Search for resonant WZ production in the fully leptonic final state in proton–proton collisions at √s = 13 TeV with the ATLAS detector

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Physics Letters B

www.elsevier.com/locate/physletb

Search for resonant W Z production in the fully leptonic final state in proton–proton collisions at √

s = 13 TeV with the ATLAS detector

.The ATLASCollaboration

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

Articlehistory:

Received5June2018

Receivedinrevisedform9October2018 Accepted11October2018

Availableonline18October2018 Editor:M.Doser

AsearchforaheavyresonancedecayingintoW Zinthefullyleptonicchannel(electronsandmuons)is performed.Itisbasedonproton–protoncollisiondata collectedbytheATLASexperimentattheLarge HadronCollider at acentre-of-mass energy of13 TeV, corresponding to an integrated luminosity of 36.1 fb⁻¹.NosignificantexcessisobservedovertheStandard Modelpredictionsandlimits areseton theproductioncrosssectiontimesbranchingratioofaheavyvectorparticleproducedeitherinquark–

antiquarkfusionorthroughvector-bosonfusion.Constraintsarealsoobtainedonthemassandcouplings ofasinglychargedHiggsboson,intheGeorgi–Machacekmodel,producedthroughvector-bosonfusion.

©^{2018TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense} (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP³.

1. Introduction

Searches for diboson resonances provide an essential test of theories of electroweaksymmetry breaking beyondthe Standard Model(BSM).VectorresonancesarepredictedinvariousBSMsce- narios,suchasinextendedgauge models [1,2],Little Higgsmod- els [3], Composite Higgs models and walking technicolor [4–6], unitarized Electroweak Chiral Lagrangian models [7], as well as intheories withextradimensions [8–10]. Inaddition,new scalar diboson resonances result from models with an extended Higgs sector [11,12].ThisLetterreportsonasearchforaW Z resonance inthe fullyleptonic decay channel ν ⁽=^{e or}μ^),produced

either by quark–antiquark (qq)¯ fusion or by vector-boson fusion (VBF).Theproton–proton collisiondatawere collectedby theAT- LAS detector [13] atthe LargeHadronCollider(LHC)ata centre- of-massenergy√

s=13 TeV.

ParameterizedLagrangians [14–16] incorporatinga heavy vec- tor triplet (HVT) permit the interpretation of searches forvector resonancesinagenericway.Here,thesimplifiedphenomenologi- calLagrangianofRef. [15] isused.Thecouplingofthenewheavy vector resonance, V, tothe Higgsboson andthe StandardModel (SM)gaugebosonsisparameterizedby gVcH andtothefermions via the combination (^g2/^gV)^cF, where g is the SM SU(2)gauge coupling. Theparameter g_V representsthetypicalstrengthofthe vector-boson interaction,whiletheparameters cH andcF are ex- pected to be of the order of unity in most models. The vector- boson scattering process, pp→V j j→W Z j j,is only sensitiveto thegaugebosoncouplingand,inthiscase, thebenchmarkmodel

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

usedtointerprettheresultsassumesnocouplingoftheheavyvec- torresonancetofermions.

The Georgi–Machacekmodel(GM) [17,18] isusedasabench- mark for a singly charged scalar resonance. The model extends the Higgs sector by including one real and one complex triplet, while preserving custodial symmetry, ensuring that the parame- ter ρ=^M2_W/(^M2_Z^cos2θW)=^{1 at treelevel. Itis less experimen-} tally constrained [19,20] than other models with higher isospin representations, such as Little Higgs models or Left–Right sym- metric models [21]. A parameter sinθH,representing the mixing ofthe vacuumexpectation values,determines thecontributionof thetripletstothemassesoftheW and Z bosons.Thetenphysi- calscalarstatesareorganizedintodifferentcustodialmultiplets:a fiveplet (^H++₅ ,^H₅⁺,^H0₅,^H₅⁻,^H−−₅ ) ^whichisfermiophobicbutcou- ples toW Z,atriplet,andtwosinglets,oneofwhichisidentified asthe 125 GeV SMHiggsboson.Assumingthat thetriplet states are heavierthan thefivepletscalars, H5 canonlybeproducedby vector-bosonfusionandthecrosssectionisproportionaltosin²θH. The singlychargedmembersofthisfivepletarethe objectofthe present search inthe VBF channel.For both modelsthe intrinsic widthofthe resonanceisbelow4%, whichislower than theex- perimentalresolutioninnearlyalltheparameterspaceexploredin thepresentanalysis.

