Measurements of properties of the Higgs boson decaying to a Wboson pair in pp collisions at root s=13 TeV

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https://www.sciencedirect.com/science/article/pii/S0370269319301169

DOI: 10.1016/j.physletb.2018.12.073

Direitos autorais / Publisher's copyright statement:

©2019

by Elsevier. All rights reserved.

DIRETORIA DE TRATAMENTO DA INFORMAÇÃO

Cidade Universitária Zeferino Vaz Barão Geraldo

CEP 13083-970 – Campinas SP

Fone: (19) 3521-6493

http://www.repositorio.unicamp.br

Contents lists available atScienceDirect

Physics

Letters

B

www.elsevier.com/locate/physletb

Measurements

of

properties

of

the

Higgs

boson

decaying

to

a

W boson

pair

in

pp collisions

at

√

s

=

13 TeV

.

The

CMS

Collaboration



CERN,Switzerland

a

r

t

i

c

l

e

i

n

f

o

a

b

s

t

r

a

c

t

Articlehistory:

Received13June2018

Receivedinrevisedform1December2018 Accepted20December2018

Availableonline20February2019 Editor: M.Doser

Keywords:

Higgs WW

Measurements ofthe productionofthe standardmodel Higgsbosondecaying toaW bosonpairare reported. The W+W− candidatesare selectedin eventswith anoppositelycharged leptonpair,large missingtransversemomentum,andvariousnumbersofjets.ToselectHiggsbosonsproducedviavector boson fusionand associated productionwith aW or Z boson,events with twojets orthreeor four leptons are also selected. The event sample corresponds to an integrated luminosity of 35.9 fb−1, collectedinpp collisionsat√s=13 TeV bythe CMSdetectorattheLHCduring2016. Combiningall channels, the observed cross sectiontimes branchingfraction is 1.28₋+0₀._.18₁₇ timesthe standard model prediction for theHiggsboson withamass of125.09 GeV.Thisis the firstobservation oftheHiggs bosondecaytoW bosonpairsbytheCMSexperiment.

©2019TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3_.

1. Introduction

In the standard model (SM) of particle physics, the origin of the massesofthe W and Z bosons isbased on thespontaneous breaking of the electroweak symmetry. This symmetry breaking isachieved throughthe introductionofa complex doubletscalar field [1–6],leadingtothepredictionoftheexistenceofone phys-ical neutral scalar particle, commonly known as the Higgs bo-son (H).The observationofa newparticle ata massof approxi-mately125 GeV withHiggsboson-likepropertieswas reportedby theATLAS [7] andCMS [8,9] Collaborationsduringthefirstrunning periodoftheCERNLHCinproton-proton(pp)collisionsat center-of-mass energies of 7 and 8 TeV. Subsequent publications from both collaborations,basedon the 7and8 TeV data sets [10–13], established that all measured properties of the new particle, in-cludingitsspin,parity,andcouplingstrengthstoSMparticles,are consistentwithintheuncertaintieswiththoseexpectedfortheSM Higgsboson.AcombinationoftheATLAS andCMSresults [14,15] furtherconfirmedtheseobservationsandresultedindetermining thebosonmasstobemH

=

125

.

09

±

0

.

21 (stat)

±

0

.

11 (syst) GeV.

The Higgsbosondecayto a pairof W bosonswas studied by the ATLAS and CMS Collaborations using the 7 and 8 TeV data sets in leptonic final states, exploring several production mech-anisms [16–18]. The probability of observing a signal atleast as largeastheoneseen,underthebackground-onlyhypothesis,

cor- _{E-mailaddress:cms-publication-committee-chair@cern.ch.}

respondedtoasignificanceof6.5and4.3standarddeviations(s.d.) forATLASandCMSrespectively,whiletheexpectedsignificancefor aSMHiggsbosonwas5.8(5.9)s.d.fortheCMS(ATLAS) collabora-tion.AlaterCMScombination [12],thatincludesHiggsboson pro-ductioninassociationwithatopquarkpair,reportedanobserved significanceof4.7s.d.forthisdecay.Thesamedecaychannelwas usedbytheATLASandCMSCollaborationstosearchfortheHiggs bosonoff-shellproduction [19,20] andtoperformfiducialand dif-ferentialcrosssectionmeasurements [21,22].

In2015, theLHC restartedat

√

s

=

13 TeV,deliveringhigh lu-minositypp collisions.Thenewdataareusedtofurtherconstrain the propertiesofthe Higgsboson: anysignificant deviationfrom theSMpredictionswouldbeaclearsignofnewphysics.This pa-per presentstheanalysisoftheH

→

WW decay at13 TeV,using a data sample corresponding to a total integrated luminosity of 35

.

9 fb−1,collectedduring2016.Thesamefinalstatewasrecently studiedbyATLAS [23] using2015and2016data.

Gluon fusion (ggH) is the dominant production mode for a Higgs boson with a mass of 125 GeV in pp collisions at

√

s

=

13 TeV. The large Higgs boson branching fraction to a W boson pair makes this channel suitable for a precision measurement of the Higgsbosonproductioncrosssection,andalsoallowsstudies of subleading production channels, such as Higgs boson produc-tionviavectorbosonfusion(VBF)andassociatedproductionwith a vector boson (VH). Thesechannels are also studiedin this pa-per,contributingtotheprecisioninthemeasurementoftheHiggs bosoncouplings.

https://doi.org/10.1016/j.physletb.2018.12.073

0370-2693/©2019TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP3_.

The leptonic decays of the two W bosons provide the clean-estdecay channel, despite the presence ofneutrinos in the final statethatpreventsthefullreconstructionoftheHiggsbosonmass. Thedifferent-flavor (DF) leptonic decay mode e

μ

has thelargest branchingfraction, isthe leastaffected by backgroundprocesses, andtherefore is the most sensitive channel of the analysis. The same-flavor(SF) e+e− and

μ

+

μ

−final statesarealsoconsidered, although their sensitivity is limited by the contamination from the Drell–Yan (DY) background with missingtransverse momen-tumduetoinstrumentaleffects.

Events with a pair of oppositely charged leptons (electrons and/or muons) and missing transverse momentum, due to the presence of neutrinos in the final state, are selected. This sig-nature is common to other SM processes that contribute to the background in this analysis. The main contribution comes from nonresonant production of W boson pairs (WW), an irreducible backgroundthat sharesthesamefinal stateandcanonlybe sep-aratedfromthe signalusingkinematicdistributions. Backgrounds comingfromtopquarkevents(tt andtW)arealsoimportant, fol-lowedby otherprocesses,suchasW

+

jets andother dibosonand triboson production processes. The DY process is the dominant source of background in the dielectron and dimuon final states, while it is subdominant in the electron-muon final state, since its contribution arisesfrom the leptonic decays ofthe

τ

leptons emergingfromZ

/

γ

∗

→

τ

+

τ

−.

Theeventsare categorizedbyjet multiplicity tobetterhandle thett background.Inaddition,dedicatedcategoriesaredesignedto enhancethesensitivitytotheVBFandVH productionmechanisms. 2. TheCMSdetector

The CMS detector is a multipurpose apparatus designed to studyhightransversemomentum(pT)physicsprocessesin

proton-proton and heavy ion collisions, and is described in detail in Ref. [24] togetherwithadefinitionofthecoordinatesystemused. Asuperconducting solenoidoccupies its central region,providing amagnetic field of 3.8 Tparallel to the beamdirection. Charged particle trajectories are measured by the silicon pixel and strip trackers,whichcoverapseudorapidity regionof

|

η

|

<

2

.

5.Alead tungstatecrystal electromagneticcalorimeter (ECAL), and a brass andscintillator hadroncalorimeter surroundthetracking volume andcover

|

η

|

<

3. The steel and quartz fiber Cherenkov hadron forward calorimeter extends the coverage to

|

η

|

<

5. The muon systemconsistsofgas-ionizationdetectorsembedded inthesteel flux-return yoke outside the solenoid, and covers

|

η

|

<

2

.

4. The first levelof the CMS triggersystem, composed of custom hard-wareprocessors,isdesignedtoselectthemostinterestingevents in less than 4

μ

s, using information from the calorimeters and muondetectors. Thehigh-level trigger processorfarm further re-ducestheeventratetoabout1000 Hz,beforedatastorage.

3. Dataandsimulatedsamples

Theeventsusedinthisanalysisareselectedbyhigh-level trig-geralgorithms that require the presence of one or two high-pT

electrons or muons passing loose identification andisolation re-quirements.Insingle-leptontriggers,relativelytightlepton identi-ficationcriteriaareapplied.The pTthresholdis25 GeV inthe

cen-tralregion(

|

η

|

<

2

.

