Search for new resonances in events with one lepton and missing transverse momentum in $\mathit{pp}$ collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

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Physics Letters B

www.elsevier.com/locate/physletb

Search for new resonances in events with one lepton and missing transverse momentum in pp 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:

Received14June2016

Receivedinrevisedform16August2016 Accepted20September2016

Availableonline28September2016 Editor:W.-D.Schlatter

AsearchforW^bosonsineventswithonelepton(electronormuon)andmissingtransversemomentum is presented. The search uses 3.2 fb⁻¹ of pp collision data collected at √

s=^{13 TeV by the ATLAS} experimentattheLHCin2015.Thetransversemassdistributionisexaminedandnosignificantexcess ofeventsabovethelevelexpectedfromStandardModelprocessesisobserved.UpperlimitsontheW^ bosoncross-sectiontimesbranchingratio toleptons areset asafunctionoftheW^ mass.Withinthe Sequential StandardModel W^ massesbelow 4.07 TeVare excludedatthe 95%confidencelevel. This extendsthelimitsetusingLHCdataat√

s=^{8 TeV byaround800 GeV.}

©^{2016TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense} (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP³.

1. Introduction

Manymodelsofphysics beyondtheStandardModel(SM)pre- dicttheexistence ofnewspin-1 gaugebosons that could bedis- covered atthe Large Hadron Collider (LHC).While the details of themodelsvary, conceptuallytheseparticlesare heavierversions oftheSM W and Z bosonsandaregenericallycalled W^and Z^ bosons.

In this letter, a search for a W^ boson is presented using 3.2 fb⁻¹ of pp collision data collected with the ATLAS detector in 2015 at a centre-of-mass energy of 13 TeV. The results are interpreted in thecontext of the benchmark SequentialStandard Model(SSM),i.e.theextended gaugemodeldescribed inRef.[1], inwhich the couplingsof the W_SSM^ tofermions are assumedto be identicalto those ofthe SM W boson. Thedecay ofthe SSM W^ to SM bosons is not allowed and interference between the SSM W^ and the SM W boson is neglected. The search is con- ductedintheW^→ νchannel,whereisanelectronoramuon.

Thesignatureisachargedleptonwithhightransversemomentum (pT) andsubstantialmissingtransversemomentum(E^miss_T )dueto theundetectedneutrino.Thediscriminanttodistinguishsignaland backgroundisthetransversemass

m_T=

2p_TE^miss_T (1−^cosφν), (1)

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

whereφν istheanglebetweentheleptonandE_T^missinthetrans- verseplane.¹ThedominantbackgroundfortheW^→ νsearchis thehigh-mTtailofthecharged-currentDrell–Yan(qq¯^→^W→ ν) process.

PrevioussearchesforW_SSM^ bosonsintheW^→^eνandW^→ μν channels were carried out by both the ATLAS and CMS col- laborations using the Run-1 data. The previous ATLAS analysis is based on data corresponding to an integrated luminosity of 20.3 fb⁻¹ taken at a centre-of-mass energy of √

s=^{8 TeV and} sets a 95% confidence level (CL) lower limit on the W_SSM^ mass of 3.24 TeV[2]. The CMSCollaboration published a search using 19.7 fb⁻¹ of √

s=^{8 TeV data from 2012 which excludes W}_SSM^ massesbelow3.28 TeV at95% CL[3].

2. ATLASdetector

The ATLAS experiment [4]atthe LHC isa multi-purposepar- ticledetectorwitha forward–backwardsymmetriccylindricalge- ometry anda near 4π coverage in solid angle. It consistsof an innertrackingdetector(ID)surroundedbyathinsuperconducting solenoidprovidinga2 Taxialmagneticfield,electromagnetic(EM) andhadroniccalorimeters,andamuonspectrometer(MS).Thein- ner trackingdetectorcoversthepseudorapidityrange|η|<2.5.It

