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Physics Letters B
www.elsevier.com/locate/physletb
Evidence for electroweak production of four charged leptons and two jets in proton-proton collisions at √
s = 13 TeV
.The CMSCollaboration
CERN,Switzerland
a rt i c l e i n f o a b s t r a c t
Articlehistory:
Received16August2020
Receivedinrevisedform14November2020 Accepted30November2020
Availableonline3December2020 Editor:M.Doser
Keywords:
CMS Physics SM ZZ VBS aQGC
Evidenceispresented forthe electroweak (EW)productionoftwo jets(jj)inassociation withtwo Z bosonsand constraintsonanomalousquarticgaugecouplingsareset.Theanalysis isbasedonadata sampleofproton-protoncollisionsat√
s=13TeV collectedwiththeCMSdetectorin2016–2018,and correspondingtoanintegratedluminosityof137 fb−1.Thesearchisperformedinthefullyleptonicfinal stateZZ→,where,=e,μ.TheEWproductionoftwojetsinassociationwithtwoZ bosonsis measuredwithanobserved(expected)significance of4.0(3.5)standarddeviations.Thecrosssections for the EW production are measuredin three fiducial volumes and the result is σEW(pp→ZZjj→ jj)=0.33+−00..1110(stat)+−00..0403(syst) fb in the mostinclusive volume,in agreement with the standard modelpredictionof0.275±0.021 fb.Measurementsoftotalcrosssectionsforjjproductioninassociation withtwoZ bosonsarealsoreported.Limitsonanomalousquarticgaugecouplingsarederivedinterms oftheeffectivefieldtheoryoperatorsT0,T1,T2,T8,andT9.
©2020TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3.
1. Introduction
In the standard model (SM), the electroweak (EW) vector bosons,like theotherfundamental particles,acquiretheir masses throughthecouplingtotheBrout-Englert-Higgsfield.Thephoton remainsmassless,withonlytwodegreesofpolarization(i.e.,trans- verse),whereas theW andZ bosonsacquire anadditionaldegree offreedom(i.e.,longitudinal),asaconsequenceoftheelectroweak symmetry breaking (EWSB) [1,2]. Thus, the scatteringof massive vectorbosonsisattheheartoftheEWSBmechanismanditsstudy can lead to significant insight into the origin ofparticle masses.
Moreover, if the couplings between the Higgs boson and vector bosons(HVV)differfromtheirSMvalues,thesubtleinterplaybe- tweenHVV,triple,andquarticgaugecouplingsaspredictedinthe SM isincomplete,andthecrosssection forthelongitudinalscat- teringdivergesatlargescatteringenergies,eventuallyviolatingthe unitarity.
Atthe CERNLHC,vectorboson scattering(VBS) istheinterac- tionoftwoEWvectorbosonsemittedbyquarks(q)fromthetwo collidingprotons.TheVBSprocessisgenerallylabeledbythetype of outgoing vector bosons. The two jets (jj) originatingfrom the scatteredquarksaretypicallyemittedintheforward-backwardre- gion ofthedetector,givingrise toeventswhose signaturein the detectorischaracterizedbyaregioninrapidity(so-called“rapidity
E-mailaddress:cms-publication-committee-chair@cern.ch.
gap”) [3,4],wherenoadditionalhadronicactivityisexpectedfrom thehardscattering. Thedecayofthevectorbosons intofermions defines the final signature of the VBS-like event. The pure VBS contributions, however,are embedded into a wider set of possi- ble two-to-six processes, with which they interfere (Fig. 1). All processes atthe order of αEW6 (tree level) are considered asEW production(Fig. 1 upperpanels and bottom left panel), whereas theprocessesattheorder α4EWαS2 whereattreelevelthejetsare inducedbyquantumchromodynamics(QCD)(lowerrightpanelin Fig.1),constitutea backgroundreferredto asQCD-inducedback- ground. Kinematic requirements on the dijet system are used to definefiducialregionsenrichedinVBS-likeeventsandwhereQCD- inducedbackgroundsaresuppressed.
