Measurement of $W^{\pm }$ and Z-boson production cross sections in $\mathit{pp}$ collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

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Contents lists available atScienceDirect

Physics Letters B

www.elsevier.com/locate/physletb

Measurement of W

^±

and Z -boson production cross sections 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:

Received31March2016

Receivedinrevisedform6June2016 Accepted10June2016

Availableonline15June2016 Editor:W.-D.Schlatter

Measurements of the W^±→ ^±ν and Z→ ⁺⁻ productioncross sections (where ^±=^e^±,μ^±) in proton–proton collisions at√_s

=^{13 TeV are}^presented ûsing^data ^recorded ^by^the ÂTLASêxperiment at the Large Hadron Collider, corresponding to a total integrated luminosity of 81 pb⁻¹. The total inclusiveW^±-bosonproductioncrosssectionstimesthesingle-lepton-flavourbranchingratiosareσ_W^tot₊₌

11.83±⁰.02 (stat)±⁰.32 (sys)±⁰.25 (lumi) nb andσ_W^tot₋₌8.79±⁰.02 (stat)±⁰.24 (sys)±⁰.18 (lumi) nb for W⁺ and W⁻, respectively. The total inclusive Z -boson production cross section times leptonic branchingratio,withintheinvariantmasswindow66<m<116 GeV,isσ_Z^tot₌1.981±⁰.007 (stat)± 0.038 (sys)±⁰.042 (lumi) nb.TheW⁺,W⁻,and Z -bosonproductioncrosssectionsand cross-section ratioswithinafiducialregiondefinedbythedetectoracceptancearealsomeasured. Thecross-section ratios benefit from significant cancellation of experimental uncertainties, resulting in σ_W^fid+/σ_W^fid−= 1.295±⁰.003 (stat)±⁰.010 (sys) andσ_W^fid_±/σ_Z^fid₌10.31±⁰.04 (stat)±⁰.20 (sys).Theoreticalpredictions, based oncalculations accurateto next-to-next-to-leading orderfor quantum chromodynamics and to next-to-leadingorderforelectroweakprocessesandwhichemploydifferentpartondistributionfunction sets,arecomparedtothesemeasurements.

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

1. Introduction

Measurementsofelectroweakvector–bosonproductionathad- ron colliders provide a benchmark for the understanding of quantumchromodynamic(QCD) andelectroweak(EW)processes.

Predictions are available up to next-to-next-to-leading-order (NNLO) accuracy in QCD and includeEW correctionsat next-to- leading-order (NLO) accuracy [1]. The cross-section predictions depend onthe parton distribution functions(PDFs)and are thus sensitive to the underlying dynamics of strongly interacting par- ticles.Therefore,measurements of W^± and Z -boson¹ production offera unique opportunity to test models ofparton dynamicsat the LargeHadron Collider’s (LHC) [2] newhigher centre-of-mass energyof√

s=^{13 TeV.}

Thispaperdescribesmeasurementsoftheinclusiveproduction crosssectionstimesleptonicbranchingratiosforthe W^±→ê^±ν, W^±→μ^±ν, Z →ê⁺ê⁻^, ând ^Z →μ⁺μ⁻ processes. Measure- mentsofthecross-sectionratiosofW⁺ toW⁻productionandof W^±toZ productionarealsopresented.Allmeasurementsareper- formedwithproton–proton(pp)collisiondatacorrespondingtoan

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

1 Throughoutthispaper, Z/γ^∗-bosonproductionisdenotedsimplybyZ -boson production.

integrated luminosity of 81 pb⁻¹, collected at√

s=13 TeV with the ATLAS detector [3]. The data were collected during the pe- riodofJune13toJuly16,2015,atwhichpointtheLHCcirculated 6.5 TeV beamswith50 nsbunch spacing. Thepeak delivered in- stantaneousluminositywasL=¹.7×¹⁰³³^cm⁻¹^s⁻¹^and^the^mean numberofpp interactionsperbunchcrossing(hardscatteringand pile-upevents)wasμ=^19.

