Search for WZ resonances in the fully leptonic channel using $\mathit{pp}$ collisions at $\sqrt{s}=8$ TeV with the ATLAS detector

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Physics Letters B

www.elsevier.com/locate/physletb

Search for WZ resonances in the fully leptonic channel using pp collisions at √

s = ^{8 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:

Received17June2014

Receivedinrevisedform30July2014 Accepted15August2014

Availableonline20August2014 Editor:W.-D.Schlatter

AsearchforresonantWZ productionintheν^^ (,^=^e,μ)decaychannelusing20.3 fb⁻¹of√ s= 8 TeV pp collisiondatacollectedbytheATLASexperimentatLHCispresented.Nosignificantdeviation fromtheStandardModelpredictionisobservedandupperlimitsontheproductioncrosssectionsofWZ resonancesfromanextendedgaugemodelW^andfromasimplifiedmodelofheavyvectortripletsare derived.Acorrespondingobserved(expected)lowermasslimitof1.52(1.49) TeVisderivedfortheW^ atthe95%confidencelevel.

©^{2014CERNforthebenefitoftheATLAS}Collaboration.PublishedbyElsevierB.V.Thisisanopen accessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/3.0/).FundedbySCOAP³.

1. Introduction

Thesearch fordibosonresonancesisan essentialcomplement totheinvestigationofthesourceofelectroweaksymmetrybreak- ing.Despitethecompatibilitybetweenthepropertiesofthenewly discoveredparticle atthe LHC [1–4] withthose expectedforthe StandardModel(SM) Higgsboson,the naturalnessproblemasso- ciated witha light Higgsboson suggeststhat the SM islikely to beaneffectivetheoryvalidonlyatlowenergies.Extensionsofthe SM, such as GrandUnified Theories [5], Little Higgs models [6], Technicolor[7–10],moregeneric CompositeHiggsmodels[11,12], ormodelsofextradimensions[13–15],predictdibosonresonances athighmasses.

This Letter presents a search for resonant WZ production in the fully leptonic decay channels WZ→ ν^^ (,^=^e, μ) us- ing20.3 fb⁻¹ ofpp collisiondatacollectedbytheATLASdetector ata centre-of-massenergy of√

s=^{8 TeV.Fourpossible leptonic} decaychannels(eνee,eνμμ, μνee and μνμμ)areconsidered.To interpretthe results,the extendedgauge model (EGM)[16] with a spin-1 W^ boson is used asa benchmark signal hypothesis. In thismodel,the couplingsoftheEGM W^ boson totheSM parti- clesareidenticaltothoseofthe W boson,exceptforitscoupling to WZ, which is suppressed with respect to the SM WWZ triple gaugecouplingby afactorof(mW/mW^)² andentailsalinearre- lationshipbetweentheresonancewidthandmass.The branching ratio BR(W^→^WZ) varies between 1% and 2% for a W^ mass range200–2000 GeV. Inother scenarios, such asfor leptophobic W^ bosons [17–19], the decayto a pairof gauge bosons can be a dominantchannel. A narrow W^ resonanceis predictedin the EGM,withanintrinsicdecaywidththatisnegligiblewithrespect

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totheexperimentalresolutionsonthereconstructedWZ invariant mass. Possible interferences betweensignal andSM backgrounds are assumedto besmallandareneglected. Undertheseassump- tions,thefinalresultspresentedherecanbereinterpretedinterms of anynarrow spin-1 resonancefor a givensignal efficiencyand acceptance.

AphenomenologicalLagrangianforheavy vectortriplets(HVT) [20]hasrecentlybeenintroduced,wherethecouplingsofthenew fields to fermions andgauge bosons are defined in termsof pa- rameters.ByscanningtheseparametersthegenericLagrangiande- scribesalargeclassofmodels.Thetripletfield,whichmixeswith theSMgaugebosons,couplestothefermioniccurrentthroughthe combinationofparameters g²cF/gV andto theHiggsandvector bosons through g_Vc_H,where g is theSU(2)L gauge coupling,the parameter gV represents the couplingstrength to vector bosons, and cF and cH allow to modify the couplings andare expected tobeclosetounityinmostspecificmodels.Twobenchmarkmod- els,providedinRef.[20],areusedhereaswell.InModelA,weakly coupledvectorresonancesarisefromanextensionoftheSMgauge group [21]. In Model B, the heavy vector triplet is produced in a strongly coupled scenario, for example in a Composite Higgs model [22]. InModel A, the branching fractionsto fermions and gaugebosonsarecomparable,whereasforModelB,fermioniccou- plingsaresuppressed.