1 ATLAS usesaright-handedcoordinatesystemwithitsoriginatthe nominal interactionpoint(IP)inthecentreofthedetectorandthe z-axisalongthebeam pipe.Thex-axispointsfromtheIPtothecentreoftheLHCring,andthe y-axis points upward.Cylindricalcoordinates(r,φ)areusedinthetransverseplane,φ beingtheazimuthalanglearoundthebeampipe.Thepseudorapidityisdefinedin termsofthepolarangleθasη= −ln tan(θ/2).

http://dx.doi.org/10.1016/j.physletb.2016.09.040

0370-2693/©^{2016TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense}(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP³.

consists of a silicon pixel detector including the newly installed insertableB-layer [5,6],followedbysiliconmicrostrip,andtransi- tionradiationtrackingdetectors.Lead/liquid-argon(LAr)sampling calorimetersprovideEMenergymeasurementswithhighgranular- ity.A hadronic (steel/scintillator-tile) calorimeter covers the cen- tralpseudorapidityrange(|η_|<1.7). Theendcapandforwardre- gionsareinstrumentedwithLArcalorimetersforboththeEMand hadronicenergy measurements up to |η_|=4.9. The muon spec- trometersurrounds the calorimeters andis based on three large air-coretoroidsuperconductingmagnetswitheightcoilseach.The field integral of the toroids ranges between 2.0 and 6.0 Tm for most of the detector. It includes a system of precision tracking chambers, over |η|<2.7, and fast detectors for triggering, over

|η|<2.4.Atwo-level triggersystemisusedtoselectevents.The first-level triggeris implemented in hardware anduses a subset ofthedetector information.Thisisfollowed by asoftware-based triggersystemthatreducestheacceptedeventratetoabout1 kHz.

3. Backgroundandsignalsimulation

Monte Carlo(MC) simulation samples are used to model the expectedsignal andbackground processes,with theexception of data-driven background estimates forevents in which one final- statejetorphotonsatisfiestheelectronormuonselectioncriteria.

Themain backgroundis duetothe charged-currentDrell–Yan (DY)process,generatedatnext-to-leadingorder(NLO)inQCDus- ing Powheg-Box v2[7]andtheCT10partondistributionfunctions (PDF) [8], with Pythia 8.186 [9] to modelparton showering and hadronisation.The samesetup isused fortheneutral-current DY (q^q¯→^Z/γ^∗→ ^{) process. In both cases, samples for all three} leptonflavoursaregenerated,andthefinal-statephotonradiation (QED FSR) is handled by Photos [10]. The DY samples are nor- malised asa function of massto a next-to-next-to-leading order (NNLO)perturbativeQCD(pQCD)calculationusing VRAP[11] and the CT14NNLO PDF set [12]. In addition, NLO electroweak (EW) correctionsbeyondQEDFSRarecalculatedwith Mcsanc[13,14]at LOinpQCD asa function ofmass.In orderto combinethe QCD andEW terms,the so-calledadditive approachisusedwherethe EWcorrectionsareaddedto theNNLOQCD cross-sectionpredic- tion.

Backgroundsfromtt and¯ singletop-quarkproductionare esti- mated atNLO using Powheg-Box.These processes use theCT10 PDFset andare interfacedto Pythia 6.428[15] forpartonshow- eringandhadronisation. Furtherbackgrounds are duetodiboson (W W , W Z and Z Z ) production. These processes are generated with Sherpa 2.1.1[16]usingtheCT10PDFset.

Signal samplesforthe W^→^eν and W^→μν processes are producedatleadingorder(LO)inQCDusing Pythia 8.183andthe NNPDF2.3LOPDFset.The W_SSM^ bosonhasthesamecouplingsto fermionsastheStandard ModelW boson andisassumednot to coupleto the SM W and Z bosons.Interference effectsbetween the W^ and the SM W boson are neglected. In this model the branchingratiotoa chargedleptonandaneutrinois 8.2%inthe entiremassrangeconsidered inthissearch.The decayW^→τ ν, wherethe τ leptonsubsequentlydecaysleptonicallyisnottreated aspartofthesignal.Ifincluded,thisdecaywouldconstituteavery smallcontribution.Thesignalsamplesarenormalisedtothesame mass-dependentNNLO pQCD calculation asused fortheDY pro- cess. TheEW correctionsbeyondQEDFSRare not applied tothe signalsamplesbecausethey dependon thecouplingsofthenew particleto W and Z bosons, andarethereforemodel-dependent.

