Search for ttH at ATLAS - University of Oxford · l For the ﬁrst ttH searches of Run-2, we are...

Search for ttH at ATLAS Stefan Guindon

State University of New York at Albany

Oxford University January 24th, 2017

Stefan Guindon University at Albany Search for ttH at ATLASJan 24, 2017

Overview

2

l The Standard Model

l The Higgs boson

l Special Properties of the Top Quark

l ATLAS Detector

l Top & H: ttH̄ production at ATLAS

l ttH̄: Beyond 2017


Standard Model

3

l Standard Model of particle physics as it exists today l Very successful unified representation of electroweak and strong forces

l Building blocks of matter: fermions (leptons and quarks) l Force carriers: bosons (gluon, photon, W+-, Z)


Standard Model

4

l Several recent additions to the SM

l Top Quark: discovered in 1995 at Fermilab

l Higgs Boson: discovered in 2012 at CERN

l Standard Model of particle physics as it exists today l Very successful unified representation of electroweak and strong forces

l Building blocks of matter: fermions (leptons and quarks) l Force carriers: bosons (gluon, photon, W+-, Z)


Discovery of a new Particle

5

l In July of 2012, both ATLAS and CMS Collaborations announced the discovery of a new boson at a mass of approximately 125 GeV

l First question: is this the elusive SM Higgs boson?l Establishing decay to fermions is not as simple

Expected SM Higgs

Observed Signal

l Decaying to either γγ and ZZ bosonsl Observation very similar to expected SM Higgs


The Higgs boson in the SM

6

l Something was responsible for electroweak symmetry breaking in the Standard Model

l Is the new boson the Higgs predicted by the Standard Model?

l Measurement of the properties is essential l Production rate (cross-section) l Couplings to other particles

l Couplings are related to the particle massl Top-quark yukawa should have the largest coupling in the SM l Any deviation could hint at new physics l May play special role in EWSB

l Only direct measurement of the top-quark yukawa coupling is via the cross-section of ttH


Production and Decay of Higgs Boson

7

l Four different production mechanisms l Strong or weak production mechanisms l Measurements of all production mechanisms vital

ggF ttH

VH VBF

l Largest rate from ggF

l ttH̄ and tH production mechanisms have the smallest production rate


ttH Production Mechanism

8

l Already indirect evidence of Higgs coupling to fermions

l Top quark largest contributor to ggF production l Contributions from top in Higgs decays to photons l Assumption: no new particles which can couple to

the Higgs


ttH Production Mechanism

9

l Already indirect evidence of Higgs coupling to fermions

l Top quark largest contributor to ggF production l Contributions from top in Higgs decays to photons l Assumption: no new particles which can couple to

the Higgs

l Direct measurement of the top-Higgs coupling with ttH̄ production measurement

l Strongest SM coupling with the top quark Yt ~ 1

l σ(ttH̄) ~ Yt2

l Additionally: single top + Higgs measurement sensitive to sign of top-Higgs coupling

l Production and decay signatures sensitive to new physics


Decay Channels of the Higgs Boson

10

l Higgs decays largely to two bottom quarks l Roughly 58 % of the time

l Second largest decay channel is to WW, which further decay into leptons and quarks

l Small branching ratio final states which are still important due to clean signatures

l γγ (discovery channel) l ZZ -> 4 leptons (golden channel)

[GeV]HM120 121 122 123 124 125 126 127 128 129 130

Bran

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LHC

HIG

GS

XS W

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bb

ττ

µµ

cc

gg

γγ

ZZ

WW

γZ


Special Top Quark

11

l Top quark is the largest known elementary particlel mass ~ 175 GeV l 35 times the mass of its partner: bottom quark

l Because it is so massive, it decays almost instantaneously l Does not hadronize l Lifetime = 10-25 s

l Bare quark, giving properties to decay products

l Top quark gives an interesting playground to study properties of quarks

l Not possible with any other quark


Decay Channels of the Top Quark

12

l Decay of top-quark pair plays a role in final state signature

l Top quark decays to a b-quark and W boson l Decay of W boson determines final state

l Three final state categories:

l alljets: both W bosons decay into quarks (4 in total)

l lepton+jets: one W boson decays into quarks and the other into a charged lepton and neutrino

l dileptons: both W bosons decay each into a charged lepton and neutrino

τ+τ 1%

τ+µ 2%

τ+e 2%

µ+µ 1%

µ+e 2%

e+e 1%

e+jets 15%

µ+jets 15%

τ+jets 15%

"alljets" 46%

"lepton+jets""dileptons"