Direct searchesforWZ resonanceshavebeenreportedby sev- eralexperiments.TheATLASCollaboration reportedonsearchesfor a W^resonanceusingapproximately1 fb⁻¹ofdatafortheν^^ channel and 4.7 fb⁻¹ ofdata forthe νj j channel, where j isa hadronic jet,both at√

s=^{7 TeV, andexcluded an EGM W bo-} sonwithmassbelow0.76 TeV[23]and0.95 TeV[24]respectively.

TheCMSCollaboration searchedfortheproductionofgenericWZ resonancesinthesemileptonicfinalstate,andobtainedupperlim- its on the production cross section asa function of signal mass

http://dx.doi.org/10.1016/j.physletb.2014.08.039

0370-2693/©^{2014CERNforthebenefitoftheATLAS}Collaboration.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/3.0/).FundedbySCOAP³.

andwidth[25].Theyalsoanalyzed dijetsignaturescontainingjets taggedasW and Z bosondecays,andexcluded EGM W^ bosons withmassesbelow1.7 TeV[26].Theadvantageofthethree-lepton WZ finalstateoveritspartialorfullyhadronicfinalstatecounter- parts isits better sensitivity atthe lower end ofthe mass spec- trumduetoitssignificantlysmallerSMbackgroundsandsuperior mass resolution. The CMS Collaboration analyzed 5 fb⁻¹ of data at √

s=^{7 TeV in the} ν^^ channel, and EGM W^ bosons with massesbelow1.143 TeV[27]wereexcluded.

2. TheATLASdetector

The ATLAS detector [28] consists of an inner tracking detec- tor (ID), electromagnetic (EM) and hadronic calorimeters, and a muon spectrometer. The ID is immersedin a 2 Taxial magnetic field, generated by a superconducting solenoid, andconsists ofa silicon pixel detector, a silicon microstrip detector, anda transi- tion radiationtracker. The ID provides a pseudorapidity coverage of|η_|<2.5.¹

The EM calorimeters are composed of interspersed lead and liquid argon, acting as absorber and active material respectively, withhighgranularityinboththebarrel(|η_|<1.475)andend-cap up to the endof the trackeracceptance (1.375<|η|<2.5), and somewhat coarsergranularityfrom|η_|=2.5 to 3.2.The hadronic calorimeter uses steel and scintillator tiles in the barrel region, whiletheendcapsuseliquidargonastheactivematerialandcop- perasanabsorber.Themuonspectrometer(MS)isbasedonthree largesuperconductingair-coretoroidsarrangedwithaneight-fold azimuthal coilsymmetry aroundthe calorimeters.Three layers of precision tracking chambers, consisting of drift tubes and cath- ode strip chambers, enable precise muon trackmeasurements in thepseudorapidityrangeof|η|<2.7,andresistive-plateandthin- gapchambersprovide muon triggering capabilityin therange of

|η_|<2.4.

3. DataandMonteCarlo modelling

The data analyzed herewere collected by the ATLAS detector attheLHC in pp collisions at√

s=8 TeV duringthe2012 data- takingrun. Events are selected usinga combination(logical OR) of isolated andnon-isolated single-lepton (e or μ) triggers. The pT thresholds are 24 GeV for isolated single-lepton triggers and 60 (36) GeVfornon-isolatedsingle-e (μ)triggers.Therequirement thatthreehigh-pTleptons areinthefinalstate givesatriggeref- ficiencyabove 99.5%.Afterdata-qualityrequirements areapplied, the total integrated luminosity is 20.3 fb⁻¹ with an uncertainty of 2.8%[29].

ThebaselineEGMW^signalsaregeneratedwithPYTHIA8.162 [30]andtheMSTW2008LO[31]partondistributionfunction(PDF) set. The production cross section times branching fraction (with W →^eν, μν, τ ν, where all τ decays are considered, and Z → ee, μμ) arescaledtotheir theoreticalpredictionsatnext-to-next- to-leading order (NNLO) using ZWPROD [32], which are 1.43 pb for mW^ =^{200 GeV, 4.12 fb for m}W^ =^{1 TeV, and 0.08 fb for} m_W=^{2 TeV.IntheW}→τ ν component,onlytheleptonic τ de- cays enter the signal acceptance, albeitslightly andonly at high signal mass, whereas the Z →τ τ component is totally negligi- ble.TheintrinsicdecaywidthsoftheEGM W^scalelinearlywith

1 ATLASusesaright-handedcoordinatesystemwithitsoriginatthenominalin- teractionpoint(IP)inthecentreofthedetectorandthez-axisalongthebeampipe.

