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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
2
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 )
3
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)
4
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 !
5
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)
6
H→µµ
7
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
8
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
Even
ts /
0.2
10
210
310
Data =1.11)µ Vbb (→VZ
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
Tp
outputVZBDT1− 0.8− 0.6− 0.4− 0.2− 0 0.2 0.4 0.6 0.8 1D
ata/
Pred
.0.5
11.5
9
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
Even
ts /
0.2
210
310
410 Data =1.11)µ Vbb (→VZ
WWVhtt
Single topW+(bb,bc,cc,bl)W+clW+llZ+(bb,bc,cc,bl)UncertaintyPre-fit background
10× Vbb →SM VZ
ATLAS Preliminary -1 = 13 TeV , 36.1 fbs
0 lepton, 2 jets, 2 b-tags 150 GeV≥ V
Tp
outputVZBDT1− 0.8− 0.6− 0.4− 0.2− 0 0.2 0.4 0.6 0.8 1D
ata/
Pred
.
0.51
1.5 outputVZBDT
1− 0.8− 0.6− 0.4− 0.2− 0 0.2 0.4 0.6 0.8 1
Even
ts /
0.2
210
310
410
Data =1.11)µ Vbb (→VZ
WWVhtt
Single topMultijetW+(bb,bc,cc,bl)W+clW+llZ+(bb,bc,cc,bl)UncertaintyPre-fit background
20× Vbb →SM VZ
ATLAS Preliminary -1 = 13 TeV , 36.1 fbs
1 lepton, 2 jets, 2 b-tags 150 GeV≥ V
Tp
outputVZBDT1− 0.8− 0.6− 0.4− 0.2− 0 0.2 0.4 0.6 0.8 1D
ata/
Pred
.
0.51
1.5
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
10
VZ, Z→bb (MVA)
• Significance: • 5.8 σ (observed) • 5.3 σ (expected)
(S/B)10
log2.5− 2− 1.5− 1− 0.5− 0 0.5
Even
ts /
0.25
1
10
210
310
410
510
610
710
810
910 Data =1.11)µ Vbb (→VZ
WWVhtt
Single topMultijetW+(bb,bc,cc,bl)W+clW+llZ+(bb,bc,cc,bl)Z+clZ+ll
ATLAS Preliminary -1 = 13 TeV , 36.1 fbs
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
tat.)
05
10
outputVHBDT1− 0.8− 0.6− 0.4− 0.2− 0 0.2 0.4 0.6 0.8 1
Even
ts /
0.13
10
210
310
410
510 Data =1.20)µ Vbb (→VH
Dibosontt
Single topMultijetW+(bb,bc,cc,bl)W+clW+llZ+(bb,bc,cc,bl)UncertaintyPre-fit background
20× Vbb →SM VH
ATLAS Preliminary -1 = 13 TeV , 36.1 fbs
1 lepton, 2 jets, 2 b-tags 150 GeV≥ V
Tp
outputVHBDT1− 0.8− 0.6− 0.4− 0.2− 0 0.2 0.4 0.6 0.8 1D
ata/
Pred
.
0.51
1.5 outputVHBDT
1− 0.8− 0.6− 0.4− 0.2− 0 0.2 0.4 0.6 0.8 1
Even
ts /
0.13
1
10
210
310
410Data
=1.20)µ Vbb (→VH DibosonZ+(bb,bc,cc,bl)Z+clZ+lltt
Single topW+(bb,bc,cc,bl)UncertaintyPre-fit background
10× Vbb →SM VH
ATLAS Preliminary -1 = 13 TeV , 36.1 fbs
2 leptons, 2 jets, 2 b-tags 150 GeV≥ V
Tp
outputVHBDT1− 0.8− 0.6− 0.4− 0.2− 0 0.2 0.4 0.6 0.8 1D
ata/
Pred
.
0.51
1.5
11
VH, H→bb (MVA)
outputVHBDT1− 0.8− 0.6− 0.4− 0.2− 0 0.2 0.4 0.6 0.8 1
Even
ts /
0.13
10
210
310
410
Data =1.20)µ Vbb (→VH
Dibosontt
Single topW+(bb,bc,cc,bl)W+clW+llZ+(bb,bc,cc,bl)Z+clZ+llUncertaintyPre-fit background
10× Vbb →SM VH
ATLAS Preliminary -1 = 13 TeV , 36.1 fbs
0 lepton, 2 jets, 2 b-tags 150 GeV≥ V
Tp
outputVHBDT1− 0.8− 0.6− 0.4− 0.2− 0 0.2 0.4 0.6 0.8 1D
ata/
Pred
.
