Measurements of Fermionic Couplings of the Standard Model ...July 5th-12th, 2017 2 Introduction •A new boson was discovered by ATLAS & CMS in bosonic decay channels •More precise

1

Measurements of Fermionic Couplings of the Standard Model Higgs Boson using the bb, ττ and µµ

Decay Channels with the ATLAS Detector

Song-Ming Wang Academia Sinica

On behalf of the ATLAS Collaboration

International Europhysics Conference on High Energy Physics

Venice, Italy July 5th-12th, 2017

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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 @ 125GeV) : • 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

Decay Channel

Measured Sign. (σ)

Expected Sign. (σ)

H→ττ 5.5 5.0

H→bb 2.6 3.7

Combined Run1 ATLAS+CMS results ( JHEP08 (2016) 045 )

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Introduction • Fermionic decay searches performed in various production channels

(ggH)

(ttH)

(WH, ZH)

• Largest producJon rate • Suitable for leptonic decay searches (e.g. μμ, ττ)

• 2 forward jet, low hadronic acJvity in between (exclude contr. from H)

• Unique topology for BG suppression • Considered in all fermionic decay search

• Leptonic decays from W,Z help in BG suppression and triggering

• Considered in ττ, bb decay searches

• Associated [ signature used to trigger events and BG suppression

• Considered in ττ, bb decay searches

σ=48.6 pb σ=3.78 pb

σ=1.37, 0.88 pb σ=0.59 pb

(Shown cross secJon values for √s=13 TeV)

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Results for this Talk

• H→µµ (13 TeV, 36 fb-1 , ATLAS-CONF-2017-014, arXiv:1705.04582)

• H→ττ (7,8 TeV, 4.5+20.3 fb-1 , JHEP 04 (2015) 117)

• H→bb : •  VBF+γ (13 TeV, 12.6 fb-1, ATLAS-CONF-2016-063) (backup slides)

•  ttH (13 TeV, 13.2 fb-1, ATLAS-CONF-2016-080) (backup slides)

•  VH (13 TeV, 36.1 fb-1, ATLAS-CONF-2017-041)

• Express measurement in terms of signal strength µ µ =σ ×BR(σ ×BR) SM

• All signal strength µ and limits are determined for mH=125 GeV

NEW !

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H→µµ • 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)

tight loose

arXiv:1705.04582 VBF Classification BDT

VBF, tight • Search for signal in m(µµ) distribution

• Simultaneous fit of m(µµ) for all categories : •  µ= -0.1 ± 1.5 •  upper limit : µ<3.0 (3.1 exp) @ 95% CL

• Combined with Run1 •  µ= -0.1 ± 1.4 •  upper limit : µ<2.8 (2.9 exp) @ 95% CL

• Limit is driven by the data statistics uncertainty

13 TeV data (36.1 fb-1)

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H→µµ

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H→ττ • Sensitive to VBF, ggF and VH productions • Reconstruct H→ττ , consider all tau decays

• τLEPτLEP , τLEPτHAD , τHADτHAD

• Di-tau invariant mass, mττMMC , constructed using

Missing Mass Calculator (MMC) method • Reconstruct mass from most probable kinematic config. of neutrinos using matrix element probability

• 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

JHEP 04 (2015) 117

• Event categorized for VBF and ggF+jet • Multivariate analysis : BDT output use as final discriminating distribution

(signal scaled to best-fit values)

• Results : Combined all categories/decay channels • Evidence of H→ττ at 4.5 σ significance ! • Expected significance of 3.4 σ

•  µ = 1.43 +0.43 -0.37

• ATLAS+CMS Run1 => 5.5 σ significance !

mH @ 125.36 GeV

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VH, H→bb

ZH → νν bb 0-lep

ATLAS-‐CONF-‐2017-‐041

• All leptonic channels require 2 b-tagged jets • Maximize sensitivity by event categorization :

• 2 jet, 3 jet (≥3 jet for 2-lep) • pT

V>150 GeV (extra cat. 75-150 GeV for 2-lep) • pT

V= , pT(W→lν), pT(Z→ll)

•  mbb correction : • µ-in-jet, b-jet energy response correction, kinematic likelihood fit

!ETmiss

mbb resolution improved by ~18-40%

NEW ! 13 TeV data (36.1 fb-1)

• Analysis Strategy : • MVA : main search technique for VH(bb) • Validation: • MVA for diboson VZ(bb) search • VH(bb) search using m(bb) main discriminant

