Study of W boson production in pPb collisions at √

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Physics Letters B

www.elsevier.com/locate/physletb

Study of W boson production in pPb collisions at √

s

= 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:

Received19March2015

Receivedinrevisedform6September2015 Accepted23September2015

Availableonline28September2015 Editor:M.Doser

Keywords:

CMS Wboson pPbcollisions

ThefirststudyofWbosonproductioninpPbcollisionsispresented,forbosonsdecayingtoamuonor electron,andaneutrino.Themeasurementsarebasedonadatasamplecorrespondingtoanintegrated luminosity of34.6 nb⁻¹atanucleon–nucleon centre-of-massenergyof√_s

NN=⁵.02 TeV,collectedby the CMSexperiment.TheWbosondifferential crosssections,leptonchargeasymmetry, andforward–

backwardasymmetriesaremeasuredforleptonsoftransversemomentumexceeding25 GeV/c,andasa functionoftheleptonpseudorapidityinthe|ηlab|<2.4 range.Deviationsfromtheexpectationsbasedon currentlyavailablepartondistributionfunctionsareobserved,showingtheneedforincludingWboson datainnuclearpartondistributionglobalfits.

©^{2015CERNforthebenefitoftheCMS}Collaboration.PublishedbyElsevierB.V.Thisisanopenaccess articleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP³.

1. Introduction

Electroweakbosonproductioninproton–nucleusandnucleus–

nucleuscollisionsattheCERNLHCoffersauniqueopportunityto probenuclearpartondistributionfunctions(nPDFs)[1–4].Leptonic decaysofelectroweakbosons are ofparticular interestsince lep- tonsdonot interactstronglywiththemediumproduced inthese collisions [5,6].As compared to thosein aproton, thenPDFs are expected to be depleted (shadowing) for partons carrying small momentumfractions x^{10−2,andenhanced} (anti-shadowing)in the5×¹⁰⁻²^x^{10−1 range[7].However,becauseofthelackof} availabledata,partondensitiesarelesspreciselyknownfornuclei thanfornucleons.Asa consequence,precise calculationsdescrib- inghardprocessesinhigh-energyheavyioncollisionsare limited byuncertaintiesinthenPDFs.ForWboson production,thedom- inant processes at LHC energies are ud→^{W+ and du}→^W−, principallyreflectinginteractions thattake placebetweenvalence quarksandseaantiquarks.AccordingtoRef.[4],PDFnuclearmod- ifications could affect theyield of W bosons inpPb collisions at theLHC by asmuch as15% in certain kinematic regions. There- fore,precise measurements ofW boson productionin heavy ion collisionsmightlead toan improveddeterminationofthenPDFs.

Moreover, asymmetries in the individual yields of W⁺ and W⁻ shouldpermit theflavour decompositionofuanddquark distri- butionsinnuclei.

The ATLAS [8,9] and CMS[10,11] Collaborations reported the observations of Z bosons in heavy ion interactions, at a centre-

 E-mailaddress:cms-publication-committee-chair@cern.ch.

of-massenergy of2.76 TeV per nucleonpair. Thesedata showed thattheZbosonyieldspernucleon–nucleon(NN)collisionarees- sentially unmodified by the medium produced in the collisions.

Although W bosons decaying to a lepton and a neutrino are more difficultto detect, their rateis abouttentimes larger than that ofZbosons decayingto leptonicfinal states.The production of W bosons in PbPb collisions was reported by CMS [12] and ATLAS [13], usingdatacorresponding toan integratedluminosity of7.3 μb⁻¹and150 μb⁻¹,collectedin2010and2011,respectively.

TheW bosonyield perNNcollisionwas showntobe compatible withtheonemeasuredinppcollisions,whentakingintoaccount isospineffectsarisingfromthemixtureofprotonsandneutronsin thecollidingnuclei.However,thepresenceof10–20%nPDFeffects on ZandW boson production could not be excluded dueto the relativelylargeexperimentalandtheoreticaluncertaintiesofthese results.