2. Methodologyofcross-sectionmeasurementandpredictions

ThetotalproductioncrosssectionfortheW^± bosontimesthe branchingratiofordecaysintoasingle-leptonﬂavour^±=^e^±, μ^±

(σ_W^tot_±, σ_W^tot+,and σ_W^tot−)canbeexpressedasaratioofthenumbers ofbackground-subtracteddataevents N totheproductofthein- tegratedluminosity of thedata L^, ân âcceptance^factor Â,ândâ correctionfactorC :

σ^tot₌ ^N

L·^A·^C. (1)

The cross sections are deﬁned similarly for the Z boson in the dileptoninvariant massrange66<m<116 GeV (σ_Z^tot).The ac- ceptancefactor A isexpressedasthefractionofdecayssatisfying theﬁducialacceptance(geometricandkinematicrequirements)at the Monte Carlo generator level. The correction factor C is the ratio of the total number of generated events which pass the http://dx.doi.org/10.1016/j.physletb.2016.06.023

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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final selectionrequirementsafterreconstruction tothetotalnum- ber of generated events within the fiducial acceptance. This factor,defined beforethe decayleptons emit photonsvia final-state radiation (Born-level FSR), includes the efficiencies for triggering on, reconstructing, and identifying the W^± and Z -boson decay products within the acceptance, andalso accounts forthe slight difference between the fiducial and reconstructed phase spaces.

Theproductioncrosssectionsdefinedwithouttheacceptancefac- tors (σ^tot_·A) arereferred to asthefiducial crosssections(σ_W^fid_±,

σ_W^fid₊, σ_W^fid−,and σ_Z^fid).Forthe W^±-bosonmeasurement,the fiducial phase space is defined by the lepton transverse momentum p_T>25 GeV, the lepton pseudorapidity² |η_|<2.5, the neutrino transversemomentumpν_T>25 GeV,andtheW^±-bosontransverse mass³mT>50 GeV.Similarly, Z -bosonproductionismeasuredin the fiducialphase spacedefined by p_T>25 GeV, |η|<2.5, and 66<m<116 GeV.

Theoretical predictions of the fiducialand total cross sections are computed using DYNNLO 1.5 [4,5] for the central value and Fewz3.1[1,6–8]forallvariations reflectingsystematicuncertain- ties, thereby providing full NNLO QCD calculations. The NLO EW corrections are calculated with Fewz 3.1 for Z bosons and with theMonte Carloprogram Sanc[9,10] for W^± bosons.The calcu- lationisdone in the Gμ EWscheme[11].The crosssectionsare calculated for vector–boson decays into leptons at Born level, to matchthedefinitionoftheC factor usedinEq.(1)forthedeter- mination of the measured cross sectionsin the data. Thus, from complete NLO EW corrections the following components are included: virtual QED and weak corrections, initial-state radiation (ISR)andinterferencebetweenISRandFSR[12].Forthe Z -boson production, all the predictions include the 66<m<116 GeV requirement. The NNLO PDFs CT14nnlo [13], NNPDF3.0 [14], MMHT14nnlo68CL [15], ABM12 [16], HERAPDF2.0nnlo [17], and ATLAS-epWZ12nnlo[18] areused inthe comparisonstodata, al- though CT14nnloisusedasthebaselineforthepredictions.

The systematicuncertainties inthe predictions are dominated bytheimperfectknowledgeoftheprotonpartondistributionfunc- tions.Theseuncertaintiesareobtainedfromthesuminquadrature ofthedifferencesbetweenthe centralPDF valuesandthe eigen- vectorsoftherespectivePDFsets.Whereappropriate,asymmetric uncertaintiesaredetermined usingseparate sumsofnegative and positivevariations.The CT14nnlouncertainties(rescaledfrom90%

to 68% confidence level (CL)) are used in the comparisonto the measured cross sections in Table 3 of Section 7. The QCD scale uncertainties are defined by the symmetrised envelope of variations in which the renormalisation (μR) and factorisation (μF) scalesarechangedbyfactorsoftwowithanadditionalconstraint of 0.5≤μR/μF≤^2. ^The ^dynamic ^scale ^m andfixed scale m_W are used as the central values for the Z boson and W^± boson predictions, respectively. A significant component of these scale uncertainties originates fromthe statisticalprecision ofthe inte- gration method used to evaluate the variations. The other systematic uncertainties under consideration (labelled as “other” in Table 3) are asfollows.The uncertainties dueto the strong cou- plingconstantareestimatedfollowingtheprescriptiongivenwith the CT14nnloPDF, varying αSby±⁰.001 tocorrespondto68% CL.

