Measurement of the production cross section for a Higgs boson in association with a vector boson in the → → Channel in Collisions at TeV with the ATLAS detector

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Measurement of the production cross section for a Higgs boson in association with a vector boson in the → → Channel in Collisions at

TeV with the ATLAS detector

ATLAS Collaboration

ADORNI BRACCESI CHIASSI, Sofia (Collab.), et al .

Abstract

A measurement of the Higgs boson production cross sections via associated WH and ZH production using H→WW⁎→ℓνℓν decays, where ℓ stands for either an electron or a muon, is presented. Results for combined WH and ZH production are also presented. The analysis uses events produced in proton–proton collisions collected with the ATLAS detector at the Large Hadron Collider in 2015 and 2016. The data correspond to an integrated luminosity of 36.1fb−1 recorded at a centre-of-mass energy of 13 TeV. The products of the H→WW⁎

branching fraction times the WH and ZH cross sections are measured to be 0.67−0.27+0.31(stat.)−0.14+0.18(syst.) pb and 0.54−0.24+0.31(stat.)−0.07+0.15(syst.) pb respectively, in agreement with the Standard Model predictions.

ATLAS Collaboration, ADORNI BRACCESI CHIASSI, Sofia (Collab.), et al . Measurement of the production cross section for a Higgs boson in association with a vector boson in the → →

Channel in Collisions at TeV with the ATLAS detector. Physics Letters. B , 2019, vol. 798, p.

134949

DOI : 10.1016/j.physletb.2019.134949

Available at:

http://archive-ouverte.unige.ch/unige:128799

Disclaimer: layout of this document may differ from the published version.

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Contents lists available atScienceDirect

Physics Letters B

www.elsevier.com/locate/physletb

Measurement of the production cross section for a Higgs boson in association with a vector boson in the H → ^{W W}

^∗

→ ν ν channel in pp collisions at √

s = ^{13 TeV} ^with ^the ^ATLAS ^detector

.ATLASCollaboration

a r t i c l e i n f o a b s t ra c t

Articlehistory:

Received26March2019

Receivedinrevisedform28August2019 Accepted16September2019

Availableonline23September2019 Editor:M.Doser

A measurementoftheHiggsboson productioncrosssections viaassociated W H and Z H production usingH→^{W W}^∗→ννdecays,wherestandsforeitheranelectronoramuon,ispresented.Results forcombinedW Hand Z H productionarealsopresented.Theanalysisuseseventsproducedinproton–

protoncollisionscollectedwiththeATLASdetectorattheLargeHadronColliderin2015and2016.The datacorrespondtoanintegratedluminosityof36.1 fb⁻¹recordedatacentre-of-massenergyof13 TeV.

TheproductsoftheH→^{W W}^∗^branching^fraction^times^the^{W H} ^and ^{Z H} ^cross^sections^are^measured tobe0.67⁺₋⁰₀^._.³¹₂₇(stat.)⁺₋⁰₀^._.¹⁸₁₄(syst.) pband0.54⁺₋⁰₀^._.³¹₂₄(stat.)⁺₋⁰₀^._.¹⁵₀₇(syst.) pbrespectively,inagreementwiththe StandardModelpredictions.

©²⁰¹⁹^TheÂuthor(s).^Published^byÊlsevier^B.V.^Thisîsânôpenâccessârticleûnder^the^CC^BY^license (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP³.

1. Introduction

Higgsboson production inassociation with a W or Z boson, whichisrespectivelydenotedby W H andZ H,andcollectivelyre- ferred toas V H associated productionin thefollowing, provides directaccesstotheHiggsbosoncouplingstoweakbosons.Inpar- ticular,intheW H mode withsubsequent H→W W^∗ decay,the HiggsbosoncouplesonlytoWbosons,atboththeproductionand decayvertices.

