepton centrality= p T / E 2-lepton m min R b b Arbitrary units s = 8 TeV 0.25 Total background Single lepton 6 j, 4 b tth (m = 125 GeV) H 0.2 ATLAS Preliminary Simulation Arbitrary units 0.6 ATLAS Preliminary Simulation s Dilepton = 8 TeV 4 j, 4 b Total background tth (m H = 125 GeV) Centrality m bb 9 / 18

10 t th t t + b b: multi-variate discriminant output Neural Network trained in each signal region Select highest rank variables with NeuroBayes Use all MC events - weight light jets with b-jet mistag rate maps 1-lepton t th 6 jets 4 b-jets 2-lepton t th 4 jets 4 b-jets Events / ATLAS Preliminary L dt = 20.3 fb, s = 8 TeV Single lepton 6 j, 4 b Data tth (125) norm tth (125) tt+light tt+cc t t+bb tt+v non-tt Total unc. Events / ATLAS Preliminary L dt = 20.3 fb, s = 8 TeV Dilepton 4 j, 4 b Data tth (125) norm tth (125) tt+light tt+cc t t+bb tt+v non-tt Total unc Data / Pred NN output Data / Pred NN output / 18

11 t t + jets control regions 1-lepton 4 jet 4 b 2-lepton 4 jet 2 b Events / 50 GeV ATLAS Preliminary L dt = 20.3 fb, s = 8 TeV Single lepton 4 j, 4 b Data tth (125) tt+light tt+cc tt+bb tt+v non-tt Total unc. Events / 30 GeV ATLAS Preliminary L dt = 20.3 fb, s = 8 TeV Dilepton 4 j, 2 b Data tth (125) tt+light tt+cc tt+bb tt+v non-tt Total unc Data / Pred [GeV] H had T = p jet T had H T Data / Pred H T [GeV] H T = p jet T + p lepton T tt + b b and tt + c c are constrained from fits to signal depleted regions 11 / 18

12 Signal and control region yields 1-lepton yields post-fit 2-lepton yields post-fit Events / bin 6 5 ATLAS Preliminary L dt = 20.3 fb, s = 8 TeV Single lepton Data tth (125) t th (125) tt+cc tt+light tt+bb non-t t tt+v Total unc. Events / bin 5 4 ATLAS Preliminary L dt = 20.3 fb, s = 8 TeV Dilepton Data tth (125) t th (125) tt+cc tt+light tt+bb non-t t tt+v Total unc Data / Pred j, 2 b 5 j, 2 b 6 j, 2 b 4 j, 3 b 5 j, 3 b 6 j, 3 b 4 b 4 j, 4 b 6 j, 5 j, 4 b Data / Pred j, 2 b 3 j, 2 b 4 j, 2 b 3 j, 3 b 4 j, 3 b 4 j, 4 b Simultaneous binned likelihood fit to 6 signal and 9 control regions Measure t th signal strength µ and background normalization µ = 1 corresponds to SM t th prediction 12 / 18

13 t th t t + b b: systematic uncertainties µ Main systematic uncertainties tt+heavy flavor normalization Jet flavor tagging tt re-weighting to data and Madgraph tt theoretical uncertainties Reduced by fitting data tt+bb tt+cc tt normalisation tt+hf tt tt LightTag1 p T MCgen RW normalisation tt+hf MG-PP LightTag2 cross section tt+hf top p T RW RW CTag1 BTag3 t t + light t t + c c t t + b b Total pre-fit ±30 ±57 ±56 Total post-fit ±14 ±25 ±14 CTag2 JES1 tt+v cross section BTag2 BTag1 ATLAS Preliminary =125 GeV m H Pull Post-fit Impact on µ (θ - θ 0 )/ θ 13 / 18

