Search for anomalous couplings in boosted W W / W Z → ℓ ν q q ‾ production in proton–proton collisions at s = 8 TeV

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Contents lists available atScienceDirect

Physics Letters B

www.elsevier.com/locate/physletb

Search for anomalous couplings in boosted WW / WZ → ν qq production in proton–proton collisions at √

s = ^{8 TeV}

.TheCMS Collaboration

CERN,Switzerland

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

Articlehistory:

Received17March2017

Receivedinrevisedform20May2017 Accepted5June2017

Availableonline12June2017 Editor:M.Doser

Keywords:

CMS Physics aTGC

ThisLetter presents asearch fornew physics manifested as anomalous triple gaugeboson couplings inWW andWZ dibosonproductioninproton–proton collisions.Thesearchisperformedusingevents containingaW bosonthatdecaysleptonicallyandaW orZ bosonwhosedecayproductsaremergedinto asinglereconstructedjet.Thedata,collectedat√

s=^{8 TeV with}^the^CMS^detectorât^the^LHC,^correspond toanintegratedluminosityof19 fb⁻¹.Noevidenceforanomaloustriplegaugecouplingsisfoundandthe following95%confidencelevellimitsaresetontheirvalues:λ([−⁰.011,0.011]^),κγ ([−⁰.044,0.063]^), andg^Z₁([−⁰.0087,0.024]^).^These^limitsâreâlso^translatedînto^theirêffective^field^theoryequivalents:

cWWW/ ²([−².7,2.7]^TeV⁻²^),^cB/ ²([−¹⁴,17]^TeV⁻²^),^and^cW/ ²([−².0,5.7]^TeV⁻²^).

©²⁰¹⁷^TheÂuthor.^Published^byÊlsevier^B.V.^Thisîsânôpenâccessârticleûnder^the^CC^BY^license (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP³.

1. Introduction

Measurementsofelectroweakdibosonproductioncanbetrans- latedintomeasurementsofgaugebosonself-couplings,whichare amongthemostfundamentalaspectsofthestandardmodel(SM).

At leading order (LO), only s-channel qq annihilation diagrams have a triple-boson vertex. In WW production, the WWγ and WWZ verticescontribute, whileinWZ productiononlytheWWZ vertexispresent.PhysicsbeyondtheSMcanmodifythecouplings atthesevertices,leadingtoobservabledifferencesinthecrosssec- tionandthekinematicdistributions offinalstate particles [1].In thesearchforanomaloustriplegaugecouplings(aTGCs),weadopt theeffectiveLagrangianandLEPparametrizationinRef. [2],with- out form factors: λ_γ =λZ=λ, κZ=g^Z₁−κ_γ tan²θW. We focus in particular on the parameters λ, κγ, and g^Z₁, where the deltas represent deviations from their respective SM values (λSM =^0). ^We âlso ^translate ^these înto ^the êquivalent ^parameters defined in an effective field theory (EFT) approach, namely cWWW/ ², cW/ ², and cB/ ², where is the scale of new physics[3].

This Letter presents a search for new physics manifested as anomalouscouplingsoftriplegaugebosonverticesinWW orWZ dibosonproductionfromppcollisionsat√

s=^{8 TeV at}^the^CERN LHC.WefocusonthecasewhereoneW bosondecaysleptonically (W_lep→ν,with=^e,μ),whiletheothervectorbosonV_hadde-

E-mailaddress:cms-publication-committee-chair@cern.ch.

cayshadronically,givingrisetoasinglemergedjet(J)intheﬁnal state. Previous searches inthischannel at the LHCcan be found inRefs.[4,5].Otherrecentsearchesintheleptonicchannelarede- scribed inRefs. [6,7]. Theadvantages ofreconstructing WVpairs inthe νqq decay mode over purelyleptonic ﬁnal states are the largerbranchingfractionsofW andZ bosonstoquarks,andinthe caseof two W bosons, the abilityto reconstructtheir transverse momenta(pT).Theseadvantages arepartially offsetbythelarger backgroundsintheνqq channel,arisingmainlyfromW+jetspro- duction.ThesensitivityofWW productionto theWWγ coupling andofbothWW andWZ productiontotheWWZ coupling,espe- ciallyathighboson pT, makestheseprocessesparticularlyuseful asaprobeofaTGCs.

Comparedto our previous search at√

s=^{7 TeV} ^[4], ^we ^have added anothercoupling parameter, g^Z₁, to theparameter space, andwefocusexclusivelyontheLorentz-boostedﬁnalstates,where V_hadisreconstructedasasinglemergedjet,sincetheseﬁnalstates arefarmoresensitivetoanaTGCsignalthantheresolvedtwo-jet states.

