Constraints on off-shell Higgs boson production and the Higgs boson total width in ZZ → 4ℓ and ZZ → 2ℓ2ν final states with the ATLAS detector

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Physics Letters B

www.elsevier.com/locate/physletb

Constraints on off-shell Higgs boson production and the Higgs boson total width in Z Z → ⁴ ând ^{Z Z} → ² ² ν ^final ^states ^with ^the ÂTLAS

detector

.TheATLAS Collaboration

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

Articlehistory:

Received3August2018

Receivedinrevisedform20September 2018

Accepted24September2018 Availableonline26September2018 Editor:W.-D.Schlatter

A measurementofoff-shell Higgsbosonproductioninthe Z Z→⁴^and ^{Z Z}→²²ν ^decay^channels,

where^stands^forêitherânêlectronôrâ^muon,îs^performedûsing^data^fromproton–protoncollisions atacentre-of-massenergyof√

s=^{13 TeV.}^The^data^were^collected^by^theÂTLASêxperimentⁱⁿ²⁰¹⁵ and 2016atthe LargeHadronCollider,andtheycorrespondtoanintegratedluminosity of36.^{1 fb}⁻¹^. Anobserved (expected)upper limitonthe off-shellHiggs signalstrength, defined as theevent yield normalised to the Standard Model prediction, of 3.8 (3.4) is obtained at 95% confidence level (CL).

Assumingtheratio ofthe HiggsbosoncouplingstotheStandardModel predictionsisindependentof themomentumtransferoftheHiggsproductionmechanismconsideredintheanalysis,acombination withtheon-shellsignal-strengthmeasurementsyieldsanobserved (expected)95%CLupperlimitonthe Higgsbosontotalwidthof14.4 (15.2) MeV.

©²⁰¹⁸^TheÂuthor.^Published^byÊlsevier^B.V.^Thisîsânôpenâccessârticleûnder^the^CC^BY^license (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP³.

1. Introduction

The observation of the Higgs boson by the ATLAS and CMS experiments [1,2] attheLargeHadronCollider (LHC)marksamile- stonetowardstheunderstandingofthemechanismofelectroweak (EW) symmetry breaking [3–5]. Furtherstudies ofthe spin, par- ityand couplings of the new particlehave shown no significant deviationfromthepredictionsfortheStandardModel(SM)Higgs boson [6–10].EffortstomeasurethepropertiesoftheHiggsboson areprimarilyfocusedonon-shellproduction.ForaHiggsbosonat amassof125 GeV [10,11],theexpectednaturalwidthoftheSM Higgsbosonis^SM_H ∼4.1 MeV [12].However,above125 GeVoff- shellproductionoftheHiggsbosonhasasubstantialcrosssection attheLHC [13–16],duetotheincreasedphasespaceasthevector bosons(V=^W,^Z⁾ând^top^quark^decay^products^becomeôn-shell withtheincreasing energyscale.Thisprovides anopportunity to studytheHiggsbosonpropertiesathigherenergyscales.Off-shell productioncan provide sensitivityto new physics that altersthe interactionsbetweentheHiggsbosonandotherfundamentalpar- ticlesinthehigh-massregion [17–24].

The measured off-shell event yield from gluon–gluon fu- sion (ggF)productionnormalisedtotheSMprediction,wherethis

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

ratioisreferredtoasthesignalstrengthμoff-shell,canbeexpressed as

μoff-shell=σ_off-shell^gg^→^H^∗^→^{Z Z} σoff-shell,SM^gg^→^H^∗^→^{Z Z}

=κ_g²_,_off-shell·κ²_Z_,_off-shell,

whereσoff-shell^gg^→^H^∗^→^{Z Z} isthe crosssectionof theoff-shellHiggsbo- sonproductionviaggFwithsubsequentdecayintoa Z Z pair,and

κg,ôff-shell andκZ,ôff-shell are theoff-shell couplingmodifiersrela- tivetotheSMpredictionsassociatedwiththegg→H^∗production and the H^∗→^{Z Z} ^decay, respectively. The off-shellHiggs boson signal cannot be treated independently of the gg →^{Z Z} ^background, assizeable negative interference effects appear [13]. The interference term is assumed to be proportional to √μoff-shell= κg,off-shell·κZ,off-shell.Similarly, μon-shell fortheon-shellHiggsbo- sonproductionviaggFisgivenby:

μon-shell=σ_on-shell^gg^→^H^→^{Z Z}^∗ σon-shell,SM^gg^→^H^→^{Z Z}^∗

=κ²_g_,_on-shell·κ_Z²_,_on-shell H/^SM_H ,

which depends on the Higgs boson total width H. A measurement of the relative off-shell and on-shell event yields,

μoff-shell/μon-shell,providesdirectinformationabout H,ifoneas- sumesidenticalon-shelland off-shellHiggsbosoncouplingmodi- https://doi.org/10.1016/j.physletb.2018.09.048

0370-2693/©²⁰¹⁸^TheÂuthor.^Published^byÊlsevier^B.V.^Thisîsânôpenâccessârticleûnder^the^CC^BY^license(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP³.

