Study of W boson production in pPb collisions at root(NN)-N-S=5.02 TeV

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DOI: 10.1016/j.physletb.2015.09.057

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DIRETORIA DE TRATAMENTO DA INFORMAÇÃO Cidade Universitária Zeferino Vaz Barão Geraldo

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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−1_{atanucleon–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.

©2015CERNforthebenefitoftheCMSCollaboration.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 x10−2,andenhanced (anti-shadowing)in the5×10−2_x10−1 _{range[7].However,becauseofthelackof} availabledata,partondensitiesarelesspreciselyknownfornuclei thanfornucleons.Asa consequence,precise calculations describ-inghardprocessesinhigh-energyheavyioncollisionsare limited byuncertaintiesinthenPDFs.ForWboson production,the dom-inant processes at LHC energies are ud→W+ and du→W−, principallyreflectinginteractions thattake placebetweenvalence quarksandseaantiquarks.AccordingtoRef.[4],PDFnuclear mod-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:[email protected].}

of-massenergy of2.76 TeV per nucleonpair. Thesedata showed thattheZbosonyieldspernucleon–nucleon(NN)collisionare es-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−1_{and150 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/©2015CERNforthebenefitoftheCMSCollaboration.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense

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 √sNN= 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 onebeingreversedbeforesummingyieldsfromthetwobeam con-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,andmeasures charged-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<|ηlab|<3.00 inthetwoendcapregions.TheCMSapparatus alsohasextensiveforwardcalorimetry,includingtwo steel/quartz-fiberCherenkovhadronforward(HF)calorimeters,whichcoverthe 2.9<|ηlab|<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 pTabove12 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],whichreconstructandclassifyindividual parti-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) associatedwiththe un-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,duetopotentially differ-entnuclearmodificationsoftheZandWbosons,andresultingina 1–3%systematicuncertaintyintheextractedWyields.The notice-abledifferencebetweenthe/ETdistributionsfortheZ→e+e−and Z→μ+μ−processesintheforwardregion(bottomplotsofFig. 1) results fromthegreater ECALcoverageallowing missed electrons with 2.4<|ηlab|<3.0 to be accountedforinthe /ET calculation. The shape of the QCD multijet background is modelled by the functionalform f(/ET)= (/ET+/ET0)α exp(β

 /

ET+/ET0).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-pTquarkonia,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−,wherethe un-certaintyisstatistical,determinedbythefitprocedure.

Inordertocorrectforinefficienciesintheleptontrigger, recon-struction, and selection,the electroweak processesW→ ν have beensimulatedusingthe pythia 6.424generator[24] witha mix-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 beam direc-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,and tech-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,the effi-ciencyfromW→ νsimulationismultipliedbycorrectionfactors,

whicharedeterminedastheratioofthesingle-leptonefficiencies measured in Z→  datato those estimated insimulations. 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 the asym-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.Errorbarsrepresentthestatistical

un-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 asafunctionofthechargedleptonpseudorapidityin thelaboratory frame,withthe leptonhaving pT>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.Errorbarsrepresentthestatistical

un-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,asafunctionofthelepton pseu-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

Table 1

ProductioncrosssectionforpPb→W+X→ ν+X forpositively(top)andnegatively(bottom)chargedleptonsofpTlargerthan25 GeV/c,innanobarns,asafunction

oftheleptonpseudorapidity.Valuesaregivenfirstformuonsandelectronsseparately,thencombined.Quoteduncertaintiesarefirststatistical,thensystematic.Theoretical

predictionswith(CT10+EPS09)andwithout(CT10)PDFnuclearmodificationsarealsogiven,withtheiruncertainties.Theglobalnormalizationuncertaintyof3.5%isnot

includedinthelisteduncertainties.

dσ dη (nb) [ηbin] [−2.4,−2.0] [−2.0,−1.5] [−1.5,−1.0] [−1.0,−0.5] [−0.5,0] μ+ 43.0±2.2±3.1 62.5±2.1±2.6 86.9±2.6±3.4 98.1±2.7±2.6 98.3±2.8±3.3 e+ 46.5±2.6±3.6 64.0±3.1±4.2 84.2±3.1±4.8 99.8±3.0±4.6 102.0±2.9±4.6 + 44.5±1.7±2.3 62.9±1.8±2.2 85.9±2.0±2.7 98.6±2.1±2.3 99.7±2.1±2.7 CT10+EPS09 42.1+₋22..68 66.0+ 3.8 −4.2 84.6+ 4.8 −5.4 93.4+ 5.3 −6.0 96.0+ 5.8 −6.3 CT10 43.4+₋22..58 65.8+ 3.7 −4.2 82.4+ 4.6 −5.2 90.5+ 5.1 −5.7 94.4+ 5.7 −6.1 dσ dη (nb) [ηbin] [0,0.5] [0.5,1.0] [1.0,1.5] [1.5,2.0] [2.0,2.4] μ+ 113.9±3.1±4.5 101.3±2.8±2.9 102.3±2.8±3.6 107.9±3.1±5.7 107.8±3.7±8.4 e+ 99.6±2.7±3.6 102.8±2.9±4.6 95.6±3.4±5.8 95.4±3.5±6.2 108.3±4.3±8.7 + 105.3±2.1±2.8 101.8±2.1±2.5 100.2±2.2±3.1 102.3±2.3±4.2 108.1±2.8±6.0 CT10+EPS09 95.9+₋66..24 95.5+ 6.6 −6.7 95.7+ 6.8 −7.5 95.3+ 7.5 −8.4 91.6+ 7.9 −8.9 CT10 97.0+5.8 −6.4 100.0+ 6.4 −6.6 103.4+ 6.3 −6.8 105.7+ 6.2 −7.2 103.6+ 6.0 −7.3 dσ dη (nb) [ηbin] [−2.4,−2.0] [−2.0,−1.5] [−1.5,−1.0] [−1.0,−0.5] [−0.5,0] μ− 74.5±3.0±5.6 84.5±2.8±4.4 89.4±2.6±3.5 81.4±2.5±2.6 80.6±2.6±2.6 e− 70.2±3.2±4.8 74.3±3.3±4.8 79.6±3.1±4.3 80.7±2.7±3.7 81.3±2.6±4.0 − 72.1±2.2±3.7 79.9±2.1±3.3 85.4±2.0±2.7 81.1±1.8±2.1 80.8±1.9±2.2 CT10+EPS09 65.2+4.0 −4.6 72.4+ 4.4 −5.0 75.9+ 4.6 −4.9 76.9+ 4.6 −5.0 76.1+ 4.9 −5.3 CT10 64.2+3.9 −4.4 70.1+ 4.2 −4.7 73.3+ 4.3 −4.8 74.8+ 4.4 −4.8 75.1+ 4.7 −5.1 dσ dη (nb) [ηbin] [0,0.5] [0.5,1.0] [1.0,1.5] [1.5,2.0] [2.0,2.4] μ− 81.7±2.5±3.0 78.8±2.5±3.3 69.8±2.3±3.0 62.9±2.1±3.3 63.1±2.8±5.1 e− 73.5±2.5±3.5 74.0±2.5±3.5 70.6±2.8±4.6 55.0±2.7±4.1 64.6±3.3±6.0 − 78.0±1.8±2.3 76.5±1.8±2.4 70.1±1.8±2.5 59.8±1.7±2.6 63.7±2.1±3.9 CT10+EPS09 73.6+5.1 −5.2 69.7+ 4.9 −5.1 64.8+ 4.5 −4.9 59.1+ 4.3 −4.8 53.4+ 4.3 −4.8 CT10 74.3+4.9 −5.2 72.4+ 4.8 −5.1 69.1+ 4.2 −4.9 64.5+ 3.8 −4.3 59.3+ 3.6 −4.0

