latest results on higgs boson analyses · 2019-03-27 · outline presentation of 2015+2016+2017 or...

Latest Results on Higgs Boson analyses

Dr. Marc Bret [email protected]

Shanghai Jiao Tong University

SJTUShanghai, China

March 27, 2019

Dr. Marc Bret Cano Latest Results on Higgs Boson analyses March 27, 2019 1 / 54

The Large Hadron Collider (LHC) at CERN (Switzerland)


Particle Collision and detection


Performance of the LHC & the ATLAS detector

The LHC & ATLAS has been working remarkably well, generallyexceeded expectations

Currently in shutdown until mid-end 2021


The Higgs Boson

Predicted in the StandardModel to solve the issue ofthe mass of the W and Zbosons

Existence of Higgs-likeparticle confirmed in 2012with a mass of 125 GeV


Introduction: Higgs Production Mechanisms(arXiv:1610.07922v2)

Largely dominated by gluon-gluonfusion

Vector boson fusion is distinguishedby the presence of widely separatedjets in the event, but statistics arevery limited

Rest of production mechanismshave an expectation below oneevent

Jet topology is used to discriminateevents into different categories


Outline

Presentation of 2015+2016+2017 or Full Run-2 results (SJTUinvolvement shown in brackets), including:

Measurement of ttH in Diphoton Final State (Prof. Liang Li,Shuyang Hu)

H→ bb̄ (Chikuma Kato & Shu Li)

The combined measurements of Higgs boson cross-sections andcouplings (Prof. Haijun Yang, Zirui Wang)

Higgs Mass Measurement

Sneak peek at latest result from the H → ZZ →4` High mass search(Prof. Haijun Yang, Marc Bret & Wanyun Su)


ttH : Overview1

The associated production of the Higgs boson with top quarksprovides a direct access to the top-Higgs coupling

The diphoton channel is one of the leading channels in the ttHmeasurements

The diphoton resonant decay is an unambiguous signature for theHiggs bosonRobust background estimation from diphoton mass sidebands

1Analysis published in ATLAS-CONF-2019-004 & http:

//moriond.in2p3.fr/2019/EW/slides/1_Sunday/2_afternoon/6_HaichenWang.pdfDr. Marc Bret Cano Latest Results on Higgs Boson analyses March 27, 2019 9 / 54

http://moriond.in2p3.fr/2019/EW/slides/1_Sunday/2_afternoon/6_HaichenWang.pdf

http://moriond.in2p3.fr/2019/EW/slides/1_Sunday/2_afternoon/6_HaichenWang.pdf

ttH : Photon Reconstruction

Photons are selected using a cut-based multivariate discriminantbased on shower shape variables in the EM calorimeter:

Reconstruction efficiency is tipically > 90% for photons coming fromHiggs decay for the chosen working point in the ttH analysis

Photons are also required to be isolated


ttH : b-tagging performance

B-jets are tagged using a multivariate discriminant combiningtracking, secondary vertex and decay chain information

The b-tagging is calibrated with a ttbar control sample

A b-tagger with a 77% efficiency is used, corresponding to a rejectionof light jet at the level of a few hundreds


ttH : Diphoton event selection

Select events with two isolated“tight” ID photons fulfilling:pTγ1/mγγ > 0.4 &pTγ2/mγγ > 0.35

Expect around 1.5 milliondiphoton events with 105 < mγγ

< 160 GeV using the Full Run-2dataset

Only 160 of those eventscorrespond to the process we areinterested on: ttH with twophotons in the final state comingfrom the Higgs decay


ttH : Analysis Strategy


ttH : Boosted Decision Tree Training

Use variables such as momentum of photons, jets and leptons, missingtransverse and its direction and whether jets are b-tagged or notTrained using ttH signal simulation and a data control sample withreversed photon ID to model the background


ttH : BDT Output

The BDT is trained for events with top quark leptonic and hadronicdecays, separately

The remaining events are classified into multiple categories withdifferent signal-to-background-ratios (S/Bs) based on the BDTscores, to maximize the sensitivityDr. Marc Bret Cano Latest Results on Higgs Boson analyses March 27, 2019 15 / 54

ttH : Yields per category

Number of data events in the different analysis BDT bins, in thesmallest diphoton mass window that contains 90% of the ttH signal


ttH : Diphoton Mass Distributions

Distributions for the best S/B categories for each hadronic andleptonic type

Contamination of non-ttH Higgs signals are strongly suppressed; ttHpurity reaches 90% level in the best categories


ttH : Observed significance and cross-section

Obs. Significance: 4.9σ

Exp. Significance: 4.2σ

Obs. σttH · BRH→γγ : 1.59+0.43−0.39 fb = 1.59+0.38

−0.36 (stat) +0.15−0.12 (exp)

