Measurement of the W ± Z boson pair-production cross section in pp collisions at √s = 13 TeV with the ATLAS detector

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Article

Reference

Measurement of the W ± Z boson pair-production cross section in pp collisions at √s = 13 TeV with the ATLAS detector

ATLAS Collaboration

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

Abstract

The production of W±Z events in proton–proton collisions at a centre-of-mass energy of 13 TeV is measured with the ATLAS detector at the LHC. The collected data correspond to an integrated luminosity of 3.2 fb−1 . The W±Z candidates are reconstructed using leptonic decays of the gauge bosons into electrons or muons. The measured inclusive cross section in

the detector fiducial region for leptonic decay modes is

σW±Z→ℓ′νℓℓfid.=63.2±3.2(stat.)±2.6(sys.)±1.5(lumi.) fb . In comparison, the next-to-leading-order Standard Model prediction is 53.4−2.8+3.6 fb . The extrapolation of the measurement from the fiducial to the total phase space yields σW±Ztot.=50.6±2.6(stat.)±2.0(sys.)±0.9(th.)±1.2(lumi.) pb , in agreement with a recent next-to-next-to-leading-order calculation of 48.2−1.0+1.1 pb . The cross section as a function of jet multiplicity is also measured, together with the charge-dependent W+Z and W−Z cross sections and their ratio.

ATLAS Collaboration, ANCU, Lucian Stefan (Collab.), et al . Measurement of the W ± Z boson pair-production cross section in pp collisions at √s = 13 TeV with the ATLAS detector. Physics Letters. B , 2016, vol. 762, p. 1-22

DOI : 10.1016/j.physletb.2016.08.052

Available at:

http://archive-ouverte.unige.ch/unige:88173

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

Measurement of the W

^±

Z boson pair-production cross section 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:

Received13June2016

Receivedinrevisedform22August2016 Accepted24August2016

Availableonline6September2016 Editor:W.-D.Schlatter

The production of W^±Z events in proton–proton collisions ata centre-of-mass energy of 13 TeV is measuredwiththeATLASdetectorattheLHC.Thecollecteddatacorrespondtoanintegratedluminosity of 3.2 fb⁻¹. The W^±Z candidates are reconstructed using leptonic decays of the gauge bosons into electrons or muons. The measuredinclusive cross sectioninthe detector fiducial regionfor leptonic decaymodesisσ_W^fid.±Z→ν=⁶³.2±³.2(stat.)±².6(sys.)±¹.5(lumi.) fb.Incomparison,thenext-to- leading-orderStandardModelpredictionis53.4⁺₋³₂^._.⁶₈fb.Theextrapolationofthemeasurementfromthe fiducialtothetotalphasespaceyieldsσ_W^tot._±_Z₌50.6±².6(stat.)±².0(sys.)±⁰.9(th.)±¹.2(lumi.) pb,in agreementwitharecentnext-to-next-to-leading-ordercalculationof48.2⁺₋¹₁^._.¹₀pb.Thecrosssectionasa functionofjetmultiplicityisalsomeasured,togetherwiththecharge-dependentW⁺ZandW⁻Z cross sectionsandtheirratio.

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

1. Introduction

TheproductionofW^±Z pairsinhadroncollisionsisanimpor- tant test of the electroweak sector of the Standard Model (SM).

The W^±Z final states arise from two vector bosons radiated by quarksorfromthedecayofavirtual W boson intoa W^±Z pair, which involves a triple gauge coupling (TGC). In addition, W^±Z pairscanbe producedinvector-bosonscatteringprocesses,which involve triple and quartic gauge couplings (QGC) and are sensi- tivetotheelectroweaksymmetrybreakingsector oftheSM.New physics could manifest in W^±Z final states asa modification of the TGC andQGC strength. Precise knowledge of the W^±Z pro- ductioncrosssectionisthereforenecessaryinthesearchfornew physics.

MeasurementsoftheW^±Z productioncrosssectioninproton–

antiprotoncollisions ata centre-of-massenergyof√

s=¹.96 TeV were publishedby the CDFandD0 Collaborations [1,2] usingin- tegratedluminositiesof7.1 fb⁻¹and8.6 fb⁻¹,respectively.Atthe LargeHadron Collider(LHC),measurementshavebeen performed inproton–proton(pp) collisions by theATLAS Collaboration [3,4]

at√

s=^{7 TeV and}8 TeV usingintegratedluminositiesof4.6 fb⁻¹ and20.3 fb⁻¹,respectively.

