Article
Reference
Measurement of the W + W − production cross section in pp collisions at a centre-of-mass energy of √s = 13 TeV with the ATLAS
experiment
ATLAS Collaboration
ANCU, Lucian Stefan (Collab.), et al .
Abstract
The production of opposite-charge W -boson pairs in proton–proton collisions at s=13 TeV is measured using data corresponding to 3.16 fb −1 of integrated luminosity collected by the ATLAS detector at the CERN Large Hadron Collider in 2015. Candidate W -boson pairs are selected by identifying their leptonic decays into an electron, a muon and neutrinos. Events with reconstructed jets are not included in the candidate event sample. The cross-section measurement is performed in a fiducial phase space close to the experimental acceptance and is compared to theoretical predictions. Agreement is found between the measurement and the most accurate calculations available.
ATLAS Collaboration, ANCU, Lucian Stefan (Collab.), et al . Measurement of the W + W − production cross section in pp collisions at a centre-of-mass energy of √s = 13 TeV with the ATLAS experiment. Physics Letters. B , 2017, vol. 773, p. 354-374
DOI : 10.1016/j.physletb.2017.08.047
Available at:
http://archive-ouverte.unige.ch/unige:96451
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Measurement of the W
+W
−production cross section in pp collisions at a centre-of-mass energy of √
s = 13 TeV with the ATLAS experiment
.TheATLASCollaboration
a r t i c l e i n f o a b s t ra c t
Articlehistory:
Received16February2017 Receivedinrevisedform2June2017 Accepted21August2017
Availableonlinexxxx Editor:M.Doser
Theproductionofopposite-chargeW-bosonpairsinproton–protoncollisionsat√
s=13 TeVismeasured usingdata correspondingto3.16 fb−1ofintegratedluminosity collectedbythe ATLASdetectoratthe CERNLargeHadronColliderin2015.CandidateW-bosonpairsareselectedbyidentifyingtheirleptonic decaysintoanelectron,amuonand neutrinos.Eventswithreconstructedjetsarenot includedinthe candidateeventsample.Thecross-sectionmeasurementisperformedinafiducialphasespacecloseto theexperimentalacceptanceandiscomparedtotheoreticalpredictions.Agreementisfoundbetweenthe measurementandthemostaccuratecalculationsavailable.
©2017PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3.
1. Introduction
The measurement of the production properties of opposite- charge W-bosonpairs (denoted by W W inthisLetter)isan im- portanttest ofthe StandardModel (SM) of particlephysics. This processis sensitiveto thestrong interactionbetweenquarksand gluonsandprobestheelectroweakgaugestructureoftheSM.
Measurements of W W production were first conducted at LEP[1]usingelectron–positroncollisions.Measurementsinhadron collisions were first carriedout atthe Tevatronby the CDF [2,3]
andDØ[4]Collaborations.AttheLargeHadronCollider(LHC),the W W production cross sections have been measured in proton–
proton collisions for centre-of-mass energies of √
s=7 TeV and
√s=8 TeV by the ATLAS [5,6] and CMS [7,8] Collaborations. In order to match the experimental precision and address discrep- ancies between data and theory reported in some of the 8 TeV results,significantprogress hasbeenmadeintheoretical calcula- tionstoincludehigher-ordercorrectionsinperturbativeQuantum Chromodynamics(pQCD)[9–14].The W W signalis composedof threeleadingsub-processes:qq¯→W W production1(inthet- and s-channels), non-resonant gg→W W production, and resonant gg→H→W W production(withboth gg-initiatedprocessesoc- curringthroughaquarkloop).Thesesub-processesareknownthe- oreticallyatdifferentordersinthestrongcouplingconstant αs.
This Letter describes a measurement of W W production in proton–proton collisions at √
s=13 TeV with the ATLAS detec- torusing the data collected during the2015 run. The cross sec-
E-mailaddress:atlas.publications@cern.ch.
1 InthisLetter,thenotationqq¯→W W isusedtoincludeboththeqq¯andqg initialstatesforW W production.
tionismeasuredwithinaphasespaceclosetothegeometricand kinematic acceptance of the experimental analysis, i.e. a fiducial phasespace,intheW W→e±νμ∓ν(denotedinthefollowingby W W→eμ)decaychannel.Inaddition,theratioofcrosssections at13 TeV and8TeV centre-of-massenergiesintherespectivefidu- cial phasespaces ispresented.Both measurements arecompared tothelatesttheoreticalpredictions.
2. TheATLASdetector
TheATLASdetector[15,16]isamulti-purposeparticledetector withacylindricalgeometry.2 Itconsistsoflayersofinnertracking detectorssurroundedbyasuperconductingsolenoid,calorimeters, andamuonspectrometer.Theinnerdetector(ID)issituatedinside a2 Tmagneticfieldgeneratedby thesolenoidandprovidespre- cisiontrackingforchargedparticleswithpseudorapidity|η|<2.5.