The beamenergyisassumedto be knownto 1% (fromRef. [19],

2 ATLASusesaright-handedcoordinatesystemwithitsoriginatthenominalin- teractionpoint(IP)inthecentreofthedetectorandthez-axisalongthebeampipe.

Thex-axispointsfromtheIPtothecentreoftheLHCring,andthey-axispoints upward.Cylindricalcoordinates(r,φ)areusedinthetransverseplane,φbeingthe azimuthalanglearoundthebeampipe.Thepseudorapidityisdeﬁnedintermsof thepolarangleθasη= −^{ln tan}(θ/2).

3 mT=

2p_Tp^ν_T[1−^cos(φ− φν)] withazimuthalangleofthechargedlepton φandazimuthalangleoftheneutrinoφν.

with an additional uncertainty to take into account the extrapo- lationofthisuncertaintyto13 TeV).The limitationsofthe NNLO calculationsareestimatedbycomparingthepredictionscalculated with DYNNLO 1.5 and with Fewz 3.1. For the total cross-section predictions, thesedifferencesarefound to be <0.2% per process andhencearenegligible.Fortheﬁducialcross-sectionpredictions, these differences are larger due to a feature of the calculations involving leptons with symmetric pT requirements, resulting in consistentlylargervaluesfrom Fewz.Thedifferencesarecalculated usingthe CT14nnloPDFasacentralvalueinbothcases,andareup to1.3%fortheW^±-bosoncrosssectionsand0.6%forthe Z -boson cross section. These differences are however not included in the predictionuncertaintiesgiveninTable 3ofSection7.

Predictions forthefiducial cross-section ratios σ_W^fid₊/σ_W^fid₋ and

σ_W^fid_±/σ_Z^fidarealsocalculated,withtheircorrespondingPDFuncer- tainties consideredasfullycorrelated,eigenvectorby eigenvector, inthe ratios.TheQCD scalevariations are notconsidered forthe ratiossincethehigher-ordercorrectionsareexpectedtoaffectboth the W^± and Z bosons in a similar mannerbut the exactcorre- lation is difficult to evaluate. The differences between Fewz and DYNNLOfor W⁺/W⁻and W^±/Z are0.4% and0.6%,respectively, andarenotincludedinthepredictionuncertaintiesofTable 3.The remaining theoretical uncertainties evaluatedinthe fiducialcross sectionsmentioned abovelargely cancelintheratioandare also neglected.

The acceptancefactors A arealsocalculatedwith DYNNLO 1.5 for the central value and Fewz 3.1 for variations accounting for systematic uncertainties.Their uncertainties are derived from the envelope of the PDF variations of CT14nnlo, NNPDF3.0, MMHT14nnlo68CL,and ABM12.Calculationsoftheacceptancefac- tors obtainedfromeitherthesignal MonteCarlosimulationused in this analysis (Powheg + ^Pythia ⁸ ^[20–23], ^fully^described ⁱⁿ Section 3) or from Fewz fall within this envelope. In addition, uncertainties due to parton showers and the hadronisation de- scriptionaretakenfromapreviouspublication[24],afterchecking theirvalidityforthe13 TeVresult,andwerederivedasthediffer- enceintheacceptancescalculatedwith Powheg-Box v1butusing differentmodelsforpartonshowerandhadronisationdescriptions, namelythe Herwig[25]or Pythia[26]programs.

3. Simulationsamples

MonteCarlosimulationsareusedtoevaluatetheselectioneﬃ- ciencyforsignaleventsandthecontributionofseveralbackground processestotheanalyseddataset.Allofthesamplesareprocessed withthe Geant4-basedsimulation[27]oftheATLASdetector[28].