WH → lν bb 1-lep ZH → l+l- bb 2-lep

outputVZBDT1− 0.8− 0.6− 0.4− 0.2− 0 0.2 0.4 0.6 0.8 1

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VhZ+(bb,bc,cc,bl)Z+clZ+lltt

Single topW+(bb,bc,cc,bl)UncertaintyPre-fit background

10× Vbb →SM VZ

ATLAS Preliminary -1 = 13 TeV , 36.1 fbs

2 leptons, 2 jets, 2 b-tags 150 GeV≥ V

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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

outputVZBDT1− 0.8− 0.6− 0.4− 0.2− 0 0.2 0.4 0.6 0.8 1

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0-‐lepton (2b, 2jet) 1-‐lepton (2b, 2jet) 2-‐lepton (2b, 2jet)

bbVZµ

1− 0 1 2 3 4 5 6 7 8

Comb.

ZZ

WZ

0.22−+0.251.11 , 0.11−

+0.12 0.19−+0.22 ( )

0.26−+0.291.13 , 0.16−

+0.16 0.20−+0.24 ( )

0.57−+0.601.02 , 0.24−

+0.24 0.52−+0.55 ( )

( Tot. ) ( Stat. , Syst. )Total Stat.

ATLAS Preliminary -1=13 TeV, 36.1 fbs

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VZ, Z→bb (MVA)

• Significance: •  5.8 σ (observed) •  5.3 σ (expected)

(S/B)10

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0+1+2 leptons2+3 jets, 2 b-tags

(S/B)10

log2.5− 2− 1.5− 1− 0.5− 0 0.5Pu

ll (s

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VH, H→bb (MVA)

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bbVHµ

1− 0 1 2 3 4 5 6 7 8

Comb.

2L

1L

0L

0.36−+0.421.20 , 0.23−

+0.24 0.28−+0.34 ( )

0.64−+0.781.90 , 0.49−

+0.51 0.42−+0.59 ( )

0.59−+0.691.43 , 0.38−

+0.40 0.45−+0.56 ( )

0.51−+0.530.45 , 0.37−

+0.39 0.34−+0.36 ( )

( Tot. ) ( Stat. , Syst. )Total Stat.ATLAS Preliminary -1=13 TeV, 36.1 fbs

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VH, H→bb (MVA, Results)

• Results : • Evidence of H→bb at 3.5 σ (3.0 σ exp.) •  µ = 1.20 +0.42

-0.36 • 3 lepton compatibility : 10.3% • σ(VH→Vbb)=1.57 +0.54

-0.47 pb •  expect : 1.31 pb

• Dominant uncertainties : signal modeling, MC statistics, b-tagging

reinterpret from μ

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VH, H→bb (Cross Check MVA results)

• Signal strength µ =1.30 +0.28-0.27 (stat) +0.37

-0.29 (syst) • Observed excess = 3.5 σ (2.8 σ expected)

• 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

[GeV]bbm40 60 80 100 120 140 160 180 200Ev

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ATLAS Preliminary -1 = 13 TeV, 36.1 fbs

Dijet mass analysis

0+1+2 leptons2+3 jets, 2 b-tagsWeighted by Higgs S/B

bbVHµ

1− 0 1 2 3 4 5 6 7 8

Comb.

WH

ZH

0.26−+0.280.90 , 0.18−

+0.18 0.19−+0.21 ( )

0.42−+0.451.21 , 0.29−

+0.30 0.30−+0.34 ( )

0.33−+0.350.69 , 0.26−

+0.27 0.21−+0.23 ( )


ATLAS Preliminary =7+8+13 TeVs-1L dt=4.7+20.3+36.1 fb∫

bbVHµ

1− 0 1 2 3 4 5 6 7 8

Comb.

Run1

Run2

0.26−+0.280.90 , 0.18−

+0.18 0.19−+0.21 ( )

0.38−+0.400.51 , 0.30−

+0.31 0.23−+0.25 ( )

0.36−+0.421.19 , 0.23−

+0.24 0.28−+0.34 ( )


ATLAS Preliminary =7+8+13 TeVs-1L dt=4.7+20.3+36.1 fb∫

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VH, H→bb (Run1+Run2 Combination) • Combined results from Run1 and Run2

• Correlate signal theoretical uncertainty • Minimum impact on results with or without correlating other uncertainties

• Signal strength : • Combined u=0.90 +0.28

-0.26 • Compatibility :

Evidence for H→bb !