Top Pair Branching Fractions


Jets and b-tagging

13

l Well understood b-tagging is crucial for measurement of ttH production

l At least 2 b-tags in all ttH̄ events l Decays from the top quarks

l b-quarks live long enough to create a secondary vertex l Displaced tracks which cross the jet axis


ATLAS and the LHC

14

l ATLAS detector is one of four large experiments at the Large Hadron Collider at CERN

l Designed to push the boundaries of the energy frontier

l Multi-layered detector

l Each section has a purpose in identifying particles

l After several years of running, it is very well understood


Data Recorded at ATLAS

15


Luminosity and Dataset

16

l So far in Run-2, LHC has performed extremely well l ATLAS has recorded 3.9 fb-1 from 2015 and 36 fb-1 in 2016

l For the first ttH searches of Run-2, we are using 3.2 fb-1 of 2015 and 10.0 fb-1 of 2016 data

l Data quality checks to ensure good data


Luminosity and Dataset

17

l So far in Run-2, LHC has performed extremely well l ATLAS has recorded 3.9 fb-1 from 2015 and 36 fb-1 in 2016

l For the first ttH searches of Run-2, we are using 3.2 fb-1 of 2015 and 10.0 fb-1 of 2016 data

l Data quality checks to ensure good data

l To acquire sufficient data, require high pile-upl Multiple interactions per event


Measurement of Signal Strengths

18

l Measurement of signal strengths at the LHC l Simultaneous fit of production cross-section x decay branching ratio

Luminosity Detector acceptance Selection efficiency

l Define signal strengths for production and branching ratio:

l Assume NWA for the Higgs boson to separate production and decay rates

(SM prediction)


l Inputs from subsidiary measurement (nuisance parameters) l In this case, Gaussian prior for expected behaviour centred around zero

l Sometimes referred to as “penalty term” or prior l statistical uncertainty of MC

Profile Likelihood

19

l Measurements made using profile likelihood ratio test statistic Λ(α⃗) l = conditional / unconditional likelihoods

Parameter(s) of interest

Nuisance parameter modelling uncertainties

l A maximum-likelihood fit is performed on all categories simultaneously to extract the parameters of interest


tt̄H Searches

20

l ttH searches are broken down by the final state signatures l Three main categories Higgs Decay Branching (%)

H → bb 58%

H -> WW / ZZ / ττ 30%

H → γγ 0.2%

l ttH̄(bb)l Largest branching fraction,

large background from tt+̄HF


l ttH̄(yy)l Small branching fraction,

clean final state

tt̄H Searches

21

l ttH Searches are broken down by the final state signatures l Three main categories Higgs Decay Branching (%)

H → bb 58%

H -> WW / ZZ / ττ 30%

H → γγ 0.2%



l ttH̄(Leptons)l Multilepton final states, small backgrounds from tt+̄V and

ttH̄+jets( with fake leptons) l Mostly decays of the Higgs boson to WW, ZZ or ττ


l ttH̄(yy)l Small branching fraction,

clean final state

tt̄H Searches

22

l ttH Searches are broken down by the final state signatures l Three main categories Higgs Decay Branching (%)

H → bb 58%

H -> WW / ZZ / ττ 30%

H → γγ 0.2%



l ttH̄(Leptons)l Multilepton final states, small backgrounds from tt+̄V and

tt+̄jets( with fake leptons) l Mostly decays of the Higgs boson to WW, ZZ or ττ

l Background understanding = precision of ttH̄ measurementl Will focus quite a bit on background understanding


tt̄H(bb)