1)and27 GeV for2

.

1

<

|

η

|

<

2

.

5 forelectrons, whileit is24 GeV formuons(

|

η

|

<

2

.

4).Inthedielectrontrigger, theminimum required pT is 23 GeV forthe leading and12 GeV

forthe subleadingelectron. Inthe dimuon trigger,the minimum

pT is17 GeV fortheleading and8 GeV forthesubleadingmuon.

Inthetwodileptone

μ

triggersusedintheanalysis,theminimum

pT requirementsare either 8 GeV for the muon and 23 GeV for

theelectron,or23 GeV forthemuonand12 GeV fortheelectron. Thecombinationofsingle-leptonanddileptontriggersprovidesan overalltriggerefficiencyinexcessof98%forselectedsignalevents. Severaleventgeneratorsare usedtooptimizetheanalysisand estimate the expected yields of signal and backgrounds, as well as their associated systematic uncertainties. Different Higgs bo-son production mechanisms are simulated. Both ggH and VBF are generated with powheg v2 [25–28], which describesthe full next-to-leadingorder(NLO)perturbativequantum chromodynam-ics(QCD)propertiesoftheseprocesses.Inaddition,the ggH pro-cessisreweighted tomatchthe Higgsboson pT andthenumber

ofassociatedjetstothepredictionof powheg nnlops [29],which provides a next-to-next-to-leading order (NNLO) description for theinclusiveHiggsbosonproduction,NLO fortheexclusiveH

+

1 jetproduction,andleading order(LO) fortheexclusiveH

+

2 jets production. The reweighting is performed by computing the ra-tiooftheHiggsboson pT distributionfromthe nnlops generator

to that from the powheg generator in each jet multiplicity bin, andapplyingthisratiotothe ggH powheg simulation.The minlo hvj [30] extension of powheg is used tosimulate the associated production of the Higgs boson withvector bosons (W+H, W−H, ZH),whichsimulatestheVH

+

0 and1jetprocesseswithNLO ac-curacy. Higgsboson productioninassociation withtop orbottom quarks,such asttH andbbH productionmechanisms,are consid-eredas well,although they only contribute to a minorextent in the phase space selected by this analysis. For the simulation of ttH production the powheg generator is used, while the Mad-Graph5_amc@nlo v2.2.2 generator [31] is used to simulate the bbH production. The Higgs boson is generated with a mass of 125

.

09 GeV and ismade to decayinto a pairof W bosons, con-sidering only leptonic W boson decays (e,

μ

, or

τ

). For Higgs bosons produced via ggH [32] and VBF [33] processes, their de-cayintotwoW bosonsandsubsequentlyintoleptonsissimulated using jhugen v5.2.5 [34,35].Fortheassociatedproduction mecha-nisms,includinggluonfusionproducedZH,theHiggsbosondecay andtheassociatedvectorbosoninclusivedecaysaresimulatedby pythia8.212 [36].Thesimulatedsignalsamplesarenormalized us-ingcrosssections [37] anddecayrates [38] computedby theLHC HiggsCross SectionWorking Group.Inparticularthemostrecent next-to-next-to-next-to-leadingordercalculationsfortheinclusive gluon fusionproductionare used [37].Additional simulated sam-ples, wherethe Higgs bosondecays intoa pair of

τ

leptons, are also produced for each of the aforementioned production mech-anisms. Unlessstated otherwise, the H

→

τ τ

eventspassing the selectionare consideredsignaleventsinthesignalyield determi-nation. However, their expected contribution in the signal phase spaceissmallcomparedtoH

→

W+W−.

The various background processesin thisstudy are simulated as follows: powheg v2 [39] is used for qq

→

WW production, whereasgg

→

WW productionisgeneratedusing mcfm v7.0 [40]. A WW simulation with two additional jets is generated with MadGraph5_amc@nlo at LOaccuracy viadiagramswithsix elec-troweak (EW)vertices, referred toas WW EWproduction.In or-der tosuppressthe topquark backgroundprocesses, theanalysis is performed defining eventcategories with different number of high-pTjets(pT

>

30 GeV).Theclassificationoftheeventsinbins

of jet multiplicity spoils the convergence of fixed-order calcula-tions oftheqq

→

WW processandrequires theuseofdedicated resummation techniques foran accurate prediction ofthe differ-ential distributions [41,42]. The simulated qq

→

WW events are thereforereweightedto reproducethe pWW_T distributionfromthe

pT-resummedcalculation.

TheLOcrosssectionforthegg

→

WW process isobtained di-rectlyfrom mcfm.Forthisprocess,thedifferencebetweenLOand NLOcrosssectionsissignificant;aK factorof1.4iscalculated [43]

andappliedtothegg

→

WW simulation.Giventhetheoretical un-certainties in the K factor, and that it is mildly sensitive to the invariant mass of the WW system(mWW) in the phase space of

interest,anmWW-independentcalculationisused.

Singletop quarkandtt processesare generatedusing powheg v2. The cross sections of the different single top quark pro-cessesareestimatedatNLO accuracy [44],whilethett cross sec-tioniscomputedatNNLO accuracy,with next-to-next-to-leading-logarithmicsoft-gluonresummation [45].

The DY production of Z

/

γ

∗ is generated using MadGraph5_ amc@nlo atNLO accuracyusingtheFxFxjet matchingand merg-ingschemewithamergingscale

μ

Q

=

30 GeV [46],andtheZ

/

γ

∗

pT distributionreweighted to matchthe distribution observedin

dataindimuonevents.

TheW

γ

∗ backgroundwas simulatedwith powheg atNLO ac-curacy,down toa minimuminvariant massofthevirtual photon of100 MeV.Theeffectofthe

γ

∗ masscutoffwasestimatedwith a MadGraph5_amc@nlo W

γ

LOsample,inwhichthephotonpair productionwassimulatedby pythia inthepartonshower approx-imation.The impactfrom eventsinwhich the

γ

∗ massis below 100 MeV was found to be one order of magnitudesmaller than the uncertainties quoted in thisanalysis, thus their contribution wasneglected.

Other multiboson processes, such as WZ, ZZ, and VVV (V

=

W

,

Z),arealsosimulatedwith MadGraph5_amc@nlo atNLO accuracy.

AllprocessesaregeneratedusingtheNNPDF 3.0 [47,48] parton distribution functions(PDFs), withtheaccuracy matching that of thematrixelementcalculations.Alltheeventgeneratorsare inter-facedto pythia fortheshoweringofpartonsandhadronization,as wellasthesimulationoftheunderlyingevent(UE)and multiple-partoninteractionsbasedontheCUET8PM1tune [49].

Toestimate thesystematic uncertainties relatedto the choice of the UE andmultiple-parton interactions tune, the signal pro-cesses andthe WW backgroundare alsogenerated with alterna-tive tunes, which are representative of the uncertainties in the CUET8PM1tuning parameters. The systematicuncertainty associ-atedwithshoweringandhadronizationisestimatedbyinterfacing thesamesampleswiththe herwig++2.7generator [50,51],using the UE-EE-5Ctune for the simulation ofUE and multiple-parton interactions [49].

For all processes, the detector response is simulated using a detailed description of the CMS detector, based on the Geant4 package [52].Additional simulatedminimumbias pp interactions from pythia areoverlappedwiththeeventofinterestineach col-lisiontoreproducethenumberofinteractionsperbunchcrossing (pileup) measured in data. The average number of pileup inter-actions is about27 per eventforthe 2016 dataset used in this analysis.

4. Analysisstrategy

A particle-flow (PF) algorithm [53] is used to reconstructthe observable particlesin theevent. Energy deposits(clusters) mea-suredbythecalorimetersandchargedparticletracksidentifiedin thecentraltrackingsystemandthemuondetectorsarecombined toreconstructindividualparticles.

Among the vertices reconstructed in the event, the one with thelargestvalue ofsummedphysics-object p2

T is takentobethe

primary pp interaction vertex. The physics objects include those returnedby a jet-findingalgorithm [54,55] applied toall charged tracksassignedtothevertex,andtheassociatedmissingtransverse momentum,definedasthenegativevectorsumofthepTofthose

objects.

Electrons are reconstructed by matching clusters in the ECAL to tracksinthesilicontracker [56].Inthisanalysis, electron can-didates are required to have

|

η

|

<

2

.

5. Additional requirements are applied to reject electrons originating from photon conver-sionsinthetrackermaterialorjetsmisreconstructedaselectrons. Electronidentificationcriteriarely onobservablessensitive tothe bremsstrahlungalong theelectron trajectory,the geometricaland momentum-energy matchingbetween theelectron trackand the associated energy cluster in the ECAL, as well as ECAL shower shapeobservablesandassociationwiththeprimaryvertex.