Theresultingcross-sectiontimesbranchingratioforW_SSM^ masses of2,3and4 TeV are153,15.3and2.25 fb,respectively.

For all samples used in this analysis, the effects of multiple interactions per bunch crossing (“pile-up”) are accounted for by

overlayingsimulatedminimum-biasevents.Theinteractionofpar- ticleswiththedetectoranditsresponsearemodelledusingafull ATLAS detectorsimulation[17] performedby Geant4[18].Differ- encesbetweendataandsimulationareaccountedforinthelepton trigger,reconstruction,identification[19,20],andisolationefficien- ciesaswellastheleptonenergy/momentumresolutionandscale [21,20].

4. Objectreconstructionandeventselection

Eventsinthemuonchannelareselectedbyatriggerrequiring that atleastonemuon with pT>50 GeV isfound. Thesemuons must be reconstructed in both the MS and the ID. In the elec- tronchannel,eventsareselectedbyatriggerrequiringatleastone electron candidate with pT>24 GeV that satisfies the medium identification criteria or a trigger requiring at least one electron with pT>120 GeV thatsatisfies the loose identificationcriteria.

Theselectioncutsusedtoselectelectroncandidatesattriggerlevel areverysimilartotheonesusedintheofflinereconstructionand wereoptimisedusingalikelihoodapproach[19].

The selected events must have a reconstructed primary ver- tex,whichistheinteractionvertexwiththehighestsumof p²_T of tracksfound intheevent. Eachvertexreconstructed intheevent consistsofatleasttwoassociatedtrackswithpT>0.4 GeV. Only datatakenduring periods whenalldetectorcomponents andthe triggerreadoutarefunctioningwellareconsidered.

Muons are reconstructed from MS tracks and matching ID trackswithin |η_|<2.5,requiringthat theMStrackshaveatleast threehitsineachofthethreeseparate layersofMS chambersto ensureoptimalresolutionforhigh-momentummuons[20].Inad- dition,thesecombinedmuonsarerequiredtopassatrackquality selectionbasedonthenumberofhitsintheID.Toreducesensitiv- ityto therelative barrel–endcapalignment intheMS, theregion 1.01<|η_|<1.10 is vetoed. Muons are rejected ifthe difference between the muon charge-to-momentumratios measured inthe IDandMS exceedsseventimesthesuminquadratureofthecor- respondinguncertainties,orifthetrackcrossespoorlyalignedMS chambers. Toensure that the muonsoriginate fromthe primary vertex, thetransverseimpactparametersignificance,whichisthe ratiooftheabsolutevalueofthetransverseimpactparameter(d0) toitsuncertainty,hastobebelowthree.Thedistancebetweenthe z-position ofthe point of closest approach of themuon track in the ID tothe beamline andthe z-coordinate ofthe primary ver- texisrequiredtobelessthan10 mm.Furthermore,onlyisolated muonsareconsidered.ThescalarsumoverthetrackpT inaniso- lationcone aroundthe muon(excludingthe muonitself)divided bythemuonpTisrequiredtobebelowapT-dependentcuttuned fora99%efficiency.TheisolationconesizeR=

(η)²+ (φ)² isdefinedas10 GeV dividedbythemuon p_Tandhasamaximum sizeofR=⁰.3.