•  Run1 vs Run2 : 20.8 % •  WH vs ZH : 34.2 %

• Observed significance 3.6 σ (4 σ expected)

•  Run1+Run2 (6-mu fit) : 7.1 %

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VH, H→bb (MVA, 1-lepton)

ETmiss=320 GeV

pT(e)=151 GeV

b-‐tagged jet

b-‐tagged jet pT(V)=450 GeV m(bb)=124 GeV

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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 • 2017 data taking has started, expect to double the total 13 TeV data sample by end of the year

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BackUp

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

• Selection : •  2 b-jets, 2 forward jets, 1 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

High sensiVvity category

• Results (95% CL limit): • Expected : µ < 6 +2.3

-1.7 • Observed : µ < 4


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13 TeV data (12.6 fb-1)

ttH, H→bb

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• Provide direct observation of ttH coupling • Also sensitive to b-quark Yukawa coupling

• Search in events with 1 W (1-lepton) or both W (2-lepton) decay leptonically

• Categorized search according to jet multiplicity and # of b-jets


→bW+

→bW-

• 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

1-‐lepton, 5 jets, 4 b-‐tag

•  µ=2.1±1.0 • Sensitivity same as Run1 (µ=1.5±1.1)!

13 TeV data 13.2 fb-1)

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VH, H→ττ • Search performed on Run1 data (8TeV, 20.3 fb-1)

• Select signals candidates in : • W→l,ν , Z→ll (l=e or µ) • H→τLEPτHAD , τHADτHAD

• Apply b-veto in WH to suppress Top background

• Discriminant variable = di-tau mass • ZH : mττ

MMC , constructed using MMC method

• WH : transverse mass m2T (cannot use MMC due to extra neutrino)

• Results : • Measured signal strength µ=2.3 ± 1.6 • Limits : µ<5.6 (expected 3.5) @ 95% CL

• Main background : • Real e,µ,τ : diboson, ttbar • Fakes : jet→τ, e/µ

PRD 93, 092005 (2016)

4 32 1

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VBF, H→bb

• 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

• Discriminant : • Fit m(bb) distribution in data of each BDT region to extract signal

• Dominant systematic : limited data statistics in the mbb sideband

• Search performed on Run1 data (8TeV, 20.3 fb-1)

• MVA analysis (BDT) : • Training variables include : VBF jet width, Δη(JJ), m(JJ)

• Divide BDT output into 4 BDT regions

• Results : • Measured signal strength µ= -0.8 ± 2.3 • Limits : µ<4.4 (expected 5.4) @ 95% CL

JHEP 11 (2016) 112

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VH, H→bb (Run1, MVA, JHEP 01 (2015) 069)

JHEP 01 (2015) 069

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VH, H→bb (Run2, MVA)

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• Observed significance : • WH = 2.4 σ • ZH = 2.6 σ

• Cross sections : •  σ(WH→Wbb) = 1.07 +0.54

-0.47 pb •  expect : 0.80 pb

•  σ(ZH→Zbb) = 0.56 +0.25-0.22 pb

•  expect : 0.50 pb bb

VHµ

1− 0 1 2 3 4 5 6 7 8

Comb.

ZH

WH

0.36−+0.421.20 , 0.23−

+0.24 0.28−+0.34 ( )

0.45−+0.501.12 , 0.33−

+0.34 0.30−+0.37 ( )

0.59−+0.681.35 , 0.38−

+0.40 0.45−+0.55 ( )


ATLAS Preliminary -1=13 TeV, 36.1 fbs

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VZ, Z→bb (Run2, MVA)

bbVZµ

1− 0 1 2 3 4 5 6 7 8

Comb.

2L

1L

0L

0.22−+0.251.11 , 0.11−

+0.12 0.19−+0.22 ( )

0.30−+0.331.10 , 0.19−

+0.20 0.23−+0.26 ( )

0.59−+0.591.13 , 0.24−

+0.24 0.55−+0.54 ( )

0.28−+0.341.12 , 0.18−

+0.18 0.22−+0.28 ( )

( Tot. ) ( Stat. , Syst. )Total Stat.ATLAS Preliminary -1=13 TeV, 36.1 fbs

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VH, H→bb (Run1+Run2 Combination)

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(S/B)10

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Documents

Measurements of Fermionic Couplings of the Standard Model ...July 5th-12th, 2017 2 Introduction •A new boson was discovered by ATLAS & CMS in bosonic decay channels •More precise