The 2013 pPb LHC run provides the best currently available data sample to look for initial-stateeffects (such asPDF modifi- cations) usingelectroweak bosons. The NN-equivalent luminosity isofthesameorderofmagnitudeasforthe2011PbPb run,and the production cross sections are approximately a factor of two greater owingtotheincreasedenergy,5.02 TeV pernucleon pair.

Furthermore,theasymmetryofthepPbcollisionsystemallowsfor themeasurement ofotherobservablessuch asforward–backward pseudorapidityasymmetries.ThisLetter reportsa studyofW bo- son productioninasample ofpPbcollisions corresponding toan integrated luminosity of (34.6±¹.2)nb⁻¹ [14], collected by the CMSexperiment.

http://dx.doi.org/10.1016/j.physletb.2015.09.057

0370-2693/©^{2015CERNforthebenefitoftheCMS}Collaboration.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP³.

2. Experimentalmethods

Thedirectionoftheprotonbeamwas initiallyopposite tothe positive directionof the CMSlongitudinal axis [15], andwas re- versedafter60%ofthedataweretaken. Thebeamenergieswere 4 TeV for protons and 1.58 TeV per nucleon for lead nuclei, re- sulting in a centre-of-mass energy per nucleon pair of √

s_NN= 5.02 TeV. As a result of the energy difference of the colliding beams, the NN centre-of-mass frame in pPb collisions was not at rest with respect to the laboratory frame. Massless particles emitted atpseudorapidity ηin the NNcentre-of-mass frame are detected at ηlab=η₋0.465 (firstproton beam orientation) and

ηlab=η+⁰.465 (second proton beam orientation) in the CMS coordinate system, as defined inRef. [15]. The results presented hereafter are expressed in the usual convention of the proton- going side defining the positive pseudorapidity. It coincides with theCMSconvention inthesecondperiodofdata taking,thefirst onebeingreversedbeforesummingyieldsfromthetwobeamcon- figurations.

Adetaileddescriptionof theCMSdetectorcan befound else- where [15]. Its central feature is a superconducting solenoid of 6 minternaldiameter,providingamagneticfieldof3.8 T.Within the field volume are the silicon pixel-and-strip tracker, a lead tungstate crystalelectromagnetic calorimeter(ECAL), and a brass and scintillator hadron calorimeter (HCAL), each composed of a barrel and two endcap sections. The silicon tracker consists of 66 Mpixeland10 Mstripsensorelements,andmeasurescharged- particletrajectories inthepseudorapidity range |ηlab|<2.5.Out- sideofthesolenoid, muonsaredetected inthe|ηlab|<2.4 range, withgas-ionization detectorplanes based on three technologies:

drift tubes,cathodestrip chambers, andresistive-plate chambers.

Electrons are identified in the ECAL, which is made of 75 848 lead tungstatecrystals andcovers |ηlab|<1.48 in thebarrel and 1.48<|η_lab|<3.00 inthetwoendcapregions.TheCMSapparatus alsohasextensiveforwardcalorimetry,includingtwosteel/quartz- fiberCherenkovhadronforward(HF)calorimeters,whichcoverthe 2.9<|η_lab|<5.2 range. For online event selection, CMS uses a two-leveltriggersystem.

SelectioncriteriasimilartotheonesdevelopedinRef.[16]are appliedtothepPbsampletoremoveeventswithelectromagnetic, beam-gas,ormultiple collisions (pileup).The W bosonyields are correctedfortheinduced(4.0±⁰.5)% signalloss.

Theprimary signature of aW boson isa hightransverse mo- mentum(pT)lepton.Thecurrentanalysisisrestrictedtoleptonsof pT greater than25 GeV/c.The muonanalysisisbasedona sam- pletriggeredbyrequiringasingle muonwith p_Tabove12 GeV/c, whiletheelectron analysisuses anECAL-triggered samplewitha transverseenergy thresholdof15 GeV.Leptons arereconstructed withthe same algorithms asin proton–proton collisions [17,18], and standard selection criteria are applied, as in Refs. [12,19].