Thispaperpresentsa measurementofthe correspondingpro- duction cross sections through the decay H→^{W W}^∗→νν, using proton–proton collisions at a centre-of-mass energy of

√s= ^{13 TeV.} ^The ^data ^correspond ^to ^an ^integrated ^luminosity of 36.1 fb⁻¹ and were recorded by the ATLAS detector at the LargeHadronCollider(LHC).Previousmeasurementsat√

s=^{8 TeV} wereperformedbytheATLAS [1] andCMS [2] Collaborations and recentlyat√

s=13 TeV with 35.9 fb⁻¹ ofdataby the CMSCol- laboration [3].Recentresultsat√

s=^{13 TeV on} ^{V H}^productionⁱⁿ otherdecaymodescanbefoundinRefs. [4–9].

Theanalysisisperformedusingeventswiththree(3) orfour (4)chargedleptons(electronsormuons)intheﬁnalstate,target- ing the W H and Z H channels respectively. Leptonicdecaysof τ

leptons,fromH→^{W W}^∗→τ ντ νorH→^{W W}^∗→τ ννorfrom the associated vector bosons, are considered as signal, while no speciﬁcselectionisperformedforeventswithhadronicallydecay- ing τ leptonsin theﬁnalstate. Eventsfrom V H productionwith

E-mailaddress:atlas.publications@cern.ch.

H→τ τ areconsideredasbackground.Theleading-orderFeynman diagramsforthe W H and Z H productionprocessesare depicted inFig.1.

In the W H channel, multivariate discriminants are used to maximise the sensitivity to the Higgsboson signal, while inthe Z H channel theanalysis is performedthrough selection requirements.ThedistributionoftheseW H discriminants,togetherwith eventcountsinbackgroundcontrolregionsandthesignalregions inthe Z Hchannel,arecombinedinabinnedmaximum-likelihood ﬁtto extract thesignal yield andthebackgroundnormalisations.

Themaximum-likelihoodﬁtprovides resultsfortheW H andthe Z H channelsseparately andfortheir combination V H,assuming theStandardModel(SM)predictionfortherelativecrosssections ofthetwoproductionprocesses.

2. ATLASdetector

TheATLAS experiment [10–12] isamulti-purposeparticlede- tector with a forward–backward symmetric cylindrical geometry and a near 4π coverage in solid angle.¹ It consists of an in-

1 ATLASuses aright-handedcoordinatesystemwith itsoriginat thenominal interactionpoint(IP)inthecentreofthedetectorandthez-axisalongthebeam pipe.Thex-axispointsfromtheIPtothecentreoftheLHCring,andthe y-axis pointsupwards.Cylindricalcoordinates(r,φ)areusedinthetransverseplane,φ beingtheazimuthalanglearoundthez-axis.Thepseudorapidityisdeﬁnedinterms ofthepolarangleθ asη_{= −}ln tan(θ/2).Angulardistance ismeasuredinunits ofR≡

(η)²+(φ)².TransversemomentumandenergyaredeﬁnedaspT= psinθandET=Esinθrespectively.

https://doi.org/10.1016/j.physletb.2019.134949

0370-2693/©²⁰¹⁹^TheÂuthor(s).^Published^byÊlsevier^B.V.^Thisîsânôpenâccessârticleûnder^the^CC^BY^license(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP³.

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Fig. 1.Tree-level Feynman diagrams for theV H(H→^{W W}^∗)topologies considered in this paper: (a) 3channel and (b) 4channel.

Table 1

MonteCarlogeneratorsusedtomodelthesignalandbackgroundprocesses.Alternativegenerators, underlyingeventandparton-showeringmodels,usedtoestimatesystematicuncertainties,areshown inparentheses.Inthelastcolumnthepredictionorderforthetotalcrosssectionisshown.“Pythia6”

referstoversion6.428,“Pythia8”referstoversions8.210or8.186.

Process Generator (alternative)

UEPSmodel (alternative)

Predictionorder fortotalcrosssection qq¯→W H Powheg-Boxv2 MiNLO Pythia8 NNLO QCD + NLO EW [16–18]

(Herwig7)

qq¯→Z H Powheg-Boxv2 MiNLO Pythia8 NNLO QCD + NLO EW [16–18]

(Herwig7)

gg→^{Z H} ^Powheg-Box^v2 ^P^ythia⁸ NLO + NLL [19]

(Herwig7)

ggFH Powheg-Boxv2 NNLOPS Pythia8 NNNLO QCD + NLO EW [20]

