Search for new resonances in Wγ and Zγ final states in pp collisions at √s=8 TeV with the ATLAS detector

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Reference

Search for new resonances in Wγ and Zγ final states in pp collisions at √s=8 TeV with the ATLAS detector

ATLAS Collaboration

ALEXANDRE, Gauthier (Collab.), et al.

Abstract

This Letter presents a search for new resonances decaying to final states with a vector boson produced in association with a high transverse momentum photon, Vγ , with V=W(→ℓν) or Z(→ℓ+ℓ−) , where ℓ=e or μ . The measurements use 20.3 fb −1 of proton–proton collision data at a center-of-mass energy of s=8 TeV recorded with the ATLAS detector. No deviations from the Standard Model expectations are found, and production cross section limits are set at 95% confidence level. Masses of the hypothetical aT and ωT states of a benchmark Low Scale Technicolor model are excluded in the ranges [275,960] GeV and [200,700]∪[750,890]

GeV , respectively. Limits at 95% confidence level on the production cross section of a singlet scalar resonance decaying to Zγ final states have also been obtained for masses below 1180 GeV.

ATLAS Collaboration, ALEXANDRE, Gauthier (Collab.), et al . Search for new resonances in Wγ and Zγ final states in pp collisions at √s=8 TeV with the ATLAS detector. Physics Letters. B , 2014, vol. 738, p. 428-447

DOI : 10.1016/j.physletb.2014.10.002

Available at:

http://archive-ouverte.unige.ch/unige:55771

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Physics Letters B

www.elsevier.com/locate/physletb

Search for new resonances in W γ and Z γ final states in 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:

Received30July2014

Receivedinrevisedform22September 2014

Accepted1October2014 Availableonline6October2014 Editor:W.-D.Schlatter

ThisLetter presentsasearchfornewresonancesdecayingtofinalstateswithavectorbosonproduced inassociationwithahightransversemomentumphoton,Vγ,withV=^W(→ν)orZ(→⁺⁻),where =^{e or}μ.Themeasurementsuse20.3 fb⁻¹ofproton–protoncollisiondataatacenter-of-massenergy of√

s=8 TeV recordedwiththeATLASdetector.NodeviationsfromtheStandardModelexpectationsare found,andproductioncrosssectionlimitsaresetat95%confidencelevel.MassesofthehypotheticalaT andωTstatesofabenchmarkLowScaleTechnicolormodelareexcludedintheranges[²⁷⁵,960]^{GeV and} [²⁰⁰,700]∪ [⁷⁵⁰,890]^GeV,respectively.Limitsat95%confidencelevelontheproductioncrosssection of asinglet scalarresonance decaying to Zγ finalstates have alsobeen obtainedfor masses below 1180 GeV.

PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/3.0/).FundedbySCOAP³.

1. Introduction

The search for diboson resonances is an essential step in ex- ploringthesourceofElectroweakSymmetryBreaking(EWSB).The observation of a Higgs boson decaying to γ γ, Z Z and W⁺W⁻ final states,reported by theATLAS andCMSCollaborations [1,2], represents a milestone in particle physics history. However, the precise nature of the observed Higgs boson is not well known, anda dynamical mechanismof EWSBandfermion massgenera- tionmayyetinvolveavarietyofheavybosonsofspin-0orspin-1.

ThisLetter presentsasearchfornarrowscalarandvectorheavy resonancesdecayingtoWγ or Zγ finalstates,wherethe W and Z bosonsdecaytoleptons(e or μ).Theexistenceofanewvector orscalar resonance couplingto a boson pair Wγ or Zγ ispre- dicted by many physics scenarios, includingvarious new physics modelswithscalarparticles[3,4]andvectorparticles[5].

TheLowScaleTechnicolor(LSTC)[5]modelisusedasabench- markmodelforthesearchforspin-1resonancesdecayingto Wγ

andZγ finalstatesexploredinthispaper.ThediscoveryofaHiggs boson,withitsparametersinagreementwiththeStandardModel (SM)predictions,doesnotexcludethefullphasespaceoftheLSTC model, and the basic phenomenology would remain valid for a Technicolor modelwith a light composite Higgs boson. To mini- mizethemodeldependenceofthesearchresults,thesignalcross sectionismeasuredwithinawelldefinedfiducialregion.Thepre-

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

dictednewboundstatesofthelightestdoubletoftechnifermions, thetechnimesonsa_T,ωT,ρT and πT,generateanewphenomenol- ogy.Themasssplittingsbetweenthetechnimesonsaresettobeas follows:mρ_T=^mωT,maT≈¹.1×^mρT andmρ_T−^mπT=^mW [6].The decaysoftechnimesonstotechnipionsare thereforekinematically forbidden.Thetechnimesonsmostlydecaytopairsofelectroweak gaugebosons,themostabundantdecaychannelsbeing ωT→^Zγ, a_T→^Wγ and ρT→^{W Z},Wγ.Thesetechnimesonresonancesare expectedtobenarrow,withtypicalvaluesΓ (ρT,ωT,aT)≈^{1 GeV.}