This Letter presents measurements of the W^±Z production crosssectionin ppcollisions atacentre-of-mass energyof√

s= 13 TeV. The data sample analysed was collected in 2015 by the

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

ATLAS experiment atthe LHC, andcorresponds to an integrated luminosity of 3.2 fb⁻¹. The W and Z bosons are reconstructed using their decay modes into electrons or muons. The inclusive production cross section is measured in a ﬁducial phase space andextrapolatedtothetotalphase space.ThisLetteralsoreports the ratio of the cross sections at 13 TeV and 8 TeV [4], aswell asthe ratioofthe W⁺Z/W⁻Z crosssections,which issensitive to thepartondistribution functions(PDF).Finally,the production crosssectionisalsomeasuredasafunctionofthejetmultiplicity.

Thisdistributionprovidesan importanttestofperturbativequan- tumchromodynamics(QCD)fordibosonproductionprocesses.The W^±Z dibosonprocessisparticularlywellsuitedforthismeasure- ment,sincethe W W ﬁnalstatehasaverylargebackgroundfrom top-quark production when associated jets are present, and the Z Z ﬁnal state has substantially fewer events.The reported mea- surementsarecomparedwiththeSM cross-sectionpredictionsat the next-to-leading order(NLO) in QCD [5,6] andthe total cross section is also compared to a very recent calculation atnext-to- next-to-leadingorder(NNLO)inQCD[7].

2. ATLASdetector

TheATLASdetector[8]isamulti-purposedetectorwithacylin- drical geometry¹ andnearly 4π coverage insolid angle.The col-

1 ATLASusesaright-handedcoordinatesystemwithitsoriginatthenominalin- teractionpoint (IP)inthe centreofthedetectorand thez-axisalongthe beam http://dx.doi.org/10.1016/j.physletb.2016.08.052

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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lisionpointissurroundedbyinnertrackingdetectors(collectively referred toasthe inner detector),followed bya superconducting solenoidprovidinga2 Taxialmagneticﬁeld,acalorimetersystem andamuonspectrometer.

The inner detector (ID) provides precise measurements of charged-particle tracks in the pseudorapidity range |η_|<2.5. It consists of three subdetectors arranged in a coaxial geometry aroundthebeamaxis:asiliconpixeldetector,asiliconmicrostrip detectorandatransitionradiationtracker.Thenewlyinstalledin- nermostlayerofpixelssensors[9,10]wasoperationalfortheﬁrst timeduringthe2015datataking.

Theelectromagneticcalorimetercoverstheregion|η_|<3.2 and is based on a high-granularity, lead/liquid-argon (LAr) sampling technology. The hadronic calorimeter uses a steel/scintillator-tile detectorinthe region|η_|<1.7 and a copper/LArdetectorin the region 1.5<|η_|<3.2. The mostforward region of the detector, 3.1<|η_|<4.9,isequippedwithaforwardcalorimeter,measuring electromagnetic and hadronic energies in copper/LAr and tung- sten/LArmodules.

The muon spectrometer (MS) comprises separate trigger and high-precision tracking chambers to measure the deﬂection of muons in a magnetic ﬁeld generated by three large superconducting toroids arranged with an eightfold azimuthal coil sym- metryaroundthecalorimeters.Thehigh-precisionchamberscover a range of |η_|<2.7. The muon trigger system covers the range

|η_|<2.4 withresistive-platechambersinthebarrelandthin-gap chambersintheendcapregions.

Atwo-leveltriggersystemisusedtoselecteventsinrealtime.

Itconsistsof ahardware-based ﬁrst-leveltrigger andasoftware- basedhigh-level trigger. The latteremploys algorithms similar to thoseusedoﬄinetoidentifyelectrons,muons,photonsandjets.

3. Phasespacedeﬁnition

Thefiducialphasespaceusedtomeasurethe W^±Z crosssec- tionisdefinedtocloselyfollowthecriteriausedtodefinethesig- nalregiondescribedinSection5.Thephasespaceisbasedonthe kinematicsofthefinal-stateleptonsassociatedwiththeW and Z bosondecays.Leptonsproduced inthedecayofahadron,a τ or theirdescendantsarenotconsideredinthedefinitionofthefidu- cialphasespace. Inthesimulation,thekinematicsofthecharged lepton after quantum electrodynamics (QED) final-stateradiation (FSR) are “dressed” at particle level by including contributions fromphotonswithan angulardistance R≡

( η)²+( φ)²<

0.1 fromthelepton.Dressedleptons,andﬁnal-stateneutrinosthat do not originate from hadron or τ decays, are matched to the W and Z boson decay products using a Monte Carlo generator- independentalgorithmicapproach,calledthe“resonantshape” algorithm[4],thattakesintoaccountthenominallineshapesofthe W andZ resonances.