+0.15−0.11 (theo)

Exp. σttH · BRH→γγ : 1.15+0.09−0.12 fb


VH : Overview2

Higgs radiated from a W /Z boson, around 4% of the total Higgsproduction cross-section:

Given its low production xsec, require the Higgs to decay to bb̄, thelargest Branching Ratio with 58%

Difficult channel despite the large BR due to the large background2Analysis published in arXiv:1903.04618


Simplified template xsec3

Aim to maximise the sensitivityof measurements and reduce itsdependence on theoreticalcalculations

All decay channels are combined

Cross-sections are measureddirectly instead of signalstrengths

Results obtained for a givenproduction mode

For VH, given low productionxsec, the only “viable” finalstate is bb̄

3https://arxiv.org/abs/1610.07922Dr. Marc Bret Cano Latest Results on Higgs Boson analyses March 27, 2019 20 / 54

https://arxiv.org/abs/1610.07922

VH : Event Selection

Select events with at most twoisolated electrons or muons andtwo b-tagged jets with transversemomentum above 45 and 20 GeV

Events classified according tonumber of leptons: 0, 1 or 2,with the 0 and 1 categoriesexpected to contain missingtransverse energy

Events in the 2-lepton categoryare required to be same-flavourand have an invariant mass closeto the Z boson mass


VH : Muon calibrations

Track reconstructed combininginformation from Inner Detectorand muon chamber hits

Correct Momentum resolutionand scale in simulation toaccount for differences withdata:

Misalignements in thedetectorSimulation of the traversedmaterialMagnetic field

Very precise knowledge of themuon momentum resolutionand scale in the Higgs massrange

Approximately 2% muon momentumresolution for ATLAS in the 20-30GeV range


VH : Signal Regions


VH : BDT Output

Generally obtain a good separation between signal and background


VH : Results with 3 and 5 POI

All measurements found to be inagreement with SM Expectationwithin the (large) uncertainties


Higgs Boson Production xsec (ATLAS-CONF-2019-005)

Obtained combining γγ, ZZ ∗, WW ∗, ττ , bb̄ and µµ final states

Significances above 5σ obtained for each production modeDr. Marc Bret Cano Latest Results on Higgs Boson analyses March 27, 2019 26 / 54

Higgs Boson Production xsec and BR ratios


Higgs Boson Couplings

Interpret the results in the κframework as a function of theparticle mass assuming no BSMcontributions to total width

Probing universal couplingstrength factors for fermions (κf )and bosons (κV )

Best fit shows values of κf and κV!= 1.0, but compatible with SMwithin uncertainties


H → ZZ ∗ → 4`: the golden channel

Many useful features:Large signal to background ratio(S/B ∼ 2) & excellent massresolution (∼ 1− 2%)

Main draw-back is the smallbranching ratio (∼ 0.02%) →limited statistics

Analysis strategy:

ATLAS uses invariant mass of thefour-lepton system (m4`) as yourobservable

Non-resonant background comingfrom ZZ (reduced via MVA)

Leptons from the Z* are low pT→ more prone to backgroundcontamination

Fake background estimated inControl regions


Measurement of the Higgs boson mass4

Combination of the results fromthe 4` and γγ final states

The Powheg-Box v2 MC eventgenerator is used for ggF and VBFproduction signal simulation

Cross-section times branching ratioare profiled

The continuum ZZ ∗ background issimulated using Sherpa 2.2

Contribution from fakebackground estimated from data

4Phys. Lett. B 784 (2018) 345Dr. Marc Bret Cano Latest Results on Higgs Boson analyses March 27, 2019 30 / 54

Higgs boson mass: Event selection

Select events with four leptons andcategorize them according to thelepton composition (4µ, 4e, 2µ2eand 2e2µ, )

Use events only in the range 110< m4` < 135 GeV

A kinematic fit is performed toconstraint the invariant mass ofthe leading lepton pair to the Zpole mass

Find generally good agreementwithin uncertainties from an SMHiggs with a mass of 125 GeV


Higgs boson mass: per-event method

m4` signal distribution modelled asthe convolution of the intrinsicHiggs boson lineshape and a fourlepton invariant mass responsefunction

The response function gives theprobability of measuring a valuemmeas

4` for a truth mass mtrue4`

Validate the method by testing itwith the Z boson


Higgs boson mass: Results

mZZ∗H = 124.79 ± 0.37 GeV

mγγH = 124.93 ± 0.40 GeV

Combining with the 7 and 8 TeV dataset: mH = 124.97 ± 0.24 GeV


Higgs boson mass: Summary


High mass analysis5: overviewSelect events with four leptons(results combined with ``νν)