Muon candidates are reconstructed in the geometrical accep-tance

|

η

|

<

2

.

4 bycombininginformationfromthesilicontracker andthemuon system.Identificationcriteriabasedonthenumber of measurements in the trackerandin the muon system, the fit qualityofthemuontrack,anditsconsistencywithitsoriginfrom theprimaryvertexareimposedonthemuoncandidatestoreduce themisidentificationrate.

Prompt leptons comingfrom EW interactions are usually iso-lated,whereas misidentifiedleptons andleptonscomingfromjets are often accompanied by charged or neutral particles, and can arisefromasecondaryvertex.Hencechargedleptonsarerequired to satisfy the isolation criterion that the pT sum over charged

PF candidates associated with the primary vertex, exclusive of the lepton itself, and neutral PF particles in a cone of a radius



R

=



(φ)

2

_{+ (}

_η

₎

2

₌

₀

_.

_{4 (0.3),where}

_φ

_{isthe azimuthal}

an-gleinradians,centeredonthemuon(electron)directionisbelow athresholdof15(6)%relativetothemuon(electron)pT.To

miti-gatetheeffectofthepileuponthisisolationvariable,acorrection basedon theaverageenergydensityintheevent [57] isapplied. Additional requirementsonthetransverse(

|

dxy

|

)andlongitudinal (

|

dz

|

) impact parameters with respect to the primary vertex are included. Electrons detected by the ECAL barrel are required to have

|

dz

|

<

0

.

10 cm and

|

dxy

|

<

0

.

05 cm, while electrons in the ECALendcapmustsatisfy

|

dz

|

<

0

.

20 cm and

|

dxy

|

<

0

.

10 cm.For muons, the

|

dz

|

parameter is required to be lessthan 0

.

10 cm, while

|

dxy

|

is required to be less than 0

.

01 cm for muonswith

pT

<

20 GeV andlessthan0

.

02 cm for pT

>

20 GeV.

The jet reconstruction starts with all PF candidates, and re-movesthechargedonesthat arenot associatedwiththeprimary vertex to mitigate the pileup impact. The remaining charged PF candidates andall neutralcandidatesare clusteredbytheanti-kT

algorithm [54] with a distance parameter of 0.4. To reduce fur-thertheresidualpileupcontaminationfromneutralPFcandidates, a correction basedonthe jetarea [57] is applied.Thejet energy is calibrated using both simulation and data following the tech-niquedescribedinRef. [58].Toidentifyjetscomingfromb quarks (b jets), amultivariate(MVA)b taggingalgorithm isused [59].In thisanalysis, thechosen workingpoint correspondsto about80% efficiencyforgenuineb jets,andtoamistaggingrateofabout10% for light-quark orgluon jetsand of 35 to 50% for c jets.A per-jet scalefactor iscomputedandappliedto accountforb tagging efficiencyandmistaggingratedifferencesbetweendataand simu-lation.

Themissingtransversemomentumvector(



pmiss_T ),whose mag-nitudeisdenotedaspmiss_T ,isreconstructedasthenegative vecto-rial sum inthe transverse plane ofall PF particlecandidate mo-menta. Sincethepresence ofpileupinduces adegradation ofthe

pmiss_T measurement,affectingmostlybackgroundswithnogenuine

pmiss_T , such as DY production, another pmiss_T that is constructed fromonlythechargedparticles(trackpmiss

T )isusedineventswith

an SFleptonpair(ee or

μμ

).Tosuppresstheremaining off-peak DY contribution in categories containing events with an SF lep-ton pair, a dedicatedMVA selection basedon a boosteddecision tree algorithm (BDT) is used,combining variables relatedto lep-tonkinematicsand



pmiss

separatelyfor differentjet multiplicitycategories, andthe output discriminator is used to define a phase space enriched in signal eventsandreducedDYbackgroundcontamination.

Eventsare required topass the single-leptonordilepton trig-gers. For each event, this analysis requires at least two high-pT

leptoncandidateswithoppositesign,originatingfromtheprimary vertex, categorized as dielectron, dimuon, or e

μ

pairs. Only jets withpT

>

30 GeV (20 GeV forb jets)and

|

η

|

<

4

.

7 (

|

η

|

<

2

.

4 forb

jets)areconsideredintheanalysis.Jetsareignorediftheyoverlap withanisolatedleptonwithinadistanceof



R

=

0

.

3.Inaddition, thefollowingkinematicselectionisapplied inthee

μ

finalstate: oneelectronandonemuonarerequiredtobereconstructedinthe eventwitha minimumpT of13 GeV fortheelectronand10 GeV

forthe muon, the higher pT threshold forthe electron resulting

from the trigger definition. One of the two leptons should also havea pTgreaterthan25 GeV.InthecaseofSFe+e− and

μ

+

μ

−

finalstates,theleadingleptonisrequiredtohave pT greaterthan

25 GeV whenitisanelectron,or20 GeV whenitisamuon.The subleading electron is required to have pT greater than 13 GeV,

while forthe muon a minimum pT of 10 GeV is required. Both

leptonsarerequiredtobewellidentified,isolated,andprompt. Giventhelarge backgroundcontributionfromtt productionin bothDFandSFfinalstates,eventsarefurthercategorizedbasedon thenumberofjetsintheevent,withthe0-jetcategorydrivingthe sensitivityoftheanalysis. Acategorization ofthe selectedevents isperformed, targetingdifferent productionmechanismsand dif-ferentflavorcompositionsoftheWW decayproducts.

5. Analysiscategories

5.1.Different-flavorggH categories

The categories described in this section target the ggH pro-duction mechanism and select the DF e

μ

final state. The main backgroundprocesses are the nonresonant WW, top quark (both single and pair production), DY to

τ

lepton pairs, and W

+

jets whena jet ismisidentified asa lepton. Smallerbackground con-tributions comefromWZ, ZZ, V

γ

,V

γ

∗,andtriboson production. TheWW backgroundprocesscanbedistinguishedfromthesignal bythedifferentkinematicpropertiesoftheleptonsystem,sinceit isdominatedbytheon-shellW bosonpairsthatdonotarisefrom ascalarresonancedecay.Thetopquarkbackgroundprocess is di-lutedby definingdifferentcategoriesthat dependonthenumber ofjetsintheevent,andreducedbyvetoinganyb-taggedjetwith

pT

>

20 GeV.

TheW

+

jets contribution(alsoreferredtoasnonpromptlepton background), where one jet mimics the signature of an isolated prompt lepton, is an important background process especially in the0- and1-jetggH-taggedDFcategories.Thisbackgroundis re-ducedby takingadvantage ofthecharge symmetryof thesignal, andthe charge asymmetry of the W

+

jets process, in which the productionofW+isfavoredoverW−.Also,thefactthatthe prob-abilitiesforajettomimicanelectronoramuonaredifferent,and thefact thatthe misidentificationrateislarger forlower-pT

lep-tons,areexploited.Followingthesephysicsmotivationsthe0- and 1-jetggH-taggedDFcategoriesarefurthersplitintofourcategories according to the lepton flavor, charge and pT ordering: e+

μ

−,

e−

μ

+,

μ

+e−,and

μ

−e+, where thefirst lepton is the one with thehigherpT.Inaddition,thefourcategoriesaredivided

accord-ingto whetherthesubleadinglepton pT (pT2) isaboveorbelow

20 GeV.Thiseight-foldpartitioningofthe0- and1-jetggH-tagged categoriesprovidesan improvementintermsoftheexpected sig-nificance ofabout15% withrespect tothe inclusive0- and 1-jet categories.

Tosuppressbackground processeswiththree ormoreleptons inthefinalstate,noadditionalidentifiedandisolatedleptonswith

pT

>

10 GeV areallowedintheeventsforthedileptoncategories.

The dilepton invariant mass (m) is required to be higher than

12 GeV,torejectlow-massresonancesandbackgroundthatcomes from events with multiple jets that all arise through the strong interaction (referredto themultijetbackground). Tosuppressthe background arising from DY events decaying to a

τ

lepton pair, whichsubsequently decaystothe e

μ

final state, andtosuppress processeswithoutgenuinemissingtransversemomentum,a min-imum pmiss

T of 20 GeV is required.In the two-lepton categories,

the DY background is further reduced by requiring the dilepton

pT (pT ) to be higher than 30 GeV, as on average e

μ

lepton

pairs fromZ

→

τ

+

τ

− decayshave lower pT than the onesfrom

H

→

WW decays.Theseselectioncriteriaalsoreducecontributions

from H

→

WW

→

τ ντ ν

andH

→

τ

+

τ

−. Finally,to further

sup-presscontributionsfromZ

→

τ

+

τ

−andW

+

jets events,wherethe subleadingleptondoesnotarisefromaW bosondecay,the trans-versemassbuiltwith



pmiss_T andthesubleadinglepton,definedas:

m2,pmissT

T

=



2pT2pmissT

[

1

−

cos

φ (

2

,



pmissT

)

],

(1)

is required to be greater than 30 GeV. Here

φ (

2

,

p



miss T

)

is

the azimuthal angle between the subleading lepton momentum and



pmiss_T .

