Measurement of Z0-boson production at large rapidities in Pb–Pb collisions at √sNN = 5.02 TeV

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Physics Letters B

www.elsevier.com/locate/physletb

Measurement of Z ⁰ -boson production at large rapidities in Pb–Pb collisions at √

s _NN = ⁵ . 02 TeV

.ALICE Collaboration

a r t i c l e i n f o a b s t ra c t

Articlehistory:

Received7December2017

Receivedinrevisedform21February2018 Accepted2March2018

Availableonline6March2018 Editor: L.Rolandi

The productionof Z⁰ bosonsat large rapidities in Pb–Pb collisions at√_s

NN=⁵.02 TeV is reported.

Z⁰candidatesarereconstructedinthedimuondecaychannel(Z⁰→μ⁺μ⁻),basedonmuonsselected with pseudo-rapidity −⁴.0<η<−².5 and pT>20 GeV/c. The invariant yield and the nuclear modificationfactor,RAA,arepresentedasafunctionofrapidityandcollisioncentrality.ThevalueofRAA for the0–20%central Pb–Pbcollisions is0.67±⁰.11(stat.)±⁰.03(syst.)±⁰.06(corr. syst.), exhibiting a deviation of 2.6σ from unity. The results are well-described by calculations that include nuclear modifications of the parton distribution functions, while the predictions using vacuum PDFs deviate fromdataby2.3σinthe0–90%centralityclassandby3σ inthe0–20%centralcollisions.

©^{2018TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense} (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP³.

1. Introduction

Z⁰ bosons are weakly interacting probes formed early in the evolution of hadronic collisions (tf ∼¹/M⁰.01 fm/c), with a typical decay time td∼⁰.1 fm/c. Their leptonic decays are of particularinterestinheavy-ioncollisions,sinceleptons donot in- teractstronglyandtheirin-mediumenergylossbybremsstrahlung isnegligible [1]. Z⁰-boson productionrates in hadroniccollisions are well-understood, and their measurement via leptonic decays therefore serves as a valuable medium-blind reference for hard processesinheavy-ioncollisions [2,3].

Z⁰-boson properties have been extensively studied at LEP (CERN),SLC(SLAC),Tevatron(FNAL)andLHC(CERN)ine⁺e⁻,pp and pp collisions [4–15]. Z⁰-boson production in hadronic colli- sions is well-described by perturbative Quantum Chromodynam- ics (pQCD) calculations at next-to-next-to-leading order (NNLO) [16,17], and their comparison with data provides constraints on Parton Distribution Functions (PDFs) [18,19]. In heavy-ion colli- sions,Z⁰-bosonproductioncan beaffectedby initial-stateeffects.

Asaresultofthedifferentbalanceofthenumberofuanddva- lence quarksin protons andin leadnuclei (isospin),the yield of Z⁰ bosons in Pb–Pb collisions at√

sNN=⁵.02 TeV is expectedto increase relative to that inpp collisions by 5–8% atlarge rapidi- ties,anddecreaseby3% atcentralrapidities [20].Modificationsof thePDFsinnuclei(nPDFs) [21–27] introducearapidity-dependent changeinyield,withadecreaseinyieldrelativetothatinppcol- lisionsof8–15%atlargerapidities,correspondingtotheBjorken-x

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

ranges x1^{10−1 and x}2^{10−3, andan increase by 3% at cen-} tralrapidity,correspondingtox_1,2∼^{10−2[20,21].Theyieldcould} alsodependuponeffectssuchasmultiplescatteringandmedium- inducedbremsstrahlungoftheinitialpartonsinlargenuclei [28].

The ATLAS,ALICE, CMSandLHCbcollaborationshavereported measurementsofW^±- andZ⁰-bosonproductioninp–Pbcollisions at√

s_NN=⁵.02TeV [29–33], withcomplementaryrapidity cover- age.Thesemeasurementsarewell-describedbynext-to-leadingor- der (NLO)pQCDcalculations [20] andbyNNLOcalculationsusing theFullyExclusiveWandZProductioncode(FEWZ) [34],utilising both nPDFs [32] and vacuumPDFs.Theforward–backward asym- metry ofW^±-bosonproduction suggeststhe presence of nuclear modification ofPDFs [31].This sensitivitytonuclear effectsindi- catestheneedtoincludethesedatainthefuturenPDFfits.