VBFH Powheg-Boxv2 Pythia8 NNLO QCD + NLO EW [21]

t¯t Powheg-Boxv2 Pythia8 NNLO+NNLL [22]

(Herwig7) (Sherpa2.2.1) (Sherpa2.2.1)

W t Powheg-Boxv1 Pythia6 NLO [23]

t¯t W/Z MG5_aMC@LO Pythia8 NLO [24,25]

t Z MG5_aMC@LO Pythia6 LO [26]

qq¯/qg→ν ^Sherpa2.2.2 Sherpa2.2.2 NLO [27]

(Powheg-Boxv2) (Herwig++)

q^q¯/qg→ ^Sherpa2.1/2.2.2 Sherpa2.1/2.2.2 NLO [27]

(Powheg-Boxv2) (Herwig++)

gg→ ^Sherpa2.1.1 Sherpa2.1.1 NLO [28]

V V V Sherpa2.2.2 Sherpa2.2.2 NLO [29]

(MG5_aMC@NLO) (Pythia8)

ner tracking detector (ID) surrounded by a thin superconducting solenoidprovidinga2 Taxialmagneticﬁeld,electromagnetic(EM) andhadroniccalorimeters,andamuonspectrometer(MS).Thein- nertrackingdetectorcoversthepseudorapidityrange|η_|<2.5.It consistsofsiliconpixel,siliconmicro-strip,andtransition-radiation tracking detectors. Lead/liquid-argon (LAr) sampling calorimeters provideelectromagneticenergymeasurementswithhighgranular- ity.Ahadronic(steel/scintillator-tile)calorimetercoversthecentral pseudorapidityrange(|η_|<1.7). Theendcapandforwardregions areinstrumentedwithLArcalorimetersforbothEMandhadronic energy measurements up to |η_|₌4.9. The muon spectrometer surrounds the calorimeters and is based on three large air-core toroidal superconductingmagnet systemsthat provide a ﬁeld in- tegralbetween 2.0and 6.0 T m across most of thedetector. The muonspectrometerincludesasystemofprecision trackingcham- berscoveringtheregion|η_|<2.7 andfastdetectorsfortriggering within the range|η_|<2.4. Atwo-level trigger systemis usedto selectevents [13].

3. SignalandbackgroundMonteCarlosimulation

Monte Carlo (MC) event generators are used to model signal andbackgroundprocesses.Allsignalsamplesweregeneratedwith aHiggsboson massof125 GeV [14,15].Formostprocesses,sep-

arate programs were used to generate the hard scattering process andto model the underlying eventand the partonshower- ing (UEPS). A description ofthe MC samples isgiven inTable 1.

They are normalised to cross-section predictions calculated with theQCDandelectroweak(EW)ordersspeciﬁedinthelastcolumn ofTable1.

The qq¯→^{W H} ^and^qq¯→^{Z H} ^processes ^were ^generated ^with Powheg-Box v2 [30] MiNLO interfacedto Pythia8 [31], withthe AZNLO set oftuned parameters (tune) [32] forunderlying event, showering and hadronisation. The gg→^{Z H} ^process ^was ^simulated with Powheg-Box v2 + Pythia8 with the AZNLO tune for underlyingevent, showeringandhadronisation.Forthe V H samples, the PDF4LHC15 parton distribution function (PDF) set [33]

was usedfor thehard scatteringprocess in Powheg-Boxv2 and the CTEQ6L1 PDF set [34] wasused forthe parton showeringin Pythia8.Herwig7 [35],withthe MMHT2014lo68clPDF set [36], was used asan alternativeparton-showering model for V H. The uncertaintyduetothePDFchoiceissmallerthantheuncertainty obtainedbyusingHerwig asan alternativepartonshowermodel (Section7).

The gluon–gluon fusion (ggF) events were generated with Powheg-Box v2 NNLOPS [37] interfaced to Pythia8 with the AZNLO tune. The vector-boson fusion (VBF) events were gener- atedwithPowheg-Boxv2,interfacedtoPythia8.FortheggFand

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VBFsamples,thePDF4LHC15 PDFsetwasusedforthehardscat- teringprocess in Powheg-Boxv2 andthe CTEQ6L1 PDF set was usedforthe partonshoweringinPythia8. Thecontribution from thet¯t H andt H productionmodesisnegligible.

