Measurements of Fermionic Couplings of the Standard Model Higgs Boson using the bb, ττ and µµ Decay Channels with the ATLAS Detector International Europhysics Conference on High Energy Physics Venice, Italy July 5 th -2 th, 207 Song-Ming Wang Academia Sinica On behalf of the ATLAS Collaboration
Introduction A new boson was discovered by ATLAS & CMS in bosonic decay channels More precise measurements in these decay modes strengthen the hypothesis the new particle is the SM Higgs boson Essential to study whether it also decays directly into fermions as predicted in SM Coupling strength proportional to the fermion mass in order to establish mass generation mechanism for fermions Fermionic decay search is challenging (mh @ 25GeV) : BR(H bb) ~ 58% Large & several background sources need well design triggers to select signal efficiently BR(H ττ) ~ 6.3% Need good tau ID performance Understand MET reconstruction under pileup BR(H µµ) ~0.022% Very small decay rate, although clean final state Combined Run ATLAS+CMS results ( JHEP08 (206) 045 ) Decay Channel Measured Sign. (σ) Expected Sign. (σ) H ττ 5.5 5.0 H bb 2.6 3.7 2
Introduction Fermionic decay searches performed in various production channels (ggh) σ=48.6 pb σ=3.78 pb Largest producjon rate Suitable for leptonic decay searches (e.g. μμ, ττ) σ=.37, 0.88 pb 2 forward jet, low hadronic acjvity in between (exclude contr. from H) Unique topology for BG suppression Considered in all fermionic decay search (tth) σ=0.59 pb (WH, ZH) Leptonic decays from W,Z help in BG suppression and triggering Considered in ττ, bb decay searches (Shown cross secjon values for s=3 TeV) Associated [ signature used to trigger events and BG suppression Considered in ττ, bb decay searches 3
Results for this Talk H µµ (3 TeV, 36 fb -, ATLAS-CONF-207-04, arxiv:705.04582) H ττ (7,8 TeV, 4.5+20.3 fb -, JHEP 04 (205) 7) H bb : VBF+γ (3 TeV, 2.6 fb -, ATLAS-CONF-206-063) (backup slides) tth (3 TeV, 3.2 fb -, ATLAS-CONF-206-080) (backup slides) VH (3 TeV, 36. fb -, ATLAS-CONF-207-04) Express measurement in terms of signal strength µ NEW! µ = σ BR (σ BR) SM All signal strength µ and limits are determined for mh=25 GeV 4
H µµ arxiv:705.04582 Search in VBF and ggf production channels Signal selection, optimized for : VBF : 2 jets Multivariate analysis (MVA) to enrich VBF signal region Categorization : loose and tight ggf : If not VBF Categorization based on pt(µµ) and η(µ) Search for signal in m(µµ) distribution Simultaneous fit of m(µµ) for all categories : µ= -0. ±.5 upper limit : µ<3.0 (3. exp) @ 95% CL Combined with Run µ= -0. ±.4 upper limit : µ<2.8 (2.9 exp) @ 95% CL Limit is driven by the data statistics uncertainty 3 TeV data (36. fb - ) VBF Classification BDT loose tight VBF, tight 5
H µµ 6