In Pb–Pb collisions, W^±- and Z⁰-boson measurements at

√s_NN=².76 TeV have been carried out at central rapidity by the ATLAS andCMScollaborations [35–38]. PreliminaryZ⁰-boson measurements in Pb–Pb collisions at √

sNN =⁵.02 TeV at cen- tral rapidityhave alsobeenreportedrecentlyby ATLAS [39]. The W^±- and Z⁰-boson nuclear modification factor, RAA, defined as the ratio of the yields in Pb–Pb collisions and the cross-section inppcollisions normalisedbythenuclearoverlapfunction ^TAA^, whichrepresentstheeffectiveoverlaparea ofthe twointeracting nuclei [40],ismeasuredtobeconsistentwithunitywithinuncer- tainties,withnocentralitydependence [37–39].

Measurementsathighcollision energyandlargerapidities are sensitive tolow Bjorken-xprocesses,andare thereforeimportant to furtherconstrain theinitial-stateeffects onelectroweak boson production andtoestablisha referenceformedium-sensitive ob- servables.

https://doi.org/10.1016/j.physletb.2018.03.010

0370-2693/©^{2018TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense}(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP³.

This paper presents the first measurement of Z⁰-boson pro- duction in Pb–Pb collisions at √

sNN=⁵.02 TeV at large rapidi- ties. Opposite-signmuon pairs from Z⁰-boson decayswith 2.5<

y<4.0¹ are measured with the ALICE detector. The yield of

μ

⁺

μ

⁻ pairsincludescontributionsfromvirtual-photonprocesses andfromtheir interferenceeffects. Thismeasurementprobes the nPDFsoflarge-xvalencequarks(x1^{10−1)andlow-xseaquarks} (x₂^{10−3) at Q2}∼^M_Z^{2. The invariant yields and R}AA are re- portedasafunctionofrapidityandcollisioncentrality.Theresults arecompared tomodel calculationsincludingnPDFs.These mea- surements complement the measurements in p–Pb collisions at

√sNN=⁵.02 TeV at large rapidities [32,33], providing increased precision andnew informationon rapidity andcentrality depen- dence.ThecombinationoftheseresultswithfutureW^±measure- ments ina similar kinematic interval will provideconstraints on the flavor dependence of nPDFs, in particular the strange quark contribution [21].

Thisletterisorganisedasfollows:theexperimentalsetup and data sample are described in Sect. 2; the analysis procedure is presented inSect. 3; the results are presentedin Sect. 4; and a summaryisgiveninSect.5.

2. Experimentalsetupanddataset

TheALICEdetectorisdescribedindetailinRef. [41].Z⁰ bosons are reconstructed via their muonic decay with the ALICE muon spectrometer,which provides muontrigger, trackingandidentifi- cationin the pseudo-rapidity range −⁴.0<

η

<−².5. The muon spectrometer,asseenfromtheinteractionpoint,consistsofafront absorberof10 interactionlengths(λint) thickness, whichreduces thecontaminationofhadronsandmuonsfromthedecayoflight particles; five tracking stations; an iron absorber with thickness 7.2 λint; and two trigger stations. Each tracking station is com- posed of two planes of multi-wire proportional chambers with cathode-planereadout, whileeach trigger stationconsistsof two planesofresistiveplatechambers.Thethirdtrackingstationislo- catedinsidethegap ofadipole magnet, whichprovides a3 T·^m magneticfield integral.Themuonspectrometeriscompletedbya beamshieldsurroundingthebeampipethatprotectstheappara- tusfromsecondaryparticlesproducedintheinteractionoflarge-

η

primaryparticleswiththepipeitself.

Theinteractionvertexis reconstructedusingthetwo cylindri- callayersoftheSiliconPixelDetector,locatedataradialdistance of3.9 and7.6 cm from the beamaxis andcovering |

η

_|<2 and

|

η

_|<1.4,respectively.TheV0detector,consistingoftwoarraysof scintillatorcounterscovering2.8<

η

<5.1 and−³.7<

η

<−¹.8, isusedfortriggeringandevaluationofcollisioncentrality.Finally, theZeroDegreeCalorimeter,placedat112.5 mfromtheinterac- tionpoint along the beamline, is used toreject electromagnetic interactions [42].

