Search for the Higgs boson produced in association with top quarks - - PowerPoint PPT Presentation

SLIDE 1

Search for the Higgs boson produced in association with top quarks and decaying into bottom quarks with the ATLAS detector

Shunsuke Honda (High Energy Physics Laboratory)

• 1

2018/03/27

SLIDE 2

Shunsuke Honda (U Tsukuba)

2018/03/27

Top Quarks and Higgs Bosons

• 2

Top Quark: Largest and unique mass value mtop = 173.3 GeV ~ v / √2 → Top-Yukawa coupling Yt ~ 1: a key parameter of the SM Yt probes BSM such as VLQ, SUSY, … The ttH production allows the direct measurement of top-Yukawa with significant sensitivities in LHC Run2.

g g H

g g t H t

Yt

Small cross sections, but distinct signatures in various decay channels in Higgs and ttbar Cross-section = 0.51 pb at 13TeV

• thers

11.6% VV,ττ 30.6% γγ 0.2% bb 57.6%

SLIDE 3

Shunsuke Honda (U Tsukuba)

2018/03/27

ttH(H→bb) Objects

• 3

e/μ

W→light-/c-jets

t t H

MET

b-jets

H→bb

W→ℓν

e/μ

t t H

MET

b-jets

H→bb

W→ℓν

e/μ

W→ℓν

ttH → 6jets(4b-jets), 1lepton, MET / 4jets(4b-jets), 2leptons, MET

Background sources: tt+jets (85-96% of total Bkgd.)

tt+bb: 4b-jets

→ same final states: irreducible

tt+cc: 2b-jets + 2c-jets / tt+light: 2b-jets + 2 light-jets


→ reducible BKG with b-jet identification (=> b-tagging)

(generated gluon instead of Higgs)

t

g

t

SLIDE 4

Shunsuke Honda (U Tsukuba)

2018/03/27

Flavor Tagging (b-tagging)

• 4

1 − 0.8 − 0.6 − 0.4 − 0.2 − 0.2 0.4 0.6 0.8 1

Arbitrary units

3 −

10

2 −

10

1 −

10 1 10

ATLAS Simulation Preliminary t = 13 TeV, t s

b jets c jets Light-flavour jets

5 4 3 2 1

mv2c10 BDT score primary vertex secondary vertex jet tracks

cτ~450 μm B-hadron decaying generated B-hadron

t t H

larger smaller

5 5 4 3 1 1

SLIDE 5

Shunsuke Honda (U Tsukuba)

2018/03/27

Region Definition

• 5

Single Lepton, ≥ 6 j SR1 SR2 SR3 CRt¯

t+b

CRt¯

t+≥1c

CRt¯

t+light

(3rd, 4th) jet b b b-tagging discriminant (5, 5) (5, 4) (5, 3) (5, 2) (4, 4) (4, 3) (4, 2) (3, 3) (3, 2) (2, 2) (5, 1) (4, 1) (3, 1) (2, 1) (1, 1) (1st, 2nd) jet b b b-tagging discriminant (5, 5) (5, 4) (4, 4) (5, 3) (4, 3) (3, 3)

t t H Using four highest btagging scores per event:

larger smaller

5 5 4 3 1 1

>30% of tt+≥1c >30% of tt+b

remaining bins:

>60% >45% >30%

• f tt+≥2b

SLIDE 6

Shunsuke Honda (U Tsukuba)

2018/03/27

Defined Regions

• 6

ATLAS = 13 TeV s Single Lepton + light t t 1c ≥ + t t 1b ≥ + t t + V t t t Non-t +light t t 5j CR 1c ≥ + t t 5j CR +b t t 5j CR 2 5j SR 1 5j SR boosted SR +light t t 6j ≥ CR 1c ≥ + t t 6j ≥ CR +b t t 6j ≥ CR 3 6j ≥ SR 2 6j ≥ SR 1 6j ≥ SR

tt+light 5 j

CR

1c ≥ tt+ 5 j

CR

tt+b 5 j

CR

2 5 j

SR

1 5 j

SR

boosted

SR

tt+light 6 j ≥

CR

1c ≥ tt+ 6 j ≥

CR

tt+b 6 j ≥

CR

3 6 j ≥

SR

2 6 j ≥

SR

1 6 j ≥

SR B / S 0.01 0.02 0.03 0.04 0.05 B / S 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 ATLAS