Thecalorimetercoversthepseudorapidityrange|η|< 4.9.Within
|η|< 2.47thefinelysegmentedelectromagneticcalorimeteriden- tifieselectromagneticshowersandmeasures theirenergyandpo- sition, providingelectron identificationtogether withthe ID.The muonspectrometer (MS)surroundsthecalorimetersandincludes three large air-core toroidal superconducting magnets witheight
2 TheATLASexperimentusesaright-handedcoordinatesystemwithitsoriginat thenominalppinteractionpointatthecentreofthedetector.Thepositivex-axis isdefinedbythedirectionfromtheinteractionpoint towardsthecentreofthe LHCring,withthepositive y-axispointingupwards,whilethebeamdirectionis alongthe z-axis. Cylindricalcoordinates(r, φ)areusedinthetransverse(x, y) plane,φbeingtheazimuthalanglearoundthebeamdirection.Thepseudorapidity isdefinedintermsofthepolarangleθfromthez-axisasη= −ln[tan(θ/2)].The distanceinη–φspacebetweentwoobjectsisdefinedasR≡
(η)2+(φ)2. TransverseenergyiscomputedasET=E·sinθ.
http://dx.doi.org/10.1016/j.physletb.2017.08.047
0370-2693/©2017PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3.
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coilseach,providingmuon identificationandmeasurementinthe region|η|<2.7andtriggeringintheregion|η|<2.4.Atwo-level triggersystemisusedtoselecteventsinrealtime.Itconsistsofa hardware-basedfirst-leveltriggerandasoftware-basedhigh-level trigger. The latter uses reconstruction software with algorithms similartotheofflineversions.
3. DataandMonteCarlosamples
TheanalysisisbasedondatacollectedwiththeATLASdetector during the 2015 data-taking period.Events with pp collisions at
√s=13 TeV andallrelevantdetectorcomponentsfunctionalhave beenused.Thisdatasamplecorrespondstoanintegratedluminos- ityofL=3.16 fb−1.
Monte Carlo (MC) event generators are used to model signal andbackgroundprocesses. The W W, W Z,and Z Z diboson pro- cesses(where Z standsfor Z/γ∗)withqq¯ initialstatesaresimu- latedatnext-to-leadingorder(NLO) inpQCD withthe POWHEG- BOX v2 eventgenerator [17–21] using the CT10 NLO [22] par- tondistribution functions(PDFs). Forthemodellingofthe parton shower and non-perturbative effects such as fragmentation and the underlyingevent, POWHEG-BOX v2 isinterfaced to PYTHIA v8.210[23] withthe AZNLO [24] set oftuned parameters and the CTEQ6L1 [25] PDF. The invariant mass of the leptons origi- nating fromthe Z boson orphoton in the Z Z and W Z samples isrequired tosatisfy m>7 GeV. A sample of W Z events gen- erated withSHERPA v2.1.1 [26] withm> 0.45 GeV isused tostudysystematicuncertainties.The crosssectionsgivenbythe eventgeneratorare atNLOinQCD whilethe W W,W Z and Z Z samples are normalised using their respective inclusive next-to- next-to-leading order (NNLO) predicted cross sections [9,27–29].
TheconfigurationofthePOWHEG-BOX v2eventgenerator,asde- scribedabove,reproducesthedistributionpredictedbyNNLOcal- culationsmatchedtoresummationcalculationsuptonext-to-next- to-leading logarithm (NNLL) [9,10] forthe transversemomentum ofthe W W system(pW WT )inthe rangerelevanttothisanalysis, sonofurtherstepsaretakentoexplicitlyincluderesummationef- fectsin the W W signal samples. The resonant gg→H→W W signalcontribution issimulatedwiththePOWHEG-BOX v2event generator[30]andnormalisedusingtheinclusivenext-to-next-to- next-to-leadingorder(N3LO)predictedcrosssection[31].Thenon- resonantgg→W W signalcontributionismodelledwithSHERPA v2.1.1atleadingorder(LO) usingOpenLoopswithup toone additionalpartoninthe finalstate [32]andnormalisedusingthe inclusiveNLOpredictedcrosssection[33].