H ττ Sensitive to VBF, ggf and VH productions Reconstruct H ττ, consider all tau decays τ LEP τ LEP, τ LEP τ HAD, τ HAD τ HAD Di-tau invariant mass, m ττ MMC, constructed using Missing Mass Calculator (MMC) method Reconstruct mass from most probable kinematic config. of neutrinos using matrix element probability Event categorized for VBF and ggf+jet Multivariate analysis : BDT output use as final discriminating distribution Main background : Z(ττ) : estimated using Z(µµ) data events, replace µ by simulated τ Fakes (jet τ, e/µ), Z(ee,µµ), tt, W+jets Results : Combined all categories/decay channels Evidence of H ττ at 4.5 σ significance! Expected significance of 3.4 σ µ =.43 +0.43-0.37 ATLAS+CMS Run => 5.5 σ significance! mh @ 25.36 GeV (signal scaled to best-fit values) JHEP 04 (205) 7 7
ATLAS- CONF- 207-04 VH, H bb NEW! 3 TeV data (36. fb - ) ZH νν bb 0-lep WH lν bb -lep ZH l + l - bb 2-lep All leptonic channels require 2 b-tagged jets Maximize sensitivity by event categorization : 2 jet, 3 jet ( 3 jet for 2-lep) p TV >50 GeV (extra cat. 75-50 GeV for 2-lep) p TV =! E T miss, p T (W lν), p T (Z ll) mbb correction : µ-in-jet, b-jet energy response correction, kinematic likelihood fit Analysis Strategy : MVA : main search technique for VH(bb) Validation: MVA for diboson VZ(bb) search VH(bb) search using m(bb) main discriminant mbb resolution improved by ~8-40% 8
VZ, Z bb (MVA) Perform similar MVA analysis to search for WZ+ZZ production WZ lνbb, ZZ llbb, ννbb : Similar final state signature as VH, cross section ~9X larger Separate BDT training for each signal region category BDT output used as final discriminating variable Performed global fit to all 8 signal and 6 control regions 0- lepton (2b, 2jet) - lepton (2b, 2jet) 2- lepton (2b, 2jet) Events / 0.2 4 0 3 0 - s = 3 TeV, 36. fb 0 lepton, 2 jets, 2 b-tags V p T 50 GeV Data VZ Vbb (µ=.) WW Vh tt Single top W+(bb,bc,cc,bl) W+cl W+ll Z+(bb,bc,cc,bl) Uncertainty Pre-fit background SM VZ Vbb 0 Events / 0.2 4 0 3 0 - s = 3 TeV, 36. fb lepton, 2 jets, 2 b-tags V p T 50 GeV Data VZ Vbb (µ=.) WW Vh tt Single top Multijet W+(bb,bc,cc,bl) W+cl W+ll Z+(bb,bc,cc,bl) Uncertainty Pre-fit background SM VZ Vbb 20 Events / 0.2 3 0 2 0 - s = 3 TeV, 36. fb 2 leptons, 2 jets, 2 b-tags V p T 50 GeV Data VZ Vbb (µ=.) Vh Z+(bb,bc,cc,bl) Z+cl Z+ll tt Single top W+(bb,bc,cc,bl) Uncertainty Pre-fit background SM VZ Vbb 0 2 0 2 0 0 Data/Pred..5 BDT VZ output 0.5 0.8 0.6 0.4 0.2 0 0.2 0.4 0.6 0.8 0.8 0.6 0.4 0.2 0 0.2 0.4 0.6 0.8 BDT VZ output Data/Pred..5 BDT VZ output 0.5 0.8 0.6 0.4 0.2 0 0.2 0.4 0.6 0.8 0.8 0.6 0.4 0.2 0 0.2 0.4 0.6 0.8 BDT VZ output Data/Pred..5 0.5 0.8 0.6 0.4 0.2 0 0.2 0.4 0.6 0.8 BDT VZ output 9