• 1

= 13 TeV, 36.1 fb s Single Lepton

12 regions for single-lepton channel (5jets, ≥6jets)

• two control regions enriched by tt+light , tt+≥1c or tt+b
• five tt+bb and signal enriched regions
• one boosted region

SLIDE 7

Shunsuke Honda (U Tsukuba)

2018/03/27

Multi-Variate Analysis: ttH and tt+bb Separation

• 7

Classification BDT output

1 − 0.8 − 0.6 − 0.4 − 0.2 − 0.2 0.4 0.6 0.8 1

Data / Pred.

0.5 0.75 1 1.25 1.5

Events / bin 50 100 150 200 250 300 350 400 450 ATLAS

• 1

= 13 TeV, 36.1 fb s Single Lepton

1

6j ≥

SR Post-Fit Data H t t + light t t 1c ≥ + t t 1b ≥ + t t + V t t t Non-t Total unc. H (norm) t t

Reconstruction kinematics LH discriminant MEM discriminant

identify jet assignments for top/Higgs candidates using MVA algorithm uses Signal / Bkgd. probability via ME calculation uses various PDFs of masses/angles, 
 considering missing object prob.

(reco BDT) [GeV]

Higgs bb

m 50 100 150 200 250 300 350 Data / Pred.

0.5 0.75 1 1.25 1.5

Events / 25 GeV 100 200 300 400 500 ATLAS

• 1

= 13 TeV, 36.1 fb s Single Lepton

1 6j ≥

SR Post-Fit Data H t t + light t t 1c ≥ + t t 1b ≥ + t t + V t t t Non-t Total unc. H (norm) t t Pre-Fit Bkgd.

Higgs mass MVA score

S/B ~ 0.23

S/B ~ 0.10

Advanced discriminants : Simple kinematic variables

ΔRbbmax.pT, mbbmin. ΔR, … b-tagging scores for each jet

tt+bb tt+bb

SLIDE 8

Shunsuke Honda (U Tsukuba)

2018/03/27

Results

• 8

SM H t t

σ /

H t t

σ = µ Best fit 1 − 1 2 3 4 5 6 Combined combined fit) µ (two- Single Lepton combined fit) µ (two- Dilepton

−0.24

1.02 + 1.05 −

(

0.54 + 0.52 − 0.87 + 0.91 −

) 0.95

0.65 + 0.62 −

(

0.31 + 0.31 − 0.57 + 0.54 −

) 0.84

0.64 + 0.61 −

(

0.29 + 0.29 − 0.57 + 0.54 −

)

ATLAS

• 1

= 13 TeV, 36.1 fb s = 125 GeV

H

m tot. stat.

tot (stat syst)

SM no signal

Observed (Expected) excess = 1.4σ (1.6σ) over SM bkgd.

determined with free-floating normalizations for tt+HF:

• tt+≥1b: 1.24±0.10, tt+≥1c: 1.63±0.23

tt+light 5j

C R

1c ≥ tt+ 5j

C R

tt+b 5j

C R

2 5j

S R

1 5j

S R

boosted

S R

tt+light 6j ≥

C R

1c ≥ tt+ 6j ≥

C R

tt+b 6j ≥

C R

3 6j ≥

S R

2 6j ≥

S R

1 6j ≥

S R Data / Pred.

0.5 0.75 1 1.25 1.5

Events / bin 10

2

10

3

10

4

10

5

10

6

10

7

10

8

10 ATLAS

• 1

= 13 TeV, 36.1 fb s Single Lepton Post-Fit Data H t t + light t t 1c ≥ + t t 1b ≥ + t t + V t t t Non-t Total unc. H t t

normalizations in all regions: consistent btw data and pred.