The reported cross sections are measured in a fiducial phase spacedefinedatparticlelevelby thefollowingrequirements:the transverse momentum pT of thedressed leptons fromthe Z boson decay is above 15 GeV, the pT of the charged lepton from theW decayisabove20 GeV,theabsolutevalueofthepseudora- pidityofthechargedleptons fromthe W and Z bosons isbelow 2.5, theinvariant massofthe two leptonsfromthe Z bosonde- cay differs at most by 10 GeV from the world average value of the Z bosonmassm^PDG_Z [11].The W transversemass,definedas

direction.Thex-axispointsfromtheIPtothecentreoftheLHCring,andthey- axispointsupward.Cylindricalcoordinates(r,φ)areusedinthetransverse(x,y) plane,φbeingtheazimuthalanglearoundthebeamdirection.Thepseudorapidity isdeﬁnedintermsofthepolarangleθasη= −^ln[^tan(θ/2)]^.

m_T^W =

2·^p^ν_T·^p_T· [¹−^cos φ (,ν)]^,^where φ (,ν) is the angle betweenthelepton andtheneutrino inthetransverse plane, isrequiredtobeabove30 GeV.Inaddition,itisrequiredthatthe angulardistance R betweenthechargedleptonsfromtheW and Z decayislargerthan0.3,andthat R betweenthetwoleptons fromthe Z decayislargerthan0.2.

The ﬁducial cross section is extrapolated to the total phase spaceandcorrectedfortheleptonic branchingfractionsofthe W andZ bosons.Thetotalphasespaceisdeﬁnedbyrequiringthein- variantmassoftheleptonpairassociatedwiththe Z bosontobe intherange66<m<116 GeV.

Forthejet multiplicity differentialmeasurement,particle-level jets are reconstructed from stable particles with a lifetime of

τ>30 psinthesimulationafterpartonshowering,hadronisation, anddecayofparticleswith τ<30 ps.Muons,electrons,neutrinos and photonsassociated with W and Z decays are excluded. The particle-leveljetsarereconstructedwiththeanti-k_t algorithm[12]

with a radius parameter R=⁰.4 and are required to have a pT above25 GeV andanabsolutevalueofpseudorapiditybelow4.5.

4. Simulatedeventsamples

MonteCarlo(MC)simulationisusedtomodelsignalandback- groundprocesses.AllgeneratedMCeventsarepassedthroughthe ATLAS detector simulation[13],based on GEANT4 [14],andpro- cessed usingthe samereconstruction softwareusedfor thedata.

The event samples include the simulation of additional proton–

protoninteractions(pile-up)generatedwithPythia8.186[15] using theMSTW2008LO PDF [16] andtheA2[17] set oftuned parameters.

Scalefactorsare appliedtosimulatedeventstocorrectforthe smalldifferences betweendata andMC simulationinthe trigger, reconstruction, identiﬁcation,isolation andimpactparametereﬃ- cienciesofelectronsandmuons[18–20].Furthermore,theelectron energy and muon momentum in simulated events are smeared to account for small differences in resolution between data and MC[20,21].

Asample ofsimulated W^±Z eventsisusedtocorrectthesig- nal yield for detector effects, to extrapolate from the ﬁducial to the total phase space, andto comparethe measurements to the theoreticalpredictions.TheproductionofW^±Z pairsandthesub- sequentleptonicdecaysofthevectorbosonsaregeneratedatNLO inQCDusingthePowheg-Boxv2[22–25]generator,interfacedto the Pythia8.210 partonshower model usingtheAZNLO [26] set oftuned parameters.TheCT10[27] PDFset isusedforthehard- scatteringprocess,whiletheCTEQ6L1[28] PDFsetisusedforthe partonshower.Thejetmultiplicitymeasurementisalsocompared to the theoretical NLO prediction from the Sherpa 2.1.1 generator [29],calculatedusing theCT10PDFset inconjunctionwitha dedicated set of tuned parameters for the parton shower devel- opedbytheSherpaauthors[30].