TheVBFprocess(pp→W Z j j)ischaracterizedbythepresence of two jetswith a large rapidity gapresulting fromquarks from whichavectorbosonhasbeenradiated.Theabsenceofthistopol- ogyisinterpretedasqq¯ production,collectivelyreferredtohereas qq.¯ Thespectrum ofthereconstructed invariantmass ofthe W Z resonance candidates isexamined forlocalized excessesover the expectedSMbackground.ResultsareprovidedfortheVBFandqq¯

https://doi.org/10.1016/j.physletb.2018.10.021

0370-2693/©^{2018TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense}(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP³.

categories separately, neglecting possible signal leakage between them.

Earlyresultsfromthe Tevatron [22,23] haveput limitsonthe mass of a W boson of an extended gauge model [2] in the W Z channel between180GeVand 690GeV. The present analy- sis extends the search forresonant W Z production beyond that in Run 1 pp collision data at √

s=8 TeV performed by the ATLAS [24] and CMS [25] collaborations. Each collaboration has combined results [26–28] from searches for heavy V V and V H resonances (V =^{W or Z) based on Run 1 data and on partial} Run2dataat√

s=^{13 TeV in thefullyhadronic(qqqq),semilep-} tonic(ν^qq,^qq,νν^{qq),andfullyleptonic(,}ν^,νν^)final

states.Morerecent resultsfrom V V and V H resonancesearches with data at √

s=13 TeV have been reported in Refs. [29–38].

Thevariousdecaychannelsgenerallydifferinsensitivityindiffer- ent mass regions. The fully leptonic channel, in spite of a lower branching ratio, is expected to be particularly sensitive to low- massresonancesasithaslowerbackgrounds.Arecentsearch [39]

by the CMS Collaboration for a charged Higgs boson produced by vector-boson fusion and decaying into W Z in the fully lep- tonicmode,using15.2 fb⁻¹ ofdatacollected at√

s=^{13 TeV,has} yieldedlimits onthe coupling parameterof the GMmodel, asa function of mass. Limits on the GM model have also been set, based onanalyses ofsame-charge W W production by CMS [40]

andopposite-chargeW W productionbyATLAS [41],usingdataat

√s=13 TeV withanintegratedluminosityof36.1 fb⁻¹.

2. ATLASdetector

TheATLASdetectorattheLHChasacylindricalgeometrywith anear 4π ^{coverage in solid angle.1 Theinner detector(ID), con-}

sisting ofsilicon pixel, siliconmicrostrip and transitionradiation detectors,is surroundedby a thinsuperconducting solenoid pro- vidinga2 Taxialmagneticfield.Itallowsprecisereconstructionof tracksfromchargedparticlesandmeasurementoftheirmomenta up to a pseudorapidity of |η|=².^5. High-granularity lead/liquid- argon (LAr) sampling electromagnetic and steel/scintillator-tile hadron calorimeters, at larger radius, provide energy measure- ments in the central pseudorapidity range |η|<¹.^{7. In the end-} cap and forward regions, LAr calorimeters for both the EM and hadronicenergy measurements extendthe region of angularac- ceptance up to |η|=⁴.^{9. Outside the} calorimeters, the muon spectrometer incorporates multiple layers of trigger andtracking chambersina magneticfield producedby a systemofsupercon- ductingtoroidmagnets,enablinganindependentprecisemeasure- ment of muon track momenta for |η|<².^{7. The ATLAS trigger} systemconsistsofahardware-based level-1triggerfollowedbya software-basedhigh-leveltrigger [42].