Althoughtheinvariant massofthe Higgsbosoncannot be re-constructed because of the undetected neutrinos, the expected kinematicpropertiesoftheHiggsbosonproductionanddecaycan be exploited. Thespin-0 natureofthe SM Higgsbosonresults in thepreferential emission ofthetwochargedleptons inthe same hemisphere. Moreover, the invariant mass of the two leptons in thesignal isrelativelysmallwithrespectto theoneexpectedfor a lepton pair arising from other processes, such as nonresonant WW andtopquark production.Onthe otherhand,severalofthe smallerremaining backgroundprocesses, suchasnonprompt lep-tons, DY

→

τ

+

τ

−,andV

γ

populatethesamem phasespaceas

the Higgsboson signal. Thesecan be partially disentangled from thesignalbyreconstructingtheHiggsbosontransversemassas:

mT

=



2p_T pmiss_T

[

1

−

cos

φ (,



pmiss_T

)

],

(2)

where

φ(,



pmiss_T

)

istheazimuthal anglebetweenthe dilepton momentumand



pmiss_T .Theseadditionalbackgroundprocesses pop-ulate different regions of the two-dimensional plane in m and

mT.Ashapeanalysisbasedonatwo-dimensionalbinnedtemplate

fitofm versusmTisperformedtoextracttheHiggsbosonsignal

intheDFggH categories.

Theobservedeventsasafunctionofm andmT areshownin

Figs. 1,2,and3, afterthe templatefit tothe (m, mT)

distribu-tion.The 0- and 1-jetcategoriesare split into pT2

<

20 GeV and

pT2

>

20 GeV subcategories, to show the different purity of the

tworegions.Inthesefiguresthepostfitnumberofeventsisshown, i.e.,eachsignalandbackgroundprocessisnormalizedtotheresult ofa simultaneous fit toall categories, assuming that therelative proportionsforthedifferentHiggsboson productionmechanisms are those predictedby theSM. The events ineach binof one of thetwo variables areobtainedby integratingover theother,and weighted usingthe ratiooffitted signal

(

S

)

tothe sumofsignal andbackground

(

S

+

B

)

.S

/(

S

+

B

)

ratioineachmTbin.Thisratio

isthenusedtoperformaweightedsumofthemdistributionsin

eachmTbin.Asimilarweightingprocedureisappliedwhen

Fig. 1. Postfit numberofweightedevents(Nw)asafunctionofmandmTforDFeventswith0jetsand pT2<20 GeV (upperrow)or pT 2>20 GeV (lowerrow).The

numberofeventsisweightedaccordingtotheS/(S+B)ratioineachbinofoneofthetwovariables,integratingovertheotherone.Thevariousleptonflavorandcharge subcategoriesarealsomergedandweightedaccordingtotheirS/(S+B)value.Thecontributionsofthemainbackgroundprocesses(stackedhistograms)andtheHiggs bosonsignal(superimposedandstackedredhistograms)remainingafterallselectioncriteriaareshown.Thedashedgraybandaccountsforallsystematicuncertaintieson thesignalandbackgroundyieldsafterthefit.

weightingprocedureisusedonlyforvisualizationpurposes,andis notusedforsignalextraction.

The full list ofDF ggH categories andtheir selection require-mentsisshowninTable1.

5.2. Different-flavorVBFcategory

The VBF process is the second largest Higgs boson produc-tion mechanism at the LHC. This mode involves the production of a Higgs boson in association with two jets with large rapid-ity separations. Afterthe common preselection, the VBF analysis requires eventswithexactly two jets with pT

>

30 GeV, a

pseu-dorapidity separation (

|

η

j j

|

) between the two jets larger than 3.5,andaninvariant mass(mj j)greater than400 GeV. The rejec-tionofeventswithmorethantwojetsreducesthett background contribution without affecting the signal efficiency, thus improv-ing thesignal sensitivity.The VBFanalysisis basedon theshape of them distribution, and is split into two signal regions, one

with400

<

mj j

<

700 GeV and the other withmj j

>

700 GeV,to profit from the higher purity of the mj j

>

700 GeV region. The

post-fit signal and background events as functions of m are

shown in Fig. 4, for the two mj j regions separately. The list of event requirements applied in this category is presented in Ta-ble2.

5.3. Different-flavorVH withtwojetscategory

The VH process involves the production of a Higgs boson in association with a W or Z boson. The 2-jet VH-tagged category targets final stateswhereone vector boson (W or Z) decaysinto two resolved jets. This category with hadronically decaying vec-tor bosonsisaffected bylarge backgroundscomparedto the lep-tonicdecays,butprofitsfromahigherbranchingfraction.The2-jet VH-taggedanalysisreversesthepseudorapidityseparation require-ment of the VBF selection (

|

η

|

<

3

.

5) and requires mj j to be between65and105 GeV.Inaddition,thetwoleading jetsare re-quired to be central (

|

η

|

<

2

.

5) to profit from more stringent b jetvetorequirements,giventhat b taggingcanonlybeperformed forcentraljets.Acut on



R

<

2 isappliedtosuppresstt

Fig. 2. Same as previous figure, for DF events with one jet.

Fig. 3. Postfit numberofweightedevents(Nw)asafunctionofmandmTforDFeventswithatleast2jets.ThenumberofeventsisweightedaccordingtotheS/(S+B)

Table 1

Analysiscategorizationandeventrequirementsforthe0-,1-,and2-jetggH-taggedcategoriesintheDFdilepton finalstate.Thephasespacesdefinedbythe0-,1-,and2-jetggH-taggedrequirementscorrespondtotheevents showninFigs.1,2,and3,respectively.

Category Subcategory Requirements

Preselection – m>12 GeV,pT1>25 GeV,pT 2>13(10)GeV fore (μ),

pmiss

T >20 GeV,pT >30 GeV

noadditionalleptonswithpT>10 GeV

electronandmuonwithoppositecharges 0-jet ggH-tagged e+μ− e−μ+ μ+e− μ−e+ ⎫ ⎪ ⎪ ⎬ ⎪ ⎪ ⎭ pT 2>20 GeV mT>60 GeV,m 2,pmiss T T >30 GeV

subleadingleptonpT>20 GeV

nojetswithpT>30 GeV

nob-taggedjetswithpTbetween20and30 GeV

e+μ− e−μ+ μ+e− μ−e+ ⎫ ⎪ ⎪ ⎬ ⎪ ⎪ ⎭ pT 2<20 GeV mT>60 GeV,m 2,pmiss T T >30 GeV

subleadingleptonpT<20 GeV

nojetswithpT>30 GeV

nob-taggedjetswithpTbetween20and30 GeV

1-jet ggH-tagged e+μ− e−μ+ μ+e− μ−e+ ⎫ ⎪ ⎪ ⎬ ⎪ ⎪ ⎭ pT 2>20 GeV mT>60 GeV,m 2,pmiss T T >30 GeV

subleadingleptonpT>20 GeV

exactlyonejetwithpT>30 GeV

nob-taggedjetswithpT>20 GeV

e+μ− e−μ+ μ+e− μ−e+ ⎫ ⎪ ⎪ ⎬ ⎪ ⎪ ⎭ pT 2<20 GeV mT>60 GeV,m 2,pmiss T T >30 GeV

subleadingleptonpT<20 GeV

exactlyonejetwithpT>30 GeV

nob-taggedjetswithpT>20 GeV

2-jet ggH-tagged eμ atleasttwojetswithpT>30 GeV

m2,p

miss T

T >30 GeV andmT>60 GeV

nob-taggedjetswithpT>20 GeV

mj j<65 GeV or105<mj j<400 GeV

Fig. 4. Postfit number of events with VBF topology as a function of m, for 400<mj j<700 GeV (left) and mj j>700 GeV (right).

thatresultsinleptonsbeingpreferentiallyemittedinnearby direc-tions.Thiskinematicpropertyisfurtherenhancedinthiscategory dueto theboost ofthe Higgsboson recoilingagainst the associ-atedvectorboson.