studyofvariousasymmetriesofthe+and− crosssections.The interest in such asymmetries is twofold. First, some of the ex-perimental(e.g. integrated luminosity) and theoretical (e.g. scale dependence)uncertaintiescancelinsuchasymmetries.Second,the various asymmetries exhibit different sensitivities to the nuclear modificationsofthePDFs,asdiscussedbelow.

Theleptonchargeasymmetry,definedas(N+_−N_−)/(N+_+N_−)

with N±_ being the efficiency-corrected lepton yields, is shown in Fig. 4, as a function of ηlab, and compared to the theoret-ical predictions. For ηlab>−1, both calculations reproduce the presentmeasurements. For ηlab<−1, however, the two calcula-tions overpredict the asymmetry values. A possible physical ori-gin of this disagreement could be a different modification of u andd quark distributions in nuclei. In proton–(anti)proton colli-sions, the W-boson charge asymmetry is known to be a sensi-tiveprobe ofthedown-to-up quark PDF ratioina proton,dp_/up

[20,31,32]. Similarly, this asymmetry in pPb collisions measured in the lead fragmentation region (i.e. ηlab<0.465) probes these quark densities in a nucleon inside the lead nucleus. Assuming the standard isospin symmetry (up=dn, un=dp), one can de-fine a similar ratio, dp/A/up/A=dp/up ×Rd/Ru, where Ri are the nPDF ratios, Ru ≡up/A/up and Rd ≡dp/A/dp. The typical quark momentum fraction probed in the Pb nucleus is given by

xMW/√sNN×exp(−ηlab+0.465)(assumingthattheW boson rapidity issimilar to that of thelepton), thereforex0.02–0.20 in the range −2<ηlab<0. In most global fit analyses of the nPDFs (as in the caseof EPS09), it is assumed that the nuclear ratiosrespect the isospin symmetry, namely Ru=Rd, essentially tominimise the numberof free parameters inthe fits. However, nophysical reasonpreventsnuclear modifications tobe different forup and down quark PDFs. Forexample, it is known that the

Fig. 4. Leptonchargeasymmetry,(N+−N−)/(N++N−),asafunctionofthe

lep-tonpseudorapidity.Errorbarsrepresentthestatisticaluncertainties,whilebrackets

show statisticaland systematicuncertainties summedinquadrature. Theoretical

predictionswith(CT10+EPS09,dashed greenline)and without(CT10,solidred

line)PDFnuclearmodificationsarealsoshown,withtheiruncertaintybands.(For

interpretationofthereferencestocolorinthisfigurelegend,thereaderisreferred

tothewebversionofthisarticle.)

shapesoftheupanddownquarkdistributionsinprotonsare dif-ferent [33]. Furthermore,the presentdisparitybetweendata and theoryisunlikelytocomefromtheprotonPDFassumption,given theexcellent agreementoflepton chargeasymmetry measuredin

Fig. 5. Forward–backwardasymmetries,N(+ηlab)/N(−ηlab),forthepositive(top)

and negative (bottom)leptons.Error barsrepresent the statistical uncertainties,

whilebracketsshowstatisticalandsystematicuncertaintiessummedinquadrature.

Theoreticalpredictionswith(CT10+EPS09,dashedgreenline)andwithout(CT10,

solidredline)PDFnuclearmodificationsarealsoshown, withtheir uncertainty

bands.(Forinterpretationofthereferencestocolorinthisfigurelegend,thereader

isreferredtothewebversionofthisarticle.)

pp collisions by CMS [32] and ATLAS [34] with NLO calculations usingCT10partondensities.

Atraditionalwaytoprobenuclearpartondensitiesis to com-pare the forward and backward W yields, that are respectively sensitivetothenPDFsatsmallandlargex.Theforward–backward asymmetriesN±_(+ηlab)/N±(−ηlab)areshowninFig. 5,separately forthepositivelyandnegativelychargedleptons,andcomparedto thesamepredictionsasmentionedabove.Giventheexperimental accuracy andthe magnitude of the differencesbetween the two setsofpredictions,themeasurementshaveapotentialto discrim-inatebetweenthem.However,althoughthenegativeleptondecay channelappearstoslightlyfavourtheCT10+EPS09predictionover theCT10calculation,thepositiveleptonchanneldoesnot,thusno firmconclusioncanbedrawn.

Another asymmetry variable, (N_+(+ηlab) − N+_(−ηlab))/

(N_−(+ηlab)−N−(−ηlab)),wasproposed inRef.[4]toreach max-imum sensitivityto nuclear modifications of PDFs.However, this asymmetryprobabilitydistributionshowsaverynon-Gaussian be-haviour, when its denominator approacheszero, andits signcan

Fig. 6. Theforward–backwardasymmetryofcharge-summedWbosons,asa

func-tionoftheleptonpseudorapidity.Errorbarsrepresentthestatisticaluncertainties,

whilebracketsshowstatisticalandsystematicuncertaintiessummedinquadrature.

Theoreticalpredictionswith(CT10+EPS09,dashedgreenline)andwithout(CT10,

solidredline)PDFnuclearmodifications arealso shown,with theiruncertainty

bands.(Forinterpretationofthereferencestocolorinthisfigurelegend,thereader

isreferredtothewebversionofthisarticle.)