Nearlyall oftheprocessesofinterest, specificallyeventscontain- ing W^±or Z bosons[29],aregeneratedwiththe Powheg-Box v2 MonteCarloprograminterfacedtothe Pythia 8.186partonshower model.The CT10PDF set[30] isused inthematrix elementand the AZNLO [31] setof generator-parameter values(tune)is used, withPDFsetCTEQ6L1[32],forthemodellingofnon-perturbative effects. The EvtGen v.1.2.0 program [33] is used for properties of the bottom and charm hadron decays, and Photos++ version 3.52[34,35] isused forQEDemissionsfromelectroweakvertices and charged leptons. Samples of top-quark pair (tt) and single- top-quarkproductionaregeneratedwiththe Powheg-Boxv2gen- erator, which uses the four-flavour scheme for the NLO matrix element calculationstogether withthe fixedfour-flavour PDF set CT10f4. For all top-quark processes, top-quark spin correlations are preserved. The partonshower, fragmentation, andunderlying event are simulated using Pythia 6.428 with the CTEQ6L1 PDF sets andthe corresponding Perugia 2012 tune (P2012) [36]. The top-quark mass is set to 172.5 GeV. The EvtGen v1.2.0 program is used for properties of the bottom and charm hadron decays.

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Diboson processesare simulatedusing the Sherpa v2.1.1 generator[37].Multipleoverlaid pp collisionsaresimulatedwiththesoft QCD processes of Pythia v.8.186using the A2tune [38] andthe MSTW2008LO PDF[39].TheMonteCarloeventsarereweightedso thattheμdistributionmatchestheobservedpile-updistribution inthe data. For the comparisonto data in the distributions, the single-bosonMonteCarlosimulationsarenormalised tothecross sectionsmeasuredbythisanalysis.Intheevaluationofthesingle- bosonEW backgroundsforthe cross-sectioncalculations, simulations are instead normalised to the results of higher-order QCD calculations, withuncertainties of5%. The remaining simulations are also normalised to the predictions of higher-order QCD calculations,withuncertaintiesof 6%forthe dibosonandtop-quark processes.

4. Eventselection

Electronandmuoncandidateeventsareselectedusingtriggers whichrequireatleastone electronormuonwithtransversemo- mentumthresholds of p_T=^{24 GeV or} ^{20 GeV,}respectively, with looseisolation requirements. Torecover possibleeﬃciency losses athighmomenta,additionalelectronandmuontriggerswhichdo notmakeanyisolationrequirementsareincludedwiththresholds ofpT=^{60 GeV and}^{50 GeV,}respectively.

Electroncandidates are required to have pT>25 GeV and to pass the “medium” likelihood-based identification requirements [40,41] optimised for the 2015 operating conditions, within the fiducialregionof|η_|<2.47,excludingcandidatesinthetransition region betweenthe barreland endcapelectromagnetic calorime- ters,1.37<|η|<1.52.Muonsarereconstructedfor|η|<2.4 with pT>25 GeV andmust passthe “medium”identificationrequire- ments [42] also optimised for the 2015 operating conditions. At leastoneoftheleptoncandidatesisrequiredtomatchthelepton thattriggeredtheevent.Theelectronsandmuonsmustalsosatisfy pT-dependentcone-basedisolationrequirements,usingbothtrack- ing detector andcalorimeter information (described in Refs. [43, 44],respectively).Theisolationrequirementsaretunedsothatthe leptonisolation efficiencyis atleast90% forall pT>25 GeV,in- creasingto99%at60 GeV.

Jetsarereconstructed fromenergydepositsinthe calorimeter using the anti-kt algorithm [45] with radius parameter R=⁰.4.

Alljets[46],withenergiescalibratedattheelectromagneticscale, musthavepT>20 GeV and|η|<4.5.Themissingtransversemo- mentum(withmagnitudeE^miss_T ),whichintheW^±-bosonanalysis actsasa proxyforthe transversemomentum of theneutrino, is definedasthenegative of theglobalvector sum ofall identified physics objects (electrons, muons, jets) as well as specific “soft terms”accountingforunclassifiedsofttracksandcalorimeteren- ergyclusters.