0.51
1.5
0-‐lepton (2b, 2jet) 1-‐lepton (2b, 2jet) 2-‐lepton (2b, 2jet)
(S/B)10
log3.5− 3− 2.5− 2− 1.5− 1− 0.5− 0 0.5
Even
ts /
0.25
1
10
210
310
410
510
610
710
810 Data =1.20)µ Vbb (→VH
Dibosontt
Single topMultijetW+(bb,bc,cc,bl)W+clW+llZ+(bb,bc,cc,bl)Z+clZ+ll
ATLAS Preliminary -1 = 13 TeV , 36.1 fbs
0+1+2 leptons2+3 jets, 2 b-tags
(S/B)10
log3.5− 3− 2.5− 2− 1.5− 1− 0.5− 0 0.5Pu
ll (s
tat.)
0
5
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
12
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
13
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
ents
/ 10
GeV
(Wei
ghte
d, b
ackg
r. su
b.)
2−
0
2
4
6
8
10
12 Data =1.30)µ Vbb (→VH
DibosonUncertainty
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 ( )
( Tot. ) ( Stat. , Syst. )Total Stat.
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 ( )
( Tot. ) ( Stat. , Syst. )Total Stat.
ATLAS Preliminary =7+8+13 TeVs-1L dt=4.7+20.3+36.1 fb∫
14
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 %
15
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
16
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
17
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
ATLAS-‐CONF-‐2016-‐063
18
13 TeV data (12.6 fb-1)
ttH, H→bb
19
• 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
ATLAS-‐CONF-‐2016-‐080
→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)
20
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
21
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
22
VH, H→bb (Run1, MVA, JHEP 01 (2015) 069)
JHEP 01 (2015) 069
23
VH, H→bb (Run2, MVA)
[GeV]missTE
150 200 250 300 350 400 450 500
Even
ts /
25 G
eV
1
10
210
310
410
510Data
=1.20)µ Vbb (→VH Dibosontt
Single topW+(bb,bc,cc,bl)W+clW+llZ+(bb,bc,cc,bl)Z+clZ+llUncertaintyPre-fit background
10× Vbb →SM VH
ATLAS Preliminary -1 = 13 TeV , 36.1 fbs
0 lepton, 2 jets, 2 b-tags 150 GeV≥ V
Tp
[GeV]missTE
150 200 250 300 350 400 450 500Dat
a/Pr
ed.
0.51
1.5 [GeV]bbm
0 50 100 150 200 250 300 350 400 450 500
Even
ts /
20 G
eV
100
200
300
400
500
600
700
800Data
=1.20)µ Vbb (→VH Dibosontt
Single topMultijetW+(bb,bc,cc,bl)W+clZ+(bb,bc,cc,bl)UncertaintyPre-fit background
5× Vbb →SM VH
ATLAS Preliminary -1 = 13 TeV , 36.1 fbs
1 lepton, 2 jets, 2 b-tags 150 GeV≥ V
Tp
[GeV]bbm0 50 100 150 200 250 300 350 400 450 500D
ata/
Pred
.
0.51
1.5
[GeV]bbm0 50 100 150 200 250 300 350 400 450 500
Even
ts /
20 G
eV
20
40
60
80
100Data
=1.20)µ Vbb (→VH DibosonZ+(bb,bc,cc,bl)tt
UncertaintyPre-fit background
5× Vbb →SM VH
ATLAS Preliminary -1 = 13 TeV , 36.1 fbs
2 leptons, 2 jets, 2 b-tags 150 GeV≥ V
Tp
[GeV]bbm0 50 100 150 200 250 300 350 400 450 500D
ata/
Pred
.
0.51
1.5
24
VH, H→bb (Run2, MVA)
25
VH, H→bb (Run2, MVA)
• 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 ( )
( Tot. ) ( Stat. , Syst. )Total Stat.
ATLAS Preliminary -1=13 TeV, 36.1 fbs
26
VH, H→bb (Run2, MVA)
27
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
28
VH, H→bb (Run1+Run2 Combination)
(S/B)10
log3.5− 3− 2.5− 2− 1.5− 1− 0.5− 0 0.5
Even
ts /
0.25
110
210
310
410
510
610
710
810
910
1010 Data =0.90)µ Vbb (→VH
Dibosontt
Single topMultijetW+(bb,bc,cc,bl)W+clW+llZ+(bb,bc,cc,bl)Z+clZ+ll
ATLAS Preliminary -1 = 7 TeV , 4.7 fbs -1 = 8 TeV , 20.3 fbs
-1 = 13 TeV , 36.1 fbs
(S/B)10
log3.5− 3− 2.5− 2− 1.5− 1− 0.5− 0 0.5Pu
ll (s
tat.)
0
5