Search for Higgs Bosons produced in association with top quarks with ATLAS detector

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Search for Higgs Bosons

produced in association with top quarks with ATLAS detector

Elizaveta Shabalina

II. Physikalisches Institut, Universität Göttingen

on behalf of the ATLAS collaboration

37th International Conference of High Energy Physics

2 - 9 July, Valencia, Spain

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E.Shabalina - Search for Higgs in association with top - ICHEP 2014 03/07/2014

Motivation

After the Higgs discovery main focus is on the measurement of its properties

‣ couplings to fermions and gauge boson

Top quark is the most strongly-coupled SM particle ( Y

t

~ 1)

2

Indirect constraints on the top- Higgs Yukawa coupling Y

t

‣ loop diagrams with top quarks

contribute to ggH production and H→γγ decays

‣ assumes no new particles

Direct measurement of Y

t

in ttH production

‣ allows probing new physics in ggH and γγH effective vertices

tH production is sensitive to the sign of Higgs Yukawa coupling Y

t

Higgsstrahlung from top quarks Higgs production

t,b,?

Higgs decay to photons

t,W,?

_

σ (t tH ¯ ) ∝ g

_t²_tH_¯

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production and decay

Production

‣ σ(ttH) is known at NLO QCD

‣ suppressed compared to other modes

‣ ≈2600 events in 20.3 fb

^-1

at 8TeV

Main background

‣ tt+X

3

t tH ¯

√s (TeV)

7 8 14

(mH = 125 GeV) (fb) (pb)

86 130 611

177 253 950

t¯tH

t t ¯

_

__

Decay mode B

H → b¯b 0.58

H → γγ 2.3 ×10⁻³ H → W W, ZZ, τ τ 0.3

‣ dominant mode but large background

‣ tiny but has clean resonant signature

‣ multi-lepton final states

dominant background for ttH(bb) 2000 times larger than signal

_ _

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E.Shabaina -- Search for Higgs in association with top - ICHEP 2014

03/07/2014 ⁴

t tH ¯ (γγ )

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Analysis strategy

Event selection (8 TeV)

‣ diphoton trigger

‣ hadronic tt category: “or” of three selections (5 or ≥6 jets, 1 or ≥2 b-jets with variable jet pT

thresholds and b-tagging WP)

‣ leptonic tt category: ≥1 lepton,

≥1 b-jets, minimum ETmiss

‣ 2 isolated high pT photons

‣ combined selection efficiency - 15.3%

Search for a resonance in m(γγ)

‣ describe m(γγ) for background with the exponential function

‣ same shape for 7 and 8 TeV and both categories

5

_

Signal is extracted via unbinned likelihood fit to both categories and 7 and 8 TeV

high purity ttH selection statistically limited

‣ small impact of systematics

Category ^√s N_S N_B ttH¯ (%)

Leptonic 8 0.60 1.0^+0.6₋_0.4 81.0 Hadronic 8 0.51 2.8^+0.8₋_0.7 83.7 Leptonic 7 0.10 0.6^+0.5₋_0.4 73.0 Hadronic 7 0.07 0.6^+0.4₋_0.4 81.2

7 TeV, 4.5 fb

^-1

, 8 TeV, 20.3 fb

^-1

_

NEW

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Results

Combination of leptonic and hadronic channels

95% CL observed (expected) limit:

6.5 × SM (4.9 × SM) for mH = 125.4 GeV

ATLAS-CONF-2014-043

6

8 TeV, leptonic 8 TeV, hadronic

= 125.4 GeV at mH

SMtH

mt H/

t

mt

95% CL limit on

0 5 10 15 20 25 30 35

Combined Leptonic Hadronic

ATLAS Preliminary 2011-2012

L dt=4.5 fb-1

=7 TeV, 0

s

L dt=20.3 fb-1

=8 TeV, 0

s

1m

=0) !

H t

mt

Expected (

2m

=0) !

H t

mt

Expected ( Observed

SM signal injected

[GeV]

a

ma

110 120 130 140 150 160

Events / 5 GeV

2 4 6 8 10 12

8 TeV hadronic category Preliminary ATLAS

Ldt = 20.3 fb-1

= 8 TeV 0

s

= 125.4 GeV mH

a, a H A , H t t Data

Background fit = 1.3

!

[GeV]

a

ma

110 120 130 140 150 160

Events / 5 GeV

1 2 3 4 5

8 TeV leptonic category Preliminary ATLAS

Ldt = 20.3 fb-1

= 8 TeV 0

s

= 125.4 GeV mH

, a a A H , H t t Data

Background fit = 1.3

!

