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

ATLAS Collaboration AKILLI, Ece (Collab.), et al.

Abstract

A search for a heavy resonance decaying into WZ in the fully leptonic channel (electrons and muons) is performed. It is based on proton–proton collision data collected by the ATLAS experiment at the Large Hadron Collider at a centre-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 36.1 fb −1 . No significant excess is observed over the Standard Model predictions and limits are set on the production cross section times branching ratio of a heavy vector particle produced either in quark–antiquark fusion or through vector-boson fusion. Constraints are also obtained on the mass and couplings of a singly charged Higgs boson, in the Georgi–Machacek model, produced through vector-boson fusion.

ATLAS Collaboration, AKILLI, Ece (Collab.), et al . Search for resonant WZ production in the fully leptonic final state in proton–proton collisions at √s = 13 TeV with the ATLAS detector.

Physics Letters. B , 2018, vol. 787, p. 68-88

DOI : 10.1016/j.physletb.2018.10.021

Available at:

http://archive-ouverte.unige.ch/unige:110367

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

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

s = 13 TeV with the ATLAS detector

.TheATLAS Collaboration

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 Hadron Collider ata centre-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.

©²⁰¹⁸^TheÂuthor(s).^Published^byÊlsevier^B.V.^Thisîsânôpenâccessârticleûnder^the^CC^BY^license (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,newscalar diboson resonances result from models with an extended Higgs sector [11,12].ThisLetterreportsonasearchforaW Z resonance inthe fullyleptonic decaychannel ν (=^e ^or μ), produced either by quark–antiquark (qq)¯ fusion or by vector-boson fusion (VBF).Theproton–protoncollisiondatawere collectedby theAT- LASdetector [13] attheLargeHadron Collider(LHC)ata centre- of-massenergy√

s=^{13 TeV.}

ParameterizedLagrangians [14–16] incorporatinga heavy vector triplet (HVT) permit the interpretation of searches forvector resonancesinagenericway.Here,thesimplifiedphenomenologi- calLagrangianofRef. [15] isused.Thecouplingofthenewheavy vector resonance, V, tothe Higgsbosonandthe StandardModel (SM)gaugebosonsisparameterizedby gVcH andtothefermions via the combination (g²/gV)cF, where g is the SM SU(2)gauge coupling.Theparameter gV representsthetypical strengthofthe vector-bosoninteraction,whiletheparameters cH andcF are expected to be of the order of unity in most models. The vector- boson scatteringprocess, pp→^{V j j}→^{W Z j j,}îs ônly ^sensitive^to thegaugebosoncouplingand,inthiscase, thebenchmarkmodel

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

usedtointerprettheresultsassumesnocouplingoftheheavyvec- torresonancetofermions.

The Georgi–Machacekmodel(GM) [17,18] isusedasa benchmark 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 parameter ρ₌M²_W/(M²_Zcos²θW)=^{1 at} ^tree^level. Îtîs ^lessêxperimen- 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 vacuumexpectationvalues,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 canonlybeproduced by 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- ogyisinterpretedasq^q¯ production,collectivelyreferredtohereas qq.¯ Thespectrum ofthereconstructed invariant massofthe W Z resonance candidates isexamined forlocalized excessesover the expectedSMbackground.ResultsareprovidedfortheVBFandqq¯

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

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

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categories separately, neglecting possible signal leakage between them.

EarlyresultsfromtheTevatron [22,23] haveput limitsonthe mass of a W boson of an extended gauge model [2] in the W Z channel between180GeVand690 GeV. The presentanaly- 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 ôr ^Z⁾ ^based ôn ^Run ¹ ^data ând ôn ^partial Run2 dataat√

s=^{13 TeV in} ^the^fully^hadronic^(qqqq), ^semilep- tonic(νqq,qq,ννqq),andfullyleptonic(,ν,νν)ﬁnal states.Morerecentresults from V V and V H resonancesearches with data at √

