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Physics Letters B
www.elsevier.com/locate/physletb
Search for new resonances decaying via WZ to leptons in proton–proton collisions at √
s = 8 TeV
.CMSCollaboration
CERN,Switzerland
a r t i c l e i n f o a b s t ra c t
Articlehistory:
Received13July2014
Receivedinrevisedform27October2014 Accepted14November2014
Availableonline18November2014 Editor: M.Doser
Keywords:
CMS Physics Technicolor
Asearchisperformedinproton–protoncollisionsat√s
=8 TeV forexoticparticlesdecayingviaWZto fullyleptonicfinalstateswithelectrons,muons,andneutrinos.Thedatasetcorrespondstoanintegrated luminosityof19.5 fb−1.Nosignificantexcessisobservedabovetheexpectedstandardmodelbackground.
Upperboundsat95%confidencelevelaresetontheproductioncrosssectionofaW bosonaspredicted byanextended gaugemodel,andontheWWZ coupling.Theexpectedand observedmasslimits for a W boson,aspredictedbythismodel,are1.55and1.47 TeV,respectively.Stringentlimitsarealsoset inthecontextoflow-scaletechnicolormodelsunderarangeofassumptionsforthemodelparameters.
©2014TheAuthors.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/3.0/).FundedbySCOAP3.
1. Introduction
Many extensions of the standard model (SM) predict heavy charged gauge bosons, generically called W, that decay into a WZ bosonpair [1–6]. These extensions includemodels withex- tended gauge sectors, designed to achieve gauge coupling unifi- cation,andtheories withextra spatial dimensions.There arealso modelsinwhichtheWcouplingstoSMfermionsaresuppressed, giving rise to a fermiophobic W with an enhanced coupling to W and Z bosons [7,8]. Further, searches for W bosons that de- cayinto WZ pairsare complementary to searchesinother decay channels [9–19], many of which assume that the W→WZ de- cay mode is suppressed. New WZ resonances are also predicted intechnicolormodels ofdynamicalelectroweaksymmetry break- ing[20–22].
ThisLetterpresentsa search forexoticparticlesdecaying toa WZ pair with W→ ν and Z→ , where is either an elec- tron (e) ora muon (μ), ν denotes a neutrino, andthe W andZ bosons are allowed to decay to differently flavored leptons. The data were collected with the CMS experiment in proton–proton collisionsatacenter-of-massenergy√
s=8 TeV attheCERNLHC andcorrespondtoanintegratedluminosityof19.5 fb−1.Previous searchesinthischannelhavebeenperformedattheTevatron[23]
andattheLHC[24–26].Theresultshavetypicallybeeninterpreted within the context of benchmark models such as an extended gauge model (EGM) [2] and low-scale technicolor (LSTC) mod-
E-mailaddress:[email protected].
els [21,22].Thesearch conductedbyCMSat√
s=7 TeV[25] ex- cludedEGMWbosonswithmassesbelow1143 GeVandsetstrin- gent LSTC limits under a range of assumptions regarding model parameters. Complementary searches have also been conducted usingthehadronicdecaysoftheW andZ bosons[27–32].
The search at√
s=8 TeV presented in thispaper focuses on the fully leptonic channel, which is characterized by a pair of same-flavor,opposite-charge,isolatedleptonswithhightransverse momentum(pT)andaninvariantmassconsistentwiththatofthe Z boson.Athird,high-pT,isolated,chargedlepton isalsopresent, along with missing transverse momentum associated with the neutrino. Background arisesfrom other sources of three charged leptons, bothgenuine andmisidentified.The primary background is the irreducible SM WZ production. Non-resonant events with nogenuine Z bosoninthefinalstate, suchastopquark pair(t¯t), multijet,W+jet,Wγ+jet,andWW+jet production,arealsocon- sidered. Only the firstof theseis expected tomake a significant contribution.AlsoincludedareeventswithagenuineZ bosonde- cayingleptonicallyandathirdmisidentifiedornonisolatedlepton, suchasZ+jets (includingZ+heavy quarks)andZγ processes.The finalbackgroundcategoryincludeseventswithagenuineZ boson decaying leptonically and a third genuine isolated lepton, dom- inated by ZZ→4 decays in which one of the four leptons is undetected.Althoughirreducible,thiscontributionisnotexpected tobe significantbecauseofthesmallZZ productioncrosssection anddileptondecaybranchingfraction.