23

l Very complex final state: l lepton+jets: 4 b’s, 2 q’s and 1 lepton l dilepton: 4 b’s and 2 leptons

l Very small Signal to Background ratiol Use background dominated regions in profile

likelihood fit to reduce uncertainties in signal regions

l Categorize events according to jet and b-tag multiplicities

l Use jet scalar sum pT (HT) in background regions and Boosted Decision Trees (BDT) in signal regions

l+jets 2 b-tags 3 b-tags 4 b-tags

4 jets HT HT HT

5 jets HT HT BDT

6 jets HT BDT BDT


Background Modelling

24

l Large background and sizeable systematics have a large impact on the sensitivity l tt+̄jets is the largest background

l Becomes more heavy flavour dominating towards the signal region

l tt ̄modelled using Powheg+Pythia6

l NLO generator + Parton Shower

l tt+̄bb would be completely modelled by the Parton Shower part of the generator

l Large uncertainty on prediction l Normalization and kinematics


tt̄+bb Modelling I

25

l Instead of using Parton shower tt+̄bb modelling, move to an NLO QCD calculationl Error reduction from 80 % —> 20-30 %

l NLO QCD brings new questions on modelling

l Two possible schemes for NLO calculations:l 5FS five flavour scheme: (mb = 0, Nf in the pdf = 5)

l Mass of the b-quark set to 0 l Divergencies at mbb —> 0 GeV, must set a cut-off for calculation l Can be calculated fully inclusively with other tt+jets contributions (b treated as any other quark)

l 4FS four flavour scheme: (mb != 0, Nf = 4) l NLO QCD Matrix Element can be calculated down to 0 GeV, fully describe gluon splitting l NLO accuracy for any observable l Need additional step to combine with inclusive tt+jets samples


tt̄+bb Modelling I

26

l Instead of using Parton shower tt+̄bb modelling, move to an NLO QCD calculationl Error reduction from 80 % —> 20-30 %

l NLO QCD brings new questions on modelling

l Two possible schemes for NLO calculations:l 5FS five flavour scheme: (mb = 0, Nf in the pdf = 5)

l Mass of the b-quark set to 0 l Divergencies at mbb —> 0 GeV, must set a cut-off for calculation l Can be calculated fully inclusively with other tt+jets contributions (b treated as any other quark)

l 4FS four flavour scheme: (mb != 0, Nf = 4) l NLO QCD Matrix Element can be calculated down to 0 GeV, fully describe gluon splitting l NLO accuracy for any observable l Need additional step to combine with inclusive tt+jets samples


tt̄+bb Modelling II

27

l Understanding of this background is still relatively poor l A lot of recent theoretical work on-going to help

understand differences in predictions


tt̄+bb Modelling II

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l ATLAS results use 4FS modelling for tt+̄bbl Combined with inclusive tt+̄jets sample

through re-weighting procedure

l Understanding of this background is still relatively poor l A lot of recent theoretical work on-going to help

understand differences in predictions


Separation of Signal and Background

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l Use of topological variables from tt ̄decay and Higgs decay


Profiling Uncertainties

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l Reduction in the normalization uncertainties after the profile likelihood fitting technique

l Largest uncertainties:l tt+̄bb modelling: include uncertainties on the normalization and four-flavour modelling l b-tagging: both from b-quarks and mis-tagged light- and c-quarks


Fit Results

31

l Combination of single lepton and dilepton channels

l 95% CL observed (expected) limit: l 4.0 x SM (1.9 x SM) for mH = 125 GeV

l Best fit signal strength: μ(ttH̄) = 2.1 ± 1.0


tt̄H(Leptons)

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l Signatures with multiples charged leptons in the final state

l Mostly decays of the Higgs boson to WW, ZZ or ττ

l Selection of final states with additional jets and b-tagged jets:

l Strategy is a cut and count final state fit to data

Channel Selection

2L SS l ≥ 5 jets ≥ 1 b-tag

2LSS + tau l ≥ 4 jets ≥ 1 b-tag

3Ll ≥ 4 jets ≥ 1 b-tag or l ≥ 3 jets ≥ 2 b-tags

4L l ≥ 2 jets ≥ 1 b-tag


tt̄H(Leptons) Backgrounds

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l Signatures of multiple leptons rare from SM processes

l Dominant backgrounds from non-prompt leptons

0.04 0.12 0.15S/B:

0.30 0.14 0.40

l Semi-leptonic decays of b-jets


tt̄H(Leptons) Backgrounds

34

l Signatures of multiple leptons rare from SM processes

l Dominant backgrounds from non-prompt leptons

0.04 0.12 0.15S/B:

0.30 0.14 0.40

l Semi-leptonic decays of b-jets

l Mis-identified charged leptons (QMisReco)l Bremsstrahlung

l SM backgrounds mostly with additional jets l tt+̄Z or tt+̄W l Diboson + additional heavy flavour


Background Estimation

35

l Fake lepton estimation is crucial for multilepton channels

l Tight selection applied to reduce the amount of fake lepton backgrounds in signal region l Tight lepton cuts on ID, pT, isolation, impact parameter, overlap removal with close-by jets

l Data-driven fake lepton estimation: fake factor methodl loose-to-tight extrapolation in the 2L channel

A (SR) B

C D

l Minimum isolation, ID and tight vertex ℓ requirement

l 1 tight and 1 loose leptonl 2 tight leptons

l High N jets

l Low N jets


Background Estimation

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l Fake lepton estimation is crucial for multilepton channels

l Tight selection applied to reduce the amount of fake lepton backgrounds in signal region l Tight lepton cuts on ID, pT, isolation, impact parameter, overlap removal with close-by jets

l Data-driven fake lepton estimation: fake factor methodl loose-to-tight extrapolation in the 2L channel

l tt+̄Z CR

A (SR) B

C D

l Minimum isolation, ID and tight vertex ℓ requirement

l 1 tight and 1 loose leptonl 2 tight leptons

l High N jets

l Low N jets


tt̄H(Leptons) Fit to data

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l Pre-fit and post-fit yields compared to data l Fit categories based on lepton number and flavour (in the case of same-signed 2L channel) l 6 bin fit

l Fit prefers an increased ttH̄ production ratel Excess pre-fit in same-signed 2L channels


tt̄H(Leptons) Systematics

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l Ranking of systematics based on impact on the signal strength uncertainty

l Largest uncertainty from jet-vertex fraction l Per-jet uncertainty (~ 2.5 %) l Due to pile-up jets

l Non-prompt lepton uncertainties also very large l Correlated between 2L and 3L channels

l ttH uncertainties correlated between all channels


tt̄H(Leptons) Results

39

l Combination of all multilepton channels

l 95% CL observed (expected) limit: l 4.9 x SM (2.3 x SM) for mH = 125 GeV

l Best fit signal strength: μ(ttH̄) = 2.5 ± 1.2

l Slight access consistent with SM expectation


tt̄H(γγ)

40

l Diphoton final state is a very rare decay of the Higgs (~ 0.2 %) l Very clean final state

l Selection of γγ candidates:l Build the diphoton primary vertex using highest ET photons passing loose selection l Require two tight and isolated photons with ET > 25 GeV and |η| < 2.37 l Leading and sub-leading photons: ET/mγγ > 0.35 and 0.25

l For ttH̄(γγ), candidates are then required to pass additional object and event selection criteria:l Split according to tt ̄decay channel

Channel Hadronic Leptonc (≥ 1 lepton)

≥ 5 jets, pT> 30 GeV pT(ℓ) >10 GeV

≥ 1 b-jet ≥ 2 jets pT>25 GeV, ≥ 1 b-jet

- Z veto (mℓℓ and meγ)

cut-based γγ selection cut-based γγ selection MET > 20 GeV (for 1bjet events)


tt̄H(γγ) Background Estimation

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l Background fraction estimated using data-driven method

l Templates for real and fake photons fit in sideband regions

l Exponential of polynomial or Bernstein polynomial

l Background mostly γγ (~ 80 %)


tt̄H(γγ) Background Estimation

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l Background fraction estimated using data-driven method

l Templates for real and fake photons fit in sideband regions

l Exponential of polynomial or Bernstein polynomial

l Background mostly γγ (~ 80 %)

l Composition of Higgs related backgrounds for ttH selection:

l ggF + heavy flavour (hadronic)

l tHjb and tWH leading backgrounds in both categories


tt̄H(γγ): Signal Regions

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l Very few events expected in mass window of signal region

l Signal strength of ttH̄(γγ) fit individually is - 0.25 +1.26 -0.99

l Dominated by statistical uncertaintyl Sensitivity of this channel will grow with data