NEW

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Yukawa coupling strength

tH production is suppressed with respect to ttH

‣ destructive interference between diagrams with Higgs emitted from W and Higgs emitted off top

In BSM theories Y

t

sign can be flipped

‣ increases tH cross section and H→γγ branching ratio

Set limit on inclusive Higgs

production cross section as a function of strength parameter κ

t

= Y

t

/ Y

tSM

‣ κt changes cross sections of all Higgs production modes and B (H→γγ)

‣ set other couplings to SM value

‣ null hypothesis: B+ttH(SM)

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Y

t

+

_

95% CL lower and upper observed (expected) limits on κt: −1.3 and +8.1 (−1.2 and +7.9)

g

_HWW

_

gt

-2 0 2 4 6 8 10

) tg)(aaAH BR(!m)aaAH BR(!m 95% CL limit on

10-2

10-1

1 10 102

103

limit CLs

Observed

limit CLs

Expected 1m

"

2m

"

Preliminary ATLAS

2011-2012

= 7 TeV s

-1, Ldt = 4.5 fb

0

= 8 TeV s

-1, Ldt = 20.3 fb

0

NEW

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candidate event

8

t ¯ tH (γγ )

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FCNC in top decay

Event selection

‣ diphoton trigger

‣ 2 isolated high pT photons

‣ hadronic W category: 4 jets, ≥1 b-jets,

‣ leptonic W category: ≥1 lepton, 2 jets, ≥1 b-jets, mTW > 30 GeV

‣ Mass cuts

‣ 156<M(γγj)<191 GeV

‣ hadronic: 130<M(jjj)<210 GeV

‣ leptonic: 135<M(lνj)<205 GeV

Search for a m( γγ ) resonance

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Flavor changing neutral current

‣ forbidden at tree level in SM

‣ suppressed at high orders in SM

BSM model allow FCNC

‣ t →cH: largest branching in 2HDM type III

tt →(Wb) (cH(γγ))

_

hadronic

95% CL observed (expected) limit on Br(t→Hc): 0.83% (0.53%)

on λtcH: 0.17 (0.14) ATL-CONF-2013-081

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E.Shabaina -- Search for Higgs in association with top - ICHEP 2014

03/07/2014 ¹⁰

t tH ¯ (b ¯ b)

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Analysis strategy

Event selection

‣ Single lepton channel: 1 lepton (e or μ), from 4 to ≥6 jets with 2 to ≥4 b-jets

‣ Dilepton channel: 2 opposite-sign leptons (ee, μμ, eμ), from 2 to ≥4 jets with 2 to ≥4 b-jets

Categorize events by jet and b-jet multiplicity

‣ take advantage of low S/B regions to constrain systematic uncertainties

‣ maximize sensitivity by separating regions with different S/√B

Build multivariate discriminant from kinematic variables to separate signal from background in signal-rich regions Extract signal by simultaneous fit in jet and b-jet multiplicity bins

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8 TeV, 20.3 fb

^-1

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Analysis strategy

Event selection

‣ single lepton triggers

‣ Single lepton channel: 1 lepton (e or μ), from 4 to ≥6 jets with 2 to ≥4 b-jets

‣ Dilepton channel: 2 opposite-sign leptons (ee, μμ, eμ), from 2 to ≥4 jets with 2 to

≥4 b-jets

Categorize events by jet and b-jet multiplicity

‣ take advantage of low S/B regions to constrain systematic uncertainties

‣ maximize sensitivity by separating regions with different S/√B

Build multivariate discriminant from kinematic variables to

separate signal from background in signal-rich regions

Extract signal by simultaneous fit in jet and b-jet multiplicity bins

12

Preliminary Simulation ATLAS

L dt = 20.3 fb-1

= 8 TeV, !

s m_H = 125 GeV

Single lepton

BS /

0.0 0.5

1.0 4 j, 2 b S/B < 0.1%

BS /

0.0 0.5

1.0 4 j, 3 b S/B = 0.2%

BS /

0.0 0.5

1.0 4 j, " 4 b S/B = 1.3%

BS /

0.0 0.5

1.0 5 j, 2 b S/B = 0.1%

BS /

0.0 0.5

1.0 5 j, 3 b S/B = 0.4%

BS /

0.0 0.5

1.0 5 j, " 4 b S/B = 2.3%

BS /

0.0 0.5

1.0 " 6 j, 2 b

S/B = 0.2%

BS /

0.0 0.5

1.0 " 6 j, 3 b

S/B = 0.9%

BS /

0.0 0.5

1.0 " 6 j, " 4 b

S/B = 3.8%

Red: signal-rich regions

Blue: signal-depleted regions

8 TeV, 20.3 fb

^-1

Systematics dominated search

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Background composition

tt+jets - dominant background is both channels (>95% in 3 b-jet regions)

‣ tt+bb - same final state as ttH signal

modelled with Powheg+Pythia

top pT and tt pT reweighted with correction factors obtained from differential cross section

measurement at 7 TeV

13

dilepton channel single lepton channel

tt+bb/cc model in Powheg+Pythia is comparable to Madgraph (ME-PS MC) tt+bb/cc rate is calibrated to data in the fit using background dominated bins in signal-rich regions

‣ 50% normalization uncertainty on tt+bb/cc

‣ uncertainties due to generator

‣ Powheg+Pythia vs Madgraph+Pythia

‣ uncertainties due to parton shower

‣ Powheg+Pythia vs Powheg+Herwig

_

_ _

_

_ _

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Background composition

tt+jets - dominant background is both channels (>95% in 3 b-jet regions)