Invariant mass is used as adiscriminant

Event categorisation:gluon-gluon fusion, furthersub-divided in:

4µ4e2µ2e

Vector boson fusion, by requiring:

two or more jets with pT > 30GeV∆η > 3.3mjj > 400 GeV

Very few events expected for VBFLarge continuous backgroundqq → ZZ

5Eur. Phys. J. C 78 (2018) 293


High mass analysis: BSM Higgs & Signal Modelling

Two Higgs Doublet models (2HDM)Models:

Introduces four Higgs masses, tanβ (ratio of vev) and mixingbetween the two neutral CP statesh (SM Higgs) and H

Four different model types

Type I : one doublet couples to bosonsonly, the other to fermions

Type II : one doublet couples to up-likequarks, the other to down-like

Type III : same couplings to leptons as intype I and to leptons as in type II

Type IV : reverse couplings from type III

Focus on Type I and II models

Signal distribution is parametrised forany value of mH :

Narrow Width Approximation

Large Width Approximation (1, 5and 10%)


High mass analysis: m4`


High mass analysis: p-value

Two excesses found: 240 and705 GeV

The excess at 705 GeV ismainly due to an excess inthe 4e category, while theone at 240 GeV is driven bythe excess

Each excess represents a2.2σ global significance

Various checks performedbut nothing seems fishy,revisit with 2017 dataset

Excess not found in the ``ννchannel


High mass analysis: 95% CL


2HDM exclusion contour tan(β) vs cos(β − α) mH=200GeV


2HDM Type I Exclusion Contour tan(β) vs mH

cos(β − α)=-0.1


Sneak Peek: 2015-17 results (I)

Events

/ 2

0 G

eV

1−10

1

10

210

310

410

Data

ZZ*

ZZ* EW

+V, VVV tt

tZ+jets, t

Uncertainty

ATLAS Internal

4l→ ZZ* →H 1

13 TeV, 79.8 fb

= 2081.81ZZn

= 20.60qqZZjjn

= 44.39ttVn

= 30.17redBkgn

= 2176.96MCn

= 2473Datan

[GeV]4lm

200 400 600 800 1000 1200 1400

Da

ta/M

C

0.5

1

1.5

2

[GeV]Hm

200 400 600 800 1000 1200 1400

4l)

[fb]

→ Z

Z

→ B

R(H

× gg

Fσ

95%

CL

limits

on

1−10

1

10 limitsCLObserved limitsCLExpected σ 1 ±Expected σ 2 ±Expected limit 2015+16sCLExpected limit 2017sCLExpected

InternalATLAS-113 TeV, 79.8 fb

NWA


2015-17 results: 2015-17 results (II)

[GeV]Hm

200 250 300 350 400 450 500 550 600

0Lo

cal p

5−10

4−10

3−10

2−10

1−10

1

σ1

σ2

σ3

σ4

2015-172015-162017


NWA

[GeV]Hm

600 700 800 900 1000 1100 1200 1300 1400

0Lo

cal p

5−10

4−10

3−10

2−10

1−10

1

σ1

σ2

σ3

σ4

2015-172015-162017


NWA

Excesses at 238 and 704 GeV with 2.48 and 2.95σ, at 704 GeV comingfrom 2015+16 dataset

Aim to publish Full Run-2 results this summer


Conclusions

Various measurements of the Higgs boson presented

Still fairly limited by statistics...

Situation won’t improve till High Lumi LHC or CEPC

So far all compatible with SM expectation

As usual, stay tuned!


BACKUP SLIDES


ttH : BDT Categories Yields


VH : Results


Higgs Couplings: Summary 6

6Published in ATLAS-CONF-2019-005Dr. Marc Bret Cano Latest Results on Higgs Boson analyses March 27, 2019 49 / 54

Higgs Couplings: Production Cross-section


Higgs Couplings: Production Cross-section Systematics


Higgs Couplings: STXS Stage 1


VH : Electron corrections

ATLAS corrects the response atreco cluster in the simulationwith an MVA and correctionsfrom Z/J/ψ analysis

Correct simulation for energyloss in inactive materials

Electrons corrected in bins of leptonη and pT using tag-and-probe


High mass analysis: Signal acceptance & systematics

Low acceptance for VBF processdue to the stringent requirements(two jets with pT > 30 GeV, mjj >400 geV & δη > 3.3)

Any event not classified as VBFenters the ggF category

The contribution from final stateswith τ leptons decaying toelectrons or muons is found to benegligible

The uncertainty on the integratedluminosity enters both in thenormalisation of the fitted numberof signal events as well as in thebackground expectation fromsimulation


latest results on higgs boson analyses · 2019-03-27 · outline presentation of 2015+2016+2017 or...

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