Electronsare formed fromclustersof cells inthe electromag- neticcalorimeterassociatedwithatrackintheID.Theelectronp_T isobtainedfromthecalorimeterenergymeasurementandthedi- rection of the associated track.The electron must be within the range |η|<2.47 and outside the transition region between the barrel and endcap calorimeters (1.37<|η|<1.52). In addition, tight identification criteria [19] need to be satisfied. The identi- fication usesalikelihood discriminantbasedonmeasurements of calorimetershower shapesandmeasurements oftrackproperties fromthe ID. Toensure that theelectrons originate fromthe pri- maryvertex,thetransverseimpactparametersignificancemustbe belowfive.Furthermore,calorimeter- andtrack-basedisolationcri- teria,tunedforanoverallefficiencyof98%,independentofpT,are applied.Thesumofthecalorimetertransverseenergydepositsin theisolation coneofsize R=⁰.2 (excluding theelectronitself)

dividedbytheelectron pT isusedinthediscriminationcriterion.

Thetrack-basedisolationisdeterminedsimilarlytothatformuons.

Thescalarsumofthe p_T ofall tracksinacone aroundthe elec- tron, divided by the electron pT hasto be below a given value.

TheconehasasizeR=^10GeV/pT(e)withamaximumvalueof R=⁰.2.

Thecalculation ofthe missingtransversemomentum isbased on the selected electrons, photons, tau leptons, muons and jets found in the event. The value of E^miss_T is evaluated by the vec- tor sumofthe pT ofthe physics objects selectedin the analysis andthe tracksnotbelongingtoanyofthesephysics objects[22].

JetsusedintheE^miss_T calculationarereconstructedfromclustersof calorimetercellswith|η|<5 usingtheanti-kt algorithm[23]with a radius parameter of 0.4. They are calibrated using the method describedinRef.[24]andarerequiredtohave pT>20 GeV.

Events areselected iftheyhaveexactly one electronormuon withpT>55 GeV. The E^miss_T value foundintheeventisrequired to exceed 55 GeV and the transverse mass has to satisfy mT>

110 GeV.Fortheseselection cutsthe acceptancetimesefficiency, definedasthefractionofsimulatedcandidateeventsthatpassthe eventselection,amountsto81%(75%)fortheelectronchanneland 53%(50%)forthemuonchannelata W^massof2 TeV (4 TeV).

5. Backgroundestimateandcomparisontodata

Thebackgroundfromprocesseswithatleastonepromptfinal- state lepton is estimated with simulated events. The processes withnon-negligiblecontributionsarecharged-currentDY(W pro- duction),tt and¯ single top-quarkproduction,inthe followingre- ferredtoas“top-quark”background,aswellasneutral-currentDY ( Z/γ^∗ production)anddibosonproduction.

Backgroundcontributionsfromeventswhereonefinal-statejet orphotonpassestheleptonselectioncriteriaaredeterminedusing a data-driven “matrix” method. This includes contributions from multijet,heavy-flavourquark and γ ₊ jet production,referredto hereafter as the multijet background. The first step of the ma- trix method is to calculate the factor f , the fraction of lepton candidates that pass the nominal lepton identificationand isola- tionrequirementsinabackground-enriched datasamplecontain- ing“loose”leptoncandidates.Theseloosecandidatessatisfyonlya subset ofthenominalcriteria, whichare stricterthan thetrigger requirements imposed. Potential contamination of prompt final- stateleptons inthebackground-enriched sample isaccountedfor using MC simulation. In addition to the factor f , the fraction of realleptonsr inthesampleoflooseobjectssatisfyingthenominal requirementsisusedinevaluatingthisbackground.Thisprobabil- ityiscomputedfromMCsimulation.

The contribution to the background from events with a fake lepton isdetermined in the following way.The relation between thenumberofrealpromptleptons(NR)orfakeleptons(NF)and the number of measured objects found in the events containing thelooseleptoncandidates(NT,NL)canbewrittenas

N_T NL



=

 r f

(1−^r) (1− ^f)

 N_R NF



, (2)

wherethesubscript T referstoleptons thatpassthenominalse- lection.ThesubscriptL correspondstoleptonsthatpasstheloose requirements described above butfail the nominalrequirements.

Thenumberofjetsandphotonsmisidentifiedasleptons(N^Multijet_T ) inthetotalnumberofobjectspassingthesignalselection(NT) is givenas

N^Multijet_T = ^{f N}F= ^f r−^f



r(NL+^NT)−^NT



. (3)

Theright-handsideofEq.(3)isobtainedbysolvingEq.(2).

