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Physics Letters B
www.elsevier.com/locate/physletb
Study of Z boson production in pPb collisions at √
s
N N= 5 . 02 TeV
.CMSCollaboration
CERN,Switzerland
a r t i c l e i n f o a b s t ra c t
Articlehistory:
Received21December2015
Receivedinrevisedform24March2016 Accepted16May2016
Availableonline18May2016 Editor:M.Doser
Keywords:
CMS Physics Heavyions
TheproductionofZbosonsinpPbcollisionsat√s
NN=5.02 TeV isstudiedbytheCMSexperimentvia theelectronandmuondecaychannels.Theinclusivecrosssectioniscomparedtoppcollisionpredictions, and foundto scale with the number ofelementarynucleon–nucleon collisions.The differential cross sectionsasafunctionoftheZ bosonrapidityandtransversemomentumaremeasured.Thoughtheyare found tobeconsistentwithinuncertainty withtheoretical predictionsbothwithand withoutnuclear effects, the forward–backwardasymmetry suggeststhe presence ofnuclear effects atlarge rapidities.
Theseresultsprovidenewdataforconstrainingnuclearpartondistributionfunctions.
©2016TheAuthor.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3.
1. Introduction
Electroweakbosonproductionisanimportantbenchmarkpro- cessin high-energy particle physics. The production of Zand W bosonshasbeenextensivelystudiedathadronande+e−colliders, atvariouscollisionenergies.Thelatestmeasurements inppcolli- sions attheLHC [1–8]are well described bythe standard model using higher-order perturbative quantum chromodynamics (QCD) andpartondistributionfunctions(PDFs).
Withits large center-of-massenergy andhighluminosity,the LHCenablesforthefirsttimethestudyofZandW bosonproduc- tion in heavy ion collisions. Electroweak bosons are unmodified by the hot and dense medium created in nucleus–nucleus col- lisions, and their leptonic decays are of particular interest since leptons pass through the medium withoutbeing affected by the stronginteraction.BoththeZandW bosonproductionweremea- suredbytheATLAS[9,10]andtheCMS[11,12]experimentsusing PbPb collisions taken in 2010 and 2011 at a center-of-mass en- ergy per nucleon pair of √
sNN=2.76 TeV, confirming that the production cross section scales with the number of elementary nucleon–nucleoncollisionswithaprecisionofabout10%.
However, in nuclear collisions, the production of electroweak bosonscanbeaffectedbytheinitialconditionsofthecollision.The free-proton PDFsare expected to be modified forprotons bound inthePbnucleus, which,together withthefact thatthe nucleus contains neutrons as well asprotons (isospin effect), can modify theobservedcross sectionsascompared topp collisions.Various groupshavestudiedthenuclearmodificationofPDFs,andseveral
E-mailaddress:[email protected].
results are available at next-to-leading-order (NLO) precision in QCD[13–15].Theseresultsareobtainedbyglobalfitstotheavail- abledeepinelasticscatteringandDrell–Yandata,whichconstrain thenuclearPDFs(nPDFs)intheregionofpartonlongitudinalmo- mentum fractionx>10−2 andfour-momentum transfer squared Q2< (10 GeV)2.
The productionofelectroweakbosonsinproton–nucleus colli- sions atthe LHC provides an opportunity to studythe nPDFs at the high Q2≈ (100 GeV)2 andlower x phasespace region [16].
The CMS experiment made the first measurement of W boson productioninpPbcollisions [17].Deviations fromthe currentex- pectationsforPDFswereobserved,showingtheneedforincluding W boson datainnPDFglobalfits. Furthermore,thedijetpseudo- rapidity distribution measured inpPbcollisions by CMS[18] and theZ bosonproductioninpPbcollisionsmeasured byATLAS[19]
show better agreement with modified PDFs. Deviations frompp expectationswerealsoseenwithchargedhadrons[20].