2. TheCMSdetector

The central feature of the CMS apparatus is a superconduct- ing solenoidof6 m internal diameter,providinga magneticﬁeld of3.8 T.Withinthesolenoidvolume area siliconpixelandstrip tracker,aleadtungstatecrystalelectromagneticcalorimeter(ECAL), andabrassandscintillator hadroncalorimeter,each composedof a barrel and two endcap sections. Muons are measured in gas- ionization detectors embedded inthe steel ﬂux-returnyoke out- http://dx.doi.org/10.1016/j.physletb.2017.06.009

0370-2693/©²⁰¹⁷^TheÂuthor.^Published^byÊlsevier^B.V.^Thisîsânôpenâccessârticleûnder^the^CC^BY^license(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP³.

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side the solenoid. The CMS detector isnearly hermetic, allowing formeasurementsofthemissingtransversemomentum(E^miss_T ) in theevent. E^miss_T isdeﬁnedasthemagnitudeofthenegativevector pTsumofallreconstructedparticlesinanevent.Atwo-tiertrigger systemselectstheevents ofinterest.Amoredetaileddescription ofthe CMSdetector,together with adeﬁnition ofthe coordinate systemusedandtherelevantkinematicvariables,canbefoundin Ref.[8].

3. Dataandsimulationsamples

The data were collected using single-lepton triggers with p_T thresholdsof 24(27)GeV for muons(electrons). Theoverall trig- ger eﬃciency is about 94% (90%) for the muon (electron) data, withasmalldependence(afewpercent)on p_Tandpseudorapid- ity η. The total integrated luminosity collected andprocessed is 19.3(19.2)fb⁻¹ formuon(electron)triggers.

Weuse theMadGraph5 1.3.30 [9]eventgeneratorto produce both the W+jets and Drell–Yan samples, with up to four addi- tionalpartonsinthematrixelementcalculation.Singletop quark and top quark–antiquark pair (tt) samples are generated with powheg 1.0[10–14]. The dibosonsamples (WW, WZ) are gener- atedon-shellatnext-to-LO(NLO)withMadGraph5_amc@nlover- sion2.0.0[15] andMadSpinversion3.2[16].ThedecaysW→τ ν

are includedforall processes.The τ lepton decaysare simulated with tauola [17]. The pythia 6.422 generator [18] provides the fragmentationandpartonshowersimulation,withtheparameters ofthe underlyingeventset tothe Z2*tune [19,20].The kT-MLM matching scheme is used to interface pythia6 with MadGraph5 atLO[21].Thesetofpartondistributionfunctions(PDFs)usedis CTEQ6L1[22]forLOgeneratorsandCT10[23]forNLOgenerators.

AGeant4-basedsimulation[24]oftheCMSdetectorisusedinthe productionof all MonteCarlo (MC) samples.The simulationalso includesmultipleproton–protoncollisionswithinabunchcrossing (pileup).Simulated eventsare reconstructed and analyzed inthe samewayasmeasured collisionevents,subjectto additionalcor- rectionsthataccountfordifferencesbetweendataandsimulation intriggerandselectioneﬃciencies,andinthevertexmultiplicity distribution.

4. Eventreconstruction

Allobservable objects, namelyleptons, jets, and E^miss_T , are reconstructed witha particle-ﬂowtechnique [25,26]that combines information from several subdetectors. Muons are reconstructed within|η_|<2.4 withtheinnertrackerandthemuonsystem[27].

Electrons are reconstructed within |η|<2.5 from tracks in the trackerpointingtoenergyclustersintheECAL,andidentifiedus- ingamultivariatediscriminator[28].Theselectionsappliedtothis discriminator are tuned to match the η-binned efficiencies used for Ref. [4]. Muons (electrons) are required to have pT greater than 25(30) GeV. The lepton candidates are required to be consistent with originatingfrom the event’s primary vertex, and to beisolatedfromotheractivityintheevent.The isolationrequire- ments formuons(electrons)are basedon theparticle-flow tech- niquewithan isolation coneof R=⁰.4(0.3),andare designed to reduce theeffects ofpileup andneutralparticles. Events with additionallooselyidentifiedleptonsarevetoedtoreducetheback- groundsfromfullyleptonicdecays,suchasthoseoriginatingfrom theDrell–Yan process anddibosonproduction. Decaysof the tau leptontoelectronsormuonsthatpassthesecriteriaareincluded aspotentialsignalevents.

