Search for heavy resonances decaying to a Z boson and a photon in pp collisions at √s = 13 TeV with the ATLAS detector

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Article

Reference

Search for heavy resonances decaying to a Z boson and a photon in pp collisions at √s = 13 TeV with the ATLAS detector

ATLAS Collaboration

ANCU, Lucian Stefan (Collab.), et al.

Abstract

This Letter presents a search for new resonances with mass larger than 250 GeV, decaying to a Z boson and a photon. The dataset consists of an integrated luminosity of 3.2 fb −1 of pp collisions collected at s=13 TeV with the ATLAS detector at the Large Hadron Collider. The Z bosons are identified through their decays either to charged, light, lepton pairs ( e+e− , μ+μ− ) or to hadrons. The data are found to be consistent with the expected background in the whole mass range investigated and upper limits are set on the production cross section times decay branching ratio to Zγ of a narrow scalar boson with mass between 250 GeV and 2.75 TeV.

ATLAS Collaboration, ANCU, Lucian Stefan (Collab.), et al . Search for heavy resonances decaying to a Z boson and a photon in pp collisions at √s = 13 TeV with the ATLAS detector.

Physics Letters. B , 2017, vol. 764, p. 11-30

DOI : 10.1016/j.physletb.2016.11.005

Available at:

http://archive-ouverte.unige.ch/unige:89836

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 heavy resonances decaying to a Z boson and a photon in pp collisions at √

s = 13 TeV with the ATLAS detector

.TheATLASCollaboration

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

Articlehistory:

Received22July2016

Receivedinrevisedform3November2016 Accepted4November2016

Availableonline11November2016 Editor:W.-D.Schlatter

ThisLetterpresentsasearchfornewresonanceswithmasslargerthan250 GeV,decayingtoaZboson andaphoton.Thedatasetconsistsofanintegratedluminosityof3.2 fb⁻¹ofpp collisionscollectedat

√s=13 TeV withtheATLASdetectorattheLargeHadronCollider.TheZbosonsareidentiﬁedthrough theirdecayseithertocharged,light,leptonpairs(e⁺e⁻,μ⁺μ⁻)ortohadrons.Thedataarefoundtobe consistentwiththeexpectedbackgroundinthewholemassrangeinvestigatedandupperlimitsareset ontheproductioncrosssectiontimesdecaybranchingratioto Zγ ofanarrowscalarbosonwithmass between250 GeVand2.75 TeV.

©²⁰¹⁶^TheÂuthor(s).^Published^byÊlsevier^B.V.^Thisîsânôpenâccessârticleûnder^the^CC^BY^license (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP³.

1. Introduction

ManymodelsofphysicsbeyondtheStandardModel(SM)intro- ducenewbosonsthrough eitheranextension oftheHiggssector oradditionalgauge fields.This suggeststhat a broadexperimen- talsurveyofphysicsbeyondtheSMcanbemadebysearchingfor newmassivebosons. Some modelspredict that thesebosons de- caytofinal statescontainingtheSM electroweakW or Z bosons orphotons[1,2].Attractivedecaysfroman experimentalperspec- tiveareto γ γ [3–6], Zγ [7,8]or Z Z [9,10]finalstates,sinceboth the Z bosons and photons in pair production can be measured well with relatively low backgrounds. If such new bosons were produced,the complete reconstruction ofthesefinal states could beusedtopreciselymeasuretheirproperties,suchastheirmass.

ThisLetter presentsasearchfor X→^Zγ resonances usingan integratedluminosity of3.2fb⁻¹ ofproton–proton(pp) collisions atacentre-of-massenergy√

sof13 TeV,collectedwiththeATLAS detectoratthe LargeHadron Collider (LHC) in2015. To enhance the sensitivity of the search, both the leptonic (Z→⁺⁻, = e,μ)¹ and hadronic (Z→^qq)¯ decay modes of the Z boson are used.The combined selection capturesabout77% of all Z boson decays.Inthefollowing,thesearchbasedontheselectionofγ

ﬁnal states is also referred to asthe leptonicanalysis, while the searchbasedontheselectionoftheq^q¯γ ﬁnalstateisalsoreferred toasthehadronicanalysis.