Table 2

Valuesoftheχ2_{testbetweenthemeasurementsandthetheoreticalpredictions,}

with(CT10+EPS09)orwithout(CT10)nuclearmodificationsofthePDFs.The

prob-ability(Prob.)tomeasureavaluegreatertothatmeasuredindataisalsogivenfor

tendegreesoffreedominthecaseofthefirstthreeobservablesandfivedegreesof

freedomforthethreeothersobservables.

Observable CT10 CT10+EPS09 χ2 _{Prob. (%) χ}2 _{Prob. (%)} dσ/dη(+) 13 25 8.6 57 dσ/dη(−) 15 14 8.2 60 (N+_−N_−)/(N+_+N_−) 15 12 11 35 N+(+ηlab)/N+(−ηlab) 3.1 68 3.2 68 N−_(+ηlab)/N−(−ηlab) 9.7 8.4 3.5 63 N(+ηlab)/N(−ηlab) 6.2 29 2.1 83

be flippedwithin the uncertainty. Adifferent asymmetry is pro-posedhere, N(+ηlab)/N(−ηlab),aforward–backwardasymmetry of the charge-summed W bosons, which achieves a similar sen-sitivity. As in the caseofthe charge asymmetry,thisasymmetry can be related to the nuclear modifications of the PDFs within the lead nucleus. Here, forward (backward) W boson production is sensitive to the PDFs of the sea quark at x∼10−3 _(valence quark at x∼10−1) in the lead nucleus. Therefore, the forward– backwardratioprobesthesmall-x modificationoftheleadnucleus PDF (shadowing)over thelarge-x modifications (anti-shadowing). Thisasymmetry isshowninFig. 6,anddeviatesfromunmodified PDFs,moreclearlyfavouringCT10+EPS09overCT10.

In orderto quantifythe agreement betweenthedata andthe expectationfromtheCT10andCT10+EPS09calculations,a χ2 _test isperformedforeach oftheabove(correlated)variables.Thefew correlations in experimental uncertainties described above, only relevantforW±bosoncrosssectionsbutnotforasymmetries,are taken into account,as well asthe correlations intheoretical un-certainties. Theresulting χ2 _{valuesandprobabilitiesare givenin}

Table 2.TheCT10+EPS09calculationsprovideabetterdescription of the data, with still a relatively low probability for the lepton chargeasymmetry,becauseofthebackwardregion.

4. Summary

The first measurement of W boson production in pPb colli-sionshasbeenreported,usingtheelectronandmuondecaymodes forleptons of pT above 25 GeV/c and |ηlab|<2.4. The differen-tialcrosssectionsasafunctionoftheleptonpseudorapidityagree withtheoreticalpredictionsassumingbothunmodified(CT10)and modified (CT10+EPS09) nPDFs, exceptin the most backward re-gion(Pbionbeamdirection),whereahintofan enhancementis seen for the W− bosons. In the same region, the related lepton charge asymmetry deviates slightly from the predictions, some-thing that could potentially arise from different nuclear modifi-cationsoftheup anddownquark PDFs.Ina relatedobservation, forward–backwardasymmetriesshowadeviationfromunmodified PDFs.Takentogether,thesemeasurements show the needfor in-cludingWbosondatainnuclearpartondistributionglobalfits.

Acknowledgements

WecongratulateourcolleaguesintheCERNaccelerator depart-ments for the excellent performance of the LHC and thank the technicalandadministrativestaffs atCERN andatother CMS in-stitutes for their contributions to the success of the CMS effort. Inaddition,wegratefullyacknowledgethecomputingcentresand personneloftheWorldwideLHCComputingGridfordeliveringso effectivelythecomputinginfrastructure essential toour analyses. Finally, we acknowledge the enduring support for the construc-tionandoperationofthe LHCandtheCMSdetectorprovided by thefollowingfundingagencies:BMWFWandFWF(Austria);FNRS and FWO (Belgium); CNPq, CAPES, FAPERJ, and FAPESP (Brazil); MES(Bulgaria);CERN;CAS,MoST,andNSFC(China);COLCIENCIAS (Colombia);MSESandCSF(Croatia);RPF(Cyprus);MoER,ERCIUT andERDF(Estonia); AcademyofFinland,MEC, andHIP (Finland); CEA and CNRS/IN2P3 (France); BMBF, DFG, and HGF (Germany); GSRT (Greece); OTKA and NIH (Hungary); DAE and DST (India); IPM (Iran); SFI (Ireland); INFN (Italy); MSIP and NRF (Republic ofKorea); LAS (Lithuania);MOE andUM (Malaysia); CINVESTAV, CONACYT,SEP,andUASLP-FAI(Mexico);MBIE(NewZealand);PAEC (Pakistan);MSHE andNSC (Poland);Faculdadede Ciênciase Tec-nologia, Universidade Nova de Lisboa (Portugal); JINR (Dubna); MON,RosAtom,RASandRFBR(Russia);MESTD(Serbia);SEIDIand CPAN(Spain);SwissFundingAgencies(Switzerland);MST(Taipei); ThEPCenter,IPST,STARandNSTDA(Thailand);TUBITAKandTAEK (Turkey);NASUandSFFR(Ukraine);STFC (UnitedKingdom);DOE andNSF(USA).

Individuals have received support from the Marie-Curie pro-gramme and the European Research Council and EPLANET (Eu-ropean Union); the Leventis Foundation; the A.P. Sloan Founda-tion; the Alexander von Humboldt Foundation; the Belgian Fed-eral Science Policy Office; the Fonds pour la Formation à la Recherchedansl’Industrieetdansl’Agriculture(FRIA-Belgium);the AgentschapvoorInnovatiedoorWetenschapenTechnologie (IWT-Belgium);the Ministryof Education,Youth andSports(MEYS) of theCzechRepublic;theCouncilofScienceandIndustrialResearch, India;theHOMINGPLUSprogrammeoftheFoundationforPolish Science, cofinanced fromEuropean Union, Regional Development Fund;theCompagnia diSanPaolo (Torino); theConsorzioper la Fisica(Trieste); MIURproject 20108T4XTM(Italy); theThalisand Aristeia programmes cofinancedby EU-ESF and the Greek NSRF; andtheNationalPrioritiesResearchProgrambyQatarNational Re-searchFund.