‣ tt+bb - same final state as ttH signal

modelled with Powheg+Pythia

top pT and tt pT reweighted with correction factors obtained from differential cross section

measurement at 7 TeV

13

dilepton channel single lepton channel

tt+bb/cc model in Powheg+Pythia is comparable to Madgraph (ME-PS MC) tt+bb/cc rate is calibrated to data in the fit using background dominated bins in signal-rich regions

‣ 50% normalization uncertainty on tt+bb/cc

‣ uncertainties due to generator

‣ Powheg+Pythia vs Madgraph+Pythia

‣ uncertainties due to parton shower

‣ Powheg+Pythia vs Powheg+Herwig

_

_ _

_

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_

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Signal discrimination

Signal-rich regions: neural network NN trained to separate ttH from tt+jets

‣ event shape variables: centrality

‣ object pair properties:

‣ object kinematics:

‣ event kinematics: ,

Dedicated NN in single lepton channel with 5 jets, 3 b-jets to separate tt+bb/cc from tt+light

14

single lepton channel dilepton channel

p^jet5_T

∆η_jj^{max ∆η}

H_T N_p^jet_T_>40

_ _

_ __

_ _ _

examples of input variables to NN

C = �

�,jets

p_T/ �

�,jets

E

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Signal discrimination

Signal-rich regions: neural network NN trained to separate ttH from tt+jets

‣ event shape variables: centrality

‣ object pair properties:

‣ object kinematics:

‣ event kinematics: ,

Dedicated NN in single lepton channel with 5 jets, 3 b-jets to separate tt+bb/cc from tt+light

14

p^jet5_T

∆η_jj^{max ∆η}

H_T N_p^jet_T_>40

_ _

_ __

_ _ _

examples of input variables to NN

C = �

�,jets

p_T/ �

�,jets

E

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NN output

Signal extracted via binned likelihood fit to 6 signal and 9 control regions under

S+B hypothesis single lepton channel

dilepton channel Uncertainties

included in the fit via nuisance parameters

Fit reduces background

uncertainty by a factor of ~5-6 in most sensitive regions

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Background modelling

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before fit

after fit

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Results

Combination of single and dilepton channels

4.1 × SM (2.6 × SM) for mH = 125 GeV Best fit: signal strength μ = 1.7 ±1.4

ATLAS-CONF-2014-011

17

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= 125.4 GeV at mH

SM H t

mt H/

t

mt

95% CL limit on

0 2 4 6 8 10 12

H combined t

t

bb (8 TeV) HA

t t

(7+8 TeV) a

a HA t t

L dt=4.5 fb-1

=7 TeV, 0

s

L dt=20.3 fb-1

=8 TeV, 0

s

1m

=0) !

H t

mt

Expected (

2m

=0) !

H t

mt

Expected ( Observed

SM signal injected

ttH combination

Combine new ttH(γγ) with ttH(bb)

‣ Likelihood fit in 19 categories

‣ 15 from ttH(bb)

‣ 4 from ttH(γγ)

Modifications to ttH(bb) result

‣ Higgs mass changed to 125.4 GeV

‣ Uncertainty on H→bb branching

‣ Theoretical uncertainty on SM ttH cross section

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3.9×SM (2.3×SM) for mH = 125.4 GeV Best fit: signal strength

Significance observed (expected): 1.5 σ(1.0 σ) µ_ttH¯ = 1.6 ± 0.6(stat.)^+1.1₋_1.0(syst.)

ATLAS-CONF-2014-043

_ _ _

_ _

_

=125.4 GeV for mH

SMtH

mt H/

t

mt H=

t

!t

best fit

-1 0 1 2 3 4 5 6 7 8 9

H combined t

t

A bb H t t

a a HA t t

-1.1

= 1.6 +1.3 H

t

!t = 1.6 _-0.6^+0.6

H t

!t

-1.4

= 1.8 +1.5 H

t

!t = 1.8 _-0.7^+0.7

H t

!t

-1.7

= 1.2 +2.6 H

t

!t = 1.2 _-1.7^+2.5

H t

!t

( tot ) ( stat ) tot.

stat.

L dt=4.5 fb-1

=7 TeV, 0

s

L dt=20.3 fb-1

=8 TeV, 0

s

NEW

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Conclusion and outlook

ATLAS performed searches of ttH production using H→γγ and H→bb decay modes

‣ ttH(bb)

4.1 × SM (2.6 × SM)

‣ ttH(γγ)

6.5 × SM (4.9 × SM)

‣ ttH combination

3.9×SM (2.3× SM)

Search in the ttH(multilepton) channel and combination of all ttH channels is well underway

ttH observation and Yukawa coupling measurement is one of the priorities for Run 2 of LHC

‣ higher energy is advantageous for ttH(bb) observation

‣ ttH cross section increases by 4.7 while tt cross section by 3.7 at 14 TeV

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