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fiers [15,25].Theaboveformalismdescribingtheratioofoff-shell to on-shell cross sections also applies to the vector-boson fu- sion (VBF)productionmode.Asinthepreviousmeasurement [26], fora measurement ofH it is necessaryto assume that the on- shell and off-shell coupling modifiers are the same, and for an upperlimitthattheon-shellcouplingmodifiersarenotlargerthan the off-shell couplings. It is also assumed that any new physics whichmodifiestheoff-shellsignalstrengthandtheoff-shellcou- plingsdoes notmodifytherelativephaseoftheinterferingsignal and background processes. Further, it is assumed that there are neithersizeablekinematicmodificationstotheoff-shellsignalnor newsizeablesignalsinthesearchregionofthisanalysisunrelated toanenhancedoff-shellsignalstrength.

TheATLASandCMSexperimentshavepresentedstudiesofthe off-shellproductionoftheHiggsbosonusingRun-1proton–proton (pp) collisions data [26–29]. ATLAS obtained an observed (expected) upper limit on the off-shellHiggs boson signal strength (μoff-shell)intherangeof5.1–8.6(6.7–11.0) [26],usingtheZ Z and W W channels. Thisrange is determined by theassumption that the gg→Z Z and gg→W W background K-factors,correspond- ing to the ratio of the next-to-leading-order (NLO) QCD predictions to the leading-order (LO) predictions, lie betweenone-half andtwice thevalue ofthe gg→^H^∗→^{Z Z}(^{W W})^signal ^K^-factor.

An observed (expected) 95% confidence level (CL) upper limit of H <²³(³³) ^MeV ^was ôbtained, âssuming ^the ^gg →^{Z Z}(^{W W}) background K-factor is equal to the gg→^H^∗→^{Z Z}(^{W W}) ^signal K-factor.CMSpresenteda similarstudy in the Z Z andW W channels, withobserved (expected) 95% CL upper limit of H <

13(26)MeV [29].Bycomparison, theprecisionofH fromdirect on-shell Higgsboson massmeasurements alone is approximately 1 GeV [9,30,31],limitedbymeasurementresolution.

ThisLetterpresentsananalysisofoff-shellHiggsbosonproduc- tioninthe Z Z→4and Z Z→22ν ^ﬁnal^states⁽=e,μ^),^using

36.^{1 fb}⁻¹ ^of^data^collected^by^the^ATLAS ^detectorⁱⁿ ^pp ^collisions at √

s=13 TeV. The off-shellregion is definedby requiringthe invariant massof the Z Z system (mZ Z) tobe above the on-shell Z Z productionthreshold,hencewellabovetheHiggsbosonmass, andtheon-shellregionis definedby amasswindow aroundthe 125 GeV resonance. This analysis adopts the same methodology used inthe Run-1 analysisreportedinRef. [26]. The analysisfor the Z Z→⁴ ^final^state^closely^follows^the^Higgs^boson^measure- ments andhigh-mass search inthe samefinal state described in Refs. [32,33].Theoff-shellHiggssignalstrengthisextractedusing amatrix-elementdiscriminant,definedinSection4,inamassre- gion220 GeV<^m4<^{2000 GeV.}^Theôn-shell^signal^strength^was measuredinthe118 GeV<m4<129 GeV regioninRef. [32].The analysisoftheZ Z→²²ν^channel,^describedⁱⁿ^Section^5,^follows

a strategy similar to that used in thesearch forheavy Z Z reso- nancesdescribedinRef. [33].Forthischannel,thesignalstrength is extracted fromthe transverse mass distribution in the 250 to 2000 GeVrange.For off-shellproductionofthe Higgsboson, the dominantprocessesofggFandVBF areconsidered.Next-to-next- to-leading-order (NNLO)QCD andNLO EWcorrectionsareknown fortheoff-shellsignalprocessgg→^H^∗→^{Z Z} ^[25].^More^recently, NLOQCDcorrectionshavealsobecomeavailableforthegg→Z Z backgroundandforthesignal–backgroundinterference [34,35],for whichadditionaldetailsaregiveninSection3.GiventhattheQCD correctionsfortheoff-shellsignalprocesseshaveonlybeencalcu- latedinclusively inthe jet multiplicity,the analysisis performed inclusivelyin jet observablesandthe eventselection is designed tominimisethedependenceonthemomentumofthe Z Z system, whichissensitivetothejetmultiplicity.