Thebackgroundsourcesinthisanalysisincludeprocesseswith two or more electroweak gauge bosons, namely Z Z, W W and V V V (V=^W,Z);processeswithtopquarks,suchastt¯ andt¯t V, singletopandt Z;orprocesseswithgaugebosonsassociatedwith jetsorphotons(Z+^j ^and^Zγ).MCsimulationisusedtoestimate the contribution from background processes with three or more prompt leptons.Background processeswithatleastone misiden- tiﬁed leptonare evaluatedusing data-driventechniquesandsim- ulated events are used to assess the systematic uncertainties in thesebackgrounds.

The q^q¯ →^{Z Z}⁽^∗⁾^, ^tt,¯ and single-top processes are generated at NLO using the Powheg-Box v2 program. The CT10 PDF set is used for the matrix-element calculations. For the Z Z pro- cessthe partonshower ismodelled withPythia 8.186, usingthe

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CTEQ6L1PDFandAZNLO setoftunedparameters. Themodelling ofthepartonshower forprocesseswithtop quarksis done with Pythia6.428 [31], usingtheCTEQ6L1PDF andPerugia 2012[32]

setoftuned parameters. TheSherpa[29,30,33–36] eventgenera- torisusedtomodelthe Zγ, V V V,andgg→^{Z Z}^(∗) ^processes^at leadingorder(LO)usingtheCT10PDFset.Finally,thet¯t V andt Z processes are generated at LO using MadGraph5_aMC@NLO [37]

with the NPDF23LO [38] PDF set, interfaced with Pythia 8.186 (t¯t V)andPythia6.428(t Z).

5. Datasampleandselections

The ppcollisiondataanalysed correspondtoan integratedlu- minosityof3.2 fb⁻¹ collected withtheATLASdetectorin2015at

√s=^{13 TeV.}^Only^data^recorded^with^stable^beam^conditions^and withallrelevantdetectorsubsystemsoperationalareconsidered.

Candidate events are selected using triggers [39] that require atleast one electron or muon with pT>24 GeV or 20 GeV, respectively, that satisfies a loose isolation requirement. Possible inefficiencies for leptons with large transverse momenta are re- ducedby includingadditionalelectronandmuontriggers thatdo not include any isolation requirements with transverse momen- tumthresholds of p_T=^{60 GeV and} ^{50 GeV,} respectively.Finally, asingle-electrontrigger requiringpT>120 GeV withlessrestric- tiveelectronidentificationcriteriaisusedtoincreasetheselection efficiencyforhigh-p_T electrons.

Events are required to have a primary vertex reconstructed fromatleasttwochargedparticletracksandcompatiblewiththe luminousregion.Ifseveralsuch verticesarepresentintheevent, theonewiththehighestsumofthe p²_T oftheassociatedtracksis selectedastheprimaryvertexoftheW^±Z production.

All ﬁnal states with three charged leptons (electrons e or muonsμ) andneutrinos from W^±Z leptonic decaysare consid- ered.In thefollowing,the differentﬁnal statesarereferred to as

μ^±μ⁺μ⁻,e^±μ⁺μ⁻, μ^±e⁺e⁻ande^±e⁺e⁻.

Muon candidatesare identified by tracks reconstructedin the muonspectrometerandmatchedtotracksreconstructedinthein- nerdetector.Muonsarerequiredtopassa“medium”identification selection,which isbasedon requirementson thenumberof hits intheIDandtheMS[20].Theefficiencyofthisselectionaveraged over pT and ηislargerthan98%. Themuonmomentumiscalcu- latedbycombiningtheMSmeasurement,correctedfortheenergy deposited in the calorimeters, and the ID measurement. The pT ofthe muonmustbegreater than15 GeVandits pseudorapidity mustsatisfy|η_|<2.5.

Electroncandidates are reconstructed from energy clustersin theelectromagneticcalorimetermatched toinnerdetectortracks.

Electronsare identiﬁed using a discriminant that is the value of alikelihoodfunctionconstructedwithinformationfromtheshape of the electromagnetic showers in the calorimeter, track proper- tiesandtrack-to-clustermatchingquantitiesofthecandidate[18].