Thetop-quarkpairproduction(tt)¯ wassimulatedwithPowheg- Box v2 [38] using the NNPDF 3.0 NNLO PDF set [39] and interfaced to Pythia8 using the NNPDF 2.3 PDF set [40] for parton showering,withtheA14tune [41].Fortt¯production,Sherpa[42]

2.2.1, with the NNPDF 3.0 PDF set, was used as an alternative generator while Herwig 7, with the MMHT2014lo68cl PDF set, was used as an alternative UEPS model. The single-top-quark production W t was generated with Powheg-Box v1 [23] inter- facedtoPythia6 [43] forpartonshoweringwiththePerugia2012 tune [44].EvtGen1.2.0 [45] wasusedforthesimulationofb-quark and c-quark decays. The tt W¯ /Z and t Z processes were gener- atedat leading order (LO) withMG5_aMC@LO[25] version2.2.2 (t¯t W/Z)and2.2.1(t Z)interfacedtoPythia8(tt W¯ /Z)andPythia6 (t Z),usingtheNNPDF2.3LOPDFset.

Theqq¯/qg→^{V V}^∗^samples^with^ﬁnal^statesνand[46]

weregeneratedwithSherpa2.2.2,withtheexceptionofthe Z Z^∗ sample in the W H analysisfor whichSherpa 2.1 was used; the CT10PDF set [47] andtheNNPDF 3.0PDF setwereused forver- sions 2.1 and 2.2.2, respectively. Powheg-Box v2 [48] was used as an alternative generator for V V^∗, with Herwig++, using the CTEQ6L1PDF set,forpartonshowering. Among theloop-induced gg-initiated diboson processes, the only relevant process in this analysisis gg→^{Z Z}^∗^,^for^which^a ^K^-factor^of^1.55^was^{used [28].}

Thisprocess wassimulatedwithSherpa2.1.1,usingtheCT10PDF set.

Thetriboson V V V sampleswere generatedwithSherpa2.2.2 andtheNNPDF3.0PDFset.MG5_aMC@NLOwasusedasanalter- nativegeneratorfor V V V,withPythia8, usingtheNNPDF2.3LO PDFset.ThesamePDFsetswereusedforthehardscatteringand thepartonshoweringinalltheSherpasamplesdescribedabove.

Allsimulatedsamples includethe effectof pile-upfrommul- tipleinteractions inthe same andneighbouring bunch crossings.

Thiswas achievedbyoverlaying minimum-biasevents,simulated using Pythia8 with the A2 tune [49] and MSTW2008LO PDF set [50].AllsampleswereprocessedthroughtheGeant4 [51] AT- LASdetectorsimulation [52].

4. Eventreconstruction

Candidatesignaleventsare selectedusingtriggersthatrequire asingleisolatedleptonwithminimumtransversemomentum(pT) thresholdsbetween24 GeV and26 GeV forelectronsandbetween 20 GeV and26 GeV formuons,depending onthedata-takingpe- riod.At least one ofthe leptons reconstructed offline isrequired to have triggered the event and to have a p_T higher than the nominal trigger threshold by at least 1 GeV. The single-lepton triggerefficiencies on theplateau are approximately70% forsingle muons with |η|<1.05, 90% for single muons in the range 1.05<|η|<2.40 andgreaterthan90%forsingle electronsinthe range|η_|<2.47. Thetriggerefficiencyforthesignal events,esti- matedafterthepreselection,is94%forW H and98.5%forZ H.

Selectedeventsarerequiredtohaveatleastoneprimaryvertex reconstructedfromatleasttwoassociatedtracks,eachwithtrans- verse momentum pT>400 MeV, as described in Ref. [53]. If an eventhasmorethanonereconstructedprimaryvertex,thevertex withthelargesttrack

p²_T isselectedfortheanalysis.