3. DataandMonteCarlosamples

Thedatausedinthisanalysiswerecollected during 2015and 2016withtheATLASdetectorinpp collisions atacentre-of-mass energy of 13 TeV at the LHC. The minimum bunch crossing in- tervalis25 ns,withameannumberof23additionalinteractions perbunchcrossing.Theeventsarerequiredtohavepassedcombi- nationsofsingle-electron or single-muontriggers.The transverse

1 ATLASusesaright-handedcoordinatesystemwithitsoriginatthe nominal interactionpoint(IP)inthecentreofthedetectorandthez-axisalongthebeam pipe.Thex-axispointsfromtheIPtothecentreoftheLHCring,andthe y-axis pointsupwards.Cylindricalcoordinates(^r,φ)^{areusedinthetransverseplane,}φ beingtheazimuthalanglearoundthez-axis.Thepseudorapidityisdefinedinterms ofthepolarangleθasη= −ln tan(θ/2).Angulardistanceismeasuredinunitsof R≡

(η)²+(φ)².

momentum thresholdof the leptons in2015 is24 GeVfor elec- tronsand 20GeV formuons satisfyinga loose isolation require- mentbasedonlyonIDtrackinformation.Duetothehigherinstan- taneousluminosityin2016thetriggerthresholdwasincreasedto 26GeVforbothelectronsandmuonsandtighterisolationrequire- ments were applied. Additional electron and muon triggers that donotincludeanyisolationrequirementswithtransversemomen- tumthresholdsofpT =60GeVand50GeV,andasingle-electron triggerrequiringpT >120GeVwithlessrestrictiveelectroniden- tificationcriteriaareusedtoincreasetheselectionefficiencywhich reachescloseto 100%.Eventsare acceptedonlyifquality criteria for detectorand data conditions are satisfied. Withthese condi- tions,theavailabledatasetscorrespondtoanintegratedluminosity of36.1 fb⁻¹.

Samples of simulated data were produced by Monte Carlo (MC) generators with the detector response obtained from the Geant4toolkit [43,44].Forsomesamples,thecalorimeterresponse is obtained from a fast parameterized simulation [45], instead of Geant4. Additionalsimulated inelastic pp collisions, generated withPythia 8.186 [46] withthe A2setoftuned parameters [47]

and the MSTW2008LO [48] parton distribution function (PDF), were overlaid inordertomodel boththein- and out-of-timeef- fectsfromadditionalppcollisions(pile-up)inthesameandneigh- bouring bunch crossings. The mean numberof pile-up events in theMCsampleswassettoreflecttheconditionsinthedata.

For the HVT interpretation, W→^{W Z samples were gener-} ated. Twobenchmark models, provided inRef. [15], are used. In Model A, weakly coupledvector resonances arise froman exten- sion of the SM gauge group [49] with an additional SU(2) sym- metry group and the branching fractions to fermions and gauge bosons are comparable. In Model B, the heavy vector triplet is producedina stronglycoupledscenario,asinaComposite Higgs model [50] andfermionic couplingsare suppressed.The parame- ter gV was set to1 forModelA andto3 forModelB. Forboth models,theparameterc_F isassumedtobe thesameforall types of fermions. Simulated signal samples for the HVT benchmark ModelA weregeneratedformasses ofvector resonancesranging from250 GeV to3 TeV withMadGraph_aMC@NLO2.2.2 [51],us- ing the model file provided by theauthors in Ref. [52] with the NNPDF23LO [53] PDFset.TheyarehadronizedwithPythia8.186.

ForinterpretationintermsofModelB,theModelAcrosssections aresimplyscaled.Thisisjustifiedsincethewidthremainswellbe- low theexperimentalresolutionandtheangulardistributionsare thesameforbothmodels.

Forthe VBFproduction channel,HVTsamples were generated with gV=1 formasses rangingfrom250 GeV to2 TeV.Thecou- pling parameter c_H was set to 1 andall other couplings of the heavytriplet,includingcF,weresetto0inordertomaximizethe VBF contribution.A dijetinvariant massof atleast150 GeV was requiredduringeventgeneration.

For the GMsignal samples, pp→^H±₅^{j j}→^{W±Z j j were pro-} ducedwithMadGraph_aMC@NLO2.2.2forthemassrange200to 900GeVinthe H5-planedefinedin [54],compatiblewithpresent limits [20,55], using GMCALC [56] and with sinθH =⁰.^{5. They} wereproducedatleadingorder,butnormalizedtonext-to-leading orderaccordingto Ref. [11],wherethecrosssectionsandwidths, which scale assin²θH,are also given. Forthesesamples, a min- imum pT of 15 GeV (10 GeV)forthe jets(leptons) was required during event generation and the pseudorapidity must be in the range|η|<^{5 forjetsand}|η|<².^{7 forleptons.}