Overview of tt̄H Combination

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Main background

γγ continuum non-prompt, QMisID, tt̄V tt̄+HF (SRs)


Combination of tt̄H Channels

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l Signal strength μttH̄ combination

l Significance improvement of 50% with respect to the most sensitive individual result

l Supersedes Run-1 ATLAS sensitivity (1.5σ)

l Systematically limited with early Run-2 dataset


Top-Yukawa Coupling

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l Top-Higgs coupling not only responsible for ttH̄ production l Single top + Higgs production possible in SM (tH)

Yt


Top-Yukawa Coupling

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Yt

gHWW


l tH production is highly suppressed in SM l Much smaller than ttH̄ l Negative interference with W-H coupling

l Sensitive to the sign of top-yukawa coupling


Top-Yukawa Coupling

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l In some BSM models Yt can be negative: l Significantly increased tH cross section and BR(H → γγ) l Destructive interference becomes constructive l Attempt to rule out such hypothesis

l Set limits as a function of κt = Yt/Yt(SM) l Inclusive Higgs production as a function of strength

to SM expectation l Also correlated to BR(H → γγ)

l All other couplings are set to SM l Null hypothesis includes SM ttH̄

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


Yt

gHWWl tH production is highly suppressed in SM

l Much smaller than ttH̄ l Negative interference with W-H coupling

l Sensitive to the sign of top-yukawa coupling

ATLAS Result

https://arxiv.org/pdf/1409.3122.pdf


Boosted tt̄H(bb)

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Boosted tt̄H(bb)

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l CMS: Boosted require both hadronic top and H tagsl Reduced combinatorics for Higgs candidate l C/A 1.5 jets with a top-tagged and boosted Higgs candidate with pT > 200 GeV l Require at least 4 jets and 2 btags

CMS HIG-16-004

SR yields for 1/fb

N(ttH̄) 0.81

N(bkg) 43.3 (34 % tt+bb) large tt+B

S/B 0.02

l Resolution of H->bb candidatel Input into a BDT

l Third most sensitive region in H->bb from CMS

https://cds.cern.ch/record/2139578/files/HIG-16-004-pas.pdf


Future Outlook

51

l LHC program is still in its infancy l A lot of work ahead in observing Higgs coupling to third generation quarks (top and bottom) l Challenges ahead with High-Luminosity LHC: large number of interactions

l Run-2 integrated luminosity ~ 150 fb-1

l Significance with 13 fb-1 2.8 σ (1.8 σ) observed (expected) l Some searches systematics limited

l 2017-2018 should be very interesting for ttH̄ searches at the LHC


tt̄H/tt̄Z to measure Top-Yukawa

52

l ttH̄ cross section measurement is most often described as a measurement of the top-Yukawa couplingl This is limited by theoretical understanding of theoretical uncertainties on ttH̄ production XS l THU NLO uncertainties for ttH̄ and ttZ̄ reported in the YR4:

l Identical production dynamics:l Correlated QCD corrections, scale dependence, and αS dependence

l mZ ~ mH —> almost identical kinematic boundariesl Correlated PDF systematics

l For a given Yt, σ(ttH̄)/σ(ttZ̄) can be predicted theoretically with a much better precisionl Most precise handle on the top-Yukawa coupling

l Still far away from having leading theory uncertainties (Run-3 and HL-LHC)l Can already be done -> no real reason not to already measure y_t with Run-2 datasetl ttH̄ systematics 3rd leading in ttH̄(bb)