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W. Waltenberger, C.-E. Wulz1 InstitutfürHochenergiephysikderOeAW,Wien,Austria

V. Mossolov,N. Shumeiko, J. Suarez Gonzalez NationalCentreforParticleandHighEnergyPhysics,Minsk,Belarus

S. Alderweireldt, S. Bansal,T. Cornelis, E.A. De Wolf,X. Janssen, A. Knutsson,J. Lauwers, S. Luyckx, S. Ochesanu,R. Rougny, M. Van De Klundert, H. Van Haevermaet,P. Van Mechelen, N. Van Remortel, A. Van Spilbeeck

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F. Blekman,S. Blyweert, J. D’Hondt,N. Daci, N. Heracleous, J. Keaveney,S. Lowette, M. Maes, A. Olbrechts, Q. Python, D. Strom, S. Tavernier,W. Van Doninck, P. Van Mulders, G.P. Van Onsem, I. Villella

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UniversitéLibredeBruxelles,Bruxelles,Belgium

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CentroBrasileirodePesquisasFisicas,RiodeJaneiro,Brazil

W. Carvalho,J. Chinellato6, A. Custódio, E.M. Da Costa, D. De Jesus Damiao,C. De Oliveira Martins, S. Fonseca De Souza, H. Malbouisson,D. Matos Figueiredo, L. Mundim, H. Nogima,W.L. Prado Da Silva, J. Santaolalla,A. Santoro, A. Sznajder,E.J. Tonelli Manganote6, A. Vilela Pereira

UniversidadedoEstadodoRiodeJaneiro,RiodeJaneiro,Brazil

C.A. Bernardesb, S. Dograa,T.R. Fernandez Perez Tomeia, E.M. Gregoresb, P.G. Mercadanteb, S.F. Novaesa, Sandra S. Padulaa

a_{UniversidadeEstadualPaulista,SãoPaulo,Brazil} b_{UniversidadeFederaldoABC,SãoPaulo,Brazil}

A. Aleksandrov, V. Genchev2, R. Hadjiiska,P. Iaydjiev, A. Marinov, S. Piperov,M. Rodozov, S. Stoykova, G. Sultanov,M. Vutova

InstituteforNuclearResearchandNuclearEnergy,Sofia,Bulgaria

A. Dimitrov,I. Glushkov, L. Litov, B. Pavlov,P. Petkov UniversityofSofia,Sofia,Bulgaria

J.G. Bian,G.M. Chen, H.S. Chen, M. Chen, T. Cheng,R. Du, C.H. Jiang, R. Plestina7, F. Romeo,J. Tao, Z. Wang

InstituteofHighEnergyPhysics,Beijing,China

C. Asawatangtrakuldee, Y. Ban,S. Liu, Y. Mao,S.J. Qian, D. Wang, Z. Xu,L. Zhang, W. Zou StateKeyLaboratoryofNuclearPhysicsandTechnology,PekingUniversity,Beijing,China

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N. Godinovic, D. Lelas,D. Polic, I. Puljak

UniversityofSplit,FacultyofElectricalEngineering,MechanicalEngineeringandNavalArchitecture,Split,Croatia Z. Antunovic,M. Kovac

UniversityofSplit,FacultyofScience,Split,Croatia

V. Brigljevic,K. Kadija, J. Luetic,D. Mekterovic, L. Sudic InstituteRudjerBoskovic,Zagreb,Croatia

A. Attikis, G. Mavromanolakis,J. Mousa, C. Nicolaou, F. Ptochos,P.A. Razis UniversityofCyprus,Nicosia,Cyprus

M. Bodlak,M. Finger,M. Finger Jr.8 CharlesUniversity,Prague,CzechRepublic

Y. Assran9,A. Ellithi Kamel10,M.A. Mahmoud11,A. Radi12,13

AcademyofScientificResearchandTechnologyoftheArabRepublicofEgypt,EgyptianNetworkofHighEnergyPhysics,Cairo,Egypt M. Kadastik, M. Murumaa, M. Raidal, A. Tiko

P. Eerola, M. Voutilainen

DepartmentofPhysics,UniversityofHelsinki,Helsinki,Finland

J. Härkönen,V. Karimäki, R. Kinnunen, M.J. Kortelainen, T. Lampén, K. Lassila-Perini,S. Lehti, T. Lindén, P. Luukka, T. Mäenpää,T. Peltola, E. Tuominen, J. Tuominiemi,E. Tuovinen, L. Wendland

HelsinkiInstituteofPhysics,Helsinki,Finland J. Talvitie, T. Tuuva

LappeenrantaUniversityofTechnology,Lappeenranta,Finland

M. Besancon, F. Couderc,M. Dejardin, D. Denegri, B. Fabbro,J.L. Faure, C. Favaro,F. Ferri, S. Ganjour, A. Givernaud, P. Gras, G. Hamel de Monchenault, P. Jarry,E. Locci, J. Malcles,J. Rander, A. Rosowsky, M. Titov

DSM/IRFU,CEA/Saclay,Gif-sur-Yvette,France

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S. Gadrat

CentredeCalculdel’InstitutNationaldePhysiqueNucleaireetdePhysiquedesParticules,CNRS/IN2P3,Villeurbanne,France

S. Beauceron,N. Beaupere, C. Bernet7,G. Boudoul2, E. Bouvier, S. Brochet, C.A. Carrillo Montoya,

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UniversitédeLyon,UniversitéClaudeBernardLyon1,CNRS-IN2P3,InstitutdePhysiqueNucléairedeLyon,Villeurbanne,France Z. Tsamalaidze8

InstituteofHighEnergyPhysicsandInformatization,TbilisiStateUniversity,Tbilisi,Georgia

C. Autermann, S. Beranek,M. Bontenackels, M. Edelhoff,L. Feld, A. Heister, K. Klein,M. Lipinski, A. Ostapchuk, M. Preuten,F. Raupach, J. Sammet, S. Schael, J.F. Schulte, H. Weber, B. Wittmer, V. Zhukov5

RWTHAachenUniversity,I.PhysikalischesInstitut,Aachen,Germany

M. Ata, M. Brodski,E. Dietz-Laursonn, D. Duchardt, M. Erdmann, R. Fischer,A. Güth, T. Hebbeker, C. Heidemann,K. Hoepfner, D. Klingebiel,S. Knutzen,P. Kreuzer, M. Merschmeyer,A. Meyer, P. Millet, M. Olschewski, K. Padeken,P. Papacz, H. Reithler,S.A. Schmitz, L. Sonnenschein, D. Teyssier,S. Thüer, M. Weber