A specialelectroncharge determination,asdescribed inRef.[20], isusedinordertoreducetheelectronchargemisidentificationtoa sub-percentlevel.Eventsarereconstructedusingparticle-flow(PF) techniques[21,22],whichreconstructandclassifyindividualparti- cleswithanoptimised combinationofallsubdetectorinformation.

Two criteriaare used to remove specific background sources.

First,eventswithtwooppositelychargedleptons,withthesecond lepton pT greater than 15 (10) GeV/c for muons (electrons) are removed, since they correspond to well-identified processes like Drell–Yan,Zbosonorhigh-pT quarkoniumproduction.Second,the leptons are required to be isolated, in order to reduce the con- taminationcomingfromjet fragmentation. The energiesof allPF candidatesaresummed withina conecentredaroundthelepton, with the exception of the lepton itself. The lepton is considered isolated if the total transverse energyin the cone is small com-

pared to its transverse momentum. Formuons, a cone of radius

R=

(η)²+ (φ)²=⁰.3 is used,whereη andφ are the pseudorapidity andazimuthal distances tothe lepton.The candi- date is rejected if the in-cone transverse energy is greater than 10% ofthe muon pT.For electrons, a cone of R=⁰.4 is used, andonly particles with pT greater than 1 GeV/c aresummed, to reduce the underlying-event enhanced contribution. The electron candidate is rejectedifthe resulting transverseenergy is greater than 11.5% (9.5%) ofthe electron 4pT, forthe ECAL barrel (end- caps).

AnimportantcharacteristicofeventscontainingaW→ ν de- cay isthemissingtransverseenergy(/ET) associatedwiththeun- detectedneutrino.Itiscomputedasthemagnitudeofthevectorial sumoftransversemomentaofallthePFcandidatesintheevent.

The analysis is performed using ten lepton pseudorapidity bins, each 0.5wideexceptforthemostforwardandbackwardregions (2<|ηlab|<2.4).Afterhavingappliedtheleptonselectioncriteria, examples ofthe resulting/ET distributionsare showninFig. 1for

μ⁺ ande⁺, inthe mostcentral (−⁰.5<ηlab<0.0) andfurthest forward(2.0<ηlab<2.4) ranges.The distributionsforother bins andforthenegativeleptonsaresimilar.

Toextract thenumberofevents witha leptoncoming froma W boson,binned fitsofthesedistributionsareperformed,includ- ingthesignalandmainbackgroundcontributions,ineach ηlabbin.

The/ET shapesassumedfortheelectroweakprocesses,namelythe W^±→ ^±ν signal as well as background from W^±→τ^±ν and Z→ ⁺⁻,aredeterminedbythesimulationsdescribedhereafter, taking into account the acceptance and efficiency. Their relative normalizationisgivenbytheunmodifiedtheoreticalcrosssections (as computedin Ref. [23]). A maximal 20% variation of theW/Z normalizationratioistakenintoaccount,duetopotentiallydiffer- entnuclearmodificationsoftheZandWbosons,andresultingina 1–3%systematicuncertaintyintheextractedWyields.Thenotice- abledifferencebetweenthe/ETdistributionsfortheZ→^e+e−and Z→μ⁺μ⁻processesintheforwardregion(bottomplotsofFig. 1) results fromthegreater ECALcoverageallowing missed electrons with 2.4<|η_lab|<3.0 to be accountedforinthe /ET calculation.

The shape of the QCD multijet background is modelled by the functionalform f(/E_T)= (/E_T+/E_T⁰)α exp(β

/

E_T+/E_T⁰).Itisshown to reproducethe/ET shape ofdataeventscontainingnon-isolated leptons, withthe /ET0, α, andβ parameters, which are observed to depend mildly andlinearly on the cone/lepton transverse en- ergy ratio. These fitted parameters are then extrapolated to the isolatedleptonsignalregimeandtheresultingfunctionisusedas the QCDbackgroundshape.The multijetbackgroundcontribution is largerintheelectron channel becausethemisidentified lepton rate ishigher, particularly dueto a contribution fromphoton-jet events.Contributionsfromothersources,suchastt productionand high-p_Tquarkonia,werefoundtobenegligible.