BoththeATLASandCMSCollaborationshaveperformedsearches for the scattering of massive vector bosons, using data from proton-proton (pp) collisions at the center-of-mass energy of 13 TeV. The ATLAS Collaboration reported the observation of EW productionof two jetsin association witha same-sign W boson pair [5],witha WZ bosonpair [6],and, recently,withaZ boson pair [7]. Results were also reported on the measurement of the EWdibosonproduction(WW,WZ,ZZ)inassociationwithahigh- massdijetsysteminsemileptonicfinalstates [8],withanobserved significanceof2.7standard deviations.TheCMSCollaborationob- servedtheproductionoftwoEW-inducedjetswithtwosame-sign W bosons [9,10] and with WZ pairs [10], andmeasured theEW production ofjets in association with ZZ [11] with an observed significanceof2.7standarddeviations.
https://doi.org/10.1016/j.physletb.2020.135992
0370-2693/©2020TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP3.
Fig. 1.RepresentativeFeynmandiagramsfortheEW- (toprowandbottomleft)andQCD-induced(bottomright)productionoftheZZjj→jj (,=e,μ)finalstate.
ThescatteringofmassivegaugebosonsasdepictedinthetoprowisunitarizedbytheinterferencewithamplitudesthatfeaturetheHiggsboson(bottomleft).
This paper presents evidence for the EW production of two jets inassociation with two Z bosons, whereboth Z bosonsde- cay intoelectrons ormuons, ZZ→ (,=e,μ). Despitea low crosssection, a smallZ→ branching fraction, anda large QCD-induced background, this channel provides a clean leptonic final state witha small experimental background, where one or morereconstructedleptoncandidatesoriginatefromthemisiden- tificationofjetfragmentsorfromnonpromptleptons.
The searchfortheEW-induced productionofthe jj final state is carried out using pp collisions at √
s=13TeV recorded with the CMS detector at the LHC. The data set corresponds to an integratedluminosity of137 fb−1 collected in2016,2017, and 2018. A discriminant based on a matrix element likelihood ap- proach (MELA) [12–16] is used to extract the signal significance and to measure the cross sectionsfor the EW and the EW+QCD production ofthe jj final state ina fiducialvolume. Finally, theselectedjj eventsareusedtoconstrainanomalousquar- ticgauge couplings(aQGC) described inthe effectivefieldtheory approach [17] by the operators T0, T1, and T2, as well as the neutral-currentoperatorsT8andT9 [18].
2. TheCMSdetector
The central feature of the CMS apparatus is a superconduct- ing solenoid of6 m internal diameter, providing a magneticfield of 3.8 T. Withinthesolenoid volume area silicon pixelandstrip tracker, a lead tungstate crystalelectromagnetic calorimeter, and a brass and scintillator hadron calorimeter, each composed of a barreland twoendcap sections.Forwardcalorimetersextend the pseudorapidity (η) coverage provided by the barrel and endcap detectors.Muonsare detected ingas-ionization detectorsembed- ded in the steel flux-return yoke outside the solenoid. The first level of the CMS trigger system, composed of custom hardware processors, usesinformationfromthecalorimetersandmuonde- tectors to select events of interest with a latency of 3.2 μs. The high-level triggerprocessor farmfurtherdecreases theeventrate fromaround100 kHztolessthan1 kHz,beforedatastorage [19].A moredetaileddescriptionoftheCMSdetector,togetherwithadef-
inition ofthecoordinate systemusedandthe relevantkinematic variables,canbefoundinRef. [20].
3. Signalandbackgroundsimulation
SeveralMonte Carlo(MC) eventgenerators are used to simu- late signal and backgroundcontributions. The simulatedsamples areemployed tooptimizethe eventselection,evaluate thesignal efficiencyandacceptance,andtomodelthesignalandirreducible backgroundcontributionsinthesignalextractionfit.
TheEWproductionoftwoZ bosonsandtwofinal-statequarks, wheretheZ bosonsdecayleptonically, issimulatedatleadingor- der (LO) usingMadGraph5_amc@nlo v2.4.6(abbreviated asMG5 inthefollowing) [21].Theleptonic Z bosondecaysaresimulated usingMadSpin[22].Thecontributionofelectronsandmuonsfrom
τ decays to the signal is very small and is therefore neglected.
The sample includes triboson processes, where the Z bosonpair isaccompanied by a third vector bosonthat decays hadronically, aswell asdiagrams involving the quartic gauge coupling vertex.