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M. Aldaya Martin,I. Asin, N. Bartosik, J. Behr, U. Behrens,A.J. Bell, A. Bethani,K. Borras, A. Burgmeier, A. Cakir,L. Calligaris, A. Campbell,S. Choudhury, F. Costanza, C. Diez Pardos,G. Dolinska,S. Dooling, T. Dorland,G. Eckerlin, D. Eckstein, T. Eichhorn, G. Flucke, J. Garay Garcia, A. Geiser, A. Gizhko, P. Gunnellini, J. Hauk, M. Hempel15,H. Jung, A. Kalogeropoulos, M. Kasemann,P. Katsas, J. Kieseler, C. Kleinwort,I. Korol, D. Krücker,W. Lange, J. Leonard, K. Lipka,A. Lobanov, W. Lohmann15, B. Lutz, R. Mankel, I. Marfin15,I.-A. Melzer-Pellmann, A.B. Meyer, G. Mittag, J. Mnich, A. Mussgiller,

S. Naumann-Emme,A. Nayak,E. Ntomari, H. Perrey,D. Pitzl, R. Placakyte, A. Raspereza,

P.M. Ribeiro Cipriano,B. Roland, E. Ron, M.Ö. Sahin,J. Salfeld-Nebgen, P. Saxena,T. Schoerner-Sadenius, M. Schröder,C. Seitz, S. Spannagel, A.D.R. Vargas Trevino, R. Walsh, C. Wissing

DeutschesElektronen-Synchrotron,Hamburg,Germany

V. Blobel, M. Centis Vignali, A.R. Draeger,J. Erfle, E. Garutti,K. Goebel, M. Görner, J. Haller,

M. Hoffmann,R.S. Höing,A. Junkes, H. Kirschenmann, R. Klanner, R. Kogler,J. Lange, T. Lapsien, T. Lenz, I. Marchesini,J. Ott, T. Peiffer, A. Perieanu, N. Pietsch,J. Poehlsen, T. Poehlsen, D. Rathjens, C. Sander, H. Schettler,P. Schleper, E. Schlieckau, A. Schmidt, M. Seidel, V. Sola,H. Stadie, G. Steinbrück,

D. Troendle,E. Usai, L. Vanelderen, A. Vanhoefer UniversityofHamburg,Hamburg,Germany

C. Barth,C. Baus, J. Berger,C. Böser, E. Butz, T. Chwalek, W. De Boer,A. Descroix, A. Dierlamm, M. Feindt,F. Frensch, M. Giffels, A. Gilbert,F. Hartmann2,T. Hauth, U. Husemann, I. Katkov5,

A. Kornmayer2, P. Lobelle Pardo, M.U. Mozer,T. Müller, Th. Müller, A. Nürnberg, G. Quast, K. Rabbertz, S. Röcker,H.J. Simonis, F.M. Stober,R. Ulrich, J. Wagner-Kuhr, S. Wayand, T. Weiler, R. Wolf

InstitutfürExperimentelleKernphysik,Karlsruhe,Germany

G. Anagnostou,G. Daskalakis, T. Geralis,V.A. Giakoumopoulou, A. Kyriakis, D. Loukas,A. Markou, C. Markou, A. Psallidas,I. Topsis-Giotis

InstituteofNuclearandParticlePhysics(INPP),NCSRDemokritos,AghiaParaskevi,Greece

A. Agapitos,S. Kesisoglou, A. Panagiotou,N. Saoulidou, E. Stiliaris UniversityofAthens,Athens,Greece

X. Aslanoglou,I. Evangelou, G. Flouris,C. Foudas, P. Kokkas, N. Manthos, I. Papadopoulos,E. Paradas, J. Strologas

UniversityofIoánnina,Ioánnina,Greece

G. Bencze,C. Hajdu, P. Hidas,D. Horvath16, F. Sikler,V. Veszpremi, G. Vesztergombi17, A.J. Zsigmond WignerResearchCentreforPhysics,Budapest,Hungary

N. Beni,S. Czellar, J. Karancsi18,J. Molnar, J. Palinkas, Z. Szillasi InstituteofNuclearResearchATOMKI,Debrecen,Hungary

A. Makovec,P. Raics, Z.L. Trocsanyi, B. Ujvari UniversityofDebrecen,Debrecen,Hungary

S.K. Swain

NationalInstituteofScienceEducationandResearch,Bhubaneswar,India

S.B. Beri,V. Bhatnagar, R. Gupta, U. Bhawandeep, A.K. Kalsi,M. Kaur, R. Kumar,M. Mittal, N. Nishu, J.B. Singh

Ashok Kumar, Arun Kumar,S. Ahuja, A. Bhardwaj, B.C. Choudhary,A. Kumar, S. Malhotra,M. Naimuddin, K. Ranjan,V. Sharma

UniversityofDelhi,Delhi,India

S. Banerjee, S. Bhattacharya, K. Chatterjee,S. Dutta, B. Gomber, Sa. Jain, Sh. Jain,R. Khurana, A. Modak, S. Mukherjee,D. Roy, S. Sarkar, M. Sharan

SahaInstituteofNuclearPhysics,Kolkata,India

A. Abdulsalam, D. Dutta, V. Kumar, A.K. Mohanty2,L.M. Pant, P. Shukla, A. Topkar BhabhaAtomicResearchCentre,Mumbai,India

T. Aziz, S. Banerjee, S. Bhowmik19,R.M. Chatterjee, R.K. Dewanjee, S. Dugad, S. Ganguly,S. Ghosh, M. Guchait,A. Gurtu20,G. Kole, S. Kumar, M. Maity19, G. Majumder, K. Mazumdar,G.B. Mohanty, B. Parida,K. Sudhakar, N. Wickramage21

TataInstituteofFundamentalResearch,Mumbai,India

H. Bakhshiansohi,H. Behnamian, S.M. Etesami22, A. Fahim23, R. Goldouzian, M. Khakzad,

M. Mohammadi Najafabadi, M. Naseri, S. Paktinat Mehdiabadi, F. Rezaei Hosseinabadi, B. Safarzadeh24, M. Zeinali

InstituteforResearchinFundamentalSciences(IPM),Tehran,Iran M. Felcini,M. Grunewald