Various models predict different nuclear modifications of the Z boson productioncross section (σ) asa function of transverse momentum (pT) and rapidity in the nucleon–nucleon center-of- massframe( ycm)[21–25].Processesmediatedbyavirtualphoton andinterferenceeffectsarealsoconsideredaspartoftheZ boson signal.TherapiditydistributionofZbosonsisparticularlysensitive to the parton content of the interacting nucleons. Consequently, thesymmetricrapidityspectrumoftheZbosonsinthecenter-of- mass frameofpp collisions ismodified by nucleareffects inpPb collisions [24]. This can be quantified through measurements of theforward–backwardasymmetryinthecenter-of-massframe:
RFB(ycm)=dσ(+ycm)/d ycm
dσ(−ycm)/d ycm, (1)
http://dx.doi.org/10.1016/j.physletb.2016.05.044
0370-2693/©2016TheAuthor.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP3.
wherebyconventionpositiverapidityvaluescorrespondtothedi- rectionoftheincomingproton.
The aimof thispaper is to studythe Z→ process (where
represents eithermuonsorelectrons) andtomeasure the pro- ductioncross sectionasfunctionsof rapidity andtransversemo- mentum.The typicalquark momentumfractionprobed inthePb nucleus is given by x=MZ/√
sNNe−ycm, and thus with 0.002<
x<0.3 in the measured rangeof −2.8<ycm<2.0. These mea- surementswillhelptoconstrainthepartoncontentofthenucle- onsinthenucleus.
2. Experimentalsetup,dataselectionandreconstruction
Adetailed descriptionof the CMSdetector andits coordinate systemcan be found elsewhere [26]. Its central feature is a su- perconducting solenoid with internal diameter of 6 m, provid- ing a magnetic field of 3.8 T. Within the solenoid volume are a silicon pixel andstrip tracker,a lead tungstate crystalelectro- magnetic calorimeter(ECAL), anda brass andscintillator hadron calorimeter (HCAL). Muons are detected in the pseudorapidity range|ηlab|<2.4 using gas-ionizationdetectorsembedded inthe steelreturnyokeoutsidethe solenoid. Electronsare measuredin theECALthatconsistsof75 848leadtungstatecrystalsprovidinga coverageinthebarrelregion of|ηlab|<1.48 andinthetwo end- capregions of1.48<|ηlab|<3.00.Extensive forwardcalorimetry complements the coverage provided by these barrel andendcap detectors.CMShasatwo-leveltriggersystem.Thefirstlevel,com- posedofcustom hardware processors, uses informationfromthe calorimeters and muon detectors to select the most interesting events.Thehigh-leveltriggerprocessorfarmfurtherdecreasesthe eventratebeforedatastorage.
TheanalysisisperformedusingthepPbcollisiondatatakenat the beginning of2013 andcorresponding to an integrated lumi- nosity of 34.6±1.2 nb−1 [27]. The beam energies were 4 TeV for protons and 1.58 TeV per nucleon for lead nuclei, resulting ina center-of-massenergy pernucleon pairof √
sNN=5.02 TeV.
As aconsequence ofthe energy differencebetween thecolliding beams, the nucleon–nucleon center-of-mass frame is not at rest withrespecttothelaboratoryframe.Masslessparticlesemittedat rapidityycm=0 inthenucleon–nucleoncenter-of-massframewill bedetected at ylab= −0.465 (clockwiseprotonbeam)or+0.465 (counterclockwise proton beam)in the laboratory frame. The re- sults presented here are expressed in the center-of-mass frame withtheproton-goingsidedefiningtheregionofpositiveycmval- ues,to respectthe usual convention ofthe protonfragmentation regionbeingprobedatpositiverapidity.Thedirectionofthehigher energyprotonbeamwasinitiallyclockwiseandwasthenreversed, producingtwocomparabledatasets.
Duringdatataking,muonandelectrontriggerswereemployed toselectandrecordalleventswithhigh-pT leptons.Themeasure- ments in the muon final state are based on a sample obtained by requiring at least one muon with pT greater than 12 GeV/c.
The muon candidates are reconstructed with an algorithm that combinesinformationfromboththesilicontrackerandthemuon system[28].Backgroundmuonsfromcosmicraysandheavy-quark semileptonicdecaysarerejectedbyapplyingasetofqualitycrite- riatoeachmuon,basedonpreviousstudiesoftheperformanceof themuonreconstruction[28].Themuonsareselectedbyrequiring atleasttwomuonstationstobematchedtothemuontrack,a low
χ2/ndf oftheglobalfit,aminimumnumberoftrackerlayersand pixelhits,andfinally,amaximumdistancefromtheprimaryver- texinthetransverseandlongitudinaldirection.