Thepredictionsfromthissamplearecross-checkedwiththoseob- tainedfromtheLOgeneratorPhantomv1.2.8 [23] withagreement intheyieldsandthedistributionsexploitedforthesignal extrac- tion.
The leading QCD-induced production of two Z bosons in as- sociationwithjets, whosecontributionwithtwo jetsinthe final state is referred to as qq¯→ZZjj, issimulated atnext-to-leading order (NLO) with MG5 with up to two extra parton emissions, and merged with the parton shower simulation using the FxFx scheme [24]. Next-to-next-to-leading order correctionscalculated withMATRIX v1.0.0 [25–27] areappliedasK factors,differentially asa function ofthe invariant mass ofthe ZZ system(mZZ).The resultingcorrectionsrangefrom9%,atvaluesofmZZ closeto180 GeV,to5%,forhighmZZvalues.AdditionalNLOEWcorrectionsare applied formZZ>2mZ,following the calculationsfromRef. [28].
ThesecorrectionsbecomelargerwithincreasingvaluesofmZZ and arebelow5%formZZ<600 GeV.
Theinterferencebetweenthe EWandQCD diagramsisevalu- atedusing dedicated samplesproduced with MG5 atLO, via the
direct generation of the interferenceterm betweenthe two pro- cesses.
The loop-inducedproduction of two Z bosons from a gluon- gluon (gg)initial state, whosecontribution with two jets in the final stateisreferredto asgg→ZZjj,issimulatedatLOwithup totwoextrapartonemissionsusingMG5byexplicitlyrequiringa loop-inducedprocess [29].Forthe1- and2-jetcontributions,app initialstateinsteadofgg isspecifiedinMG5toalsoincludeinitial- state radiation contributions where a gluon involved inthe hard processisemittedfromaninitialquark.Finally,thesampleswith0 to2extrapartonsaremergedwithpartonshowersimulationusing theMLM matchingscheme [30,31].An NLO/LO K factor,whichis extractedfromRefs. [32,33],isusedtonormalizethisprocess.
Backgroundprocessesthatcontainfourprompt,isolatedleptons and additional jets in the final state, namely t¯tZ and VVZ (V= W,Z),aresimulatedwithMG5atNLO.
ThesimulationoftheaQGCprocessesisperformedatLOusing MG5 andemploys matrix element reweighting to obtain a finely spaced grid for each of the five anomalous couplings probed by theanalysis.
Thepythia8.226and8.230 [34] packageversionsareusedfor parton showering, hadronization and the underlying event sim- ulation, with parameters set by the CUETP8M1 tune [35] (CP5 tune [36]) for the 2016 (2017 and 2018) data-taking period.
The NNPDF3.0 (NNPDF3.1) set of parton distribution functions, PDFs [37],isusedforthe2016(2017and2018)data-takingperiod.
Unlessspecified otherwise,thesimulatedsamplesare normalized tothecrosssectionsobtainedfromtherespectiveeventgenerator.
Thedetectorresponseissimulatedusingadetaileddescription oftheCMSdetectorimplementedinthe Geant4package [38,39].
Thesimulatedeventsarereconstructedusingthesamealgorithms usedforthedata,andincludeadditionalinteractions inthesame andneighboringbunchcrossings, referredtoaspileup.Simulated eventsareweightedsothatthepileupdistributionreproducesthat observedinthedata,whichhasanaverageofabout23(32)inter- actionsperbunchcrossingin2016(2017and2018).
4. Eventreconstructionandselection
The final state consists of at least two pairs of oppositely charged isolated leptons and at least two hadronic jets. The ZZ selectionissimilartothatusedintheCMSH→ZZ→mea- surement [40].
The primary triggers requirethe presence ofa pair ofloosely isolated leptons, whose exact requirements depend on the data- taking year. Triggers requiring three leptons withlow transverse momentum (pT), as well as isolated single-electron and single- muon triggers, help to recoverefficiency. The overall triggereffi- ciency foreventsthat satisfythe ZZ selectiondescribed belowis
>98%.
Events are reconstructed using a particle-flow algorithm [41]
that identifies each individual particlewith an optimized combi- nation of all subdetector information. The candidate vertex with thelargest valueofsummed physics-object p2T istheprimary pp interaction vertex. The physics objects are the jets, clusteredus- ing the jet finding algorithm [42,43] with the tracks assigned to candidateverticesasinputs,andtheassociatedmissingtransverse momentum(pmissT ),takenasthenegativevectorsumofthe pT of thosejets(whichincludetheleptons).