The analysis is based on the shape of the m discriminant

distribution,presentedinFig.5.Thelistofeventrequirements ap-pliedispresentedinTable3.

5.4. Same-flavorggH categories

SimilarlytotheDFggH-taggedanalysisdescribedinSection5.1, an analysistargetingggH in the SF e+e− and

μ

+

μ

− channelsis performed. The main challenge inthisfinal state isthe large DY

backgroundcontribution.Inordertocontrolit,aBDTistrainedto buildadiscriminator,calledDYMVA,toidentifyDYevents.

AcategorizationbasedonthepT ofthesubleadingleptonis

in-troducedtobettercontrolthenonprompt leptonbackground,and a categorization in thenumber ofjetsis usedto control the top quarkbackgrounds.Thefulllistofeventrequirementsisshownin Table4.

Thisisanevent-counting analysis,andtheeventrequirements are chosen to maximize the expectedsignal significance in each category. The DY background estimations in these channels are basedexclusivelyoncontrolsamplesindata,asdescribed in Sec-tion6.

Table 2

Analysiscategorizationandeventrequirementsforthe2-jetVBF-taggedcategory,intheDFdileptonfinalstate.The phasespacesdefinedbythe2-jetVBF-taggedrequirementscorrespondtotheeventsshowninFig.4.

Category Subcategory Requirements

Preselection – m>12 GeV, pT 1>25 GeV, pT 2>13(10)GeV for e(μ)

pmiss

T >20 GeV, p



T >30 GeV

no additional leptons with pT>10 GeV

electron and muon with opposite charges 2-jet VBF-tagged eμlow mj j exactly two jets with pT>30 GeV

60<mT<125 GeV

leptonsηbetween the two leading jets 400<mj j<700 GeV and|ηj j| >3.5

no b-tagged jets with pT>20 GeV

eμhigh mj j exactly two jets with pT>30 GeV

60<mT<125 GeV

leptonsηbetween the two leading jets

mj j>700 GeV and|ηj j| >3.5

no b-tagged jets with pT>20 GeV

Table 3

Analysiscategorizationandeventrequirementsforthe2-jetVH-taggedcategory,intheDFdileptonfinalstate.The phasespacedefinedbythe2-jetVH-taggedrequirementscorrespondstotheeventsshowninFig.5.

Category Subcategory Requirements

Preselection – m>12 GeV, pT 1>25 GeV, pT 2>13(10)GeV for e (μ)

pmiss

T >20 GeV, pT >30 GeV

no additional leptons with pT>10 GeV

electron and muon with opposite charges 2-jet VH-tagged eμ at least two jets with pT>30 GeV

two leading jets with|η| <2.5 60<mT<125 GeV andR<2

no b-tagged jets with pT>20 GeV

65<mj j<105 GeV and|ηj j| <3.5

Fig. 5. Postfit numberofeventsasafunctionofmfor DFeventsinthe2-jets

VH-taggedcategory.

5.5.AssociatedWH productionwiththreeleptonsinthefinalstate

The three-lepton WH-tagged analysis selectsevents that have theleadingleptonwith pT1

>

25 GeV,thesubleadinglepton with

pT2

>

20 GeV, andthe trailing lepton with pT3

>

15 GeV. Events

withafourthleptonwithpT

>

10 GeV arediscarded.Avetois

ap-plied to events withSF lepton pairs of opposite charge that are compatiblewithcomingfromthedecayofaZ boson.Events con-tainingjetswith pT

>

30 GeV orb-taggedjetswith pT

>

20 GeV

arealsovetoed,tosuppressthett background.Theazimuthal

an-glebetween



pmiss_T andthethree-leptonsystempT,

φ(,



pmissT

)

,

is used to reduce the contamination of nonprompt lepton back-grounds. The rest of the three-lepton WH-tagged selection is in commonwith theother categories. These requirementsare sum-marizedinTable5.

The eventsare further divided into two categories:same-sign SF (SSSF) lepton pairs,

μ

±

μ

±e∓

/

e±e±

μ

∓, and opposite-sign SF (OSSF) lepton pairs,

μ

∓

μ

±e∓

/

e∓e±

μ

∓. The two selections have different signal-over-background ratios, with the SSSF being the purestofthetwo.Themainbackgroundcontributioninbothcases is the contamination from nonprompt leptons. In the OSSF cate-gory,eventsarerequiredtohave pmiss_T

>

50 GeV toreduce theDY background.

Theanalysisisbasedontheminimum



R betweenoppositely charged leptons. The distribution of this variable is presented in Fig.6,separatelyfortheSSSFandOSSFcategories.

5.6. AssociatedZH productionwithfourleptonsinthefinalstate

TheZH finalstateistargetedbyrequiringexactlyfourisolated leptonswithtightidentificationcriteriaandzerototalcharge,and large pmiss

T fromtheundetectedneutrinos.Themajor background

processesareZZ andttZ production.

Among the four leptons, the pair ofSF leptons withopposite charge,andwiththeinvariantmassclosesttotheZ bosonmass,is chosen astheZ boson candidate.Theremaining dileptonsystem, denotedasX,canbeeitherSForDF.Eventsarethereforedivided intotwocategories,distinguishingbetweenthecasesinwhichthe X candidatecontainstwoDFleptons(XDF)ortwoSFleptons(XSF), asshowninTable6.

Thesignalfractionisequallydistributedinthetworegions. In the XSF region, ZZ, DY, and ttZ production are the major back-groundsources,whileinthe XDFregion, ttZ andZZ backgrounds

Table 4

Analysiscategorizationandselectionsforthe0- and1-jetggH-taggedcategoriesintheSFdileptonfinalstate. Category Subcategory Requirements

Preselection – m>12 GeV, pT 1>25(20)GeV for e (μ), pT 2>13(10)GeV for e (μ),

track pmiss

T >20 GeV, pT >30 GeV

no additional leptons with pT>10 GeV

two electrons or two muons with opposite charges 0-jet ggH-tagged e+e− pT 2<20 GeV

μ+μ− pT 2<20 GeV

DYMVA>0.991, m<55 GeV, mT>50 GeV,

pT 2<20 GeV,φ<1.7

no jets with pT>30 GeV

no b-tagged jets with pT>20 GeV

e+e− pT 2>20 GeV

μ+μ− pT 2>20 GeV

DYMVA>0.991, m<55 GeV, mT>50 GeV,

20 GeV<pT 2<50 GeV,φ<1.7

no jets with pT>30 GeV

no b-tagged jets with pT>20 GeV

1-jet ggH-tagged e+e−

μ+μ−

DYMVA>0.95, m<57 GeV, 50<mT<155 GeV,

pT 1<50 GeV,φ<1.75

exactly one jet with pT>30 GeV

no b-tagged jets with pT>20 GeV

Table 5

AnalysiscategorizationandeventrequirementsfortheWH-taggedcategory,inthethree-leptonfinalstate.Here, min–m+− isthe minimumm betweenthe oppositelychargedleptons.Forthe Z boson veto,the

opposite-signsame-flavorpairwiththemclosesttotheZ bosonmassisconsidered.Eventsthatfulfillthethree-lepton

WH-taggedrequirementscorrespondtothesignalphasespaceshowninFig.6. Category Subcategory Requirements

Preselection – pT 1>25 GeV, pT 2>20 GeV, pT 3>15 GeV

no additional leptons with pT>10 GeV

min–m+−>12 GeV, total lepton charge sum±1 3-lepton WH-tagged OSSF no jets with pT>30 GeV

no b-tagged jets with pT>20 GeV

pmiss

T >50 GeV, min–m+−<100 GeV

Z boson veto:|m−mZ| >25 GeV

φ(,pmiss T ) >2.2

SSSF no jets with pT>30 GeV

no b-tagged jets with pT>20 GeV

φ(,pmiss T ) >2.5

Fig. 6. PostfitRdistribution for events in the three-lepton WH-tagged category, split into the OSSF (left) and SSSF (right) subcategories.

aredominant. Backgroundswithtwo Z bosonsfallpredominantly intotheXSF region,andentertheXDFselectiononlythroughthe leptonicdecaysofthe

τ

leptons.ThismakestheXDFregionmuch cleanerthantheXSFone.

Giventhelow expectedsignal yieldsintheXDFandXSF cate-gories,the resultinthiscaseisextractedfromevent-countingin eachcategory.

6. Backgroundestimation

6.1. Nonpromptleptonbackground

Events inwhich a single W boson is produced in association withjetsmaypopulatethesignal regionwhenajet is misidenti-fiedasa lepton.Theseeventscontainagenuine leptonand pmiss

Table 6

AnalysiscategorizationandeventrequirementsfortheZH-taggedcategory,inthefour-leptonfinalstate.Here,X isdefinedastheremainingleptonpairaftertheZ bosoncandidateischosen.ThecomponentleptonsofX canbe eithersame-flavor(XSF)ordifferent-flavor(XDF).