The event selection for the W^±-boson signature requires exactly one identiﬁed electron or muon. The event is required to have E^miss_T >25 GeV, and the transverse mass of the W^± boson calculatedusing themissing transversemomentum vector is requiredto satisfy mT>50 GeV. In order forthe W^±-boson se- lectionto be consistent withthe missing transverse momentum reconstruction methodology, an overlap removalalgorithm is applied to the selection for events with jets and leptons found at adistance ofR=

(η)²+ (φ)²<0.4 of each other,removing eitherone or the other object. Afterthe full W→ ν selection,a total of 462,950 W^±-boson candidates (256,858 W⁺ and 206,092 W⁻) pass all requirements in the electronchannel, and 475,208 W^±-boson candidates (266,592 W⁺ and 208,616 W⁻) passtherequirementsinthemuonchannel.

Eventscontaining a Z -bosoncandidate areselected by requir- ing exactly two selected leptons of the same ﬂavour but of op-

posite charge with invariant mass of 66<m <116 GeV. No overlap removal is applied in the Z -boson analysis, as missing transverse momentumis not requiredinthe selection. A totalof 35,009 candidates pass all requirements in the electron channel and44,898 candidatesinthemuonchannel.

5. Evaluationofbackgrounds

Contributionsfromtheelectroweak(single-bosonanddiboson) andtop-quark(single-topandtop-quarkpair) componentsof the backgroundareestimatedfromtheMonteCarlosamplesdescribed earlier.The W→τ ν and Z→τ τ processeswiththesubsequent leptonic decayofthe τ are treatedasbackground.The dominant contributions, givenas percentagesof the totalnumber ofsimu- lated events passing the signal selection in each analysis, are as follows: the W →τ ν and top-quark production contribute ap- proximately 2% and1%, respectively, in the W^±-boson analyses, the Z→ê⁺ê⁻ând ^Z→μ⁺μ⁻ processescontribute1%and5%in W→êνandW→μν,respectively,whilethetotalbackgroundin Z→ ⁺⁻isapproximately0.5%,dominatedbytt production(the sumofallelectroweakbackgroundsis0.2%).

Eventsinvolvingsemileptonicdecaysofheavy quarks,hadrons misidentiﬁed as leptons, and, in the case of the electron channel,electronsfromphotonconversions(allreferredto collectively as “multijet events”) are a sizeable source of background in the W^±-bosonanalysis.Themultijetbackgroundinthe Z -bosonanalysis is estimated from simulation to be < 0.1% and is therefore neglected.

Themultijetcontributiontotheelectronandmuonchannelsof the W^±-bosonanalysisisestimatedwitha data-drivenapproach, performing maximum-likelihood ﬁts on the data with template distributions to exploit the discriminating power between signal and background in certain kinematic distributions. The discrimi- nant variables used in the multijet evaluation are mT, E^miss_T , p_T, and φ betweenthe lepton and transverse missingmomentum.

Twofitregionsareusedtoextractthemultijetnormalisation.The first fit region is defined as the full event selection but removing the m_T requirement, and the second one is defined as the full eventselection but removing the E^miss_T requirement. Several multijet-enricheddatasamples(multijettemplates)arebuiltfrom eventspassingallselectionrequirementsineachfitregionexcept lepton isolation. Mutually exclusive requirements (“intervals”) in eithertracking- orcalorimeter-basedisolationvariablesarechosen to createstatisticallyindependentmultijet templates. Thesesam- plesaredesignedtobeprogressivelyclosertothesignal-candidate selection by fixing one of the isolation criteria to that of the signal region and varying the other one; four such samples are builtforeachisolation type intheelectronchannel andfour(for tracking-basedisolation)orsix(forcalorimeter-basedisolation)in themuonchannel.Templatesaresimilarlyconstructedfromsimu- lationforW^±signalandelectroweakandtop-quarkbackgrounds, to account for potential contaminationsin the multijet template.

Foreachisolation interval,the normalisationofthemultijettem- plate is extracted with a maximum-likelihood fit to the data in the two fit regions and separately for each one ofthe discrimi- nantvariables andchargedleptonsamples.Ineachfitregion,the normalisation of the signal template derived from simulation is left free to float while the remaining background templates are normalisedtotheir expectedvalues,basedonthemeasuredinte- grated luminosity and the predictedcross sections (but are per- mittedtovarywithin5%oftheirexpectedvalues,asdescribed in Section3).Itwasverifiedthatthevalueofthesignalnormalisation extractedfromthisfithasnosignificantimpactonthemultijetes- timate.