NLO QCD+EW XS [fb] @ 13 TeV Scale PDF αS

tt̄H 507.1 + 5.8 - 9.2 % +/- 3.0 % +/- 2.0 %

tt̄Z 839.3 + 9.6 - 11.3 % +/- 2.8 % +/- 2.8 %

https://twiki.cern.ch/twiki/bin/view/LHCPhysics/LHCHXSWGTTH

M. Mangano - Training Lectures on FCC

https://twiki.cern.ch/twiki/bin/view/LHCPhysics/LHCHXSWGTTH


CP mixture states of the Higgs

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l Statistical p-value on different observablesl Longer term projects - though should be open to surprises

arXiv:1501.03157l Though, still unobserved ttH at the LHC, need to keep in mind

important future measurements l Any CP violating instances in the top-Yukawa couplingl Still allowed given current EDM constraints l Gauge boson coupling CP violations suppressed and non-

universality of fermion couplings possible (H->tautau) in NP models

l ttH/A would be an interesting place to study CP violation in the Higgs sector due to its coupling to top quarks

l Mixture of scalar and pseudo- scalar

l Change in expected cross section based on mixturel Kinematic differences

l Spin correlation of ttbar decays

https://arxiv.org/pdf/1501.03157v2.pdf


Back-up

54


Summary of Run-1 Results

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ATLAS + CMS Couplings Combination

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l Signal strength measurements of Higgs production mechanisms from ATLAS+ CMS combination

l ttH signal strength 2.3 times SM expectation

Higgs Production


ttH(bb) Systematic Uncertainties

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l List of systematic uncertainties based on impact on signal strength


CMS ttH(bb): Fit Regions

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l 4 fit regions in lepton+jets channel and 3 in dilepton onlyl Dropped several from 2015 analysis


CMS ttH(bb): Backgrounds

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l Background fractions in 6j4b the most sensitive signal region

CMS ATLAS

l Larger fraction of tt+>=1 c in CMS than ATLAS

l b-tagging rejection is not as high as ATLAS (also slight difference in labelling)

l S/B and S/sqrt(B) higher in ATLAS

CMS ATLAStt+>=1b 58.5% 79.8%tt+>=1c 23.0% 8.5%tt+>=1l 11.3% 4.0%

Total bkg 764.8 840Total signal 26.9 44.9


CMS ttH(bb): Final Discriminants

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Stefan Guindon University at Albany CMS ttH(bb) AnalysisNov 16, 2016

CMS ttH(bb): Systematics

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l no ttH shape systematics

l Only 1 JES

l Contamination in CRs

l 50 % normalization on 6 HF categories (3 b and 1 c)

l Largest impact on signal strength

l PS scale l Only tt+bb shape modelling

uncertainty in the analysis l No feeling for how large this

is in the final discriminants l Normalization is a few %


CMS ttH(bb): Fit Results

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l Individually, very slight excesses in signal-enriched regions

l Combination pulls down result l JES related pull correlated

across all regions

l Very little shape change in the final discriminant

l Increase in tt+LF post-fit


CMS ttH(bb): S/B

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l As expected from individual vs total fit result, the lower fitted mu is the result of small S/B bins and not from the most sensitive SR

l Same effect in both lepton+jets and dilepton


CMS Results: ttH(Leptons) I

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l Multivariate approach to separate ttH from fake lepton and SM backgrounds l Binned fit in 2D space formed by BDT (ttH̄-vs-tt)̄ and BDT (ttH̄-vs-ttV̄)

l Input variables: kinematics, jet multiplicity, matrix element (for 3ℓ, ~8% improvement) l Not included: lepton ID

l Fit categories based on: l light ℓ flavour, hadronically-decaying 𝜏, presence of b-jets, lepton charge

l Total: (97 fit bins)

l MVA to discriminate prompt from non-prompt leptons

l BDT output of data and MC for ttH and tt (with fake leptons)