14 t th t t + b b: results ATLAS Preliminary s=8 TeV, L dt=20.3 fb ATLAS Preliminary s=8 TeV, L dt=20.3 fb Dilepton tot. stat. ( tot ) ( stat ) Dilepton 2.9 ± 2.3 ( 1.4 ) Lepton+jets Expected ± 1σ Expected ± 2σ Lepton+jets 1.3 ± 1.6 ( 0.8 ) Combination Observed Expected ( µ=1) Combination 1.7 ± 1.4 ( 0.7 ) % CL limit on σ/σ SM at m H =125 GeV best fit µ=σ/σ SM for m H =125 GeV Observed (expected ) 95% CL limit 4.1 SM (2.6 SM) for m H = 125 GeV Best fit µ = 1.7 ± 1.4 ATLAS-CONF / 18

15 t th t t + γγ candidate event 15 / 18

16 t th t t + γγ: signal regions Hadronic tt channel: 6 jet 2 b - N signal = 0.36 Leptonic tt channel: 1-lepton, 1 b - N signal = 5 2 isolated high p T photons - search for m γγ resonance Model m γγ for background with exponential function Simultaneous fit to signal and control regions with relaxed kinematic cuts Events / 5 GeV Leptonic Data ATLAS preliminary SR+CR background fit SM signal (m = GeV) H SR-only background fit Signal region Leptonic channel 2000 Control region s = 8 TeV Ldt = 20.3 fb m γγ [GeV] Events / 5 GeV Hadronic Data ATLAS preliminary SR+CR background fit SM signal (m = GeV) H SR-only background fit Signal region Hadronic channel 40 Control region s = 8 TeV Ldt = 20.3 fb m γγ [GeV] 16 / 18

17 t th t t + γγ: results tth /σ SM 95% CL limit on σ tth Observed CL s limit Expected CL s limit ± 1σ H γγ tth channels comb. ATLAS preliminary ± 2σ Data 2012 s = 8 TeV Ldt = 20.3 fb [GeV] m H Observed 95% CL limit: 5.3 SM (6.4 SM) for m H = GeV 92% uncertainty on σ(t th) and 5% uncertainty on BR(H γγ) 4-6% uncertainty on photon id and isolation in multi-jet environment 5-% uncertainty on jet energy scale and b-tagging in hadronic channel ATLAS-CONF / 18

18 Summary and outlook t th t t + b b Observed (expected) 95% CL limit: 4.1 SM (2.6 SM) t th t t + γγ Outlook Observed (expected) 95% CL limit: 5.3 SM (6.4 SM) Publications of improved analyses of these final states are forthcoming Results of multi-lepton analyses will be also published t th cross-section increases by 3.9 at s = 13 TeV compared to 8 TeV ATLAS pixel detector upgrade: added Insertable B-Layer Expect to reach SM sensitivity in Run 2 18 / 18

19 BACKUP 18 / 18

20 t th t t + b b analysis strategy 1-lepton: t th 6 jets 4 b-jets 1 electron or muon with p T > 25 GeV 4 jets 2 b-jets 2-lepton: t th 4 jets 4 b-jets 2 electron or muon with p T > 25(15) GeV 2 jets 2 b-jets eµ: H T > 130 GeV ll: m ll > 15GeV and Z veto Exactly 2 b ll: m ll > 60 GeV Jets with p T > 25 GeV in η < % b-tagging working point with 1% mis-tag 18 / 18

21 t th t t + b b: multi-variate discriminant output Neural Network trained in each signal region Select highest rank variables with NeuroBayes Use all MC events - weight light jets with b-jet mistag rate maps 1-lepton 5 jet 4 b 1-lepton 6 jet 3 b 1-lepton 6 jet 4 b Events / ATLAS Preliminary L dt = 20.3 fb, s = 8 TeV Single lepton 5 j, 4 b Data tth (125) norm tth (125) tt+light tt+cc t t+bb tt+v non-tt Total unc. Events / ATLAS Preliminary Data L dt = 20.3 fb, s = 8 TeV tth (125) norm tth (125) Single lepton tt+light j, 3 b tt+cc t t+bb tt+v non-tt Total unc. 300 Events / ATLAS Preliminary L dt = 20.3 fb, s = 8 TeV Single lepton 6 j, 4 b Data tth (125) norm tth (125) tt+light tt+cc t t+bb tt+v non-tt Total unc Data / Pred NN output Data / Pred NN output Data / Pred NN output 18 / 18