Run1:

SM H t t

σ /

H t t

σ = µ Best fit 1 − 1 2 3 4 5 6

1.5 ± 1.1 ( ±0.8 ±0.7)

tot (stat syst)

→40% improvements from Run1

Single-lepton regions

SLIDE 9

Shunsuke Honda (U Tsukuba)

2018/03/27

Systematic Uncertainties (15 Unc. with Largest Impacts on μ-value)

• 9

Less impacts from tt+cc , other Bkgd. modelings But still large impact by tt+bb modeling also in new results.

• 1
• 0.5

0.5 1

PDF (MSTW) b +b t t light-jet tagging 2 jet energy scale 3 renormalisation scale b +b t t reweighting

T

top p c +c t t b-jet tagging 1 reweighting

T

p t t c +c t t light-jet tagging 1 jet energy scale 2 shower recoil scheme b +b t t +V cross section t t

bb

scale choice m renormalisation b +b t t normalisation c +c t t jet energy scale 1 normalisation b +b t t

µ ∆

• 1
• 0.5

0.5 1 θ ∆ )/ θ

• θ

(

• 1.5
• 1
• 0.5

0.5 1 1.5

Pull µ Pre-fit Impact on µ Post-fit Impact on

ATLAS

=125 GeV

H

, m

• 1

= 8 TeV, 20.3 fb s

θ Δ )/ θ

• θ

(

2 − 1.5 − 1 − 0.5 − 0.5 1 1.5 2

b-tagging: efficiency NP II : soft-term resolution

miss T

E b-tagging: mis-tag (c) NP I b-tagging: efficiency NP I Wt: diagram subtr. vs. nominal +light: PS & hadronization t t Jet energy resolution: NP II 1c: ISR / FSR ≥ + t t 1b: shower recoil scheme ≥ + t t 5F vs. nominal

HERPA

1c: S ≥ + t t 3b normalization ≥ 1b: tt+ ≥ tt+ H: cross section (QCD scale) t t Jet energy resolution: NP I 0.10 ± 1b) = 1.24 ≥ k(tt+ b-tagging: mis-tag (light) NP I H: PS & hadronization t t 1b: ISR / FSR ≥ + t t 1b: PS & hadronization ≥ + t t 4F vs. nominal

HERPA

1b: S ≥ + t t 5F vs. nominal

HERPA

1b: S ≥ + t t

µ Δ 1 − 0.5 − 0.5 1

: µ Pre-fit impact on θ Δ + θ = θ θ Δ

• θ

= θ : µ Post-fit impact on θ Δ + θ = θ θ Δ

• θ

= θ

• Nuis. Param. Pull

ATLAS

• 1

= 13 TeV, 36.1 fb s

Run1 This result

more impact more impact

SLIDE 10

Shunsuke Honda (U Tsukuba)

2018/03/27

Comparison btw. ATLAS and CMS

• 10

Classification BDT output 1 − 0.8 − 0.6 − 0.4 − 0.2 − 0.2 0.4 0.6 0.8 1 Data / Pred.

0.5 0.75 1 1.25 1.5

Events / bin 50 100 150 200 250 300 350 400 450 ATLAS

• 1

= 13 TeV, 36.1 fb s Single Lepton

1 6j ≥

SR Post-Fit Data H t t + light t t 1c ≥ + t t 1b ≥ + t t + V t t t Non-t Total unc. H (norm) t t

SM H t t

σ /

H t t

σ = µ Best fit 1 − 1 2 3 4 5 6 Combined combined fit) Single Lepton combined fit) Dilepton

−0.24

1.02 + 1.05 −

(

0.54 + 0.52 − 0.87 + 0.91 −

) 0.95

0.65 + 0.62 −

(

0.31 + 0.31 − 0.57 + 0.54 −

) 0.84

0.64 + 0.61 −

(

0.29 + 0.29 − 0.57 + 0.54 −

)

ATLAS

• 1

= 13 TeV, 36.1 fb s = 125 GeV

H

m tot. stat.

tot (stat syst)

most sensitive region

different treatments for tt+jets modeling but smaller unc. in CMS. good consistencies in all bins.