The dataset used in this analysis consists of Pb–Pb events at

√sNN =⁵.02 TeV selected with a dimuon trigger that requires the coincidence of a minimum-bias (MB) trigger and a pair of tracks with opposite sign in the muon spectrometer, each with p_T^{1 GeV/c. The MB trigger is defined by the} coincidence of thesignalsfrombotharraysoftheV0.TheMBtriggerisfullyef- ficientforevents within the 0–90% centralityinterval, which are usedinthisanalysis.Themuontriggerefficiencyhasa plateauof about98%for muonswith pT>5 GeV/c.The resulting efficiency forpairs ofopposite-sign muons, withmuon p_T>20 GeV/c and

1 IntheALICEreferenceframethemuonspectrometercoversanegativeηrange and,consequently,anegativeyrange.However,sincethePb–Pbsystemissymmet- ricinrapidity,apositiveynotationisusedtopresenttheresults.

−⁴.0<

η

<−².5,is95%.Afteralleventselectioncuts,thedataset correspondstoanintegratedluminosityofabout225 μb⁻¹. 3. Analysisprocedure

The procedure for Z⁰-boson signal extraction in this analysis is the same as that used in the analysis of p–Pb collisions at

√sNN=⁵.02 TeV [32].Tracksarereconstructedinthemuonspec- trometer using the algorithm described in Ref. [43]. Tracks are selectedforanalysisiftheyhavepseudorapidity−⁴.0<

η

<−².5 and polarangle 170^◦< θ_abs<178^◦, measured atthe end of the frontabsorber.Thisselectionrejectsparticles thatcrossthehigh- densityregionofthefrontabsorber andundergosignificant mul- tiple scattering. Tracks reconstructed in the tracking stations are identified asmuonsifthey matcha tracksegment inthetrigger stations,placeddownstreamtheironwall.Thecontaminationfrom backgroundtracksthatdonotpointtotheinteractionvertexisre- ducedbyutilisingtheproductofthemomentumandthedistance ofclosestapproachtotheinteractionvertex.Thiscutremoves88%

ofall tracksforevents inthe0–90% centrality interval,while re- taining all signal candidates with negligible residual background contribution.

Only muons with p_T>20 GeV/c are used in this analysis.

Thisselection reduces thecontribution ofmuonsfromthe decay ofcharm,beautyandlow-mass resonances(seebelow). Z⁰-boson candidatesareformedbycombiningpairsofopposite-signmuons.

The candidatesare further selectedby requiringthat their rapid- ity,calculatedusingthemeasuredinvariantmass,isintheinterval 2.5<y<4.0.Fig.1presentsthe

μ

⁺

μ

⁻ invariant massdistribu- tioninthecentralityintervals0–90% inFig.1(a),0–20%inFig.1(b), and20–90%inFig. 1(c).The distributionforthe0–90% centrality interval is comparedwiththe resultof aMonte Carlo(MC) sim- ulation obtained using the POWHEG [44] event generator paired withPYTHIA 6.4.25 [45] forthepartonshower.Thepropagationof the particles throughthe detectoris simulatedwith theGEANT3 code [46]. The isospin of the Pb-nucleus is accounted for by a weightedaverageofneutronandprotoninteractions,butnomod- ification of the nucleon PDF was applied to account fornuclear effects.The simulations account forvariations inthe detectorre- sponsewithtimeandin-situalignmenteffects.Adata-drivende- scriptionofthemuonmomentumresolution isalsoimplemented (seeRef. [32] fordetails).Theshapeofthe

μ

⁺

μ

⁻invariant mass distribution, which is mainly affected by the momentum resolu- tion,issimilarindataandMC.

Various background sources contribute to the

μ

⁺

μ

⁻ invari- antmassdistribution.Contaminationfromthedecayoftt (tt−→

μ

⁺

μ

⁻X)and

τ

(Z⁰−→

τ τ

_−→

μ

⁺

μ

⁻X)pairsisestimatedwith POWHEGsimulations [10,44,47] andfoundtobesmallerthan0.5%

ofthesignalyield,whichisconsideredasasystematicuncertainty.

Thecontributionofopposite-signmuonpairsfromthedecayofcc (cc−→

μ

⁺

μ

⁻X) andbb (bb−→

μ

⁺

μ

⁻X) pairswas studied in p–Pb collisions [32] andfound to be smaller thanthat of tt and

τ

pairs.InPb–Pb collisions,the presenceofhigh-pT muonsfrom thedecayofheavy-flavourpairsisexpectedtobefurtherreduced duetothein-mediumenergylossofheavy quarks.Thiscontribu- tion was therefore neglected. Finally,the combinatorial contribu- tionfromtherandom pairingofmuonsintheeventisevaluated vialike-signmuonpairs(

μ

^±

μ

^±).Thiscombinatorialcontribution isfoundtobesmall(onecandidateinthe20–90% centralityinter- val)andissubtractedfromthesignalestimate.