UniversityCollegeDublin,Dublin,Ireland

M. Abbresciaa,b, C. Calabriaa,b,S.S. Chhibraa,b, A. Colaleoa, D. Creanzaa,c,L. Cristellaa,b,

N. De Filippisa,c, M. De Palmaa,b,L. Fiorea,G. Iasellia,c,G. Maggia,c,M. Maggia, S. Mya,c, S. Nuzzoa,b, A. Pompilia,b,G. Pugliesea,c,R. Radognaa,b,2, G. Selvaggia,b, A. Sharmaa, L. Silvestrisa,2, R. Vendittia,b, P. Verwilligena

a_{INFNSezionediBari,Bari,Italy} b_{UniversitàdiBari,Bari,Italy} c_{PolitecnicodiBari,Bari,Italy}

G. Abbiendia,A.C. Benvenutia,D. Bonacorsia,b, S. Braibant-Giacomellia,b, L. Brigliadoria,b, R. Campaninia,b, P. Capiluppia,b,A. Castroa,b,F.R. Cavalloa, G. Codispotia,b, M. Cuffiania,b,

G.M. Dallavallea,F. Fabbria, A. Fanfania,b, D. Fasanellaa,b,P. Giacomellia,C. Grandia,L. Guiduccia,b, S. Marcellinia, G. Masettia, A. Montanaria, F.L. Navarriaa,b,A. Perrottaa, A.M. Rossia,b,T. Rovellia,b, G.P. Sirolia,b,N. Tosia,b, R. Travaglinia,b

a_{INFNSezionediBologna,Bologna,Italy} b_{UniversitàdiBologna,Bologna,Italy}

S. Albergoa,b,G. Cappelloa, M. Chiorbolia,b, S. Costaa,b, F. Giordanoa,2,R. Potenzaa,b, A. Tricomia,b, C. Tuvea,b

a_{INFNSezionediCatania,Catania,Italy} b_{UniversitàdiCatania,Catania,Italy} c_{CSFNSM,Catania,Italy}

G. Barbaglia, V. Ciullia,b,C. Civininia, R. D’Alessandroa,b,E. Focardia,b,E. Galloa,S. Gonzia,b,V. Goria,b, P. Lenzia,b,M. Meschinia,S. Paolettia, G. Sguazzonia,A. Tropianoa,b

a_{INFNSezionediFirenze,Firenze,Italy} b_{UniversitàdiFirenze,Firenze,Italy}

L. Benussi, S. Bianco, F. Fabbri,D. Piccolo INFNLaboratoriNazionalidiFrascati,Frascati,Italy

R. Ferrettia,b,F. Ferroa,M. Lo Veterea,b,E. Robuttia, S. Tosia,b

a_{INFNSezionediGenova,Genova,Italy} b_{UniversitàdiGenova,Genova,Italy}

M.E. Dinardoa,b, S. Fiorendia,b, S. Gennaia,2, R. Gerosaa,b,2,A. Ghezzia,b, P. Govonia,b,M.T. Lucchinia,b,2, S. Malvezzia, R.A. Manzonia,b, A. Martellia,b, B. Marzocchia,b,2, D. Menascea,L. Moronia,

M. Paganonia,b,D. Pedrinia, S. Ragazzia,b, N. Redaellia,T. Tabarelli de Fatisa,b

a_{INFNSezionediMilano-Bicocca,Milano,Italy} b_{UniversitàdiMilano-Bicocca,Milano,Italy}

S. Buontempoa, N. Cavalloa,c,S. Di Guidaa,d,2, F. Fabozzia,c,A.O.M. Iorioa,b, L. Listaa,S. Meolaa,d,2, M. Merolaa, P. Paoluccia,2

a_{INFNSezionediNapoli,Napoli,Italy} b_{UniversitàdiNapoli‘FedericoII’,Napoli,Italy} c_{UniversitàdellaBasilicata,Potenza,Italy} d_{UniversitàG.Marconi,Roma,Italy}

P. Azzia, N. Bacchettaa, D. Biselloa,b, A. Brancaa,b,R. Carlina,b,P. Checchiaa, M. Dall’Ossoa,b, T. Dorigoa, U. Gasparinia,b,A. Gozzelinoa,K. Kanishcheva,c, S. Lacapraraa,M. Margonia,b,A.T. Meneguzzoa,b, J. Pazzinia,b, N. Pozzobona,b, P. Ronchesea,b,F. Simonettoa,b, E. Torassaa,M. Tosia,b, S. Vaninia,b, S. Venturaa,P. Zottoa,b,A. Zucchettaa,b,G. Zumerlea,b

a_{INFNSezionediPadova,Padova,Italy} b_{UniversitàdiPadova,Padova,Italy} c_{UniversitàdiTrento,Trento,Italy}

M. Gabusia,b_{,S.P. Ratti}a,b_{, V. Re}a_{,C. Riccardi}a,b_{,P. Salvini}a_{,P. Vitulo}a,b

a_{INFNSezionediPavia,Pavia,Italy} b_{UniversitàdiPavia,Pavia,Italy}

M. Biasinia,b,G.M. Bileia,D. Ciangottinia,b,2,L. Fanòa,b, P. Laricciaa,b, G. Mantovania,b, M. Menichellia, A. Sahaa, A. Santocchiaa,b,A. Spieziaa,b,2

a_{INFNSezionediPerugia,Perugia,Italy} b_{UniversitàdiPerugia,Perugia,Italy}

K. Androsova,25_{,P. Azzurri}a_{,G. Bagliesi}a_{,J. Bernardini}a_{,T. Boccali}a_{,G. Broccolo}a,c_{,R. Castaldi}a_, M.A. Cioccia,25,R. Dell’Orsoa, S. Donatoa,c,2,G. Fedi, F. Fioria,c,L. Foàa,c,A. Giassia, M.T. Grippoa,25, F. Ligabuea,c,T. Lomtadzea,L. Martinia,b, A. Messineoa,b, C.S. Moona,26, F. Pallaa,2, A. Rizzia,b, A. Savoy-Navarroa,27,A.T. Serbana, P. Spagnoloa, P. Squillaciotia,25,R. Tenchinia,G. Tonellia,b, A. Venturia,P.G. Verdinia,C. Vernieria,c

a_{INFNSezionediPisa,Pisa,Italy} b_{UniversitàdiPisa,Pisa,Italy}

c_{ScuolaNormaleSuperiorediPisa,Pisa,Italy}

L. Baronea,b, F. Cavallaria,G. D’imperioa,b,D. Del Rea,b, M. Diemoza,C. Jordaa, E. Longoa,b,

F. Margarolia,b, P. Meridiania,F. Michelia,b,2,G. Organtinia,b, R. Paramattia,S. Rahatloua,b, C. Rovellia, F. Santanastasioa,b, L. Soffia,b,P. Traczyka,b,2