A small charge misidentification correction (less than 0.2%) is applied to the electron yields; this correction is negligible for muons. All fits are of good quality, as illustrated by the bottom panels of Fig. 1 that show the ratio of the data to the fit out- come. The observed numbers of leptons coming from W boson decaysovertheentirepseudorapidityrangeare:11660±¹¹¹μ⁺, 9459±⁹⁹μ⁻,9892±^{116 e+,and7872}±^{101 e−,wheretheun-} certaintyisstatistical,determinedbythefitprocedure.

Inordertocorrectforinefficienciesintheleptontrigger,recon- struction, and selection,the electroweak processesW→ ν have beensimulatedusingthe pythia 6.424generator[24] withamix- tureofppandpninteractionscorrespondingtopPbcollisions.The detector response to each pythia signal event is simulated with Geant4 [25] and then embedded in a minimum bias pPb back- ground event. These background events are produced with the hijing eventgenerator [26] andpassed through Geant4 as well.

Fig. 1. MissingtransverseenergydistributionforW⁺→μ⁺ν (left)andW⁺→^e+ν (right)eventswithinthe−⁰.5<ηlab<0.0 (top)and2.0<ηlab<2.4 (bottom)ranges.

Binnedfitstothedata(redpoints)areperformedwithfourcontributions,stackedfrombottomtotop:multijet(QCD,blue),W⁺→τ⁺ν (brown),Z→ ^(white)and W⁺→ ⁺ν(yellow).Theηlabregionsaredefinedsuchthattheprotonismovingtowardspositiveηlabvalues.Errorbarsrepresentstatisticaluncertainties.Thelowerpanels displaythedatadividedbytheresultofthefit,withthebandrepresentingthestatisticaluncertaintiesonthesumofthefitcomponents,foreach/ETbin.(Forinterpretation ofthereferencestocolorinthisfigurelegend,thereaderisreferredtothewebversionofthisarticle.)

Eachsimulationis done twice, onceforeach proton beamdirec- tion,andincludesaboosttoreproducethe0.465rapidityshift.The embeddingisdoneatthelevelofdetectorhits,andthesignaland backgroundeventssharethesamegenerated vertexlocation. The embeddedeventis thenprocessedthrough thetrigger emulation andthefulleventreconstructionchain.Theresultingreconstructed events are then reweighted to match the distributions observed indata of the event vertexand activity (as measured in the HF calorimeters).Theobtainedefficienciesvarywith ηlab(withhigher efficienciesatmid-rapidity),from59%to89%formuons,andfrom 51%to84%forelectrons.

Thevariouscomponentsofthesingle-leptonefficiencyarealso directlycomputedfrompPbdata,usingZ→ ^{samples,andtech-} niquesdescribedinRef.[23].Theseefficienciesarethencompared to the corresponding efficiencies computed from simulations. In thecaseoftriggerandreconstruction efficiencies, theyare found to be consistent. The isolation criterion rejects more leptons in data,becausethelocal activityoftheunderlying eventis greater thaninthesimulation.Toaccountforsuchdiscrepancies,theeffi- ciencyfromW→ νsimulationismultipliedbycorrectionfactors,

whicharedeterminedastheratioofthesingle-leptonefficiencies measured in Z→  ^{datato those estimated in}simulations. The so-called“tag-and-probe” methodused forthis estimationis de- scribedinRef.[27].Thesecorrectionfactorsarecomputedinbins of ηlabandforpositivelyandnegativelychargedmuonsseparately.

Inthe electroncase, thelow statisticalprecisionmotivatesa cor- rectionfactorestimatedforelectronsandpositronscombined.