Category Subcategory Requirements

Preselection – four tight and isolated leptons, with zero total charge

pT>25 GeV for the leading lepton

pT>15 GeV for the second leading lepton

pT>10 GeV for the remaining two leptons

no additional leptons with pT>10 GeV

Z dilepton mass>4 GeV X dilepton mass>4 GeV no b-tagged jets with pT>20 GeV

4-lepton ZH-tagged XSF |m−mZ| <15 GeV

10<mX<50 GeV

35<pmiss T <100 GeV

four-lepton invariant mass>140 GeV XDF |m−mZ| <15 GeV

10<mX<70 GeV

pmiss T >20 GeV

from the W boson decay as well as a second nonprompt lep-ton from a misidentified jet, likely arising from a B hadron de-cay. Asimilar backgroundarises fromsemileptonicdecaysof top quark pairs,especially inthe1- and 2-jetscategories. At a lower rate,multijetproductionandfullyhadronictop quarkpairdecays alsocontribute. These backgrounds are particularlyimportantfor eventswithlow-pT leptonsandlowm,andhenceinthesignal

regionoftheanalysis.

Thenonprompt lepton backgroundis suppressedby the iden-tification and isolation requirements imposed on the electrons andmuons,whiletheremainingcontributionisestimateddirectly fromdata.Acontrolsample isdefinedusingeventsinwhichone leptonpassesthe standardlepton identificationandisolation cri-teria andanother lepton candidate failsthesecriteria butpasses alooserselection,resultinginasampleof“pass-fail”leptonpairs. Thepass-fail sampleis dominatedbynonprompt leptons.The ef-ficiency(

misID)forajetthatsatisfiesthislooserselectiontopass

the standard selection is estimated directly from data in an in-dependent sample dominated by eventswith nonprompt leptons frommultijetprocesses.Thecontaminationofpromptleptonsfrom electroweakprocessesinsuchasampleisremovedusingthe sim-ulation. The uncertainty from this subtraction is propagated to

misID.The efficiency misID isparameterized asafunction ofthe

pT and

η

ofthe leptons,andisused toweightthe eventsinthe

pass-fail sample by

misID

/(

1

−

misID

)

, to obtain the estimated

contributionfrom thisbackgroundin the signal region.The con-taminationofpromptleptonsinthe“pass-fail”sampleiscorrected forusingtheirprobabilitytopassthestandardselectiongiventhat they pass the looser selection, as measured in a Drell–Yan data controlsample.Thesystematicuncertaintyassociatedwiththe de-terminationof misIDisdominantandarisesfromthedependence

of

misID on the composition ofthe jetthat is misidentified asa

lepton.Itsimpactisestimatedintwoindependentways,whichare combinedto yield a conservative result. First, a closuretest per-formed on simulated W

+

jets events with

misID estimated from

simulatedQCD multijet eventsprovides an overall normalization uncertainty.Second,ashapeuncertaintyisderivedbyvaryingthe jet pT thresholdinthe differentialmeasurementof misID inbins

ofthe

η

andpTofthelepton.Thethresholdisvariedbyaquantity

thatreflectsthedifferenceinthefakeleptonpTspectrumbetween

W

+

jets and tt events. The total uncertainty in

misID, including

thestatistical precision ofthe control sample, is about40%. This uncertaintyfullycoversanydata/simulationdifferencesincontrol regionsinwhichtwosame-signleptonsarerequested.

Table 7

Data-to-simulation scalefactors for the top quark backgroundnormalization in sevendifferentcontrolregions.

Final state Category Scale factor DF 0-jet ggH-tagged 0.94±0.05 1-jet ggH-tagged 0.94±0.03 2-jet ggH-tagged 0.98±0.02 2-jet VH-tagged 0.98±0.03 2-jet VBF-tagged 1.01±0.04 SF 0-jet ggH-tagged 1.03±0.06 1-jet ggH-tagged 0.98±0.02

6.2. Topquarkbackground

Backgroundcontamination fromsingle top quark processes,in particulartW associatedproduction,andfromtt production,arises becauseoftheinefficiencyofb jetidentificationandtherelatively large top quark cross sections at 13 TeV. The shapes of the top quark background distributions in the various categories are ob-tained from simulation, taking into account the measured b jet identificationinefficiencies.The normalizationsare obtainedfrom controlregionsenrichedintopquarkevents.Thebackground esti-mation isobtainedseparatelyforthe0-,1- and2-jet ggH-tagged categories,the2-jetVBF- andVH-taggedcategories,andforDFand SFfinalstates.

Thecontrolregionforthe0-jetggH-taggedcategoryisdefined thesamewayasthesignalregion,exceptfortherequirementthat atleastone jetwith20

<

pT

<

30 GeV is identifiedasab jetby

means of the b tagging algorithm. Forthe 1-jet ggH-tagged top quarkenrichedregion,exactlyonejetwithpT

>

30 GeV identified

asab jet isrequired.Inthe2-jettop quarkenriched regions (ei-ther ggH-,VH-, orVBF-tagged), two jetswith pT

>

30 GeV must

bepresentintheeventandatleastonehastobeidentifiedasab jet.Toreduceotherbackgroundsinthetopquarkcontrolregions, the dileptonmassis requiredto be higherthan 50 GeV. The de-rivedscalefactorsareshowninTable7.Thenormalizationofthe top quark background inthe three- and four-lepton categories is takenfromsimulationwithitsNNLOcrosssectionuncertainty.

Thetopquark pT intt eventsisreweighted insimulated

sam-ples inorder to have a better description of the pT distribution

observedindata,asdescribedinpreviousCMSanalyses [60].The differencebetweenapplyingthisreweighting,ornot,istakenasa systematic shape uncertainty. The theoretical uncertainty related to the single top quark and tt cross sections is also taken into account. It is evaluated by varying the ratio between the single

Table 8

Data-to-simulationscalefactorsfortheDY→τ+τ−backgroundnormalizationin theDFcontrolregions.

Final state Category Scale factor DF 0-jet ggH-tagged 0.94±0.06 1-jet ggH-tagged 1.02±0.05 2-jet ggH-tagged 0.99±0.09 2-jet VH-tagged 0.99±0.13 2-jet VBF-tagged 1.04±0.16

top quark and tt crosssection by its uncertainty, which is 8% at 13 TeV [18].A1% theoreticaluncertaintyarising fromPDF uncer-tainties and QCD scale variations affects the uncertainty on the signal region to control region ratio. All the experimental uncer-tainties described in Section 7 are also included asuncertainties onthetopquarkbackgroundshape.

6.3. Drell–Yanbackground

TheDY

→

τ

+

τ

− backgroundisrelevantforDFcategoriesand, likethesignal,populatesthelow-mTandlow-mphasespace.The

kinematicvariablesofthisbackgroundarepredictedbythe simu-lation after reweighting the Z boson pT spectrum to match the

distributionmeasuredinthedata.Thenormalizationisestimated in data control regions by selecting events with mT

<

60 GeV

and30

<

m

<

80 GeV.Normalizationscale factorsareextracted,

separately for the 0-, 1-, 2-jet ggH-tagged, the 2-jet VBF- and VH-taggedcategories,andareshowninTable8.

Theeffectofmissinghigher-ordercorrectionsintheDY simula-tionisestimatedby varyingtherenormalizationandfactorization scales by a factor oftwo up and down. Thiseffect is treated as a shape uncertainty andamounts to 1–2% inthe DY yield.A 2% theoretical uncertainty arising from PDF uncertainties and scale variations affects the uncertainty on the signal region to control regionratio.Allexperimentaluncertainties describedinSection 7 areconsideredasshapeuncertaintiesforthisbackgroundprocess.

IntheSFcategories,adominantsourceofbackgroundisDY

→

e+e− and DY

→

μ

+

μ

−. The contribution of the DY background outside the Z boson mass region (dubbed the out region, which corresponds to the signal region of the analysis) isestimated by countingthenumberofeventsintheZ bosonmassregionindata (in region),subtracting thenon-Z-bosoncontributionfromit,and scalingtheyieldbyaratioRout/in.Thisratioisdefinedasthe

frac-tionofeventsoutsideandinsidetheZ bosonmassregioninMonte Carlo(MC)simulation,Rout/in

=

NMCout

/

NMCin .

The Z boson massregion is defined as

|

m

−

mZ

|

<

7

.