The search presented here followsthe method applied in the previous analysis [25], whereby a counting experiment is used to compare the number of observed events to the number of http://dx.doi.org/10.1016/j.physletb.2014.11.026
0370-2693/©2014TheAuthors.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/3.0/).Fundedby SCOAP3.
expected signal and background events. However, the new anal- ysisbenefitsfromtheincrease incenter-of-massenergyto8 TeV andalso from improvementsin lepton identification, particularly at high pT. An increase in sensitivity is achieved at high W massesby usingoptimizedisolation criteriathatsuccessfullytake into account collimated leptons from highly boosted Z bosons.
The larger center-of-mass energy alone increases the signal pro- duction crosssection by roughly 45–70% forW massesbetween 1000–1500 GeV,whiletheimprovedleptonisolationcriteriacon- tribute a 50% increase in signal efficiency over the same range.
Additional improvements relatedto the optimizationof selection criteria are also incorporated. Finally, as in the previous anal- ysis [25], the results are interpreted within the context of W bosons in extended gauge models and vector particles in LSTC models.
2. TheCMSdetector
The central feature of the CMS apparatus is a superconduct- ing solenoidof6 m internaldiameter, providinga magneticfield of 3.8 T. Withinthe superconducting solenoid volume are a sili- conpixelandstriptracker,aleadtungstatecrystalelectromagnetic calorimeter(ECAL), and a brass andscintillator hadron calorime- ter(HCAL), each composed of a barreland two endcap sections.
Muons are measured in gas-ionization detectors embedded in thesteelflux-returnyokeoutsidethesolenoid. Extensive forward calorimetrycomplementsthecoverageprovidedbythebarreland endcapdetectors.
TheECALenergyresolutionforelectronswithtransverseenergy ET≈45 GeV from Z→ee decaysis betterthan2% inthecentral region ofthe ECALbarrel (|η|<0.8), andisbetween 2% and5%
elsewhere.Forlow-bremsstrahlung electrons,where 94%or more oftheir energy iscontained within a 3×3 array ofcrystals,the energyresolutionimprovesto1.5%for|η|<0.8[33].
Muons are measured in the pseudorapidity range |η|<2.4, withdetectionplanesmadeusingthree technologies:drifttubes, cathode strip chambers, and resistive-plate chambers. Matching muons to tracks measured in the silicon tracker results in a pT resolutionbetween1and5%,forpTvaluesupto1 TeV[34].
Theparticle-flowmethod[35,36]consistsinreconstructingand identifyingeach singleparticlewithan optimizedcombinationof allsubdetector information.The energyofphotonsis directlyob- tainedfromtheECALmeasurement,correctedforzero-suppression effects. The energy of electrons is determined from a combi- nation of the track momentum at the main interaction vertex, the corresponding ECAL cluster energy, and the energy sum of all bremsstrahlung photons attached to the track. The energy of muonsisobtainedfromthecorrespondingtrackmomentum.
AmoredetaileddescriptionoftheCMSdetector,togetherwith adefinitionofthecoordinatesystemused andthe relevantkine- maticvariables,canbefoundelsewhere[37].
3. Eventsimulation
The pythia 6.426 eventgenerator [38] and the CTEQ6L1 [39]
partondistribution functions(PDFs)were used forproducingthe EGM W and LSTC signal samples. For the detailed simulation of the W samples, pythia was used for parton showering and hadronization with the Z2* tune [40] for the underlying event simulation.Crosssectionsarescaledtonext-to-next-to-leadingor- der(NNLO) valuescalculatedwith fewz 2.0[41],andrangefrom 27.96 fb to 0.33 fb for W massesbetween1000 and1500 GeV.