Theleptonicanalysisuseseventscollectedusingleptontriggers and is performed in the X boson mass (mX) range 250 GeV–

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

1 Inthefollowing,⁺⁻ﬁnalstatesarereferredtoasforsimplicity.

1.5 TeV. The hadronic analysis is performed in the mX range 700 GeV–2.75 TeV. Due to the large value of mX, the Z bosons from X→^Zγ are highly boosted andthe two collimated sprays of energetic hadrons, called jets in the following, that are produced in Z →^qq¯ decays are merged into a single, large-radius, jet J.Theeventsusedforthehadronicanalysisarecollectedusing single-photontriggers. Duetothe larger Z bosonbranching ratio tohadrons,theboostedhadronicanalysisdominatesthesensitivity athighmX,wherethenumberofeventsisvery small, whilethe leptonicanalysis,withitshighersignal-to-backgroundratio,domi- natesthesensitivityatlowmX.

Previous searches for non-SM bosons decaying into Zγ ﬁnal stateswerecarriedoutattheTevatronandtheLHC.TheD0Collab- orationsetlimits[11]on X→^Zγ productionusing pp¯ collisions at √

s=¹.96 TeV. At the LHC, the ATLAS Collaboration used pp collisions collected in2011and2012at√

s=7 and 8 TeV toex- tendthemassrangeandsensitivityofX→^Zγ searches[7,8].The analysesassumedanarrowwidthforthe X bosonandusede⁺e⁻ and μ⁺μ⁻decaysofthe Z boson.Nosignalswere observedand limitsonthe productoftheproductioncross section σ(pp→^X) timesthebranching ratio B R(X→^Zγ) weredetermined forval- uesofmX intherange≈²⁰⁰^to^{1600 GeV.}

Theanalysespresentedheresearchforalocalizedexcessinthe reconstructedinvariantmassdistributionoftheﬁnalstate,eithera photonandtwoleptonsoraphoton andaheavy,large-radius jet.

Inthe leptonicanalysis, themain backgroundarisesfromcontin- uumproductionofaZ bosoninassociationwithaphoton,or,toa lesserextent,withahadronicjetmisidentiﬁedasaphoton.Inthe hadronic analysis, the background is dominated by non-resonant SM productionofγ₊jet events,withsmallercontributionsfrom dijet events with a jet misidentiﬁed as a photon, and from SM http://dx.doi.org/10.1016/j.physletb.2016.11.005

0370-2693/©²⁰¹⁶^TheÂuthor(s).^Published^byÊlsevier^B.V.^Thisîsânôpenâccessârticleûnder^the^CC^BY^license(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP³.

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V+γ events(V =^W,Z). Theinvariant mass distributionofthe background should be smoothly and steeply decreasing withthe mass.It isparameterizedby a smooth functionwithfree parameters, whichare adjusted to the data.The intrinsic width ofthe heavybosonisassumedtobesmallcomparedtotheexperimental resolution.Thebosonisassumedtobeaspin-0particleproduced viagluonfusion.

2. TheATLASdetector

The ATLAS detector is a multi-purpose particle detector with approximatelyforward–backwardsymmetriccylindricalgeometry.² Its original design [12] hasbeen complemented with the instal- lation,prior tothe 2015 data-taking, ofa new innermost silicon pixellayer[13].

Atwo-leveltriggersystem[14]selectseventstoberecordedfor offlineanalysis.Thefirst-leveltriggerishardware-based,whilethe second,high-leveltriggerisimplementedinsoftwareandemploys algorithmssimilartothoseusedofflinetoidentifyleptonandpho- toncandidates.

3. Datasample

Data were collected in 2015 during pp collisions at a centre- of-massenergyof13 TeV. Thebunch spacingwas 25nsandthe average number of inelastic interactions per bunch crossing was 13.

The search in the γ ﬁnal state is performed in events recorded usingthe lowest-threshold unprescaledsingle-lepton or dileptontriggers.Thesingle-muontriggerhasanominaltransverse momentum(pT)thresholdof20 GeVandalooserequirementon thetrackisolation.Thisquantity,deﬁnedasthesumofthetrans- verse momenta ofthe tracks in the inner detector(ID) found in a cone of size R≡

(η)²+(φ)²=⁰.2 around the muon, excluding the muon track itself, is required to be less than 12%

ofthe muon pT. Only trackswith longitudinalimpact parameter z0 within 6 mm of that from the muon track are considered in the calculation. An additional single-muon trigger with a higher p_T threshold(50 GeV)but noisolation requirement isalso used.

Thedimuon triggerhasa p_T thresholdof10 GeV forbothmuon candidatesandappliesnoisolationcriteria.Thesingle-electron(di- electron)triggerhasanominal pT thresholdof24 GeV(12 GeV).