Thex-axispointsfromtheIPtothecentreoftheLHCring,andthey-axispoints upward.Cylindricalcoordinates(r,φ)areusedinthetransverseplane,φbeingthe azimuthalanglearoundthebeampipe.Thepseudorapidityisdefinedintermsof thepolarangleθasη= −^{ln tan}(θ/2).Theseparationbetweenfinal-stateparticles isdefinedas R=

( η)²+ ( φ)^{2.Thetransversemomentumisdenotedbyp}T.

Table 1

OverviewoftheprimaryMCsamples.Thebackgroundsfrommisidentifiedjetsare estimatedfromthedata.

Process Generator Parton Shower PDF

W^ PYTHIA PYTHIA MSTW2008LO

WZ POWHEG-BOX PYTHIA

Z Z POWHEG-BOX PYTHIA CT10

Zγ ^{SHERPA SHERPA}

t¯t+^W/Z MadGraph PYTHIA CTEQ6L1

m_W athigh massand are 5.5 GeV form_W=^{200 GeV, 36 GeV} form_W=^{1 TeV,and72 GeV form}W^=^{2 TeV. Thesearesignifi-} cantlylessthantheexperimentalresolutions,whichhaveGaussian widths of theorder of 25 GeV form_W =^{200 GeV,} 100 GeV for mW^ =^{1 TeV, and} 180 GeV for mW^=^{2 TeV. MC samples were} produced for the EGM W^ signal from 200 GeV to 400 GeV at intervals of 50 GeV and from 400 GeV to 2 TeV at intervals of 200 GeV.Aninterpolationprocedureisadoptedtoobtainthedis- tributions for mass points between 200 GeV and 400 GeV with 25 GeV stepsize andfrom400 GeV to2 TeV with 50 GeV step size.

The dominant SM WZ background is modelled by POWHEG- BOX[33–36],a next-to-leading-order(NLO) eventgeneratorcom- bined with the NLO CT10 PDF set [37]. Background events aris- ing from Z Z aremodelledwithPOWHEG-BOX,while thosefrom t¯t+^W/Z processes are generated with MadGraph 5.1.4.8 [38]

togetherwiththeCTEQ6L1[39]PDFset.Alltheseeventsareinter- facedwithPYTHIA,usingtheAU2tune[40]forpartonshowering.

Asecondcategoryofbackgroundarisesfromphotonsmisiden- tified aselectrons, mainly from Zγ production.A photon can be misreconstructedasanelectronifitliesclosetoachargedparticle trackorifthephoton convertstoe⁺e⁻ afterinteractingwiththe materialinfrontofthecalorimeter.Thiscontributionisestimated usingsimulatedZγ MCeventsgeneratedwithSHERPA1.4.0[41].

Finally, a third category of background includes all other sources where one or more jets are misidentified as an isolated lepton. The contributions from these fake backgrounds are esti- mated usingadata-drivenmethodasdescribed inSection 6.The contributionfromeventswithonlyonejetmisidentifiedasaniso- latedleptonisfoundtobedominantwhilethosewithmorethan onearefoundtobenegligible.Thus,inthisanalysisthefake back- groundsaredenotedby^+^jets.

An overview of the major MC samples used is presented in Table 1.

MonteCarlo(MC) eventsare processedthroughthefulldetec- tor simulation [42] using geant4 [43], and their reconstruction is performed with the same software used to reconstruct data events.Correctionfactorsforleptonreconstruction andidentifica- tionefficienciesareappliedtothesimulationtoaccountfordiffer- ences with respect to data. The simulated lepton four-momenta are tuned, via calorimeter energy scaling and momentum reso- lution smearing, to reproduce the distributions observed in data fromleptonicW ,Z and J/ψdecaysaftercalibration.Furthermore, additionalinelastic pp collisioneventsare overlaid withthe hard scattering process in the MC simulation and then reweighted to reproducetheobservedaveragenumberofinteractionsperbunch- crossingindata.

4. Objectreconstruction

Electroncandidates are reconstructedin theregion ofthe EM calorimeter with|η_|<2.47 bymatching the calorimeterclusters to the tracks inthe ID. The transitionregion betweenthe barrel and endcap calorimeters (1.37<|η_|<1.52) is excluded. Candi- date electrons must satisfy the medium quality definition [44]