5 GeV.

Such a tight masswindow is chosen to reduce the non-Z-boson backgroundcontributions,whichcan be splitintotwo categories. Thefirstoneiscomposedofthebackgroundprocesses,suchastop quarkpairandW+W−production,withequaldecayratesintothe fourlepton-flavor finalstates(ee, e

μ

,

μ

e,and

μμ

). Their contri-butions tothe Z bosonmassregion indata, Nbackground_ |in, canbe estimatedfromthenumberofeventsinthee±

μ

∓finalstate,Neinμ,

applyingacorrectionfactorthataccountsforthedifferencesinthe detectionefficiencybetweenelectronsandmuons(keeandkμμ):

Nbackground|in_

=

1

2k

(

N

in

eμ

−

Nineμ

(

VV

)),

(3)

where



standsforee or

μμ

. Nin

eμ

(

VV

)

isthenumberofevents,

estimated from simulation, arising from WZ and ZZ decays and contributingto the e

μ

final state. The factorof 1

/

2 comes from therelativebranchingfractionbetweenthe



ande

μ

finalstates. The second category is composed of background processes, such

Table 9

ScalefactorsforthenonresonantWW backgroundnormalization.

Final state Category Scale factor DF 0-jet ggH-tagged 1.16±0.05 1-jet ggH-tagged 1.05±0.13 2-jet ggH-tagged 0.8±0.4 2-jet VH-tagged 0.6±0.6 2-jet VBF-tagged 0.5±0.5 SF 0-jet ggH-tagged 1.13±0.07 1-jet ggH-tagged 1.03±0.18

as WZ and ZZ (denoted asVV) production,with subsequent de-caymostly intoSFfinal statesviatheon-shellZ boson,whichare determined from simulation. The number of events arising from these backgroundprocesses contributing to thesame flavor final stateisdenotedasNin_

(

VV

)

.

Finally, the number of DY events in the signal region is esti-matedfromthenumberofeventsintheSF finalstate, Nin

,

sepa-ratelyforelectronsandmuonsaccordingtothefollowingformula:

N_Z→out

=

Rout/in



Nin_

−

Nbackground_ |in

−

Nin_

(

VV

)

.

(4)

The differenceofthe Rout/in valuesfromthedataandsimulation

istakenasasystematicuncertainty,andamountsto10–25%.

6.4. TheWZ andW

γ

∗background

The W

γ

∗ EWproductionisincludedinthesimulationaspart oftheWZ production, andthetwo processesareseparatedusing a 4 GeV threshold on the Z

/

γ

∗ mass at the generator level.For thefinalstateswithtwoleptons,theWZ andW

γ

∗ processesmay contribute tothesignalregionwheneveroneofthethreeleptons isnot identified.Therefore,itisimportanttoobservetheprocess indatatovalidatethesimulation.

The yield of the WZ background is measured in data by se-lecting events withthreeisolated leptons, two electrons andone muon (ee

μ

), ortwo muonsandone electron (

μμ

e).The SF lep-ton pairis identified as the Z bosoncandidate, andits invariant mass isrequiredto bewithin the Z bosonmass windowdefined in Section 6.3. This phase space is used to derive a scale factor fortheWZ simulation,whichisfoundtobe1

.

14

±

0

.

18,fromthe weightedaverageofthescalefactorsintheee

μ

and

μμ

e regions withtheirstatisticaluncertainties.

A W

γ

∗-enriched control region is definedby selecting events with twomuons withinvariant massbelow 4 GeV, likely arising froma

γ

∗ decay,andathirdisolatedelectronormuonpassing a tightidentificationrequirement.Thedimuoninvariantmassregion closeto theJ

/ψ

resonancemassis discarded.Thiscontrol region is usedtoderive a scalefactorforthe W

γ

∗ simulation, whichis foundtobe0

.

9

±

0

.

2,withtheuncertaintycomingfromtheevent countsinthe

μμ

e and

μμμ

samples.

All experimental uncertainties described inSection 7 are con-sidered as shape and yield uncertainties for the WZ and W

γ

∗

background determination.Moreoverthe effectsofscale andPDF uncertainties on thenormalization (3% fromscale variations and 4%forPDFvariations)andacceptance(3%)areincluded.

6.5. NonresonantWW andotherbackgrounds

The nonresonant WW background populates the entire two-dimensional phase space in m andmT, while the Higgs boson

signal is concentrated at low m values, and mT values around

theHiggsbosonmass.Theyieldofthisbackgroundishence esti-mateddirectlyfromthefitprocedure,separatelyforeachcategory. ThederivedscalefactorsareshowninTable9.

Intheqq

→

WW process,the pWW

T spectrum insimulationis

reweightedtomatchtheresummedcalculation [41,42].The mod-eling ofthe shape uncertainties related to missinghigher orders isdone intwopieces:thefirstvariesthefactorizationand renor-malizationscales by a factorof two up anddown andtakes the envelope;thesecond independentlyvariestheresummationscale bya factorof two upanddown. The crosssection ofthe gluon-induced WW process is scaled to NLO accuracy and the uncer-taintyonthisK factoris15% [61].Incategorieswithatleasttwo jets,theEWWW productionisalsotakenintoaccount.The theo-reticaluncertaintyintheLOcrosssectionofthisprocessamounts to11%, andisestimatedby varying therenormalizationand fac-torizationscales by a factorof two up and down,including also theeffectofPDFvariations.

TheWZ and Z

γ

∗ backgroundsinthe three-lepton WH-tagged analysisareestimatedusingdedicatedcontrolregionsfromwhich thescale factorsof1

.

09

±

0

.

06 and 1

.

61

±

0

.

18,respectively, are derived.TheZZ backgroundinthefour-lepton ZH-taggedanalysis isalsoestimatedusingacontrolregion fromwhichascalefactor of0

.

96

±

0

.

07 isderived.

Allremainingbackgrounds fromdibosonandtriboson produc-tion are estimated according to their expected theoretical cross sectionsandtheshapeistakenfromsimulation.

7.Statisticalprocedureandsystematicuncertainties

The statistical methodology used to interpret subsets of data selectedfortheH

→

WW analysisandtocombinetheresultsfrom theindependentcategorieshasbeendevelopedbytheATLASand CMSCollaborations inthecontext ofthe LHC HiggsCombination Group.Ageneraldescriptionofthemethodologycanbe foundin Ref. [62].

Thenumberofeventsineach categoryandineachbinofthe discriminantdistributionsusedtoextractthesignalismodeledas aPoissonrandomvariable,withamean valuethat isthesumof thecontributionsfromtheprocessesunderconsideration. System-atic uncertainties are represented by individual nuisance param-eters with log-normal distributions. The uncertainties affect the overall normalizations of the signal and backgrounds, as well as the shapesof the predictions across the distributions ofthe ob-servables.Correlationsbetweensystematicuncertainties in differ-entcategoriesaretakenintoaccount.

Thevariouscontrol regions describedinSection 6are usedto constrainindividualbackgroundsandareincludedinthefitinthe formofsingle bins,representingthenumberofeventsineachof thecontrolregions.

The remaining sources of systematic uncertainties of experi-mentalandtheoretical natureare describedbelow.Effects dueto theexperimentaluncertaintiesareestimatedbyscalingor smear-ing the targeted variable in the simulation andrecalculating the analysisresults.Allexperimentalsourcesofsystematicuncertainty, except for the integrated luminosity, have both a normalization andashapecomponent.The followingexperimental uncertainties aretakenintoaccount:

•

Theuncertaintyinthemeasuredluminosity,whichis2.5% [63].

•

The trigger efficiencyuncertainty associatedwiththe combi-nationofsingle-leptonanddileptontriggers,whichis2% [64].

•

The uncertainties inthe lepton reconstructionand identifica-tionefficiencies,whichvarywithin2–5%forelectrons [56] and 1–2%formuons [65],dependingon pTand

η

.

•

The muon momentum andelectron energyscale and resolu-tionuncertainties,whichamountto0.6–1.0%forelectronsand 0.2%formuons.

•

The jet energy scale uncertainties, which vary in the range 1–13%,dependingonthepT and

η

ofthejet [66].

•

The pmiss_T resolution uncertainty includes the propagation of lepton and jet energy scale and resolution uncertainties to

pmiss_T ,aswellastheuncertaintiesontheenergyscalesof par-ticlesthat are not clusteredintojets, andtheuncertaintyon theamountofenergycomingfrompileupinteractions.

•

The scale factors correcting the b tagging efficiency and

mistaggingrates,which are variedwithin their uncertainties. The associated systematic uncertainty, which varies between 0.5–1.0% [59],affects,in ananticorrelatedway,the topquark controlregionsandthesignalones.