VZ, Z bb (MVA) WZ ZZ Total Stat. - s=3 TeV, 36. fb ( Tot. ) ( Stat., Syst. ) +0.60 0.57.02 (, ) +0.29 0.26 +0.24 0.24 +0.6 0.6 +0.55 0.52 +0.24 0.20.3 (, ) Events / 0.25 9 0 Data 8 VZ Vbb (µ=.) 0 - WW s = 3 TeV, 36. fb Vh 7 0 tt 0++2 leptons Single top 6 0 2+3 jets, 2 b-tags Multijet W+(bb,bc,cc,bl) 5 0 W+cl W+ll Z+(bb,bc,cc,bl) 4 0 Z+cl Z+ll 3 0 2 0 Comb. +0.25 0.22 +0.2 0. +0.22 0.9. (, ) 0 2 3 4 5 6 7 8 Significance: 5.8 σ (observed) 5.3 σ (expected) µ bb VZ Pull (stat.) 0 0 5 0 2.5 2.5 0.5 0 0.5 log (S/B) 0 0
Events / 0.3 4 0 3 0 2 0 - s = 3 TeV, 36. fb 0 lepton, 2 jets, 2 b-tags V p T 50 GeV Data VH Vbb (µ=.20) Diboson tt Single top W+(bb,bc,cc,bl) W+cl W+ll Z+(bb,bc,cc,bl) Z+cl Z+ll Uncertainty Pre-fit background SM VH Vbb 0 Events / 0.3 5 0 4 0 3 0 2 0 VH, H bb (MVA) 0- lepton (2b, 2jet) - lepton (2b, 2jet) 2- lepton (2b, 2jet) - s = 3 TeV, 36. fb lepton, 2 jets, 2 b-tags V p T 50 GeV Data VH Vbb (µ=.20) Diboson tt Single top Multijet W+(bb,bc,cc,bl) W+cl W+ll Z+(bb,bc,cc,bl) Uncertainty Pre-fit background SM VH Vbb 20 Events / 0.3 4 0 3 0 2 0 0 - s = 3 TeV, 36. fb 2 leptons, 2 jets, 2 b-tags V p T 50 GeV Data VH Vbb (µ=.20) Diboson Z+(bb,bc,cc,bl) Z+cl Z+ll tt Single top W+(bb,bc,cc,bl) Uncertainty Pre-fit background SM VH Vbb 0 0 0 Data/Pred..5 BDT VH output 0.5 0.8 0.6 0.4 0.2 0 0.2 0.4 0.6 0.8 0.8 0.6 0.4 0.2 0 0.2 0.4 0.6 0.8 BDT VH output Data/Pred..5 0.5 0.8 0.6 0.4 0.2 0 0.2 0.4 0.6 0.8 BDT VH output Data/Pred..5 BDT VH output 0.5 0.8 0.6 0.4 0.2 0 0.2 0.4 0.6 0.8 0.8 0.6 0.4 0.2 0 0.2 0.4 0.6 0.8 BDT VH output Events / 0.25 8 0 Data VH Vbb (µ=.20) 7 - Diboson 0 s = 3 TeV, 36. fb tt Single top 6 0++2 leptons 0 Multijet 2+3 jets, 2 b-tags W+(bb,bc,cc,bl) 5 W+cl 0 W+ll Z+(bb,bc,cc,bl) 4 0 Z+cl Z+ll 3 0 Pull (stat.) 2 0 0 5 log (S/B) 0 0 3.5 3 2.5 2.5 0.5 0 0.5 3.5 3 2.5 2.5 0.5 0 0.5 log (S/B) 0
VH, H bb (MVA, Results) Total Stat. - s=3 TeV, 36. fb ( Tot. ) ( Stat., Syst. ) 0L +0.53 0.5 +0.39 0.37 +0.36 0.34 0.45 (, ) L +0.69 0.59 +0.40 0.38 +0.56 0.45.43 (, ) 2L +0.78 0.64 +0.5 0.49 +0.59 0.42.90 (, ) Comb. +0.42 0.36 +0.24 0.23 +0.34 0.28.20 (, ) Results : Evidence of H bb at 3.5 σ (3.0 σ exp.) µ =.20 +0.42-0.36 3 lepton compatibility : 0.3% σ(vh Vbb)=.57 +0.54-0.47 pb reinterpret from μ expect :.3 pb Dominant uncertainties : signal modeling, MC statistics, b-tagging 0 2 3 4 5 6 7 8 µ bb VH 2
VH, H bb (Cross Check MVA results) Di-jet m(bb) mass as discriminant variable m(bb) after subtract all backgrounds except diboson weighted by S/B of each mbb bin signal scaled to the best fit µ value Signal strength µ =.30 +0.28-0.27 (stat) +0.37-0.29 (syst) Observed excess = 3.5 σ (2.8 σ expected) Events / 0 GeV (Weighted, backgr. sub.) 2 Data - s = 3 TeV, 36. fb VH Vbb (µ=.30) Diboson 0 0++2 leptons Uncertainty 2+3 jets, 2 b-tags 8 6 4 2 0 2 Weighted by Higgs S/B Dijet mass analysis 40 60 80 00 20 40 60 80 200 m bb [GeV] 3