Thebackgroundsources inthisanalysisincludeprocesseswith twoormoreelectroweakgaugebosons,namelyV V and V V V as well asprocesseswithtopquarks,such ast¯t,t¯t V,single topand t Z,andprocesseswithgaugebosonsproducedinassociationwith jetsorphotons(Z+jand Zγ^{).MCsimulationisusedtoestimate}

the contribution from background processes withthree or more promptleptonswhiledata-driventechniquesareusedforthecase of background processes withat least one misidentified or non- promptlepton.Simulatedeventsareusedforcrosschecksandto assessthesystematicuncertaintiesinthesebackgrounds.

ThedominantW Z SMbackgroundprocessoforder(α²αs²)in- volving colour-exchange diagrams, here referred to as QCD W Z, was modelled using Sherpa 2.2.2 [57] at next-to-leading order (NLO),andincludeshard-scattering, partonshower,hadronization andthe underlyingevents. Up tothree additional partonsgener- atedattreelevelweremergedwiththepartonshower.Inorderto estimate anuncertaintydueto thepartonshower modelling,two alternativeW Z sampleswereproducedusingPowheg-Boxv2 [58]

interfaced with Pythia 8.186 and Herwig++ [59], respectively.

A sampleofthepurelyelectroweakprocessW Z j j→ν j j(la- belled W Z j j) witha matrix-elementb-quark veto(at zero order in αs) was generatedseparately with Sherpa2.2.2. Contributions from W Z jb→ν bj (labelled W Z bj) are included in the t Z sample described below. To estimate an uncertainty due to the partonshower modellinganalternativeMadgraph+Pythia8sam- plewasproduced.ThisMadgraphsampleincludesb-quarksinthe initial state and was split to provide a sample without (with) a b-quark inthefinal state tomodelthe W Z j j (t Z+^{W Z bj) back-} ground.

Samplesof q^q¯→^{Z Z}→^{4 orqq}¯→^{Z Z}→ νν ^{were gener-}

ated by Powheg-Boxv2 at NLO, interfaced to Pythia 8.186 and normalizedtoNNLObyK-factorsevaluatedinRef. [60].Thegg→ Z Z andtribosons were generatedwithSherpa2.1.1.The tt V¯ and t Z processesweregeneratedatLOusingMadgraph_aMC@NLO,in- terfaced withPythia 8.186(t¯t V) and Pythia6.428 (t Z). The tt V¯ sampleswerenormalizedtoNLOpredictions [11].

Finally samplesofSM backgroundswithatleastonemisiden- tified or non-prompt lepton, including Zγ^{, W}γ^{, Drell–Yan Z}→ , W →ν ^{as well as top-pair and single-top were gener-}

atedtoassistinthefake/non-promptleptonbackgroundestimate.

Events with Zγ ^{and W}γ ^{in the final state were generated with}

Sherpa2.1.1. Drell–Yan Z→, W→ν ^{aswell astop-pairand}

single-topproductionchannelswere generatedwithPowheg-Box v2andhadronizedwithPythia.Toavoiddoublecountingthe Zγ

events,Z eventsproducedbytheDrell–Yanprocesswithaphoton from final-state radiation with p_T>10 GeV were removed. The partonshower for processeswith topquarks was modelled with Pythia 6.428. Madgraph_aMC@NLO andPythia 8.186were used forbackgroundprocessesinvolvingapairoftopquarksaccompa- nied by a W boson orby a pair of chargedleptons. The Z and single-top cross sections were normalized to NNLO by K-factors evaluatedinRef. [60,61].

SM backgrounds with Higgs bosons (H,^t¯t H,^{V H) contribute} less than 0.1% of the total background because of the low cross section andtherequirementofawell reconstructed Z bosonde- cayingleptonically.Thesebackgroundsareneglected.

4. Reconstructedobjects

Events are requiredto have at least one primary vertex with at least two associated tracks, each with transverse momentum p_T>⁰.^{4 GeV. Ifthere is more than one vertex}reconstructed in the event, the one withthe largest track

p²_T is chosen asthe hard-scatter primary vertexandis subsequently usedfor the re- constructionofelectrons,muons, jetsandmissingtransversemo- mentum.