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

Thevariousdecaychannelsgenerallydifferinsensitivityindiffer- ent massregions. The fully leptonic channel, in spite ofa 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 ofdatacollectedat√

s=^{13 TeV,}^has yieldedlimitson the couplingparameter of the GMmodel,as a function of mass. Limits on the GM model have also been set, basedon analyses ofsame-charge W W production by CMS [40]

andopposite-chargeW W productionbyATLAS [41],usingdataat

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

2. ATLASdetector

TheATLAS detectorattheLHChasacylindricalgeometrywith anear 4π coverage in solidangle.¹ Theinner detector(ID), con- sistingof silicon pixel,silicon microstrip and transitionradiation detectors,is surroundedby a thinsuperconducting solenoidpro- vidinga2 Taxialmagneticﬁeld.Itallowsprecisereconstructionof tracksfromchargedparticles andmeasurementoftheirmomenta 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 measurements in thecentral pseudorapidity range|η_|<1.7. In the end- cap and forward regions, LAr calorimeters for both the EM and hadronicenergy measurements extendthe region ofangular ac- ceptance up to |η_|₌4.9. Outside the calorimeters, the muon spectrometer incorporates multiple layers of trigger andtracking chambersinamagnetic ﬁeld producedby asystemof supercon- ductingtoroidmagnets,enablinganindependentprecisemeasure- ment of muon track momenta for |η_|<2.7. The ATLAS trigger systemconsistsofahardware-based level-1triggerfollowedby a software-basedhigh-leveltrigger [42].

3. DataandMonteCarlosamples

Thedatausedinthisanalysiswerecollected during 2015and 2016withtheATLASdetectorinppcollisionsatacentre-of-mass energy of 13 TeV at the LHC. The minimum bunch crossing in- tervalis25 ns,withameannumberof23additionalinteractions perbunchcrossing.Theeventsarerequiredtohavepassedcombi- nationsofsingle-electronorsingle-muon triggers.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 is 24GeVfor elec- tronsand 20GeV formuons satisfyinga loose isolation require- mentbasedonlyonIDtrackinformation.Duetothehigherinstan- taneousluminosityin2016thetriggerthresholdwasincreasedto 26GeVforbothelectronsandmuonsandtighterisolationrequire- ments were applied. Additional electron and muon triggers that donotincludeanyisolationrequirementswithtransversemomen- tumthresholdsofpT =⁶⁰^GeVând⁵⁰^GeV,ândâsingle-electron triggerrequiringpT >120GeVwithlessrestrictiveelectroniden- tificationcriteriaareusedtoincreasetheselectionefficiencywhich reachescloseto 100%.Events areacceptedonlyifquality criteria for detectorand data conditions are satisfied. Withthese conditions,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 A2setof tunedparameters [47]

and the MSTW2008LO [48] parton distribution function (PDF), were overlaid inordertomodel boththe in- andout-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 ^generated. Twobenchmark models, provided in Ref. [15], are used. In Model A, weakly coupledvector resonances arise froman exten- sion of the SM gauge group [49] with an additional SU(2) symmetry 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 parameter gV was setto 1 forModelA andto3 forModel B.Forboth models,theparametercF isassumedtobethesameforall types of fermions. Simulated signal samples for the HVT benchmark ModelA weregeneratedformassesofvector resonances ranging from250 GeV to3 TeV withMadGraph_aMC@NLO2.2.2 [51],using the model ﬁle provided by the authorsin Ref. [52] with the NNPDF23LO [53] PDFset.Theyarehadronized withPythia8.186.

ForinterpretationintermsofModelB,theModelAcrosssections aresimplyscaled.Thisisjustiﬁedsincethewidthremainswellbe- low theexperimentalresolutionandtheangulardistributionsare thesameforbothmodels.

Forthe VBFproduction channel,HVTsamples were generated with g_V=^{1 for}^masses^ranging^from^{250 GeV to}^{2 TeV.}^The^coupling parameter cH was set to 1 and all 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. Forthese samples,a minimum 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 andV V V as well asprocesseswithtopquarks,such ast¯t,t¯t V,single topand t Z,andprocesseswithgaugebosonsproducedinassociationwith jetsorphotons(Z+^j^and ^Zγ).MCsimulationisusedtoestimate