2. ATLASdetector

The ATLAS experiment is described in Ref. [36]. ATLAS is a multipurposedetectorwithaforward–backwardsymmetriccylin- drical geometry and a solid-angle¹ coverage of nearly 4π^. ^The

inner tracking detector, covering the region |η|< ^2.5, ^consists of a silicon pixel detector, a silicon microstrip detector and a straw-tubetransition-radiation tracker.Theinnermostlayerofthe pixel detector, the insertable B-layer [37], was installed between Run 1 and Run 2 of the LHC. The inner detector is surrounded byathinsuperconductingsolenoidprovidinga2T magneticﬁeld, andbyaﬁnelysegmentedlead/liquid-argon(LAr)electromagnetic calorimeter covering the region |η|<3.2. A steel/scintillator-tile hadroniccalorimeterprovidescoverageinthecentralregion|η|<

1.^7.^Theêndcapând^forward^regions,^covering ^thepseudorapidity range 1.⁵<|η|<⁴.^9, âreinstrumentedwithelectromagneticand hadronicLArcalorimeters,withcopperortungstenastheabsorber material. Amuon spectrometer systemincorporatinglarge super- conducting toroidal air-core magnets surrounds the calorimeters.

Three layers of precision wire chambers provide muon tracking in the range |η|< ^2.7, ^while ^dedicated ^fast ^chambers âre ûsed for triggeringin theregion |η|< ^2.4. ^The^trigger ^systemîs^com- posedoftwostages [38].Thefirststage,implementedwithcustom hardware,usesinformationfromcalorimetersandmuonchambers to reduce the eventrate from about 40 MHz to a maximum of 100 kHz. The second stage, called the high-level trigger, reduces thedataacquisitionratetoabout1 kHzonaverage.Thehigh-level triggerissoftware-basedandrunsreconstructionalgorithmssimi- lartothoseusedintheofflinereconstruction.

3. MonteCarlosimulationandhigher-ordertheorycorrections Monte Carlo (MC) samplesof gg→(H^∗→)Z Z events,which include the SM Higgs boson signal, gg →^H^∗ →^{Z Z,} ^the ^continuum background, gg → ^{Z Z}^, ând ^the signal–background interference contribution, were generated with the MC generator Sherpa-v2.2.2+ÔpenLoops^[39–42].^Matrixêlements^were^calcu- latedforzerojetsandonejetatLOandmergedwiththeSherpa parton shower [43]. The NNPDF30NNLO [44] PDF set was used, andthe QCD renormalisationandfactorisationscales were setto mZ Z/2.

The K-factor forthe gg→H^∗→Z Z process is known up to NNLO inQCDasafunction ofm_{Z Z} [12,25]. Morerecently,aNLO QCD calculation whichincludesthe gg→^{Z Z} ^continuum^process hasbecomeavailable [34,35] allowingmZ ZdifferentialK-factorsto becalculatedwithanexpansionintheinversetopmass (1/^mt)be- low2mt,andassumingamassless-quarkapproximationabovethis threshold.ThisNLOQCD calculationwasusedtocorrectall three componentswithseparate K-factorscomputedforthesignalgg→ H^∗→^{Z Z} ^(K^S(^mZ Z)^), ^the ^background ^gg→^{Z Z} ⁽^K^B(^mZ Z)⁾ ând theinterference (KÎ(mZ Z)).SincetheNNLOQCDcorrectionisonly known differentiallyin mZ Z forthe gg→^H^∗→^{Z Z} ^process ând notforallthreecomponentsintheoff-shellregion,anoverallcor- rectionis appliedbyscaling thedifferentialNLO QCDreweighted cross section by an additional factorof 1.2, which isassumed to be thesameforthesignal,backgroundandinterference.Thisad- ditional constantscale factorisjustifiedby theconstantNNLO to

1 TheATLASexperimentusesaright-handedcoordinatesystemwithitsoriginat thenominalinteractionpoint(IP)inthecentreofthedetectorandthez-axisalong thebeampipe.Thex-axispoints fromtheIPtothecentreoftheLHCring,and the y-axispointsupward.Cylindricalcoordinates(r,φ)areusedinthetransverse plane,φbeingtheazimuthalanglearoundthez-axis.Thepseudorapidityisdeﬁned intermsofthepolarangleθasη= −ln tan(θ/2).