The Z(→ee/μμ/τ τ)+jets productionprocessesaresimulated withtheMadgraph5_aMC@NLO v2.2.2[34]eventgeneratorin- terfacedtoPythia v8.186.Thematrixelementsfor Z produc- tion withup to fourassociated partons arecalculated atLO and theNNPDF2.3 LOPDFset[35]isused.ThePHOTOS++program version3.52[36]isusedforQEDemissionsfromelectroweakver- tices andchargedleptons. Alternativesamplesof Z(→τ τ)+jets are produced withdifferentMC event generators forthe estima- tion of systematic uncertainties in the modelling: POWHEG-BOX v2withNLOmatrixelementsinterfacedtoPYTHIA v8.210,and SHERPA v2.2.0 with NLO matrix-element accuracy up to two associatedpartonsandwithLOaccuracyforthreeandfourasso- ciatedpartons.The Z+jets eventsarenormalisedusingtheNNLO Z productioncrosssection[37].
The SHERPA v2.1.1 event generator is used to model the Wγ and Zγ processes withLO matrix element calculations for eventswithup to3 partonsinthe final state matchedto parton shower, usingtheCT10 NLO PDFset andwiththeγ transverse momentumgreaterthan10 GeV.
The POWHEG-BOX v2 eventgenerator [38,39]withthe CT10 NLO PDF is used for the generation of tt¯ and single top quarks in the W t channel. Parton shower, fragmentation, and the un- derlying event are simulated using PYTHIA v6.428 [40] with the CTEQ6L1 PDF and the Perugia 2012 [41] set of param- eters. The top-quark mass is set to 172.5 GeV. Alternative sam- plesaregeneratedwithdifferentsettingstoassesstheuncertainty in modelling top-quark events. For estimating the effect of par- ton shower and hadronisation modelling an alternative sample is generated with the POWHEG-BOX v2 event generator inter- faced toHERWIG++[42].Acomparisonbetweenthissampleand a differentone produced withMadgraph5_aMC@NLO interfaced to HERWIG++ is used to estimate the uncertainty associated to thematrix-elementimplementationandthematchingtothepar- ton showers.Separatealternativesamplesarealsogeneratedwith POWHEG-BOX v2interfacedtoPYTHIA v6.428withextrajetra- diation emitted inthematrix elementandin thepartonshower.
Inaddition,themodellingoftheoverlapatNLObetweenW tand t¯t diagrams [43] is studied. The effect is assessed by generating W t events withdifferent schemes foroverlap removalusing the POWHEG-BOX v2eventgeneratorinterfacedtoPYTHIA v6.428 for the simulation of parton showeringand non-perturbative ef- fects.Thesesamplesaresimulatedfollowingtherecommendations documentedinRef.[44].Thett¯samplesarenormalisedusingthe NNLO+NNLLsoft-gluonresummationprediction[45],whiletheW t samplesarenormalisedusingtheNLO+NNLLprediction[46].
The EvtGen v1.2.0[47] programis usedfortheproperties ofthebottom andcharmhadrondecaysinall samplesgenerated usingthePOWHEG-BOX v2andMadgraph5_aMC@NLO v2.2.2 programs.Thegeneratedsamplesarepassedthroughasimulation of the ATLAS detector based on GEANT4 [48,49]. They are over- laidwithadditionalproton–protoninteractions(pile-up)generated with PYTHIA v8.210andthe distribution ofthe averagenum- berofinteractionsperbunchcrossingisreweightedtoagreewith the corresponding datadistribution.The simulatedeventsare re- constructed and analysed with the same algorithms as the data and are corrected withdata-driven correction factors to account fordifferencesbetweendataandsimulationinleptonandjet re- constructionandidentification.
4. Eventreconstructionandselection
TheW W eventcandidatesareselectedbyrequiringexactlyone electron andone muon ofopposite charge inthe event, andsig- nificantmissingtransversemomentum,asdescribedbelow.Events with a same-flavour lepton pair are not used because they have largerbackgroundfromtheDrell–Yanprocess.
Candidate events are preselected by either a single-muon or single-electrontriggerrequiringtransversemomentumpT>20 or 24GeV respectively.TheefficiencyofthetriggerforselectingW W eventsisapproximately99% foreventsthatpasstheofflineselec- tion.
Leptonsare requiredtooriginate fromtheprimary vertex, de- finedasthereconstructedvertexwiththelargestsumofthep2Tof the associated tracks. The longitudinal impact parameter ofeach leptontrack,definedasthedistancealongthebeamlinebetween thetrackandthepointofclosestapproachofthetracktothepri- maryvertex,multipliedbythesineofthetrackθangle,isrequired tobelessthan0.5mm.Furthermore,thesignificanceofthetrans- verse impactparametercalculated withrespecttothebeamline,
|d0/σd0|,isrequiredtobelessthan3.0(5.0)formuons(electrons).
Electroncandidatesarereconstructed fromthecombinationof aclusterofenergydepositsintheelectromagneticcalorimeterand a trackin the ID [50]. Candidate electrons mustsatisfy the Tight quality definitiondescribed in Ref. [50]. Muon candidatesare re-