Electronsare reconstructedfromclustersofenergydepositsin theEMcalorimetermatchedtoIDtracks,andareidentiﬁedusing criteriabasedonthecalorimetershowershape,thequalityofthe matchbetweenthetrackandtheclusterandtheamountoftransi- tionradiationemittedintheID,asdescribedinRef. [54].Electrons

are required to satisfy |η|<2.47, excluding 1.37<|η|<1.52, whichcorrespondstothetransitionregionbetweenthebarreland the endcap EM calorimeters. The efficiency for electron identification rangesfrom88% to94%, depending onelectron p_T andη. MuonsarereconstructedbycombiningIDandMStrackswithcon- sistent trajectories andcurvatures. Anoverall fit ofhitsfrom the IDtrack,energylossinthecalorimeterandthehitsofthetrackin themuonsystemisusedtoformmuoncandidates,asdescribedin Ref. [55].Theefficiencyformuonidentificationiscloseto95%over thefull instrumented ηrange.Tosuppressparticlesmisidentified as leptons, several identification requirements as well asimpact parameter, calorimeterandtrackisolation criteria [54,55] areap- plied.

Jetsare reconstructed usingthe anti-kt algorithm withradius parameterR=⁰.4 [56,57].Thefour-momentaofjetsarecorrected for the effects of calorimeter non-compensation, energy loss in non-instrumented regions, and contributions from pile-up [58].

Jetsarerequiredtohave|η_|<4.5,withpT>25 GeV fortheregion

|η_|<2.5 and pT>30 GeV fortheregion2.5<|η_|<4.5.Amulti- variate selection [59] is used to suppress jetswith p_T<60 GeV and |η_|<2.4 originated from pile-up. Furthermore, to suppress pile-up jetsin the forward region,jet shapesandtopological jet correlationsinpile-upinteractionsareexploited [60].Jetswith p_T

>20 GeV and|η_|<2.5 containingb-hadrons(b-jets)areidentiﬁed usingamultivariatetechnique [61] withaneﬃciencyof85%,esti- matedfromsimulatedt¯t events.The multivariatetechnique gives rejectionfactorsagainstjetsoriginatingfromalightquarkorgluon andjetscontainingc-hadronsof33and3,respectively.

Themissingtransversemomentum^p_T^miss^with^magnitude^E^miss_T ineacheventiscalculatedfromthenegativevectorialsumofthe transverse momenta of electrons, muons, and jets. It uses both track-basedandcalorimeter-basedmeasurements [62].

5. Eventselection

In the W H channel, exactly three isolated leptons with p_T>

15 GeV are requiredwith a total charge of ±^1. ^The ^lepton ^with unique charge is labelled 0, the lepton closest to 0 in angular distance R is labelled1, andthe remaining lepton is labelled 2.Insignal eventsleptons0 and1 aremostlikelytooriginate fromthe H→^{W W}^∗ ^decay,^withprobabilitiesof99% and85% respectively.

The most prominent background processes to the W H chan- nelareW Z/Wγ^∗productionandtop-quarkprocesseswitheither three prompt leptons, e.g. t¯t V, or two prompt leptons and one non-promptleptonfromab-hadrondecay,e.g.t¯t.Otherimportant background processes are Z Z^∗ (including Zγ^∗), Zγ and Z+jets production; they may satisfy the signal selection requirements if a lepton is undetected, in the case of Z Z^∗, or if they contain a misidentifiedornon-promptlepton,inthecaseof Zγ and Z+jets production.Processeswiththreepromptleptonsinthefinalstate such as tribosons, in particular W W W, also contribute to the background.Contributionsfrombackgroundprocessesthatinclude more than one misidentified lepton, such as W+jets production and inclusive bb¯ pair production, are negligible. The background fromtop-quarkproductionissuppressedbyvetoingeventsifthey containanyb-taggedjet.

TheanalysisoftheW H channelseparateseventswithatleast one same-ﬂavour opposite-sign charge (SFOS) lepton pair from events withzero SFOS lepton pairs, which havedifferent signal- to-background ratios. Due to the presence of Z→ decays as a dominant background, the former is hereafter referred to as the Z-dominatedcategory, while the latter is referred to as the Z-depletedcategory.IntheZ-dominatedcategory,themajorback- ground processes are those involving Z bosons. Events are ve-

Measurement of the production cross section for a Higgs boson in association with a vector boson in the → → Channel in Collisions at TeV with the ATLAS detector

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