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NLOratiooftheQCDpredictionsover thedataregionconsidered intheanalysis.UsingthesescaledNLO K-factors,thecrosssection forthe gg→(^H^∗→)^{Z Z} ^process ^withâny ôff-shell^Higgs ^boson signal strengthμoff-shell canbe obtainedfroma parameterisation ofthreeSMMCsamples:thegg→H^∗→Z Zsignal(σgg^SM→^H^∗→^{Z Z}), the gg→^{Z Z} ^continuum ^background ⁽σ_gg^SM_→_{Z Z}_,_cont⁾ ând ^the ^full

processwithsignal,backgroundandinterferencegg→(H^∗→)Z Z (σgg^SM→(^H^∗→)^{Z Z}),wherethelastsampleisrequiredtoderivethein- terferencesample:

σgg→(^H^∗→)^{Z Z}(μoff-shell)

=μoff-shell·1.2·K^S(mZ Z)·σgg^SM→^H^∗→^{Z Z}

+√μoff-shell·1.2·K^I(mZ Z)·σ_gg^SM_→_{Z Z}_,Interference (1) +1.2·K^B(mZ Z)·σgg^SM→^{Z Z},^cont,

σ_gg^SM_→_{Z Z}_,Interference

=σ_gg^SM_→(_H∗→)Z Z−σ_gg^SM_→_H∗→Z Z−σ_gg^SM_→_{Z Z}_,_cont. ⁽²⁾

Theelectroweak pp→^{V V}+²^j ^processes^containing ^both^the VBF-likeeventsandeventsfromassociatedHiggsproductionwith vector bosons (V H), which includes on-shell Higgs boson production, were simulated using MadGraph5_aMC@NLO [45] with matrix elements calculated at LO. The QCD renormalisation and factorisation scales were set to mW following the recommenda- tion in Ref. [46] and the NNPDF23LO PDF set [47] was used.

Pythia 8.186 [48] was used for parton showering and hadronisation, with the A14set of tuned parameters forthe underlying event [49].DuetothedifferentH dependence,the on-shelland off-shell Higgs boson production processes are separated when weighting MC events asin Eqs. (1) by requiringthat the generated Higgs boson mass satisfy |m^gen._H −125 GeV|<1 GeV. This requirement is fully eﬃcient in selecting the on-shell V H process. The cross section σpp→^{V V}+²^j(μoff-shell) ^for^the electroweak pp→V V+2jprocessforanyoff-shellHiggsbosonsignalstrength

μoff-shell is parameterised in the same way as for the gg → (^H^∗→)^{Z Z} ^process.

The qq¯→Z Z background was simulated with Sherpa v2.2.2, usingthe NNPDF30NNLOPDF setforthe hard-scatteringprocess.

NLO QCD accuracy is achievedin the matrix-element calculation for0- and 1-jetﬁnalstatesandLOaccuracyfor2- and 3-jetﬁnal states.ThemergingwiththeSherpapartonshowerwasperformed usingtheMePs@NLOprescription.NLOEWcorrectionsareapplied asafunctionoftheparticle-levelmZ Z [50,51].

TheW W andW Z backgroundsweresimulatedatNLOinQCD usingthePowheg-Boxv2eventgenerator [52] withtheCT10NLO PDFset [53] andPythia8.186forpartonshoweringandhadronisa- tion.Thenon-perturbativeeffectswere modelledwiththeAZNLO setoftunedparameters [54].The interferencebetweentheqq¯→ Z Z andqq¯→^{W W} ^processes^for^the²²ν ^ﬁnal ^state^is ^found^to

benegligibleandthusisnotconsidered.

Events containing a single Z bosonwith associatedjets (Z + jets)weresimulatedusingtheSherpav v2.2.1eventgenerator.Ma- trixelements were calculated forup to two partons atNLO and four partons at LO using the Comix [55] and OpenLoops [41]

matrix-element generators and merged with the Sherpa parton shower [43] usingtheMePs@NLOprescription.TheNNPDF30NNLO PDF set was used in conjunction with dedicated parton-shower tuningdevelopedbytheSherpaauthors.The Z + ^jets^events^are normalisedusingtheNNLOcrosssections [56].

The triboson backgrounds Z Z Z, W Z Z, and W W Z withfully leptonic decays and at least four prompt charged leptons were modelled using Sherpav v2.2.1. The contribution from triboson backgroundswithone W or Z bosondecayinghadronicallyisnot

includedinthesimulation,buttheimpactontheanalysisisfound to be negligible. For the fully leptonic t¯t+^Z background, with four prompt charged leptons originating from the decays of the topquarksand Z boson,MadGraph5_aMC@NLOwasused.Thet¯t background, as well as the single-top and W t production, were modelled using Powheg-Box v2 interfaced to Pythia 6.428 [57]

with the Perugia 2012 [58] set of tuned parameters for parton showering, hadronisation and the underlying event, and to Evt- Gen v1.2.0 [59] for properties of the bottom and charm hadron decays.