a_{INFNSezionediRoma,Roma,Italy} b_{UniversitàdiRoma,Roma,Italy}

N. Amapanea,b,R. Arcidiaconoa,c, S. Argiroa,b, M. Arneodoa,c,R. Bellana,b,C. Biinoa,N. Cartigliaa, S. Casassoa,b,2, M. Costaa,b,R. Covarelli,A. Deganoa,b,N. Demariaa, L. Fincoa,b,2,C. Mariottia, S. Masellia,G. Mazzaa,E. Migliorea,b, V. Monacoa,b, M. Musicha, M.M. Obertinoa,c, L. Pachera,b, N. Pastronea,M. Pelliccionia,G.L. Pinna Angionia,b, A. Romeroa,b, M. Ruspaa,c,R. Sacchia,b, A. Solanoa,b,A. Staianoa, U. Tamponia

a_{INFNSezionediTorino,Torino,Italy} b_{UniversitàdiTorino,Torino,Italy}

S. Belfortea,V. Candelisea,b,2, M. Casarsaa,F. Cossuttia, G. Della Riccaa,b,B. Gobboa,C. La Licataa,b, M. Maronea,b,A. Schizzia,b, T. Umera,b,A. Zanettia

a_{INFNSezionediTrieste,Trieste,Italy} b_{UniversitàdiTrieste,Trieste,Italy}

S. Chang, A. Kropivnitskaya,S.K. Nam KangwonNationalUniversity,Chunchon,RepublicofKorea

D.H. Kim,G.N. Kim, M.S. Kim,D.J. Kong, S. Lee, Y.D. Oh,H. Park, A. Sakharov, D.C. Son KyungpookNationalUniversity,Daegu,RepublicofKorea

T.J. Kim, M.S. Ryu

ChonbukNationalUniversity,Jeonju,RepublicofKorea J.Y. Kim, D.H. Moon,S. Song

ChonnamNationalUniversity,InstituteforUniverseandElementaryParticles,Kwangju,RepublicofKorea

S. Choi, D. Gyun,B. Hong, M. Jo,H. Kim, Y. Kim,B. Lee, K.S. Lee, S.K. Park,Y. Roh KoreaUniversity,Seoul,RepublicofKorea

H.D. Yoo

SeoulNationalUniversity,Seoul,RepublicofKorea M. Choi,J.H. Kim, I.C. Park, G. Ryu UniversityofSeoul,Seoul,RepublicofKorea

Y. Choi,Y.K. Choi, J. Goh,D. Kim, E. Kwon, J. Lee, I. Yu SungkyunkwanUniversity,Suwon,RepublicofKorea

A. Juodagalvis VilniusUniversity,Vilnius,Lithuania

J.R. Komaragiri, M.A.B. Md Ali

NationalCentreforParticlePhysics,UniversitiMalaya,KualaLumpur,Malaysia

E. Casimiro Linares, H. Castilla-Valdez, E. De La Cruz-Burelo,I. Heredia-de La Cruz, A. Hernandez-Almada,R. Lopez-Fernandez, A. Sanchez-Hernandez

CentrodeInvestigacionydeEstudiosAvanzadosdelIPN,MexicoCity,Mexico S. Carrillo Moreno, F. Vazquez Valencia UniversidadIberoamericana,MexicoCity,Mexico

I. Pedraza, H.A. Salazar Ibarguen BenemeritaUniversidadAutonomadePuebla,Puebla,Mexico A. Morelos Pineda

UniversidadAutónomadeSanLuisPotosí,SanLuisPotosí,Mexico D. Krofcheck

P.H. Butler,S. Reucroft

UniversityofCanterbury,Christchurch,NewZealand

A. Ahmad, M. Ahmad, Q. Hassan,H.R. Hoorani, W.A. Khan,T. Khurshid, M. Shoaib NationalCentreforPhysics,Quaid-I-AzamUniversity,Islamabad,Pakistan

H. Bialkowska,M. Bluj, B. Boimska,T. Frueboes, M. Górski, M. Kazana, K. Nawrocki, K. Romanowska-Rybinska,M. Szleper, P. Zalewski

NationalCentreforNuclearResearch,Swierk,Poland

G. Brona,K. Bunkowski, M. Cwiok,W. Dominik,K. Doroba, A. Kalinowski,M. Konecki, J. Krolikowski, M. Misiura, M. Olszewski

InstituteofExperimentalPhysics,FacultyofPhysics,UniversityofWarsaw,Warsaw,Poland

P. Bargassa,C. Beirão Da Cruz E Silva, P. Faccioli, P.G. Ferreira Parracho,M. Gallinaro, L. Lloret Iglesias, F. Nguyen, J. Rodrigues Antunes,J. Seixas, J. Varela, P. Vischia

LaboratóriodeInstrumentaçãoeFísicaExperimentaldePartículas,Lisboa,Portugal

S. Afanasiev,P. Bunin,M. Gavrilenko,I. Golutvin, I. Gorbunov, A. Kamenev,V. Karjavin, V. Konoplyanikov, A. Lanev,A. Malakhov,V. Matveev28, P. Moisenz, V. Palichik,V. Perelygin, S. Shmatov, N. Skatchkov, V. Smirnov,A. Zarubin

JointInstituteforNuclearResearch,Dubna,Russia

V. Golovtsov,Y. Ivanov, V. Kim29,E. Kuznetsova, P. Levchenko, V. Murzin, V. Oreshkin, I. Smirnov, V. Sulimov,L. Uvarov, S. Vavilov, A. Vorobyev,An. Vorobyev

PetersburgNuclearPhysicsInstitute,Gatchina(St.Petersburg),Russia

Yu. Andreev,A. Dermenev,S. Gninenko, N. Golubev, M. Kirsanov, N. Krasnikov, A. Pashenkov, D. Tlisov, A. Toropin

InstituteforNuclearResearch,Moscow,Russia

V. Epshteyn,V. Gavrilov, N. Lychkovskaya,V. Popov, I. Pozdnyakov,G. Safronov, S. Semenov, A. Spiridonov,V. Stolin, E. Vlasov,A. Zhokin