Theelectronmeasurementsarebasedonacandidatephotonor electronsamplecollectedbyrequiringatleastoneECALtransverse energy deposit of ET>15 GeV and online identification criteria
thatarelooserthantheelectronselectionappliedoffline.Electrons arereconstructedbymatchingECALclusterstotracksmeasuredin thesilicontracker.Thismatchingisusedtodifferentiateelectrons fromphotons[29].Theidentificationcriteriaarechosen tomatch thoseusedforppcollisions[30].Theelectronsareselectedbyre- quiring a match between the η and φ coordinates of the track and the ECAL cluster, a narrow width of the ECAL cluster in η, a low HCAL energy measured in the ECAL cluster direction and byrejecting electronswithapartnertrackconsistentwitha pho- tonconversion.Inthismeasurement,noisolationrequirementsare imposedontheleptons.
3. Analysisprocedure
The Z boson production cross section is calculated using the followingequation:
σ= S−B
α Lint, (2)
where S is thenumberofZcandidates, B is theestimatedback- ground, α is theacceptance, is theefficiency,including correc- tionfactorsderivedfromdata,andLint istheintegratedluminos- ity. The phase spaceregion considered in the analysisis defined byrequiringtwoleptons withpT>20 GeV/c andwithpseudora- pidity inthelaboratoryframe |ηlab|<2.4 in ordertoensurethat thetriggersaremaximallyefficientandarewithinthegeometrical coverage of themuon detectors. This fiducialregion ofthe mea- surementisextrapolatedtothefull phasespaceover pT and ηlab
by the acceptance correction. Each component of Eq. (2)is pre- sentedandsystematicuncertaintiessummarizedbelow.
3.1. Signalandbackground
The Z candidate events are selected by requiring a same- flavor,oppositely-chargedleptonpairwithaninvariantmassinthe 60–120 GeV/c2 range,where both leptons satisfy the acceptance and quality requirements and at least one of them corresponds to the lepton that triggered the event. Fig. 1 shows the invari- ant massdistribution ofthe selected lepton pairs comparedto a combinationof pythia 6and hijing (pythia 6+hijing)MonteCarlo (MC) simulations. The pN→Z→ process is simulated using the pythia 6 [31] generator (version 6.424, tune Z2[32]) witha mixtureofppandpninteractionscorrespondingtopPbcollisions.
Each pythia 6signal event isembedded in aminimum bias pPb backgroundeventwhichisproducedwiththe hijing eventgener- ator version 1.383 [33].The detectorresponse foreach produced eventis simulated with Geant4[34].The signal and background events havethe same generated vertexlocation andare boosted to have the correctrapidity distribution in thelaboratory frame.
Theembedding isdone atthelevelofdetectorhitsandthen the eventsareprocessedthroughthetriggeremulation andtheevent reconstructionchains.Thereconstructedlongitudinalprimaryver- texandoverallmultiplicity distributions arereweighted tomatch thoseobservedindata.
An electron energyscale correction is extractedby fitting the energy to momentum ratio of electrons in a very pure W→eν
controlsample[29].Afterfixingtheshapeofthedistributionfrom MC,theenergytomomentum ratioindataisfittedtoderivethe differenceoftheenergyscalebetweendataandMC,andthenthe data is corrected forthis difference. A correction of theelectron energyresolutionisappliedtoMC bycomparing themassdistri- butionofelectronpairsbetweendataandMC.Suchcorrectionsare alsoestimatedfortheZ→μ+μ− channelandfoundtobenegli- gible.
Fig. 1. Invariantmassofselectedmuon(top)andelectron(bottom)pairscompared to pythia 6+hijing simulatedpN→Z→ eventswithN= (p,n)accordingtothe numberofnucleonsinthePbnucleus.TheMCsampleisnormalizedtothenumber ofeventsinthedata.
The rawyield, S, ofZ bosoncandidates in the pPbsample is determinedby countingthenumberofoppositely-chargedlepton pairs in the 60–120 GeV/c2 mass region that fulfill the accep- tanceandquality requirements.Thisnumberis foundtobe2183 inthe muon channel and 1571in the electron channel. The dif- ference betweenthetwo channelsis dueto thetighter selection criteria applied to the electrons in order to suppressthe higher background.Acharge misidentificationcorrectionof1%isapplied tothe dielectronyields;thiscorrection isnegligiblefordimuons.