Electronsmustsatisfy a “medium”likelihood requirement, which provides an overall identiﬁcation eﬃciency of 90%. The electron momentum is computed from the cluster energy andthe direction of the track. The p_T of the electron must be greater than 15 GeVandthepseudorapidityoftheclustermustbeintheranges

|η_|<1.37 or1.52<|η_|<2.47.

Electronand muon candidates are required to originate from the primary vertex. Thus, the signiﬁcance of the track’s trans- verseimpactparameter calculatedwithrespecttothe beamline,

|^d0/σd0|^,^must^be^smaller^than^three^for^muonsând^less^than^five forelectrons, andthelongitudinal impact parameter, z₀ (the dif- ferencebetweenthevalueofz ofthepointonthetrackatwhich d0 isdefinedandthelongitudinalpositionoftheprimaryvertex), isrequiredtosatisfy|^z0·^sin(θ )|<0.5 mm.

Electronsandmuonsarerequiredtobeisolatedfromotherpar- ticles.Theisolationrequirementisbasedonbothcalorimeterand trackinformationandistunedforaneﬃciencyofatleast95%for p_T>25 GeV andatleast99%for p_T>60 GeV[20].

Jets are reconstructed from clusters of energy deposition in the calorimeter [40] using the anti-kt algorithm [12] with a radius parameter R=⁰.4. Events with jets arising from detector noise or other non-collision sources are discarded [41]. All jets must have pT>25 GeV and be reconstructed in the pseudorapidity range |η|<4.5. A multivariate combinationof track-based variables is usedto suppressjets originatingfrompile-up in the ID acceptance[42]. Theenergyofjetsiscalibrated andcorrected fordetectoreffectsusingacombinationofsimulatedeventsandin situmethodsin13 TeV data,similartotheproceduredescribedin Ref.[43].

The transverse momentum of the neutrino is estimated from the missingtransversemomentum inthe event, E^miss_T , calculated as the negative vector sum of the transverse momentum of all identiﬁed hardphysics objects(electrons,muons,jets),aswell as an additional soft term. A track-based measurement of the soft term[44],whichaccountsforlow-pTtracksnotassignedtoahard object,isusedintheanalysis.

Events are requiredtocontain exactlythree lepton candidates satisfyingtheselectioncriteriadescribedabove.Toensurethatthe triggereﬃciencyiswelldetermined,atleastoneofthecandidate leptons is requiredto have p_T>25 GeV and to be geometrically matchedtoaleptonthatwasselectedbythetrigger.

To suppress background processes with at least four prompt leptons, events with a fourth lepton candidate satisfying looser selection criteriaare rejected. Forthislooser selection,the pT of the leptons is lowered to pT>7 GeV and “loose” identiﬁcation requirementsareusedforboththeelectronsandmuons.Theiso- lationrequirementusesIDtrackinformationonlyandislessstrin- gent.

Candidateeventsarerequiredtohaveatleastonepairoflep- tonsofthesameﬂavourandofoppositecharge,withaninvariant mass that is consistent with the nominal Z boson mass [11] to within 10 GeV. Thispair isconsidered to be the Z boson candidate.Ifmorethanonepaircanbeformed,thepairwhoseinvariant massisclosesttothenominalZ bosonmassistakenasthe Z bo- soncandidate.

Theremaining third leptonisassignedtothe W boson decay.

The transverse mass of the W candidate, computed using E^miss_T andthe pT oftheassociatedlepton,isrequiredtobegreaterthan 30 GeV.

Backgrounds originating from misidentiﬁed leptons are sup- pressed by requiring the lepton associatedwith the W boson to satisfy morestringent selectioncriteria. Thus, thetransverse momentum of theseleptons is requiredto be greater than 20 GeV.

Furthermore, electrons associated with the W boson decay are required to pass the “tight” likelihood identification requirement [18], which has an overall efficiency of 85%. Finally, these electronsmustalsopassatighterisolationrequirement,tunedfor anefficiencyofatleast90% (99%)forpT>25(60)GeV.

6. Backgroundestimation

The backgroundsources areclassiﬁedinto twogroups: events whereat leastoneof thecandidateleptons is nota prompt lepton (reducible background) and events where all candidates are prompt leptons or are produced inthe decayof a τ (irreducible background). Candidates that are not prompt leptons are called also“misidentiﬁed”or“fake”leptons.

The reduciblebackground, whichrepresents abouthalf ofthe total backgrounds, originates from Z+ ^j, ^Zγ, tt¯, W t and W W