ThenumberofZ⁰ candidatesisestimatedusingtheprocedure described in Ref. [32], by countingthe entries in the

μ

⁺

μ

⁻ in- variant mass interval 60<Mμμ<120 GeV/c² after subtracting the contribution from like-sign pairs for each centrality and ra- pidity interval. A total of 64 candidates is found in the 0–90%

Fig. 1.Invariantmassdistributionofμ⁺μ⁻pairsforPb–Pbcollisionsat√_s

NN=⁵.02 TeV,reconstructedusingmuonswith−⁴.0<η<−².5 andpT>20 GeV/c(black points).Thepanelspresentthedistributionindifferentcentralityintervals.Theerrorbarsareofstatisticaloriginonly.Theinvariantmassdistributionoflike-signmuonpairs isalsoshown(redopenpoints).Onlyonelike-signcandidateisfoundinthe20–90%centralityinterval.ThesolidbluelinedrawninFig.1(a)representsthedistribution fromaPOWHEGsimulationforPb–PbcollisionswithoutnuclearmodificationofPDFs(seetextfordetails).(Forinterpretationofthecoloursinthefigure(s),thereaderis referredtothewebversionofthisarticle.)

centrality bin, of which 37 are in the 0–20% bin and 27 in the 20–90% bin.Asafunctionofrapidity,33candidatesareinthein- terval2.5<y<3.0,and31areintheinterval 3.0<y<4.0.The raw yields are corrected for the detector acceptance and forre- construction andselection efficiency(A·

ε

). The value of A·

ε

is 75% for events in the 0–90% centrality interval, estimated using the POWHEG [44] simulations described previously. The depen- denceof theefficiency on thedetector occupancy was evaluated byembeddingthegeneratedZ⁰ signalinrealMBPb–Pbdata.The A·

ε

termis constantas afunction of centralityfromperipheral tosemi-centraleventsanddecreasesinthemostcentralcollisions.

Thevalueof A·

ε

is78%inthe20–90%centralityintervaland74%

inthe0–20%interval, withcentrality-independent systematicun- certaintyof5%,asdiscussedbelow.

To evaluate the invariant yields (dN/dy), the raw dimuon- triggered mass distribution must be normalised by the factor Fⁱμ-trig/MB, which is the inverse of the probability to observe a dimuon pair in a MB event for the centrality class i. The value of Fμⁱ-trig/MB iscalculated in two different ways, by applyingthe dimuonselectioncriteriontoMBevents,andbytherelativecount- ing rate of the two triggers [48]. The variation in Fⁱμ-trig/MB de- termined by these two methods is 0.5% and contributes to the systematicuncertainty.

ThenuclearmodificationfactorRAArequiresthedetermination ofthecollisioncentrality,whichistypicallyquantifiedbytheaver- agenumberofnucleonsparticipatingintheinteractionforagiven

Table 1

Valuesoftheaveragenuclearoverlapfunction,TAA,thenumberofparticipating nucleons,Npart,andthenumberofbinarynucleon–nucleoncollisions,Ncoll,for eachcentralityinterval.Theaveragenumberofparticipantsasweightedbytheav- eragenumberofcollisions,^NpartN_coll,isalsoreported.

Centrality ^TAA(mb⁻¹) ^Npart ^Ncoll ^NpartNcoll

0–90% 6.2±⁰.2 126±^{2 435}±^{41 263}±³

0–20% 18.8±⁰.6 311±^{3 1318}±^{130 322}±³

20–90% 2.61±0.09 73±1 183±15 141±2

centralitybin,N_part.However,therateofhardprocessesisknown to scale with the average number of nucleon–nucleon collisions Ncoll. The average centrality for hard processes is therefore pre- sentedastheaveragenumberofparticipantnucleonsweightedby thenumberofcollisions^NpartNcoll.Table1showstheestimatesof theaveragenuclearoverlapfunction^TAA^{,thenumberofpartici-} patingnucleons^Npart^{andthenumberofbinary}nucleon–nucleon collisions ^Ncoll^{, which are obtained via a Glauber model fit of} thesignal amplitudeinthetwoarraysoftheV0detector [49,50].