Electroncandidates are reconstructed fromenergydeposits in the EM calorimeter which are matched to a well-reconstructed ID track originating from the primary vertex. The electron iden- tification is based on a likelihood evaluated from a multivariate

discriminant.Theyarecategorizedassatisfyingthemediumorthe tight reconstruction quality requirements, asdefined inRef. [62].

OnlyelectronswithtransverseenergyET>25 GeV inthepseudo- rapidityrange|η|<2.47 areconsideredinthisanalysis.Thecandi- dateelectronsarerequiredtopassanisolationcondition:anupper valueofthescalarsumofthetransversemomentumofthetracks with p_T>⁰.^{4 GeV in a cone of size R}=^min(⁰.²,^{10 GeV}/^ET) around the electron,excluding the trackof the electron itself, is chosen suchthattheefficiencyisconstantat99%forelectrons in Z→^{eeevents.Fortightelectrons,anisolation}requirementisim- posed,basedoncalorimeteraswellastrackvariables,whichvaries asafunctionoftransverseenergyandyieldsanefficiencybetween 95% and99% forelectrons with pT in the range25–60 GeV. For a pair of electrons sharing the same ID-track, the electron with higherclusterETiskept.

Muons are reconstructed by combining tracks fromthe inner detector with tracks from the muon spectrometer. They are re- quiredtosatisfy mediumortightquality requirements,asdefined in Ref. [63]. Only muons with pT>^{25 GeV and} |η|<².^{7 are} considered in this analysis. Isolation requirements are also ap- pliedtoall muons,basedontheratio p^varcone_T /^pμ_T^{,where pvarcone}_T is the scalar sumof the transverse momenta of the tracks with pT>1 GeV in acone ofsize R=min(10 GeV/pμ

T,0.3) around themuon,excludingthemuontrackitself.Thisisolationgives99%

efficiency,independentlyofη^orpμT,in Z→μμ^samples.

Electron and muon candidates are required to originate from theprimaryvertex. Thus,thesignificanceofthetrack’stransverse impact parameter calculated relative to the beam line, |^d0/σd0|^, mustbelessthanthreeformuonsandlessthanfiveforelectrons, andthelongitudinalimpactparameter,z0 (thedifferencebetween the value of z of the point on the track at which d0 is defined andthelongitudinalpositionoftheprimaryvertex),isrequiredto satisfy|z0·sin(θ)|<0.5 mm.

Jetsarereconstructedfromclustersofenergydepositioninthe calorimeter [64] usingtheanti-kt algorithm [65] witharadiuspa- rameter R=⁰.^{4. Events with jetsarising fromdetector noise or} other non-collisionsourcesarediscarded [66].Thissearchconsid- ers jets with p_T>^{30 GeV in the range} |η|<⁴.^5. Furthermore, to mitigate the pile-up contamination, a jet vertex tagger [67], based on information about tracks associated with the primary vertex and pile-up vertices, is applied to jets with pT<^{60 GeV} and |η|< ^{2.4. The selected working point provides at least92%}

efficiency. The energy ofeach jet is calibrated and corrected for detector effects using a combination of simulated events and in situmethodsin13 TeV data [68].

As lepton and jet candidates can be reconstructed from the same detector information,a procedure to resolve overlap ambi- guities is applied. If an electron anda muon sharethe same ID track,themuonisselected.Reconstructedjetswhichoverlapwith electronsormuonsinaconeofsize^R=⁰.^{2 areremoved.}

Jetscontainingb-hadronsareidentifiedasb-jetsbytheMV2c10 b-tagging algorithm [69], which uses information such as track impact-parameter significances and positions of explicitly recon- structed secondarydecayvertices.Aworking pointcorresponding to85%b-taggingefficiencyonasampleoftt¯eventsischosen [70], withalight-flavourjetrejectionfactorofabout 34andac-jetre- jectionofabout3.Correctionfactorsareappliedtothesimulated event samples to compensate for differences between data and simulation in theb-tagging efficiencyfor b-jets, c-jets andlight- flavourjets.

The missing transverse momentum, p^miss_T , and its magnitude E^miss_T , are calculated from the imbalance in the sum of visible transverse momenta of reconstructed physics objects: electrons, muonsandjets,aswellasa“soft”termreconstructedfromtracks