The total systematic uncertainty in the lepton yields is esti- mated by adding the different contributions in quadrature. The

ηlab-dependent sources of systematic uncertainty arise from the methodused forthe estimationof multijetbackground(0.1–2.0%

formuons, 0.5–3.8%forelectrons),thenormalizationofthe elec- troweakbackground(1–3%formuonsandelectrons),theefficiency correctionfactors(2.2–7.5%formuons,2.6–7.4%forelectrons),and the energy scale of electrons (0.1–2.0%). The uncertainty in the momentum scale of muons is found to be negligible. The inte- gratedluminositymeasurementuncertainty(3.5%[14])affectsonly the W bosonproduction crosssections andcancelsin theasym- metry measurements, as does the additional global uncertainty arising from the efficiency of the filter rejecting pileup events

Fig. 2. ProductioncrosssectionsforW⁺→ ⁺ν (top)andW⁻→ ⁻ν (bottom), asafunctionoftheleptonpseudorapidity.Errorbarsrepresentthestatisticalun- certainties,whilebracketsshowstatisticalandsystematicuncertaintiessummedin quadrature.Thegloballuminosityuncertaintyof±3.5%isnotincluded.Toimprove visibility,the muon(electron)measurements, inredcircles (bluesquares),have beenshifted by−⁰.05 (+⁰.05)inpseudorapidity.(Forinterpretationofthe ref- erencestocolorinthisfigurelegend,thereaderisreferredtothewebversionof thisarticle.)

(0.5% for both channels). Though the common electron/positron correction factors cancel, a residual systematicuncertainty of 3%

isassignedtothechargeasymmetry,basedonsimulationstudies and ηlab-integratedefficiencies determined fromZ→^{e+e− data.}

Noothersystematicuncertaintycancellationsareassumedforthe asymmetryresults.

3. Results

Fig. 2 shows the production cross sections for pPb→^W±+ X→ ^±ν+^{X asafunctionofthechargedlepton}pseudorapidityin thelaboratory frame,withthe leptonhaving p_T>25 GeV/c.The crosssectionsaredetermined by dividingtheefficiency-corrected leptonyieldsbytheintegratedluminosity.

Since the cross sections measured in the electron and muon channelsarefoundtobeingoodagreementwitheachother,they are combined using the BLUE method [28]. Fig. 3 compares the combinedcrosssectionswithnext-to-leading-order (NLO)pertur-

Fig. 3. Productioncrosssectionsfor W⁺→ ⁺ν (top)and W⁻→ ⁻ν (bottom), asafunctionoftheleptonpseudorapidity.Errorbarsrepresentthestatisticalun- certainties,whilebracketsshowstatisticalandsystematicuncertaintiessummedin quadrature.Thegloballuminosityuncertaintyof±3.5%isnotdisplayed.Theoreti- calpredictionswith(CT10+EPS09,dashedgreenline)andwithout(CT10,solidred line)PDFnuclearmodificationsarealsoshown,with theuncertaintybands.The bottompanelsshowtheratioofthedata(blackpoints)andCT10+^EPS09(dashed greenline)totheCT10baseline.AlltheoryuncertaintybandsincludescaleandPDF uncertainties,excepttheEPS09ofthebottompanelswhichonlyincludestheEPS09 PDFuncertainties.(Forinterpretationofthereferencestocolorinthisfigurelegend, thereaderisreferredtothewebversionofthisarticle.)

bative QCD predictions provided by theauthors of Ref. [4]using CT10 [29] proton parton distribution functions (PDF) without or withEPS09[30] nPDFcorrections,termedCT10andCT10+^EPS09, respectively. Their uncertainties are estimated as prescribed in Refs. [29,30]. Table 1gives the measured cross sections for each channelseparatelyandcombined,asafunctionoftheleptonpseu- dorapidity, for positive andnegative leptons. The theoretical pre- dictionsandtheiruncertainties(comingfromthePDFsetandfrom the renormalisation and factorisation scales) are also given. The agreement between the data and both theoretical predictions is withintheuncertainties,althoughasmallexcessofW⁻candidates appearsatnegative ηlab,i.e. inthePbionbeamdirection.

The comparison between the CT10 and CT10+^{EPS09 calcula-} tions shows that the predicted modifications of the PDFs are of thesameorderasthetheoreticaluncertainties.Thisindicatesthat cross sectionsalonelackdiscriminatingpower, andmotivatesthe