Characteristicsignalwidthsarebetween100and168 GeVforthe same mass range and are dominated by the detector resolution, sincethenaturalwidthsvaryfrom33to54 GeV.
For the LSTC study we assume that the technihadrons ρTC and aTC decay to WZ. Since thesetwo statesare expected to be nearly mass-degenerate [22], they wouldappear as a single fea- ture in the WZ invariant mass spectrum,and we hereafter refer to them collectively as ρTC. Since we do not expect a difference in the kinematics between the W andLSTC signals, we use the Wsamplesasthedefaultfortheanalysis,withthecrosssections for LSTC asgiven by pythia. We consider the same relationship between the masses of the ρTC and πTC technihadrons as used in Refs. [25] and[42], MπTC= 34MρTC−25 GeV, andalso investi- gate the dependenceof the results on the relative values of the
ρTC and πTC masses. Therelationship betweenthemassessignif- icantly affects the ρTC branching fractions [42].If MρTC<2MπTC, the decay ρTC→WπTC dominates,such that the branching frac- tion B(ρTC→WZ)<10%. However, if the ρTC→WπTC decay is kinematicallyinaccessible,B(ρTC→WZ)approaches100%.Follow- ingRef.[42]wealsoassumethattheLSTCparametersinχisequal to 1/3.Changesinthisparameteraffectthebranchingfractionsfor decayintoWZ andWπTC.
The MadGraph 5.1[43]and powheg 1.1[44–47]generatorsare interfacedto pythia forpartonshowering,hadronization,andsim- ulationoftheunderlyingevent. TheSMWZ process,whichisthe dominantirreduciblebackground,was generatedwithMadGraph. The ZZ process, which contributes when one of the leptons is either outside the detector acceptance or misreconstructed, was generatedusing powheg.Theinstrumentalbackgroundswerepro- ducedusing MadGraph andincludeZ+jets,t¯t,Zγ,WW+jets,and W+jets.The backgroundcontribution fromQCD multijetevents andfromWγ eventswasalsostudiedinthesimulationandfound to be negligible. Next-to-leading order (NLO) cross sections are usedwiththeexceptionoftheW+jets process,wheretheNNLO cross section is used. The W signal and SM processes used to estimate backgroundweremodeledusingafull Geant4[48] sim- ulationoftheCMSdetector.
Forallthesimulatedsamples,theadditionalproton–protonin- teractionsineachbeamcrossing(pileup)weremodeledbysuper- imposing minimumbias interactions(obtained using pythia with the Z2*tune)onto simulatedevents,withthe multiplicity distri- butionmatchingtheoneobservedindata.
4. Objectreconstructionandeventselection
The WZ→3ν decay is characterized by a pair of same- flavor, opposite-charge,high-pT isolated leptonswithaninvariant mass consistent with a Z boson, a third, high-pT isolated lep- ton, and a significant amount of missing transverse momentum associatedwiththeescapingneutrino. The analysis,therefore,re- lies on the reconstruction of three types of objects: electrons, muons, and EmissT . The magnitude ofthe negative vector sum of transversemomentaofallreconstructedcandidatesisusedtocal- culate EmissT . The events are reconstructed using a particle-flow approach[35,36]andthedetailsoftheselectionare providedbe- low.
Candidate events are required to have at least three recon- structed leptons (e, μ) within the chosen detectoracceptance of
|η|<2.5(2.4)forelectrons(muons). Theeventsare selectedon- lineusingadouble-electronordouble-muontriggerforfinalstates withtheZ bosondecayingintoelectronsormuons,respectively.