22 t th t t + b b: multi-variate discriminant output Neural Network trained in each signal region Select highest rank variables with NeuroBayes Use all MC events - weight light jets with b-jet mistag rate maps 1-lepton 5 jet 4 b 1-lepton 6 jet 3 b 1-lepton 6 jet 4 b Events / ATLAS Preliminary L dt = 20.3 fb, s = 8 TeV Dilepton 3 j, 3 b Data tth (125) norm tth (125) tt+light tt+cc t t+bb tt+v non-tt Total unc. Events / ATLAS Preliminary L dt = 20.3 fb, s = 8 TeV Dilepton 4 j, 3 b Data tth (125) norm tth (125) tt+light tt+cc t t+bb tt+v non-tt Total unc. Events / ATLAS Preliminary L dt = 20.3 fb, s = 8 TeV Dilepton 4 j, 4 b Data tth (125) norm tth (125) tt+light tt+cc t t+bb tt+v non-tt Total unc Data / Pred NN output Data / Pred NN output Data / Pred NN output 18 / 18

23 1-lepton variables Variable Centrality H1 Definition Sum of the p T divided by sum of the E for all jets and the lepton Second Fox-Wolfram moment computed using all jets and the lepton mbb Mass of the combination of two b-tagged jets with the smallest R N jet 40 Number of jets with p T 40GeV R avg bb Average R for all b-tagged jet pairs m max p T jj Mass of the combination of any two jets with the largest vector sum p T Aplanarity b jet 1.5λ 2, where λ 2 is the second eigenvalue of the momentum tensor built with only b-tagged jets HT had mjj Rlep bb mbj m max p T bj muu p jet5 T R max p T bb mbb max m pt,uu m jjj Ruu m max p T bb Scalar sum of jet p T Mass of the combination of any two jets with the smallest R R between the lepton and the combination of two b-tagged jets with the smallest R Mass of the combination of a b-tagged jet and any jet with the smallest R Mass of the combination of a b-tagged jet and any jet with the largest vector sum p T Mass of the combination of two untagged jets with the smallest R Fifth leading jet p T R between two b-tagged jets with the largest vector sum p T Mass of the combination of two b-tagged jets with the largest invariant mass Scalar sum of the p T s of the pair of untagged jets with the smallest R Mass of the jet triplet with the largest vector sum p T Minimum R between two untagged jets Mass of the combination of two b-tagged jets with the largest vector sum p T 18 / 18

24 Variable m b b H T p jet3 T p jet4 T Centrality Aplanarity jet H1 H4 Rhl max R Rhl 2-lepton variables Definition Mass of the two b-tagged jets from the Higgs candidate system Scalar sum of jet p T and lepton p T s Third leading jet p T Fourth leading jet p T Sum of the p T divided by sum of the E for all jets and both leptons 1.5λ 2, where λ 2 is the second eigenvalue of the momentum tensor built with all jets Second Fox-Wolfram moment computed using all jets and both leptons Fifth Fox-Wolfram moment computed using all jets and both leptons R between the Higgs candidate and the closest lepton R between the Higgs candidate and the furthest lepton N Higgs 30 Number of Higgs candidates within 30 GeV of the defined Higgs mass mjj closest Dijet mass between any two jets closest to the defined Higgs mass max η η jj Maximum η between any two jets in the event mjj min m Minimum dijet mass between any two jets m max p T jj Mass of the combination of any two jets with the largest vector sum p T mbb Mass of the combination of two b-tagged jets with the smallest R Rbj Minimum R between a b-tagged jet and any jet Rlj Minimum R between any lepton and jet R max p T bb R between two b-tagged jets with the largest vector sum p T Rbb max m R between two b-tagged jets with the largest invariant mass 18 / 18

25 Variable ( 6j, 4b) ( 6j, 3b) (5j, 4b) (5j, 3b) R avg bb mbb Centrality H p jet5 T Aplanarity b jet muu R max p T bb Rlep bb 9 - m max p T bj N jet mbj m max p T jj HT had mjj mbb max m pt,uu m jjj Ruu m max p T bb Table: The lists and rankings of the variables considered in each of the regions in the single lepton channel where a NN is used. 18 / 18