VH, H bb (Run+Run2 Combination) Combined results from Run and Run2 Correlate signal theoretical uncertainty Minimum impact on results with or without correlating other uncertainties Total Stat. s=7+8+3 TeV L dt=4.7+20.3+36. fb - ( Tot. ) ( Stat., Syst. ) Total Stat. s=7+8+3 TeV L dt=4.7+20.3+36. fb - ( Tot. ) ( Stat., Syst. ) Run2 +0.42 0.36 +0.24 0.23 +0.34 0.28.9 (, ) ZH +0.35 0.33 +0.27 0.26 +0.23 0.2 0.69 (, ) Run +0.40 0.38 +0.3 0.30 +0.25 0.23 0.5 (, ) WH +0.45 0.42 +0.30 0.29 +0.34 0.30.2 (, ) Comb. +0.28 0.26 +0.8 0.8 +0.2 0.9 0.90 (, ) 0 2 3 4 5 6 7 8 Signal strength : Combined u=0.90 +0.28-0.26 Compatibility : Run vs Run2 : 20.8 % WH vs ZH : 34.2 % Observed significance 3.6 σ (4 σ expected) µ bb VH Comb. +0.28 0.26 +0.8 0.8 +0.2 0.9 0.90 (, ) 0 2 3 4 5 6 7 8 Run+Run2 (6-mu fit) : 7. % Evidence for H bb! µ bb VH 4
VH, H bb (MVA, -lepton) E T miss =320 GeV b- tagged jet p T (e)=5 GeV b- tagged jet p T (V)=450 GeV m(bb)=24 GeV 5
Summary ATLAS is actively exploring direct fermionic decays of SM Higgs boson New results with Run 2 data Improved limits on H µµ Explore the new production channel VBF H+γ Observe evidence of H bb in the VH associated production channe Updates from other analyses (e.g. H ττ, tth, VBF ) are underway 207 data taking has started, expect to double the total 3 TeV data sample by end of the year 6
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ATLAS- CONF- 206-063 VBF+γ, H bb Search for H bb in VBF events with a γ High pt photon provides : extra handle for trigger, suppress multijet BG 3 TeV data (2.6 fb - ) Selection : 2 b-jets, 2 forward jets, central γ Separate signal from background with MVA (BDT) Divide BDT output into different sensitivity categories Fit m(bb) distribution in each category to extract signal Results (95% CL limit): Expected : µ < 6 +2.3 -.7 Observed : µ < 4 High sensivvity category 8
ATLAS- CONF- 206-080 tth, H bb bw + Provide direct observation of tth coupling Also sensitive to b-quark Yukawa coupling Search in events with W (-lepton) or both W (2-lepton) decay leptonically Categorized search according to jet multiplicity bw - and # of b-jets Dominant background : tt+jets 4 b-jets in final state, need to find right pair for H bb MVA to discriminate signal from background, BDT output use as discriminating variable - lepton, 5 jets, 4 b- tag µ=2.±.0 Sensitivity same as Run (µ=.5±.)! 3 TeV data 3.2 fb - ) 9
VH, H ττ Search performed on Run data (8TeV, 20.3 fb - ) PRD 93, 092005 (206) Select signals candidates in : W l,ν, Z ll (l=e or µ) H τ LEP τ HAD, τ HAD τ HAD Apply b-veto in WH to suppress Top background Main background : Real e,µ,τ : diboson, ttbar Fakes : jet τ, e/µ Discriminant variable = di-tau mass ZH : m ττ MMC, constructed using MMC method WH : transverse mass m 2 T (cannot use MMC due to extra neutrino) Results : Measured signal strength µ=2.3 ±.6 Limits : µ<5.6 (expected 3.5) @ 95% CL 20