Result

~ same results in both analysis +0.24/-0.28 for CMS +0.46/-0.46 for ATLAS

SLIDE 11

Shunsuke Honda (U Tsukuba)

2018/03/27

Conclusion

• 11

The ttH production allows the direct measurement of top-Yukawa: largest and unique coupling value = a key for SM/BSM physics. Searched for ttH production with H->bb Key point: large systematic uncertainties in tt+jets modeling Run1 result: 1.4σ observed (1.1σ expected) Developed MVAs for optimized event categorization → μ=σttHobs/σttHSM = 0.8±0.6 : 1.4σ observed (1.6σ expected)

SLIDE 12

Backup

Shunsuke Honda (U Tsukuba)

2018/03/27

• 12

SLIDE 13

Backup

Shunsuke Honda (U Tsukuba)

2018/03/27

ttH Combination Results

Observed (Expected) excess = 4.2σ (3.8σ) over SM bkgd. : evidence for ttH production

• 13

Uncertainty Source ∆µ t¯ t modeling in H → b¯ b analysis +0.15 −0.14 t¯ tH modeling (cross section) +0.13 −0.06 Non-prompt light-lepton and fake τhad estimates +0.09 −0.09 Simulation statistics +0.08 −0.08 Jet energy scale and resolution +0.08 −0.07 t¯ tV modeling +0.07 −0.07 t¯ tH modeling (acceptance) +0.07 −0.04 Other non-Higgs boson backgrounds +0.06 −0.05 Other experimental uncertainties +0.05 −0.05 Luminosity +0.05 −0.04 Jet flavor tagging +0.03 −0.02 Modeling of other Higgs boson production modes +0.01 −0.01 Total systematic uncertainty +0.27 −0.23 Statistical uncertainty +0.19 −0.19 Total uncertainty +0.34 −0.30

2 − 2 4 6 8 10

=125 GeV

H

for m

H t t

µ Best-fit

H combined t t VV → H, H t t b b → H, H t t γ γ → H, H t t τ τ → H, H t t

0.6 − +0.6

1.5 ,

0.4 − +0.4 0.4 − +0.5

( )

0.6 − +0.6

0.8 ,

0.3 − +0.3 0.5 − +0.5

( )

0.6 − +0.7

0.6 ,

0.6 − +0.7 0.2 − +0.2

( )

1.0 − +1.2

1.5 ,

0.8 − +0.9 0.6 − +0.8

( )

0.3 − +0.3

1.2 ,

0.2 − +0.2 0.2 − +0.3

( )

( tot. ) ( stat. , syst. ) total stat.

ATLAS

• 1

=13 TeV, 36.1 fb s

SM no signal

Largest impact by tt+bb modeling also seen in combined result.

• tHqb, WtH and other Higgs prod. treated as bkgd. and fixed to SM pred.
• Higgs decays are fixed to SM pred.

SLIDE 14

Backup

Shunsuke Honda (U Tsukuba)

2018/03/27

CMS Latest Results (H->bb)

• 14

Large different treatments for tt+jets modeling unc. from ATLAS.