26 Variable ( 4j, 4b) ( 4j, 3b) (3j, 3b) max η η jj mbb m b b Rhl N Higgs R max p T bb Aplanarity jet mjj min m max R Rhl mjj closest - H T Rbb max m Rlj - - Centrality m max p T jj H p jet3 T Table: The lists and rankings of the variables considered in each of the regions in the dilepton channel where a NN is used. 18 / 18

27 t t + jets signal region composition regions tth, H bb 1-lepton 4 j, 2 b 4 j, 3 b 4 j, 4 b ATLAS Preliminary Simulation = 125 GeV m H s = 8 TeV 5 j, 2 b 5 j, 3 b 5 j, 4 b tth, H bbb tth, H WW tth, H ττ tth, H gg tth, H cc tth, H ZZ tth, H others 6 j, 2 b 6 j, 3 b 6 j, 4 b Single lepton 2-lepton 2 j, 2 b ATLAS Preliminary Simulation = 125 GeV m H s = 8 TeV 3 j, 2 b 3 j, 3 b tth, H bbb tth, H WW tth, H ττ tth, H gg tth, H cc tth, H ZZ tth, H others 4 j, 2 b 4 j, 3 b 4 j, 4 b Dilepton 18 / 18

28 ( 6j, 4b) systematic uncertainties Pre-fit t th (125) t t + light t t + c c t t + b b Jet energy scale ±6.5 ±14 ± ±8.2 Jet efficiencies ±1.6 ±5.4 ±2.5 ±2.4 Jet energy resolution ±0.1 ±8.5 ±4.1 ±4.3 b-tagging efficiency ±9.0 ±5.8 ±5.1 ±9.2 c-tagging efficiency ±1.9 ±7.3 ±14 ±2.8 Light jet-tagging efficiency ±1.0 ±17 ±4.4 ±1.5 t t modelling: reweighting ±11 ±13 ±13 t t modelling: parton shower ±7.5 ±1.8 ± t t heavy-flavour: normalisation ±50 ±50 Total pre-fit ±12 ±30 ±57 ±56 Total post-fit ±7.2 ±14 ±25 ±14 18 / 18

29 5 t t + jets signal regions ATLAS Preliminary s=8 TeV, L dt=20.3 fb Dilepton Lepton+jets Combination Dilepton Lepton+jets Expected ± 1σ Expected ± 2σ Observed Expected ( µ=1) ATLAS 95% CL limit on σ/σ SM at m H =125 GeV Preliminary tot. stat. s=8 TeV, L dt=20.3 fb ( tot ) ( stat ) 2.9 ± 2.3 ( 1.4 ) 1.3 ± 1.6 ( 0.8 ) Events / bin Data / Bkgd ATLAS s=8 TeV Preliminary L dt =20.3 fb Comb. Single lepton and Dilepton tth (µ =1.7) + Bkgd. fit tth (µ =4.1) + Bkgd. 95% excl. Data 2012 tth (µ =1.7) fit tth (µ =4.1) 95% excl. Bkgd log (S/B) Combination 1.7 ± 1.4 ( 0.7 ) best fit µ=σ/σ SM for m H =125 GeV 18 / 18

30 t th t t + γγ: analysis strategy Hadronic tt channel: 6 jet 2 b - N signal = 0.36 Leptonic tt channel: 1-lepton, 1 b - N signal = 5 Aim for high efficiency to maximize signal yields 80% b-tagging working point with 4% light quark jet rejection 2 isolated high p T photons - 40 (30) GeV Jets with p T > 25 GeV in η < 2.5 Arbitrary units 0.4 ATLAS Preliminary Non-isolated, non-tight γ ID γγ + l + jets (at least 1b) 0.35 L dt = 20.3 fb γγ jets (at least 1b) s = 8 TeV 0.3 γγ jets (b veto) Leptonic Channel 0.25 Arbitrary units 0.4 ATLAS Preliminary Non-isolated, non-tight γ ID γγ + >=6j 0.35 L dt = 20.3 fb γγ + 5j (b veto) 0.3 s = 8 TeV γγ + 5j (1b) Hadronic Channel m γγ [GeV] m γγ [GeV] 18 / 18