Theparticle-leveleventsproducedbyeachMCeventgenerator wereprocessedthroughtheATLASdetectorsimulationwithinthe Geant4framework [60,61] orthefastdetectorsimulationpackage Atlfast-II [61].Additional pp interactionsin thesame andnearby bunchcrossings(pile-up)areincludedinthesimulation.Thepile- upeventsweregeneratedusingPythia8withtheA2setoftuned parameters [62] and theMSTW2008LO PDF set [63]. The simula- tionsampleswereweightedtoreproducetheobserveddistribution ofthemeannumberofinteractionsperbunchcrossinginthedata.

4. Z Z→4analysis

The analysis for the Z Z →⁴ ^final ^state ^closely ^follows ^the on-shell Higgsbosonmeasurements andhigh-mass search inthe same final state described in Refs. [32,33], with the same event reconstruction, triggerandeventselections, andbackgroundesti- mationmethods.Amatrix-element-based (ME-based)discriminant computedatLOisconstructedtoenhancetheseparationbetween the gg→^H^∗→^{Z Z} ^signal ând^the ^gg→^{Z Z} ând^q^q¯→^{Z Z} ^backgrounds, andthis discriminant is subsequently used in a binned maximum-likelihood fit for the final result. To minimise the de- pendenceofthegg→^{Z Z} ^kinematicsônhigher-orderQCDeffects, theanalysisis performedinclusively,ignoringthe numberofjets intheevents.

The analysisis split into three channels(4μ^, ^2e2μ^, ^4e). ^Each

electron (muon)musthavetransversemomentump_T>⁷(⁵)^GeV and be measured in the pseudorapidity range |η|<².^{47 (}|η|<

2.^7). ^The ^highest-pT lepton in the quadruplet must satisfy pT>

20 GeV, and the second (third) lepton in pT order is required to have pT>15 GeV (pT>10 GeV). Lepton pairs are formed fromsame-flavour opposite-chargeleptons. Foreach channel,the quadrupletwitha leptonpairwhosemassisclosest tothe Z boson mass iskept. This pairis referred to astheleading dilepton pairanditsinvariantmass,m12,isrequiredtobebetween50 GeV and106 GeV.Thesecond(subleading)pairischosenfromthere- maining leptons asthe pair closest in mass to the Z boson and intherange50 GeV<m34<115 GeV.The off-shellregionisde- finedastherange220 GeV<^m4<^{2000 GeV,}^while^theôn-shell regionisdefinedas118 GeV<^m4<^{129 GeV.}

Thedominantbackgroundinthe Z Z→⁴^channel ârises^from qq¯→^{Z Z} êvents.^Thisîs^modelledûsing^MCsimulation,accurateto NLOQCDandNLOEWcorrectionsasexplainedinSection3.Other backgrounds,such astribosonproduction,tt V, Z + jets,andtop quark production,constituteless than2% ofthetotal background intheoff-shellsignalregion,andareeithertakenfromsimulation orfromdedicateddatacontrolregions.

Fig.1(a)shows theobservedandexpecteddistributionsofm4 combiningallleptonchannelsintheoff-shellregion.Thedataare in agreement with the SM predictions, with two small excesses atm₄around240 GeV and700 GeV,eachhavingasignificanceof abouttwostandarddeviations (2σ^),âsêvaluated^by^the^high-mass

resonancesearchreportedinRef. [33].Table1showstheexpected and observed numbers of events in the signal region and addi- tionallyin the400 GeV<m4< 2000 GeVmassrange,which is

Constraints on off-shell Higgs boson production and the Higgs boson total width in ZZ → 4ℓ and ZZ → 2ℓ2ν final states with the ATLAS detector

Physics Letters B

Constraints on off-shell Higgs boson production and the Higgs boson total width in Z Z → 4 and Z Z → 2 2 ν ﬁnal states with the ATLAS

detector

Constraints on off-shell Higgs boson production and the Higgs boson total width in ZZ → 4ℓ and ZZ → 2ℓ2ν final states with the ATLAS detector

Constraints on off-shell Higgs boson production and the Higgs boson total width in Z Z → ⁴ ând ^{Z Z} → ² ² ν ^final ^states ^with ^the ÂTLAS