The anti-kT (AK) [29,30] andCambridge–Aachen (CA) [29–31]

clusteringalgorithmsareusedtoreconstructjetsintheevent.The AKalgorithmusesadistanceparameterof R=⁰.5 (AK5).TheCA

jetsareclusteredwithR=⁰.8 (CA8)andareusedforreconstruct- ingVhad,wheretheVbosondecayproductsaremergedintoasin- gle jet.The combinedsecondary vertexalgorithmatthemedium operatingpointisusedtotagAK5jetsasbjets[32].Weassignthe E^miss_T measured inthe eventtothe neutrinocandidateandcom- binethiswiththeidentiﬁedleptontoreconstructW_lep.BoostedW eventsareselectedbyrequiringpT>200 GeV forW_lep.

We requireone CA8jetwith pT>200 GeV,andnoadditional CA8jetswithp_T>80 GeV,intheregion|η|<2.4.TheE^miss_T isre- quiredtobeabove50(70)GeV forthemuon(electron)channelto suppressmultijetbackgrounds.Weensurethatthetwobosonsare back-to-backbyrequiringR(,J)>π/2,φ (E^miss_T ,J)>2.0,and φ (Wlep,J)>2.0. Wevetoeventsbased onthe presenceofany b-taggedAK5jetswith p_T>20 GeV andoutsidetheCA8jetcone toreducethett background.Afterthekinematicselections,weap- ply jetsubstructuretechniques.Improvedseparationbetweenthe signalandthemultijetbackgroundisobtainedinthejetmassob- servable by means of a “pruning” algorithm [33,34] designed to remove soft gluon radiation and pileup contributions from jets.

The “N-subjettiness”variable[35]isa jetsubstructureobservable thatdeﬁnesameasure, τN,forajettohaveNsubjets.Werequire

τ2/τ1,whichistheratioof2-subjettinessto1-subjettiness,ofthe leadingCA8jettobelessthan0.55todiscriminateagainstW+jets backgrounds.

5. Backgroundandsignalmodeling

Afterallselectionsthebackgroundcomprisesthreemaincom- ponents:W+jets,topquark(tt andsingletopquark),andSMdibo- sonproduction.Multijets,Z+jets,ZZ,Zγ,H(125)→^WW^∗^,^and^fully hadronic and leptonic WW decay mode backgrounds were esti- matedanddeterminedtobenegligible.

FortheaTGCsearchweselectthemergedjetp_T,p_T^J,astheob- servable, whichfor dibosonpairs isthe pT of Vhad. We take the binned shapeofthe p_T^J distributionforeach contributingprocess from MC samples. However, since the LO W+jets predictionfalls below the data, we choose to extract the normalizations of the largestbackgroundcomponentsfirstfromanunbinnedmaximum- likelihoodfitto thedatadistributionofthe mergedjetmass,mJ. The dibosonmJ shapeinthefitregionisunaffectedbytheaTGC signalatthelevelofsensitivityofthisanalysis.

5.1. NormalizationextractionsfromthemJﬁt

Forthispartoftheanalysisweemployatwo-stageprocedure:

firstwefitthedistributioninsimulationforeachprocessindivid- ually.TheMCtemplatesusedinthe7 TeV analysisarereplacedby analytical functions, whichprovide additionalflexibility to model the data accurately. Second, we utilize the results from the first setoffitstoperformanunbinnedmaximum-likelihoodfittodata thatincludesallcomponents.Duetothedifferencesinbackground compositions andshapes, the fitto data is performedseparately for the muon andelectron channels. All fits are performed over the mass range40<mJ<140 GeV. Withineach fit to data,the normalizationforeachbackgroundprocessiseitherfreetofloator allowed tovaryaroundacentralvaluesubjecttoaGaussiancon- straint.Somecomponentshavebeencombinedbecauseofsimilar- ityinshape,orbecausetheW andZ bosonsarenotwell-resolved inmJ.Finally,theyieldsusedtonormalizebackgroundp_T^J compo- nentsareextractedfromthesignalregionof70<mJ<100 GeV.