SLIDE 15

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Shunsuke Honda (U Tsukuba)

2018/03/27

Region Definition in 5J

• 15

Single Lepton, 5 j SR1 SR2 CRt¯

t+b

CRt¯

t+≥1c

CRt¯

t+light

(3rd, 4th) jet b b b-tagging discriminant (5, 5) (5, 4) (5, 3) (5, 2) (4, 4) (4, 3) (4, 2) (3, 3) (3, 2) (2, 2) (5, 1) (4, 1) (3, 1) (2, 1) (1, 1) (1st, 2nd) jet b b b-tagging discriminant (5, 5) (5, 4) (4, 4) (5, 3) (4, 3) (3, 3)

SLIDE 16

Backup

Shunsuke Honda (U Tsukuba)

2018/03/27

Region Definition in Dilepton Channel

• 16

Dilepton, 3 j CRt¯

t+≥1b

CRt¯

t+light

3rd jet b b b-tagging discriminant 5 4 3 2 1 (1st, 2nd) jet b b b-tagging discriminant (5, 5) (5, 4) (4, 4) (5, 3) (4, 3) (3, 3) Dilepton, ≥ 4 j SR1 SR2 SR3 CRt¯

t+≥1c

CRt¯

t+light

(3rd, 4th) jet b b b-tagging discriminant (5, 5) (5, 4) (5, 3) (5, 2) (4, 4) (4, 3) (4, 2) (3, 3) (3, 2) (2, 2) (5, 1) (4, 1) (3, 1) (2, 1) (1, 1) (1st, 2nd) jet b b b-tagging discriminant (5, 5) (5, 4) (4, 4) (5, 3) (4, 3) (3, 3)

SLIDE 17

Backup

Shunsuke Honda (U Tsukuba)

2018/03/27

Defined Regions in Dilepton Channel

• 17

ATLAS = 13 TeV s Dilepton + light t t 1c ≥ + t t 1b ≥ + t t + V t t t Non-t

+light t t 3j

CR

1b ≥ + t t 3j

CR

+light t t 4j ≥

CR

1c ≥ + t t 4j ≥

CR

3 4j ≥

SR

2 4j ≥

SR

1 4j ≥

SR

tt+light 3 j

CR

1b ≥ tt+ 3 j

CR

tt+light 4 j ≥

CR

1c ≥ tt+ 4 j ≥

CR

3 4 j ≥

SR

2 4 j ≥

SR

1 4 j ≥

SR Data / Pred.

0.5 0.75 1 1.25 1.5

Events / bin 10

2

10

3

10

4

10

5

10

6

10

7

10 ATLAS

• 1

= 13 TeV, 36.1 fb s Dilepton Post-Fit Data H t t + light t t 1c ≥ + t t 1b ≥ + t t + V t t t Non-t Total unc. H t t

t t + l i g h t 3j

CR

1 b ≥ t t + 3j

CR

t t + l i g h t 4j ≥

CR

1 c ≥ t t + 4j ≥

CR

3 4j ≥

SR

2 4j ≥

SR

1 4j ≥

SR B / S 0.01 0.02 0.03 0.04 0.05 B / S 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 ATLAS

• 1

= 13 TeV, 36.1 fb s Dilepton

7 regions for single-lepton channel (5jets, ≥6jets)

• two control regions enriched by tt+light ,
• ne control regions enriched by tt+≥1c or tt+b
• three signal enriched regions

SLIDE 18

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Shunsuke Honda (U Tsukuba)

2018/03/27

Higgs Mass

• 18

[GeV]

Higgs candidate

m 50 100 150 200 250 300 350 Data / Pred.

0.5 0.75 1 1.25 1.5

Events / 25 GeV 50 100 150 200 250 300 350 ATLAS

• 1

= 13 TeV, 36.1 fb s Single Lepton

boosted

SR Post-Fit Data H t t + light t t 1c ≥ + t t 1b ≥ + t t + V t t t Non-t Total unc. H (norm) t t Pre-Fit Bkgd. (reco BDT) [GeV]

Higgs bb

m 50 100 150 200 250 300 350 Data / Pred.

0.5 0.75 1 1.25 1.5

Events / 25 GeV 100 200 300 400 500 ATLAS

• 1

= 13 TeV, 36.1 fb s Single Lepton

1 6j ≥

SR Post-Fit Data H t t + light t t 1c ≥ + t t 1b ≥ + t t + V t t t Non-t Total unc. H (norm) t t Pre-Fit Bkgd. (reco BDT) [GeV]

Higgs bb

m 50 100 150 200 250 300 350 Data / Pred.