Aphenomenologicalmodeldescribingasingletscalarparticleφ [7,8]ischosenasanotherbenchmarkinthesearchforspin-0res- onances decaying to Zγ. The neutralscalar could be composite, producedbyahypotheticalnewstronginteraction.Itcouldbethe pseudo-Goldstone bosonplayingan importantrole inthedynam- ical EWSB.Thislow energyeffectivemodelisindependentofthe underlyingdynamicaldetailsanditsLagrangiancanbewrittenas follows:

Leff=^cg

4π αs

Λ φG^a_μνG^aμν+^cW

4π αem

Λ·^sin2θ_WφW_μν^a W^aμν +cB

4π αem

Λ·cos²θW

φB_μνB^μν. (1)

Here αs andαem are,respectively, thecouplingsof thestrong andelectromagnetic interactions,Λ isthe cutoffscale andθW is theWeinbergangle.Moreover,cg,cW andcB arethecouplingco- efficients betweenthe scalarfield φ andthe gluon field strength G^aμν, the SU(2) field strength Wμν^a and the U(1) field strength Bμν, respectively. The scalar field φ interacts directly with the http://dx.doi.org/10.1016/j.physletb.2014.10.002

0370-2693/PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/3.0/).FundedbySCOAP³.

gauge bosonpairs via the dimension-5 operators rather than via theloop-induced processes,and could lead to enhanced produc- tion andbranching ratio to Zγ if Λis in the TeV scale. No SM Yukawainteractionofφ withfermionsisallowed sothatthereis nodecaytobb or¯ t¯t finalstates.TheW W and Z Z decaysofφare suppressedcomparedwiththeSMHiggsboson.

Previous limits on new resonances decaying to Wγ and Zγ

final states from p^{p and}¯ pp production have been obtained at theTevatronbytheDØ Collaboration[9]andattheLargeHadron Collider(LHC)bytheATLASCollaboration[10].AtATLAS,thepro- duction of a_T and ωT for masses below 703 GeV and 494 GeV, respectively, wasexcludedwithintheLSTCbenchmarkparameters.

ThemoststringentlimitsonLSTChavebeensetbytheCMS[11]

Collaboration, excluding the production of ρT for masses below mρT<1.14 TeV, but using a slightly differentchoice of parame- ters.

2. ATLASdetectoranddatasample

The ATLAS detector [12] is composed ofan inner tracker de- tector(ID)surroundedbyathinsuperconductingsolenoidprovid- inga2 Taxialmagneticfield,electromagnetic(EM)andhadronic calorimeters(HC),andamuonspectrometer(MS)immersedinthe magneticfield produced by a systemof superconducting toroids.

The ID consists of three subsystems: the pixel and silicon mi- crostripdetectorscoverthepseudorapidity¹range|η_|<2.5,while thetransitionradiationtracker,madeofstrawtubes,hasanaccep- tancerangeof |η|<2.0.The calorimetersystemcoverstherange

|η_|<4.9.Thehighly segmentedelectromagneticcalorimeterplays acrucialroleinelectronandphotonidentification.Itiscomposed oflead absorberswith Liquid Argon (LAr) asthe active material and spans |η|<3.2. In the region |η|<1.8, a pre-sampler de- tector usinga thinlayer ofLAris used tocorrect fortheenergy lost by electrons and photons upstream of the calorimeter. The hadronictile calorimeter(|η|<1.7)isa steel/scintillating-tilede- tector and is located directly outside the envelope of the barrel electromagneticcalorimeter. The end-capsand forwardcalorime- tersuseLArastheactivematerial,withcopper(EM)andtungsten (HC)asabsorbermaterials.TheMSiscomposedofthreelargesu- perconductingair-coretoroidmagnetsofeightcoilseach,asystem ofthreestationsofhighprecisiontrackingchambersintherange

|η|<2.7,anda muontriggersystemwhich covers|η|<2.4.The precisionmeasurementisensuredbymonitoreddrifttubesand,at largepseudorapidities(|η_|>2),fortheinnermostlayer,bycathode strip chambers. The muon trigger system is composed of resis- tiveplatechambersinthebarrelregion(|η|<1.05),andthingap chambersintheend-cap(1.05<|η|<2.4).