Theuncertaintiesinthesignalandbackgroundproductionrates due to the limited knowledge of the processes under study in-cludeseveralcomponents,whichareassumedtobeindependent: the choicesofPDFsandthe strongcouplingconstant

α

S,theUE andpartonshowermodel,andtheeffectsofmissinghigher-order correctionsviavariations oftherenormalizationandfactorization scales. As most of the backgrounds are estimated from control regions in data, these theoretical uncertainties mostly affect the Higgs boson signal and they are implemented as normalization-onlyuncertaintiesunlessstatedotherwise.

The PDFs and

α

S uncertainties are further split between the cross section normalization uncertainties computed by the LHC HiggsCross Section Working Group [38] for the Higgsboson sig-nalandtheireffectontheacceptance [67].Thesignalcrosssection normalizationuncertaintiesamount to3%fortheggH and2% for the VBF Higgs boson production mechanism, between 1.6% and 1.9%forVH processes,and3.6%forttH production.Theacceptance uncertaintiesarelessthan1%forallproductionmechanisms.

TheeffectofmissinghigherorderQCDcorrectionsontheggH productionmechanismissplitintonineindividualcomponentsas identified inRef. [37], chapterI.4.Each componentispropagated suchthatboththeintegratedeffectandthecorrelationsacross dif-ferentcategoriesareproperlytakenintoaccount.Theoveralleffect ontheggH crosssectionisabout10%.Theeffectofmissing higher-order correctionsinthe VBFandVH simulations is lessthan1%, whileitamountstoabout8%forthettH simulation.

TheUEuncertaintyisestimatedbyvaryingtheCUET8PM1tune ina rangecorresponding totheenvelopeof thesingle tuned pa-rameters post-fit uncertainty, as described in Section 3. The de-pendence onthepartonshower(PS)modelis estimatedby com-paringsamplesprocessedwithdifferentprograms,asdescribedin Section 3.Theeffectonthe expectedggH signalyieldsafter pre-selection is about5% forthe UE tuning andabout7% forthe PS description, andis partially accountedfor by the lepton identifi-cationscale factorsanduncertainties.The remaining contribution ismigrationbetweenjet categoriesandisanticorrelatedbetween the0-jetcategoryandthecategorieswithjets.Sucheffectsareof the orderof15-25% fortheparton shower(VBF categoriesbeing themostaffected)and5-17%forUE(2-jetVH-taggedcategory be-ing themostaffected). Theanticorrelationbetweenjet categories reducestheimpactoftheseuncertaintiesonthefinalresults.

Finally, the uncertainties arising from the limited number of events in the simulated samples are included independently for eachbinofthediscriminantdistributionsineachcategory. 8. Results

Thesignal strengthmodifier (

μ

), definedastheratiobetween the measuredsignal cross section andtheSM expectationin the H

→

WW

→

2



2

ν

decay channel, is measured by performing a binned maximumlikelihoodfitusingsimulatedbinned templates forsignalandbackgroundprocesses.

The combined results obtained using all the individual anal-ysis categories are described in this section. A summary of the expected fraction of different signal production modes in each category is shown in Fig. 7, together with the total number of expected H

→

WW events. The chosen categorization proves ef-fectiveintacklingthedifferentproductionmechanisms,especially ggH,VBF,andVH.ThemeasurementsassumeaHiggsbosonmass ofmH

=

125

.

09 GeV,asreportedintheATLASandCMScombined

Higgsboson massmeasurement [14]. The resultsreported below show avery weakdependenceon theHiggsbosonmass hypoth-esis, with the expectedsignal yield varying within 1% when the signalmasshypothesisisvariedwithinitsmeasureduncertainty.

Thenumberofexpectedsignalandbackgroundevents,andthe numberofobservedeventsindata,ineach categoryafterthefull eventselectionareshowninTables10and11.

Postfitevent yields are alsoshownin parentheses, and corre-spondtotheresultofasimultaneousfittoallcategories,assuming that the relative proportions of the different production mecha-nismsarethosepredictedbytheSM.

Fig. 7. Expected relativefractionofdifferentHiggsbosonproductionmechanismsin eachcategoryincludedinthecombination,togetherwiththeexpectedsignalyield.

8.1. Signalstrengthmodifiers

Thesignalstrengthmodifierisextractedbyperforminga simul-taneousfittoallcategoriesassumingthattherelativeproportions of the different productionmechanisms are the same asthe SM ones. As such, the value of

μ

provides an insight intothe com-patibility between this measurement andthe SM. The combined observedsignalstrengthmodifieris:

μ

=

1

.

28₋+0₀._.18₁₇

=

1

.

28

±

0

.

10 (stat)

±

0

.

11 (syst)+₋₀0_..10₀₇(theo)

,

(5) where thestatistical, systematic,andtheoretical uncertaintiesare reportedseparately.Thestatisticalcomponentisestimatedby fix-ingallthenuisanceparameterstotheirbestfitvaluesand recom-puting the likelihoodprofile. The breakdownof a givengroup of uncertainties (systematic or theoretical) is obtained by fixing all thenuisanceparameters inthegrouptotheir bestfitvalues,and recomputingthelikelihoodprofile.The correspondinguncertainty is then taken as the difference in quadrature between the total uncertainty and the one obtained fixing the group of nuisance parameters.Theexpectedandobservedlikelihoodprofilesas func-tions ofthesignalstrengthmodifierareshowninFig.8,withthe 68% and95% confidencelevel(CL)indicated.The observed signif-icance in the asymptoticapproximation [68] of the Higgs boson production forthecombinationof allcategories is9

.

1 s.d.,to be compared withthe expectedvalueof 7

.

1 s.d. Assuch, thisisthe firstobservationoftheHiggsbosondecaytoW bosonpairswith theCMSexperiment.

A breakdown of the impact on

μ

of the different systematic uncertainties is showninTable 12.The contributions ofthe nor-malizationsthatareleftfloatinginthefitenterthestatisticalerror on

μ

.

Inordertoassessthecompatibilityoftheobservedsignalwith theSMpredictionsineachcategoryoftheanalysisandtoascertain the compatibility between the different categories, a simultane-ous fit in which the signal strength modifier is allowed to float independently in each category is performed. The observed sig-nal strength modifier foreach category usedin the combination is reported in Fig. 9 (left). Results are generally consistent with unity,withthelargestdeviationshowingupinthe2-jetVH-tagged Table 10

Numberofexpectedsignalandbackgroundeventsandnumberofobservedeventsinthe0- and1-jetcategoriesafterthefulleventselection.Postfiteventyieldsarealso showninparentheses,correspondingtotheresultofasimultaneousfittoallcategoriesassumingthattherelativeproportionsforthedifferentproductionmechanismsare thosepredictedbytheSM.Theindividualsignalyieldsaregivenfordifferentproductionmechanisms.Thetotaluncertaintyaccountsforallsourcesofuncertaintyinsignal andbackgroundyieldsafterthefit.

0-jet DF ggH-tagged 1-jet DF ggH-tagged 0-jet SF ggH-tagged 1-jet SF ggH-tagged ggH 483.1 (642.1) 269.1 (339.3) 231.2 (324.6) 82.0 (92.8) VBF 5.6 (7.4) 22.1 (29.4) 1.5 (2.5) 5.9 (9.3) WH 12.4 (16.4) 15.8 (20.6) 3.3 (4.3) 2.9 (3.8) ZH 5.2 (6.9) 5.0 (6.7) 2.6 (3.4) 1.4 (1.8) ttH <0.1 (<0.1) 0.2 (0.2) <0.1 (<0.1) <0.1 (<0.1) bbH 3.4 (4.4) 1.5 (2.0) 1.7 (2.3) 0.5 (0.7) Signal 509 (677) 313 (398) 240 (337) 93 (108) ±total unc. (±31) (±19) (±24) (±13) WW 7851 (9088) 3553 (3727) 1596 (1805) 373 (365) Top quark 2505 (2422) 5395 (5224) 334 (339) 452 (443) Nonprompt 1555 (1006) 781 (482) 301 (260) 111 (97) DY 154 (154) 283 (302) 437 (459) 178 (216) VZ/Vγ∗ 368 (385) 327 (338) 101 (104) 43 (43) Vγ 213 (210) 137 (128) 23 (26) 17 (19) Other diboson 5.1 (5.3) 3.5 (3.7) 9.3 (9.4) 2.0 (2.1) Triboson 9.3 (9.6) 16 (17) 1.2 (1.2) 1.3 (1.3) Background 12660 (13280) 10496 (10222) 2803 (3004) 1177 (1186) ±total unc. (±141) (±178) (±97) (±83) Data 13964 10591 3364 1308