The double-electrontrigger requires two clustersin the ECAL with ET>33 GeV. The lateral spread in η of the energy de- posits comprising the cluster is required to be compatible with that of an electron. The trigger also requires that the sum of the energydetected in theHCAL in acone of R<0.14,where
R=
(φ)2+ (η)2,centeredon thecluster,be nomorethan 15%(10%)oftheclusterenergyinthebarrel(endcap)regionofthe
ECAL.Finally,theclustersarematchedin ηandφ toatrackthat includeshitsinthepixeldetector.
The double-muon trigger requires a global muon with pT>
22 GeV and a tracker muon with pT>8 GeV. The global muon isreconstructedusinganoutside-in approachwherebyeachmuon candidateinthemuonsystemismatchedtoatrackreconstructed inthetrackerandaglobalfitcombiningtrackerandmuonhitsis performed[34].Thetrackermuonisreconstructedusinganinside- out approachin which all tracks that are considered aspossible muon candidates are extrapolatedout tothe muon system. Ifat leastone muonsegment matchestheextrapolatedtrack, itquali- fiesasa trackermuon.The trigger requirementsdescribed above have been changed from those in Ref. [25] wherein two global muons were required to pass the online selection. The new re- quirementsimprovesensitivityforcollimated muonsfrom highly boostedZ bosons.
Simulatedeventsareweighted accordingtotriggerefficiencies measured, in both observed and simulated data, using the “tag- and-probe” technique [49] with a large Z→ sample. In the electronchannel,weapplyaparametrizationbasedontheturn-on curvemeasured withobserved electronsandfindtriggerefficien- ciestobeabove 99%.Muontriggerefficiencies abovetheturn-on aretypicallymeasured tobe above90% inobservedevents.Scale factors are also applied to the simulated samples to account for differencesbetween the observed and simulated trigger efficien- cies.Theseareapproximatelyunityforboththeelectronandmuon channels.
Candidates for leptons from the W and Z boson decays are alsorequiredtopassa seriesofidentificationandisolation crite- riadesignedtoreducebackgroundfromjetsthataremisidentified as leptons. Electron candidates are reconstructed from a collec- tionofelectromagneticclusterswithmatchedtracks.Theelectron momentum is obtained from a fit to the electron track using a Gaussian-sumfilteralgorithm[50] alongits trajectorytakinginto account the possible emission of bremsstrahlung photons in the silicontracker.WerequirepT>35(20)GeV fortheelectronsfrom the Z (W) boson decay. We also require |η|<2.5 and exclude the barreland endcap transition region (1.444<|η|<1.566) as electronreconstruction in thisregion isnot optimal. In compari- sonwiththerequirementsimposed onelectrons fromthe W bo- son decays, a looser set of identification requirements, primarily basedon thespatial matchingbetweenthetrackandtheelectro- magnetic cluster, is imposed forthe electrons from the Z boson decays.Electron candidates are alsorequired to be isolated with particle-flow-basedrelativeisolation,Irel,lessthan0.15,whereIrel is defined as the sum of the transverse momenta of all neutral andchargedreconstructed particle-flow candidates inside a cone ofR<0.3 aroundtheelectron in η–φ spacedividedby the pT ofthe electron. The Irel computation includes an event-by-event correctionappliedtoaccount fortheeffectofpileup[51].Finally, ifanelectromagneticclusterassociatedwithaphotonfrominter- nalbremsstrahlunginW andZ bosondecayshappenstobeclosely alignedwithamuontrack,itmaybemisreconstructedasanelec- tron.Inordertoremovesuchinstancesofmisreconstruction,elec- tronsare rejectedifthey are withina coneofR<0.01 around a muon. Observed-to-simulated scale factors for these identifi- cationandisolation requirements,measured using tag-and-probe andparametrizedasafunctionofelectron pT and|η|,areapplied ascorrectionstothesimulatedsamples.