CMS Results: ttH(Leptons) II

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2ℓSS

3ℓ

2ℓSS

3ℓ

x

x

largest BDT separation

x

Bins to fit


CMS Results: ttH(Leptons) III

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l Results of the CMS ttH(leptons) fit to data for each of the separate channels + combined l Slight excess compared to SM expectation

l Largest uncertainties related to the selection efficiency l Muon fake rates constrained by the fit to data

l Dimuon channel most sensitive

SM


CMS Results: ttH(yy)

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l Similar selection as ATLAS analysis

l BDTγγ discriminate H→γγ from diphoton background

l Input variables: pTγ(1,2)/mγγ, cos(Φγ1-Φγ2), relative diphoton mass resolution, BDT γID, BDT VTX Prob

l Built to be mass independent

• μ(tt̄H) = 1.91 + 1.5 – 1.2


Profiling

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•We can reduce uncertainties in signal bin by profiling •Constrain uncertainty in another bin, propagate this knowledge to signal bin

•Given: +1 sigma, nominal and -1 sigma of each systematic

•Assumption of interpolation of uncertainty between [-1,1] sigma

•Assume full correlation between bins

•What are we doing: Measuring the uncertainty in our dataset


κ Framework

69

l κ framework developed by LHC Higgs Cross Section WG l Modifiers which scale Higgs boson couplings, both fermion and boson

l Production cross-sections and decay rates for each process are parameterized with a degree of freedom κ2

l For tree-level LO processes, the coupling modifiers (κ2 ) are labeled as: l Measurements of fermions: κf —> κt, κb, κτ l Measurements of bosons: κV —> κW, κZ

Higgs width assuming possible BSM

l For processes with loops at LO, κ2 are labeled as: l Can be expressed as a polynomial of: κt, κW, … l Or assign an effective κ for that process: κg, κγ, κH

l Important note that although the parameters κ correspond to the LO degrees of freedom in each process, the SM cross-section it multiplies (σSM, BSM) is usually NLO or NNLO both in QCD and EW.


top-Higgs Yukawa in the combination

70

l Fit of the coupling modifiers accessible by the LHCl Loop coupling strengths are resolved in expressions

of tree-level coupling assuming the SM physics l One coupling parameter per SM particle l No Beyond-Standard-Model decays

l All couplings are compatible with SM l (ε = 0 and M = 246 GeV)

F,i = ⌫ ·m✏F,i/M

1+✏ and V,i = ⌫ ·m2✏V,i/M

1+2✏

✏ = 0.023+0.029�0.027 and M = 233+13

�12 GeV

Z , W , t, ⌧ , b, µ


κ Framework Loops

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l Parameterization of the ggF → H → γγ

l Starting with the production ggF -> H l Can be broken into three components with coupling modifiers t / b

�(ggF ! H) = 2t�tt + 2

b�bb + tb�tb

2g = �(ggF!H)

�SM= 2

t�tt+2b�bb+tb�tb

�tt+�bb+�tb

⇡ 1.062t + 0.012

b � 0.07tb

l Effective Hgg coupling:

l Decay H -> γγ : including contributions from top quark and W boson

2� = ��

�SM��

=2t�

tt��+2

W�WW�� +tW�tW

��

�tt��+�WW

�� +�tW��

⇡ 0.072t + 1.592

W � 0.66tW

t W


Parameterizations

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l Production, decays and partial width of the Higgs l Parameterizations for κ modifiers including higher-order QCD and EW corrections


Parameterizations

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l Interference involving top quarksl Sensitive to the relative sign of

the coupling



Parameterizations

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l Assume κt > 0l b,Z,W couplings sign can be

constrained via this assumption

gHtt

gHWW


the coupling

l Example of tH, destructive in SM, constructive with κt = -1x κW



Parameterizations

75


the coupling

l Example of tH, destructive in SM, constructive with κt = -1x κW

l Limited to no sensitivity:l κμ, κc, κs l Assumed they behave as: κμ -> κτ, κc -> κt, κs -> κb

l Not included (irrelevant): κe, κu, κd


Search for ttH at ATLAS - University of Oxford · l For the ﬁrst ttH searches of Run-2, we are...

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Transcript of Search for ttH at ATLAS - University of Oxford · l For the ﬁrst ttH searches of Run-2, we are...