TheATLAS triggersystem hasthree distinct levels, L1,L2 and theeventfilter,whereeachtriggerlevelrefinesthedecisionsmade attheprevious level.Thedatausedforthe presentanalysiswere collected in 2012 from pp collisions at a center-of-mass energy of8 TeVatthe LHC.The total integratedluminosity is20.3 fb⁻¹ withan uncertainty of 2.8% [13]. Events are selected by triggers requiringatleastone identifiedelectronormuon. Thetransverse energy(ET) thresholdforthesingle-leptontriggeris24 GeV. The leptontriggerefficienciesaremeasured using Z bosoncandidates asa functionof thetransversemomentum p_T and η. Thetrigger

1 ATLASusesaright-handedcoordinatesystemwithitsoriginatthe nominal interactionpoint(IP)inthecenterofthedetectorandthez-axisalongthebeam pipe.Thex-axispointsfromtheIPtothecenteroftheLHCring,andthe y-axis pointsupward.Cylindricalcoordinates(r,φ)areusedinthetransverse(x,y)plane, φbeingtheazimuthalanglearoundthebeampipe.Thepseudorapidityisdefined intermsofthepolarangleθ asη= −^{ln tan}(θ/2).Thedistance R inthe η–φ spaceisdefinedasR=

(η)²+(φ)².

efficienciesfortheleptonsareapproximately70%formuonswith

|η_|<1.05,90% formuonsintherange1.05<|η_|<2.4[14],and 95%forelectronsintherange|η_|<2.4.

3. Signalandbackgroundsimulatedsamples

Monte Carlo (MC) event samples, including a full simula- tion [15] of the ATLAS detector with Geant4 [16], are used to compare the datato the signal andbackground expectations.All MC samples are simulated with additional pp interactions (pile- up)inthesameandneighboringbunchcrossings.

The productionanddecay ofneutral (ωT→^Zγ) andcharged technimesons (aT→^Wγ) inthe LSTCmodel are handledby the Pythia6.426generator [17].The followingparameters areused in theeventgeneration:numberoftechnicolorsNTC=^4;techniquark charges QU=^{1 andQ}D=^{0 forthe Z}γ finalstateand QU=¹/2 andQD= −¹/2 fortheWγ finalstate.Withthisparameterization ofthetechniquarkcharges,thedominantcontributiontotheWγ

final stateisfroma_T decay.Byremoving the ρT contribution,the modeldependencethatcouldresultfromhavingtwonearbypeaks inthebenchmarksignalisfurtherreduced.Thesineofthemixing angle betweenthe technipions andthe electroweakgauge boson longitudinalcomponentissetto1/3.Asstatedintheintroduction, the mass splittings betweenthe technimesons are set to: mρ_T= mωT=^maT/1.1,andmρT−^mπT=^mW.

Simulationsofthesignalsforsingletscalarparticlesaregener- ated using Madgraph5 [18] interfaced to Pythia8 [19] for par- ton shower and fragmentation processes. The generation uses the leading-order (LO) parton distribution function (PDF) set MSTW2008LO[20].Sincethekinematicdistributionofthe Zγ de- cayofascalarbosonisdeterminedbyitsspinandCPpropertiesin thenarrow-width approximation,the gluon–gluon fusionproduc- tionandthedecayaresimulatedbyusingtheSMHiggsprocessin Madgraph5,fixingtheresonancewidthto5.75 MeV.

Thecrosssectionofthesingletscalarprocess gg→φ→^Zγ is calculatedbychoosingtwosetsofcouplingparametersinEq.(1).

Inthe parameterset(a),the couplingcoefficientsare setto cg= c_W = −^cB =¹/4π, as suggested by Naive Dimensional Analysis (NDA) [21]. Here, the relative phasesare chosen to enhance the Zγ coupling.IfalargenewcolornumberN_c=^{4 oftheunderlying} strongdynamicsisconsideredinNDA[22,23],andlargercouplings with the electroweakgauge bosons are assumed [8],then larger values ofthe coefficientscan also be possible,depending onthe underlying theory. Thus another parameter set (b) is selected as follows: c_g=¹/2π, c_W = −^cB=¹/π. In both parameterizations, the Zγ decayrateis dominantover the γ γ decay rate. Theun- derlying dynamicsscaleisset tobe Λ=^{6 TeV,motivatedby the} Higgs boson couplings measurements [24,25]. The width of the singletscalarforeach ofset(a)and(b)iswellbelowtheexperi- mentalresolutionoverthefullmassrangestudied.

The main background processes, SM pp→⁺⁻γ and pp→ νγ production, are modeled using the Sherpa (1.4.1) genera- tor [26]. An invariant mass cut of m(⁺⁻)>40 GeV is ap- plied at the generator level when simulating the pp→⁺⁻γ

process. The CT10 [27] PDF is used for samples generated with Sherpa. In the Wγ analysis, the Z(⁺⁻) and Z(τ⁺τ⁻) back- grounds are modelled with Pythia8. The final state radiation of photonsfromchargedleptonsistreatedinPythia8usingPhotos. Tauola(1.20)[28] isusedto model τ lepton decays.Eventswith oneormorehardphotonsatthegeneratorlevel(pT>10 GeV)in Z(⁺⁻)and Z(τ⁺τ⁻) MC simulations are removed, in order to avoidoverlapswith Zγ MCsamples.Inthe Zγ analysis, Z(⁺⁻) andZ(τ⁺τ⁻)MCsamplesareusedtocrosscheckthedata-driven Z+jet backgroundestimation.