The simulated top-quarkand diboson samples aswell as the data-driven background estimate are statistically limited atlarge mT.Therefore,theexpectednumberofeventsisextrapolatedinto the high-mT region using parameterisations of themT shape fit- tedtotheexpectedbackgroundinthelow-m_T region.Severalfits are carriedout based onthe functions f(m_T)=^amb_T^{mc log m}_T ^{T and} f(m_T)=^a/(m_T+^b)^c.Thesefitsexplorevariousfitrangestypically starting between 140 and 200 GeV and extending up to 600 to 900 GeV. Thefitwiththebest χ² per degreeoffreedom isused asthe extrapolatedbackgroundcontribution,withan uncertainty evaluatedusingtheenvelopeofallperformedfits.

Finally,theexpectednumberofbackgroundeventsiscalculated asthesumofthedata-drivenandsimulatedbackgroundestimates.

The backgroundisdominated bythe charged-currentDY produc- tion for all values ofmT, as can be seen in the upper panel of Fig. 1. Forexample, the contribution fromcharged-current DY is about90%forbothchannelsatm_T>1 TeV.Inbothchannels, the numberofobserved eventsagreeswiththebackground estimate, asshownintheuppertwopanelsofFig. 1andinTable 1.Ascan be seen in the middlepanels, the data are systematically above the predictedbackgroundatlowmT butarewithin the±¹σ un- certaintyband,whichisdominatedbytheE^miss_T relatedsystematic uncertainties inthis region.The lower panels ofFig. 1 show the ratioofthedatatotheadjustedbackgroundthat resultsfromthe statisticalanalysisdescribedinSection7.Thedataagreewellwith theadjustedbackgroundprediction.

6. Systematicuncertainties

Experimental systematic uncertainties arise from the back- groundandluminosityestimates,thetriggerselection, thelepton reconstruction, identificationandisolationcriteria[19,20],aswell as effects ofthe energy/momentum scale and resolution [21,20].

Thesystematicuncertaintiesforthetwochannelsaresummarised inTable 2.Atlargem_T,thedominantsourceofuncertaintyisdue tothe backgroundextrapolations inthe electronandmuon chan- nels, described inSection 5, andtothe momentum resolutionin the muon channel. The extrapolation uncertainties are shown in Table 2forthedata-drivenmultijetbackgroundandthecombined top-quarkanddibosonbackgrounds.Themultijetbackgroundun- certaintyintheelectronchannelincludesa25%contributionfrom the data-driven estimate,which is dueto thedependenceof the factor f (seeSection5)onthespecificselectionusedtoderivethe background-enrichedsample.Noadditionaluncertaintyisassigned inthemuoncaseasthemultijetbackgroundissmall.

Theelectronandmuonreconstruction,identificationandisola- tion efficienciesaswellastheircorresponding uncertainties were evaluated fromdatausingtag-and-probe methodsin Z bosonde- cays up to a pT ofO(^{100 GeV}).The ratioof theefficiencymea- suredindatatothatoftheMCsimulationisthenusedtocorrect theMCprediction.Forelectrons,theseratiosaremeasuredfollow- ing the prescriptions of Ref. [25], withadjustments forthe 2015 runningconditions.Forhigher-p_T electrons,an additionalsystem- aticuncertaintyof2.5%isassignedtotheidentificationefficiency.

Thisisbasedondifferencesobservedbetweendataandsimulation, andtheirpropagationtothesimulatedelectrons.Fortheisolation efficiency,anadditionaluncertaintyof2%isattributedtohigh-pT electronsfromthevariationofthemeanvaluesoftheratioofthe isolation efficiencies between data and simulation in various p_T and η bins.For muons,no significant dependenceofthe ratioof the efficiencies measured in dataover the onesmeasured inMC simulationasafunctionofpTisobserved[20].Forhigh-pTmuons an upper limit on the uncertainty of 2–3% per TeV is extracted fromsimulation.Fortheisolationcriterionanextrapolationofthe