VBF, H bb Search performed on Run data (8TeV, 20.3 fb - ) Event topology : two forward scattered quark jets : large Δη(JJ) Higgs colour singlet, no colour line to b-quarks Little hadronic activities between VBF jets MVA analysis (BDT) : Training variables include : VBF jet width, Δη(JJ), m(jj) Divide BDT output into 4 BDT regions Discriminant : Fit m(bb) distribution in data of each BDT region to extract signal Dominant systematic : limited data statistics in the mbb sideband Results : Measured signal strength µ= -0.8 ± 2.3 Limits : µ<4.4 (expected 5.4) @ 95% CL JHEP (206) 2 2 3 4 2
VH, H bb (Run, MVA, JHEP 0 (205) 069) JHEP 0 (205) 069 22
VH, H bb (Run2, MVA) Events / 25 GeV 5 0 4 0 3 0 2 0 - s = 3 TeV, 36. fb 0 lepton, 2 jets, 2 b-tags V p T 50 GeV Data VH Vbb (µ=.20) Diboson tt Single top W+(bb,bc,cc,bl) W+cl W+ll Z+(bb,bc,cc,bl) Z+cl Z+ll Uncertainty Pre-fit background SM VH Vbb 0 Events / 20 GeV 800 700 600 500 400 300 - s = 3 TeV, 36. fb lepton, 2 jets, 2 b-tags V p T 50 GeV Data VH Vbb (µ=.20) Diboson tt Single top Multijet W+(bb,bc,cc,bl) W+cl Z+(bb,bc,cc,bl) Uncertainty Pre-fit background SM VH Vbb 5 0 200 00 Data/Pred..5 0.5 50 200 250 300 350 400 450 500 [GeV] miss E T Data/Pred..5 m bb [GeV] 0 0.5 50 00 50 200 250 300 350 400 450 500 0 50 00 50 200 250 300 350 400 450 500 [GeV] m bb Events / 20 GeV 00 80 - s = 3 TeV, 36. fb 2 leptons, 2 jets, 2 b-tags V p T 50 GeV Data VH Vbb (µ=.20) Diboson Z+(bb,bc,cc,bl) tt Uncertainty Pre-fit background SM VH Vbb 5 60 40 20 Data/Pred..5 m bb [GeV] 0 0.5 50 00 50 200 250 300 350 400 450 500 0 50 00 50 200 250 300 350 400 450 500 [GeV] m bb 23
VH, H bb (Run2, MVA) 24
VH, H bb (Run2, MVA) WH ZH Comb. Total Stat. - s=3 TeV, 36. fb ( Tot. ) ( Stat., Syst. ) +0.68 0.59 +0.50 0.45.2 (, ) +0.42 0.36 +0.40 0.38.35 (, ) +0.34 0.33 +0.24 0.23 +0.55 0.45 +0.37 0.30 +0.34 0.28.20 (, ) 0 2 3 4 5 6 7 8 µ bb VH Observed significance : WH = 2.4 σ ZH = 2.6 σ Cross sections : σ(wh Wbb) =.07 +0.54-0.47 pb expect : 0.80 pb σ(zh Zbb) = 0.56 +0.25-0.22 pb expect : 0.50 pb 25
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VZ, Z bb (Run2, MVA) Total Stat. - s=3 TeV, 36. fb ( Tot. ) ( Stat., Syst. ) 0L +0.34 0.28 +0.8 0.8 +0.28 0.22.2 (, ) L +0.59 0.59 +0.24 0.24 +0.54 0.55.3 (, ) 2L +0.33 0.30 +0.20 0.9 +0.26 0.23.0 (, ) Comb. +0.25 0.22 +0.2 0. +0.22 0.9. (, ) 0 2 3 4 5 6 7 8 µ bb VZ 27
VH, H bb (Run+Run2 Combination) Events / 0.25 0 0 9 0 8 0 7 0 6 0 5 0 4 0 - s = 7 TeV, 4.7 fb - s = 8 TeV, 20.3 fb - s = 3 TeV, 36. fb Data VH Vbb (µ=0.90) Diboson tt Single top Multijet W+(bb,bc,cc,bl) W+cl W+ll Z+(bb,bc,cc,bl) Z+cl Z+ll 3 0 2 0 0 Pull (stat.) 5 0 3.5 3 2.5 2.5 0.5 0 0.5 log (S/B) 0 28