Electronsareidentifiedusingamultivariateclassifier,whichin- cludes observablessensitiveto bremsstrahlungalongthe electron trajectory, the geometrical and energy-momentum compatibility between the electron track and the associated energy cluster in theelectromagneticcalorimeter,theshape oftheelectromagnetic shower, isolationvariables,andvariablesthat discriminateagainst electronsoriginatingfromphotonconversions [44].
Muons are reconstructed by combining information from the silicontrackerandthemuonsystem [45].The matchingbetween the muon-system and tracker tracks proceeds either outside-in, starting froma track inthe muon system, or inside-out, starting fromatrackinthesilicontracker.Themuonsareselectedfromthe reconstructedmuontrackcandidatesbyapplyingminimalrequire- mentsonthetrackinboththemuonsystemandsilicontracker.
To further suppress electrons from photon conversions and muons originating from in-flight decays of hadrons, the three- dimensionalimpactparameterofeachleptontrack,computedwith respecttotheprimaryvertexposition,isrequiredtobe lessthan fourtimestheuncertaintyintheimpactparameter.
Leptonsarerequiredtobe isolatedfromother particlesinthe event.Therelativeisolationisdefinedas
Riso=
charged hadrons
pT +max 0,
neutral hadrons
pT +
photons
pT − pPUT pT,
(1) wherethescalarsumsrun overthechargedandneutralhadrons, as well as the photons, in a cone defined by R ≡
(η)2+(φ)2=0.3 aroundtheleptontrajectory, where ηand φ denote the azimuthal angle and pseudorapidity of the parti- cle,respectively.Tominimizethecontributionofchargedparticles frompileup to the isolation calculation, charged hadronsare in- cludedonlyifthey originatefromtheprimaryvertex.The contri- butionof neutralparticles frompileup pPUT isevaluated forelec- tronswiththejetarea methoddescribedinRef. [46].Formuons, pPUT istakenashalfthepT sumofallchargedparticlesinthecone originating from pileup vertices. The factor of one-half accounts fortheexpectedratioofchargedtoneutralparticleproductionin hadronicinteractions. MuonswithRiso<0.35 areconsidered iso- lated, whereas for electrons, the Riso variable is included in the multivariateclassifier.
Thelepton reconstructionandselection efficiencyismeasured inbins of pT and η using the tag-and-probe technique [47] on eventswithsingleZ bosons.Themeasuredefficienciesareusedto correctthesimulation.Themuon(electron)momentumscalesare calibratedinbinsof pT and ηusingtheJ/ψ mesonandZ boson (Z bosononly)leptonicdecays.
Jetsare reconstructed from particle-flow candidates usingthe anti-kT clustering algorithm [42], as implemented in the Fast- Jet package [43], with a distance parameter of 0.4. To ensure a good reconstruction efficiency and to reduce the instrumental background,aswellasthecontaminationfrompileup,looseiden- tification criteriabasedon themultiplicities andenergyfractions carriedbychargedandneutralhadronsareimposed onjets [48].
Onlyjetswith|η|<4.7 areconsidered.
Jet energy corrections are extracted fromdata andsimulated eventstoaccountfortheeffectsofpileup,uniformityofthedetec- torresponse,andresidualdifferencesbetweenthejetenergyscale indataandsimulation.Thejetenergyscalecalibration [49,50] re- lieson correctionsparameterizedin termsof theuncorrected pT andηofthe jet,andis appliedasa multiplicative factor,scaling thefour-momentumvectorofeachjet.Toensurethatjetsarewell measured and to reduce the pileup contamination, all jets must haveacorrectedpT>30GeV.Jetsfrompileuparefurtherrejected usingpileup jet identification criteriabased on thecompatibility oftheassociatedtrackswiththeprimaryvertexinsidethetracker acceptance andon the topology of the jet shape in the forward region [51].
A signal event must contain at least two Z candidates, each formed from pairs of isolated electrons or muons of opposite charges.Onlyreconstructedelectrons(muons)withpT>7(5)GeV are considered. At least two leptons are required to have pT>