Global muon candidates are reconstructed using information fromboththesilicontrackerandthemuonsystem.Candidatesare requiredtohaveatleastonemuonchamberhitthatisincludedin theglobalmuontrackfitandatleasttwomatchedsegmentsinthe muonsystem.Werequiremuonswith|η|<2.4 andleading(sub- leading)muon pT>25(10)GeV forthemuonsfromthe Z decay
andpT>20 GeV forthemuonsfromtheW decay.Wealsorequire δpT/pT<0.3 forthetrackusedforthemomentumdetermination, whereδpTistheuncertaintyonthemeasuredtransversemomen- tum, andwe eliminate cosmic ray background by requiring that thetransverseimpactparameter ofthemuon withrespectto the primary vertex position be less than 2 mm. Particle-flow-based relative isolation, withpileup correctionsapplied [52],is defined using a cone ofsize R<0.4 around the primary muon and is requiredtobe lessthan0.12. Theabove identificationcriteriaare modified for muonscoming fromthe Z boson decay:one of the muonsisallowed tobeatrackermuononlyandtherequirement on the number of muon chamber hits is removed. Additionally, the isolation variable for each muon is modified to remove the contributionof theother muon.These modificationsimprove the signal efficiency and hence the overall sensitivity for high-mass W bosons. Simulated samples are corrected using observed-to- simulated scale factors that are parametrized as a function of muon|η|.
Opposite-sign,same-flavorleptonpairswithinvariantmassbe- tween71and111 GeV,consistentwiththeZ bosonmass,areused to reconstruct Z bosoncandidates. Inthe caseof morethan one Z boson candidate,wherethe two candidatessharea lepton,the candidatewiththemassclosest tothenominalZ bosonmass[7]
isselected.EventswithtwodistinctZ bosoncandidates,wherethe candidatesdonotsharealepton,arerejectedinordertosuppress the ZZ background.The charge misidentificationrateforthe lep- tonsconsideredintheanalysisisverysmallandthusneglected.
AcandidateforthechargedleptonfromthedecayofaW bo- son,inthefollowingreferredtoasaW lepton,isthenselectedout oftheremaining leptons.When severalcandidatesarefound, the one withthe highest pT isselected. Neutrinos fromthe leptonic W boson decays escape from the detector without registering a signal andresultinsignificant EmissT inthe event. Inorderto in- creasethepurityoftheselectionofW bosondecays,the EmissT in the eventis requiredto belarger than30 GeV. Thisrequirement discriminates against both high-pT jets misidentified as leptons andphoton conversions,wherethesourceofthemisidentifiedjet orphotoncancomefromZ+jets orZγ events,respectively.
Inordertosuppresseventswherefinal-stateradiationproduces additional leptons (via photon conversion) that are identified as theW lepton,weapplytwoadditionalrequirementsontheevent aftertheW leptonselection.First,eventswiththetrileptoninvari- antmassm3<120 GeV arerejectedtoremoveeventswherem3 is close to the Z bosonmass. Second, events where the R be- tween eitherleptonfromtheZ bosondecayandtheW lepton is lessthan0.3arerejected.ThisremovescaseswheretheW lepton candidatecomesfromaconvertedphotonandisunlikelytooccur intheboostedtopologyofamassiveWbosondecay.
After the W and Z candidate selection, the two bosons are combined into a WZ candidate. The invariant mass of this can- didatecannot bedetermined uniquelysince thelongitudinal mo- mentum of the neutrino is unknown. We follow the procedure used in the previous CMS analysis [25] and assume the W bo- son to have its nominal mass, thereby constraining the value of theneutrinolongitudinalmomentum tooneofthe twosolutions ofaquadraticequation.Detectorresolutioneffectscanresultina reconstructed transversemass larger than theinvariant W boson mass, MW,leading tocomplexsolutions fortheneutrinolongitu- dinal momentum. In thesecases, a real solution is recovered by settingMW equaltothemeasuredtransversemass.Thisresultsin two identical solutions forthe neutrino longitudinal momentum.
Insimulatedeventswithtwo distinct,realsolutions,thesmaller- magnitudesolutionwas foundtobe correctinapproximately70%
of the cases,and thissolution was therefore chosen for all such events. Fig. 1 (top) shows the WZ invariant mass distributions,