Electroncandidatesarerequiredtosatisfylikelihood-basedidenti- ficationcriterialooserthanthoseappliedofflineanddescribed in Section5.The electronidentificationlikelihoodiscomputedfrom both the propertiesof the trackreconstructed in the ID andthe energydepositedintheelectromagnetic(EM)calorimeter.

The search in the Jγ ﬁnal state uses eventsrecorded by the lowest-pTthresholdunprescaledsingle-photontrigger.Thistrigger requiresatleastonephotoncandidatewithp_T>120 GeV passing looseidentiﬁcationrequirementsbasedontheshapeoftheshower intheEMcalorimeterandontheenergyleakingintothehadronic calorimeter[15].

Thetrigger eﬃciencyforeventssatisfyingthe oﬄineselection criteriadescribed inSection 5isgreaterthan99%intheeeγ and Jγ channels and is about 96% in the μμγ channel due to the reducedgeometricacceptanceofthemuontriggersystem.

2 ATLASusesaright-handed coordinatesystemwith itsoriginat thenominal interactionpoint(IP)inthecentreofthedetectorandthez-axisalongthebeam pipe.Thex-axispointsfromtheIPtothecentreoftheLHCring,andthe y-axis pointsupward.Cylindricalcoordinates (r,φ)areusedinthe transverseplane,φ beingtheazimuthalanglearoundthez-axis.Thepseudorapidityisdeﬁnedinterms ofthepolarangleθasη= −^{ln tan}(θ/2).

Theintegratedluminosityafterthetriggeranddataqualityre- quirementsisLint=³.2 fb⁻¹.

4. MonteCarlosimulation

Simulatedsignalandbackgroundsamplesweregeneratedwith a MonteCarlo(MC) technique.Theyare usedtooptimizethese- lection criteria and to quantify the signal eﬃciency of the ﬁnal selection. SuchMC samplesare alsousedto testthe analyticpa- rameterization of the Zγ invariant mass spectra of signal and background,while theestimate ofthebackgroundyield afterthe selectionisestimatedinsitufromthedata.

All MC samples are generated assuming a centre-of-mass pp collision energy of 13 TeV. The samples are passed through a detailed simulation of the ATLAS detector response [16] based on Geant4 [17]. Multiple inelastic proton–proton collisions (referred to as pile-up) are simulated with the soft QCD processes of Pythia 8.186 [18] using the A2 set of tuned parameters (A2 tune) [19] and the MSTW2008LO parton distribution function (PDF) set [20],and are overlaid on each MC event. The distribution of the number of pile-up interactions in the simulation is reweightedtomatchthedata.Thesimulatedsignalsinthedetec- tor are passed through the eventreconstruction algorithms used for the data. The simulation is tuned to take into account small differenceswithdata.Theseincludecorrectionstophoton, lepton andjet reconstructionandselection eﬃciencies,andtheir energy or momentum resolutionandscale. The correctionsare obtained either from control samples selected in early √

s=13 TeV data or from 8 TeV data with additional systematic uncertainties in- troducedto coverthedifferentconditions betweenthe2012and 2015data-taking.

In the signal simulation, a scalar boson X is produced in pp collisionsviagluonfusion,anddecaystoaphotonanda Z boson.

Monte Carlo samples are produced for different mX hypotheses between 200 GeV and 3 TeV. The width of the boson X is set to 4 MeV, which ismuch smaller than the experimental resolution,regardlessoftheresonancemass.Duetotheassumednarrow widthofthe X bosonandthe smallcontributionofgluon fusion tothenon-resonantSMproductionof Z+γ [21],theinterference betweenthe gg→^X→^Zγ signalprocess andthe SM gg→^Zγ

backgroundisneglectedinthesimulation.Thesignal samplesare generated with POWHEG-BOX [22,23] interfaced to Pythia 8.186 fortheunderlyingevent,partonshoweringandhadronization.The CT10[24]PDFsetandtheAZNLOtune[25]oftheunderlyingevent areused.