No eventsare found withmore than oneZ boson candidate.For the differential cross sections, the measurement is performedin thedileptontransversemomentumorrapiditybins,wherethera- pidityiscalculatedinthecenter-of-massframe.
Possible background contributions to the Z→ production are QCD multijetevents,tt pairs andelectroweakprocesses such as W+jets, diboson (WW, WZ, ZZ), and Z→τ τ production. Al- though theexpected backgroundcontamination is small, an esti- matebased ondata isused tosubtract its contributionfromthe dilepton raw yield. For tt, bb, WW, and Z→τ τ processes, two electron–muon events are expected for each dimuon or dielec-
tron event, because of lepton universality. In the Z boson mass range, the oppositely-charged electron–muon pairs are counted and translated into the expected number of muon or electron pairs, taking into account the differences inthe muon andelec- tron reconstruction and selection efficiencies. This background is subtracted fromthedileptonrawyieldandaccountsforthemain electroweak and tt backgrounds, as well as for the part of QCD multijetbackground(suchasbb decays)thatproducesoppositely- charged leptons. The background from random combinations of other leptons in the event is estimated by counting the same- charge pairs. Additional electroweak contributions from W+jets and diboson production are found to be negligible via MC sim- ulations. The fraction of background events subtracted from the raw yield is 2.4% (2.9%) in the muon (electron) channel, where thedominantbackgroundcontributioncomesfromQCDprocesses, sincenoisolationrequirementsareimposedontheleptons.
3.2. Efficiencyandacceptance
The efficiency, ,forZbosonsisdefinedasthenumberofre- constructedZcandidates,wherebothleptonsfulfilltheacceptance and quality requirements, divided by the number of generated Z bosons where bothleptons fulfill theacceptance requirements.
Thiscombinedreconstruction,leptonidentification,andtriggeref- ficiencyiscalculatedfromthe pythia 6+hijing simulationsamples sothattheeffectsofthepPbenvironmentaretakenintoaccount.
FortherapidlyfallingdileptonpT spectrum,anunfoldingtech- nique basedon theinversion ofa responsematrix similarto the oneusedinRef.[4]isfirstappliedtothedatabeforeapplyingthe efficiencycorrection.Theresponse matrixisconstructedfromthe pythia6+hijing simulationtotakeintoaccountthedetectorreso- lution effects.Thedilepton pT resolution isabout0.5–1.5 GeV/c, which resultsina maximumbin-to-bin spill ofabout30% inthe lowest pTbinschosenforthisanalysis.Inthemeasurementofthe dileptonrapidity,theunfoldingisnotnecessaryastheshapeofthe ycm spectrumisalmost flatandtheresolutionisasmallfraction of theanalysisbin size.Instead,the resolutioneffects inrapidity aretakenintoaccountintheefficiencycorrections.
In order to correct for possible differences betweendata and simulation,amethodderivedfromdataisusedtodeterminecor- rection factors to the baseline efficiency from simulation. These correctionfactorsaredeterminedasafunctionoflepton ηandpT byapplyingthetag-and-probe methodtobothdataandsimulation tocalculatesingle leptonefficienciesforreconstruction,identifica- tion, andtriggering,similar tothe methoddescribed inRef. [28].
Theratioofeachefficiencyfromdataoverthecorrespondingeffi- ciencyinthesimulationisthenappliedtoreweightthesimulation on alepton-by-lepton basis.Theefficiencyforthe Z bosons,after correcting forthesmalldifferencesbetweendataandsimulation, isfoundto be0.878±0.015 inthedimuonand0.605±0.015 in thedielectrondecaychannel.Thesourcesofsystematicuncertain- tiesaredescribedinSection3.3.
Theacceptance, α,isdefinedasthenumberofgenerateddilep- ton eventswherebothleptons fulfilltheacceptancerequirements (pT>20 GeV/c, |ηlab|<2.4) dividedby thenumberofall gener- ated dileptoneventsin the60–120 GeV/c2 massrange.It iscal- culated using simulatedevents.The eventgeneration is provided by the powheg generator[35–38] withthe CT10freeprotonPDF set [39],interfacedwith pythia 6 partonshower, andthe events areboostedtothelaboratoryframe(powheg+pythia 6).Final-state photonradiationisalsosimulatedby pythia 6. Theintegratedac- ceptanceisfoundtobe0.516±0.026 inbothdecaychannels.