0.5 0.75 1 1.25 1.5

Events / 25 GeV 20 40 60 80 100 ATLAS

• 1

= 13 TeV, 36.1 fb s Dilepton

1 4j ≥

SR Post-Fit Data H t t + light t t 1c ≥ + t t 1b ≥ + t t + V t t t Non-t Total unc. H (norm) t t Pre-Fit Bkgd.

32% 47%

efficiency for correctly Higgs reconstructed:

32% ≥6jets, SR1 ≥4jets, SR1 boosted

SLIDE 19

Backup

Shunsuke Honda (U Tsukuba)

2018/03/27

Single-Lepton Plots

• 19

Pre-fit Post-fit

6jCR(ttc) 6jSR3 6jSR2 6jSR1

SLIDE 20

Backup

Shunsuke Honda (U Tsukuba)

2018/03/27

Single-Lepton Plots

• 20

Pre-fit Post-fit

5jCR(ttc) 5jSR2 5jSR1 Boosted SR

SLIDE 21

Backup

Shunsuke Honda (U Tsukuba)

2018/03/27

Dilepton Plots

• 21

Pre-fit Post-fit 4jSR3 4jSR2 4jSR1

SLIDE 22

Backup

Shunsuke Honda (U Tsukuba)

2018/03/27

tt+b tt+bb tt+B 3b tt+ Fraction of events

2 −

10

1 −

10 1 8

YTHIA

+P

OWHEG

P 4F

HERPA

S ATLAS Simulation tt + b tt + bb tt + B 3b ≥ tt + 8

YTHIA

+P

OWHEG

P 4F

HERPA

S 0.5 1 1.5 2

BackGround: Normalization Reweights on tt+≥1b

PowhegPythia8(5FS) reweighted to SherpaOL(4FS)

• 22

reweight

too big!!! → additional systematics : ±50% for tt+≥3b

t t t t t t t t

SLIDE 23

Backup

Shunsuke Honda (U Tsukuba)

2018/03/27

ttbar Modeling Systematic Unc.

• 23

SLIDE 24

Backup

Shunsuke Honda (U Tsukuba)

2018/03/27

S/B Sorted Plot

• 24

SLIDE 25

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Shunsuke Honda (U Tsukuba)

2018/03/27

BDT Input Variables

• 25

SLIDE 26

Backup

Shunsuke Honda (U Tsukuba)

2018/03/27

Boosted Region

• 26

t t H ℓ ν

small jet (btagged@85%)

R=1.0 R=0.4 (btagged) R=0.4 (non-btagged) R=0.4 (btagged)

ReClustered Jet: using calibrated normal (small-R) jets to construct the large-R jet → High momentum Higgs(→bb) and top(→bjj) objects can be tagged!

R=0.4 (btagged) R=0.4 (btagged) R=1.0

Top tagged rcjet Higgs tagged rcjet BDT input variables

SLIDE 27

Backup

Shunsuke Honda (U Tsukuba)

2018/03/27

MEM and LHD

• 27

LHD

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Data / Pred.

0.5 0.75 1 1.25 1.5

Events / bin 50 100 150 200 250 300 350 400 450 ATLAS

• 1

= 13 TeV, 36.1 fb s Single Lepton

1

6j ≥

SR Post-Fit Data H t t + light t t 1c ≥ + t t 1b ≥ + t t + V t t t Non-t Total unc. H (norm) t t Pre-Fit Bkgd.

• 4))

D1

1 / (1 + exp(-MEM

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Data / Pred.

0.5 0.75 1 1.25 1.5

Events / bin 50 100 150 200 250 300 350 400 450 ATLAS

• 1

= 13 TeV, 36.1 fb s Single Lepton

1

6j ≥

SR Post-Fit Data H t t + light t t 1c ≥ + t t 1b ≥ + t t + V t t t Non-t Total unc. H (norm) t t Pre-Fit Bkgd.