Events fromSM processescontaining a photonanda Z or W boson (V +γ), a Z boson produced in association with jets, or a promptphoton produced inassociationwithjets(γ ₊jets) are simulated using the Sherpa 2.1.1 [26] generator. The matrix ele- ments forSM V +γ (γ ₊jets)productionare calculatedforreal emission of up to three (four) partons at leading order (LO) in thestrong couplingconstant αS andaremergedwiththeSherpa parton shower [27] using the ME+^PS@LO prescription [28]. The matrixelementsofeventscontainingZ bosonswithassociatedjets arecalculatedforuptotwopartonsatnext-to-leadingorder(NLO) andfourpartonsatLOandmergedwiththepartonshowerusing theME+^PS@NLOprescription[29].Thematrixelementsarecalcu- latedusingtheComix[30]andOpenLoops[31]generators.Forall thebackgroundsamples,theCT10PDF setisusedinconjunction withdedicatedpartonshowertuningdevelopedbytheSherpaau- thors. The γ ₊jets and V +γ samplesare generatedin binned rangesofthetransversemomentumofthephoton toensurepre- cisepredictionsoverthefullspectrumrelevantfortheseanalyses.

Similarly, Z +jets events are generated in binned ranges of the dileptonpairp_T fromthe Z bosondecays.

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5. Eventselection

Eventswithatleastoneprimary vertexcandidatewithtwoor moretracks with p_T>400 MeV are selected. In each event, the primary vertex candidate with the largest sum of the p²_T of the associatedtracksischosenasthehardinteractionprimaryvertex.

Events are required to contain at least one photon candidate and one Z boson candidate. In the leptonic analysis, the Z bo- soncandidateisformedfromapairofopposite-sign,same-ﬂavour electronsormuons.Inthehadronicanalysis,Z bosonsarerequired torecoilagainst a high-momentumphoton (pT>250 GeV);asa consequence of the Z boson’s large Lorentz boost, the two jets fromthehadronization of thetwo quarks are reconstructed asa single, relativelyheavy, large-radius jet.Jet-substructure variables andthejet massare then usedto discriminatebetweena Z bo- sondecayandjetsfromsinglequarksorgluons[32].Events with oneormoreelectron ormuoncandidates satisfyingtheselection describedbelowarevetoedinthehadronicanalysis.Inthefollow- ing,theselection ofphotons,leptons, large-radiusjetsandofthe ﬁnal X→^Zγ candidatesisdescribed.

Unconverted photons, photon conversions to electron-positron pairs,andelectrons arereconstructed fromclustersofenergyde- positsintheEMcalorimetercellsfoundbyasliding-windowalgo- rithmandfromtracksreconstructedintheIDandextrapolatedto thecalorimeter[33,34].

Photoncandidatesarerequiredtohaveapseudorapiditywithin the regions |η|<1.37 or 1.52<|η|<2.37, where the ﬁrst calorimeter layer has high granularity. In the leptonic analysis, the transverse momentum of photon candidates is initially required to pass a loose preselection, pT>15 GeV, whereas the ﬁnal photon pT requirement is applied when a Zγ candidate is reconstructed,asdescribedlater.Inthehadronicanalysis,thepho- tontransversemomentum isrequiredtobe largerthan 250 GeV.

Toreduce background fromhadronic jets, photon candidates are required to satisfy a set of requirements on the shower leakage in the hadronic calorimeter and on the transverse shower pro- file measured with the first two layers of the electromagnetic calorimeter [33]. The requirements were optimized using simu- latedsamplesofphotonsandhadronicjetsproducedin13 TeVpp collisions.Theefficiencyoftheidentificationcriteriaisabout98%

forconverted photoncandidatesand94%forunconverted photon candidateswith p_T>100 GeV. Backgroundfrom hadronicjetsis further reduced by requiring the transverse energy measured in thecalorimeterinaconeofsizeR=⁰.4 aroundthephotondi- rection(ET,iso[35],alsocalledcalorimeterisolationinthefollowing) tobelessthan2.45GeV+⁰.022×^pT.

Electron candidates are required to have pT>10 GeV and

|η_|<2.47,excludingthetransitionregionbetweenthebarreland endcaps in the EM calorimeter (1.37<|η|<1.52). To suppress background from hadronic jets, electron candidates are required tosatisfylikelihood-basedidentificationcriteria[36].Suchrequire- mentsprovideapproximately85%identificationefficiencyforelec- tronswithatransverse momentum of20 GeV,increasing to 95%

forpT>80 GeV.

Muons with |η|<2.5 are reconstructed by combining tracks in the ID with tracks in the muon spectrometer (MS) [37]. The acceptance is extended to the region 2.5< |η_|<2.7 by also selecting muons whose trajectory is reconstructed only in the MS.Muon candidatesarerequiredto havetransversemomentum above 10 GeV. Background muons, originatingmainly from pion andkaondecays,arerejectedbyapplyingasetofqualityrequire- mentson thenumber ofhitsinthe muonspectrometer and(for

|η_|<2.5) on the compatibility betweenthe ID andMS momen- tummeasurements. The muon identiﬁcation eﬃciency is around 97%fortransversemomentaabove10 GeV.