Toassistinthebackgrounddetermination,wedeﬁnea control sample intended to isolatepure top quark eventsforcomparison with simulation[36].The sample isconstructed by inverting the selectiononthenumberofb-taggedAK5jetsoutsidetheCA8jet, thus requiring that there be at least one AK5 b-taggedjet. This

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Fig. 1.Post-ﬁtdistributionsofthemergedjetinvariant massformuons(top)andelectrons (bottom)withthe estimatesofthe relevantbackgrounds.Themergedjet invariantmassisplottedforallevents(left),aftersubtractionofallcomponentsexceptthediboson(center),andthesubsequentnormalizedresidualorpulldistributions:

(data−ﬁt)/(ﬁt uncertainty)(right).Theerrorbarsrepresentstatisticaluncertainties.Thedashedverticallinesmarkthesignalregionof70<mJ<100 GeV,fromwhichthe pTdistributionnormalizationsareextracted.

controlsample issubsequentlyreferredtoasthetopcontrolsam- ple.

Thedibosonprobabilitydensityfunction(pdf)inmJ isparam- etrizedbya sumoftwoGaussian functionscorresponding tothe W and Z resonances. The position and width of the Z Gaussian are ﬁxed withrespect to those ofthe W Gaussian, which is ini- tially taken from simulation. The relative fractions of WW (84%

of the total) and WZ (16%) are also taken from simulation. The broad background from jets misassigned to Vhad is modeled by anerror functiontimesan exponential function.The W Gaussian parametersaresubsequentlycorrectedwithMC-to-datascalefac- torsdeterminedfromthetop controlsample, inordertoaccount formismodelingofthemerged-jetmassinsimulation.Alldiboson shapeparametersarethenﬁxedduring theﬁtstothedata,while thenormalizationsarefreeparameterstobemeasured.

FortheW+jetsprocess,theshapeofthem_J distributionisde- scribed by a kinematicturn-on at lower masses(error function) followedbyarapidlyfallingtail(exponential).Thepre-ﬁtnormal- izationissettotheLOMadGraph+pythia6crosssectiontimesan empiricalfactor of 1.3. Thisfactor provides an initial estimate of thedifference betweendata andsimulationinthetopologies, ef- fectivelyaccountingfortheexpectedincreaseintheinclusivecross sectionfromNNLOcorrections,andgivenaloose±^50%constraint.

Theshapeparametersofthefunctionareallowedtovaryintheﬁt tothedatawithoutconstraint.

Thetopquarkbackgroundisacombinationoftt andsingletop quarkproductionprocesses.Thetop quarkmodelisparametrized byasumofanerrorfunctiontimesanexponentialfunctionanda doubleGaussianfunction, corresponding tobothmerged andun- merged jetsfrom hadronic W decays. The top control sample is used to correct the W resonance shape parameters, to estimate theexpectedyield andyielduncertainties byextrapolating tothe signalregion,andtoadjustthetopnormalizationuncertainty.All topshapeparametersareﬁxedintheﬁttothedata,andthenor- malizationisconstrainedtoaGaussianwithawidthof8(10)%for

muons(electrons).Thesecomefromacombinationoftheoryun- certainty anduncertainties associatedwithuseofthetopcontrol sample.

Fig. 1showstheresultsofoneofthefits tothedata.Theleft plots show the observed mJ distributions, together with the fitted contributions of the three largest SM processes. The central plots show the same distribution after subtracting all SM contributions from data except for diboson events. The right plots showthepulldistribution,i.e.,thenormalizedresidualdefinedas (data−^fit)/(fit uncertainty),wherethefituncertaintyiscomputed ateach datapointby propagatingthe uncertaintyinthenormal- izationcoefficients.

Theindividual processyields, asdetermined bythefit,are re- portedin Table 1.The acceptancetimes efficiency(Aε) is deter- mined fromthe diboson MC.The electron channel has asmaller Aε because ofits higherkinematic threshold.The top quark re- sultsreflect theinabilityofthe fitto furtherconstrain thisback- ground. The W+jets yields are about 20% higher than the prefit value of 1.3times the LO prediction, which exhibits our limited knowledge of this boostedregime. For the diboson process, 1.35 (2.23) times the expected event count is observed in the muon (electron) channel. This excess isstatistically consistent withthe SM NLO prediction [15]. Overall, the approach produces a high quality model of the data (Fig. 1 (left)), with pull distributions consistent withzero(Fig. 1(right)), thatallows usto extract the dibosoncontributiontotheV_hadresonance(Fig. 1(center)).

5.2. Fitvalidation

We validate the fit procedure by performing pseudo-exper- iments.Foreachexperiment,wegeneratethemJ pseudo-datafor the SM processes using the fitted pdf, taking into account the correlations between the yields, and then perform a fit to each pseudo-datamJ distribution asifitwere the realdata.Likewise, weensurethat theparametrizationusedissufficientlygeneralby