InstituteforTheoreticalandExperimentalPhysics,Moscow,Russia

V. Andreev,M. Azarkin30,I. Dremin30, M. Kirakosyan, A. Leonidov30,G. Mesyats, S.V. Rusakov, A. Vinogradov

P.N.LebedevPhysicalInstitute,Moscow,Russia

A. Belyaev,E. Boos,A. Demiyanov, A. Ershov, A. Gribushin, O. Kodolova,V. Korotkikh, I. Lokhtin, S. Obraztsov,S. Petrushanko,V. Savrin, A. Snigirev, I. Vardanyan

SkobeltsynInstituteofNuclearPhysics,LomonosovMoscowStateUniversity,Moscow,Russia

I. Azhgirey,I. Bayshev,S. Bitioukov, V. Kachanov, A. Kalinin, D. Konstantinov,V. Krychkine, V. Petrov, R. Ryutin, A. Sobol,L. Tourtchanovitch, S. Troshin, N. Tyurin, A. Uzunian,A. Volkov

StateResearchCenterofRussianFederation,InstituteforHighEnergyPhysics,Protvino,Russia P. Adzic31,M. Ekmedzic, J. Milosevic,V. Rekovic UniversityofBelgrade,FacultyofPhysicsandVincaInstituteofNuclearSciences,Belgrade,Serbia

J. Alcaraz Maestre, C. Battilana,E. Calvo, M. Cerrada,M. Chamizo Llatas, N. Colino, B. De La Cruz, A. Delgado Peris,D. Domínguez Vázquez, A. Escalante Del Valle, C. Fernandez Bedoya,

J.P. Fernández Ramos,J. Flix, M.C. Fouz,P. Garcia-Abia,O. Gonzalez Lopez, S. Goy Lopez, J.M. Hernandez, M.I. Josa,E. Navarro De Martino, A. Pérez-Calero Yzquierdo, J. Puerta Pelayo, A. Quintario Olmeda, I. Redondo,L. Romero, M.S. Soares

CentrodeInvestigacionesEnergéticasMedioambientalesyTecnológicas(CIEMAT),Madrid,Spain C. Albajar, J.F. de Trocóniz, M. Missiroli, D. Moran UniversidadAutónomadeMadrid,Madrid,Spain

H. Brun, J. Cuevas,J. Fernandez Menendez, S. Folgueras, I. Gonzalez Caballero UniversidaddeOviedo,Oviedo,Spain

J.A. Brochero Cifuentes, I.J. Cabrillo, A. Calderon, J. Duarte Campderros,M. Fernandez, G. Gomez, A. Graziano, A. Lopez Virto,J. Marco, R. Marco,C. Martinez Rivero, F. Matorras, F.J. Munoz Sanchez, J. Piedra Gomez, T. Rodrigo,A.Y. Rodríguez-Marrero,A. Ruiz-Jimeno, L. Scodellaro,I. Vila,

R. Vilar Cortabitarte

InstitutodeFísicadeCantabria(IFCA),CSIC-UniversidaddeCantabria,Santander,Spain

D. Abbaneo, E. Auffray, G. Auzinger, M. Bachtis,P. Baillon, A.H. Ball, D. Barney, A. Benaglia, J. Bendavid, L. Benhabib,J.F. Benitez, P. Bloch, A. Bocci, A. Bonato, O. Bondu,C. Botta, H. Breuker, T. Camporesi, G. Cerminara, S. Colafranceschi32,M. D’Alfonso, D. d’Enterria, A. Dabrowski, A. David,F. De Guio, A. De Roeck, S. De Visscher, E. Di Marco,M. Dobson, M. Dordevic, B. Dorney, N. Dupont-Sagorin, A. Elliott-Peisert, G. Franzoni, W. Funk, D. Gigi,K. Gill, D. Giordano, M. Girone,F. Glege, R. Guida, S. Gundacker, M. Guthoff,J. Hammer, M. Hansen,P. Harris,J. Hegeman, V. Innocente, P. Janot, K. Kousouris,K. Krajczar, P. Lecoq,C. Lourenço, N. Magini, L. Malgeri,M. Mannelli, J. Marrouche,

L. Masetti, F. Meijers, S. Mersi,E. Meschi, F. Moortgat, S. Morovic, M. Mulders, L. Orsini, L. Pape,E. Perez, A. Petrilli, G. Petrucciani,A. Pfeiffer, M. Pimiä, D. Piparo, M. Plagge, A. Racz,G. Rolandi33, M. Rovere, H. Sakulin, C. Schäfer, C. Schwick,A. Sharma,P. Siegrist, P. Silva,M. Simon, P. Sphicas34,D. Spiga,

J. Steggemann,B. Stieger, M. Stoye,Y. Takahashi, D. Treille, A. Tsirou, G.I. Veres17,N. Wardle, H.K. Wöhri, H. Wollny, W.D. Zeuner

CERN,EuropeanOrganizationforNuclearResearch,Geneva,Switzerland

W. Bertl,K. Deiters, W. Erdmann, R. Horisberger, Q. Ingram, H.C. Kaestli, D. Kotlinski,U. Langenegger, D. Renker, T. Rohe

PaulScherrerInstitut,Villigen,Switzerland

F. Bachmair, L. Bäni, L. Bianchini, M.A. Buchmann,B. Casal, N. Chanon, G. Dissertori, M. Dittmar, M. Donegà, M. Dünser,P. Eller, C. Grab,D. Hits, J. Hoss,W. Lustermann, B. Mangano,A.C. Marini, M. Marionneau, P. Martinez Ruiz del Arbol, M. Masciovecchio, D. Meister,N. Mohr, P. Musella, C. Nägeli35,F. Nessi-Tedaldi, F. Pandolfi, F. Pauss,L. Perrozzi, M. Peruzzi,M. Quittnat, L. Rebane, M. Rossini,A. Starodumov36, M. Takahashi, K. Theofilatos,R. Wallny, H.A. Weber

InstituteforParticlePhysics,ETHZurich,Zurich,Switzerland

C. Amsler37, M.F. Canelli,V. Chiochia,A. De Cosa, A. Hinzmann, T. Hreus, B. Kilminster, C. Lange, J. Ngadiuba,D. Pinna, P. Robmann, F.J. Ronga,S. Taroni, M. Verzetti, Y. Yang

UniversitätZürich,Zurich,Switzerland

M. Cardaci, K.H. Chen,T.H. Doan, C. Ferro,C.M. Kuo, W. Lin,Y.J. Lu, S.Y. Tseng,R. Volpe, S.S. Yu NationalCentralUniversity,Chung-Li,Taiwan