Iftwo electroncandidates sharethesame track, orhaveclus- ters in the calorimeter separated by |η_|<0.075 and |φ|<

0.125, only the candidate with the higher energy measured by thecalorimeteriskept.Inaddition,ifthetrackassociatedwithan electroncandidateis withina distanceR=⁰.02 fromthetrack associated with a muon candidate, the electron candidate is re- jected.

Track and calorimeter isolation requirements are further ap- pliedto theselected leptons.Forelectrons, combinedcriteriaare applied to the calorimeter isolation, E_T_,_iso, in a cone of radius R=⁰.2,andtothetrackisolation,

trackspT,inaconeofradius R=⁰.2 forelectrontransversemomentapT<50 GeV andofra- diusR=(10 GeV)/p_T forp_T>50 GeV.Inthecalculationofthe trackisolation,thecontributionfromtheelectrontrackitselfisnot included.Thecriteriaarechosen toprovideaneﬃciencyofabout 99%independentoftheelectrontransversemomentumandpseu- dorapidity, asdetermined in a control sample of Z→^ee ^decays selected with a tag-and-probe technique [36]. For muons, com- binedcriteriaareimposed on E_T_,_iso inaconeofradiusR=⁰.2 andon

trackspTinsideaconeofradiusR=⁰.3 formuontrans- versemomenta pT<33 GeV andofradiusR=(10 GeV)/pT for p_T>33 GeV. The eﬃciency of these criteria increases with the muontransverse momentum,reaching95% at25 GeVand99%at 60 GeV,asmeasuredin Z→μμeventsselectedwithatag-and- probemethod[37].

In the hadronic analysis, topological clustersof energy in the calorimeterthatwere locallycalibratedandassumedtobe mass- less[38]areusedasinputstoreconstructlarge-radiusjets, based ontheanti-kt algorithm[39]withradiusparameter R=¹.0[40].

Within the large-radius jets, smaller “subjets” are reconstructed using the k_⊥ algorithm [41,42] with a radius parameter R= R_sub=⁰.2.Thelarge-radiusjet istrimmed[43] byremovingsub- jets that carry fractional pT less than fcut =^{5% of} ^the ^pT of the original jet. The pseudorapidity, energy and mass of these trimmedlarge-radius jetsarecalibrated usinga simulation-based calibrationscheme[44].Thelarge-radiusjetsarerequiredtohave p_T>200 GeV and |η|<2.0. Large-radius jets within R=¹.0 fromselectedphotonsarediscarded.ApT–dependentrequirement on the substructure observable D^(β=₂ ¹⁾ [45], deﬁned as the ratio e^(β=₃ ¹⁾/

e^(β₂ ⁼¹⁾

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ofN-pointenergycorrelationfunctionse^(β=_N ¹⁾of the jet constituents [46], is used to select hadronically decaying bosonswhilerejectingjetsfromsinglequarksorgluons.Theratio makes useof the sensitivity of the eN functionsto the “prongi- ness” characterofthejet.Inparticular,it reliesonthesensitivity of e₂ to radiationaround a single hard core, andof e₃ to radia- tionwithtwocores.Thepowersofthee2 ande3 functionsinthe ratioarechosentooptimizethediscrimination betweenone- and two-prongjetsfollowingananalysisofthe(e₂,e₃)phase-spaceof thesetwotypesofjets.

The jet mass mJ, computed from its topological cluster con- stituentsthat remainafterthetrimming procedure,isrequiredto beintherange80 GeV<mJ<110 GeV.Thejetisrequiredtobe associatedwithlessthan30trackswithpT>500 MeV originating from the hard-interaction primary vertex (before trimming).The eﬃciencyofthe D^(β=₂ ¹⁾,m_J andnumber-of-trackrequirements is around22% forthesignal jetand2.2%forjetsfromsingle quarks orgluons.

Aftertheselectionofphotons,leptonsandlarge-radiusjetcan- didates,the Zγ candidateischosen.Ifaneventhasmultiplepho- tonorjetcandidates,onlythephotonorjetcandidatewithhigh- est transverse momentum is kept. In the leptonic analysis, only Z → candidates with invariant massm within ±^{15 GeV of} the Z boson mass[47] are retained; incaseofmultiple dilepton candidates,onlytheonewithinvariantmassclosest tothe Z bo-