The resulting NpartNcoll is also shown. The classification of the events in givencentrality intervalshas an associated uncertainty of1.5–2.3%(centralitydependent),thatwasestimatedbycompar- ing the number of candidates selected by varying the centrality rangesby±⁰.5%,toaccountforthecentralityresolution [49,50].

The sources of systematicuncertainties in the yields and RAA aresummarisedinTable2.Thesystematicuncertaintyinthetrack-

Table 2

RelativesystematicuncertaintiesintheyieldsandRAA.Therangesquotedfor^TAA andthecentralitylimits, representtheuncertaintyvariationwithcentrality.The centrality-dependentcorrelateduncertaintiesaremarkedbythesymbol(),while theuncertaintysourcesthatarecorrelatedasafunctionofrapidityareindicated by ( ).

Source Relative systematic uncertainty

Background contamination <1.0%

Tracking efficiency 3.0% ()

Trigger efficiency 1.5% ()

Tracker/trigger matching 1.0% ()

Alignment 3.5% ()

Fμ-trig/MB 0.5% ( )

σpp 4.5% ()

^TAA ^{3.2–3.5% ( )}

Centrality limits 1.5–2.3% ( )

ingefficiencyis3%,obtainedfromthecomparisonoftheefficiency estimated in data and MC by exploiting the redundancy of the trackingchamber information [51]. The systematicuncertaintyof thedimuontriggerefficiencyis1.5%,evaluatedbypropagatingthe uncertaintyof the efficiency of the detection elements, which is estimated from data using the redundancy of the trigger cham- ber information. In addition, the choice of the

χ

² cut used to match the tracker and trigger tracks introduces 1% uncertainty, obtainedfromthe difference betweendata andsimulation when applyingdifferent

χ

² cuts. The uncertainties inthe trackresolu- tion andalignment are estimated by comparing the A·

ε

values obtainedwithtwodifferentsimulations.Inthefullsimulation,the alignmentismeasuredusingtheMILLEPEDE [52] packageandthe residualmisalignmentistakenintoaccount.Inthefastsimulation, thetracker response is basedon a parameterisation of the mea- suredresolution ofthe clustersassociatedwith a track [32]. The resultingsystematicuncertaintyis3.5%.

Thetotal systematicuncertainty in theyield and RAA are de- termined by summing in quadrature the uncertainty from each source,listedinTable2.Alluncertaintiesexceptthosedueto^TAA andthecentralitybinboundariesareindependentofcollisioncen- trality.Correlationsincentralityorrapidityoftheuncertaintiesof differentsources areindicated inTable 2.The relative systematic uncertaintyintheproton–protonreference

σ

pp,whichaffectsthe RAA,correspondsto4.5%andisestimatedbyvaryingthefactoriza- tionandrenormalisationscales andaccountingfortheuncertain- tiesinthePDFs [20].

4. Results

Theinvariantyieldof

μ

⁺

μ

⁻ fromZ⁰ bosons in2.5<y<4.0, divided by ^TAA^{, is 6}.11±⁰.76(stat.)±⁰.38(syst.) pb for the 0–90% centralityinterval. The comparisonwith theoretical calcu- lationsatNLOisshowninFig.2.TheCT14 [53] predictionutilises free proton and neutron PDFs, with relative weights to account forthe isospin of the Pbnucleus. The uncertaintyon the model includetheuncertainty onthe NLO calculationsandofthe mea- surementsconsideredinthePDF fit.Themeasuredinvariantyield deviatesfromthelower limit of thispredictionby 2.3

σ

. Forthe description of nuclear PDFs, two different approaches were con- sidered. The standard approach evaluates the nPDF as the free PDFmultipliedbyaparameterisationofnuclearmodifications.The calculations obtained with the EPS09 [54] and the more recent EPPS16 [22] parameterisations are shown. Inthe other approach, thenPDFsareobtainedbyfittingthenucleardatainasimilarway asdoneforfreeprotondata,butusingaparameterisationthatde- pends on the atomic mass of the nucleus. The results obtained with the nCTEQ15 nuclear PDFs [21,55] are also presented. The

Fig. 2.Invariantyieldofμ⁺μ⁻fromZ⁰productionin2.5<y<4.0 dividedbythe averagenuclearoverlapfunctioninthe0–90%centralityclass,consideringmuons with−4.0<η<−2.5 andpT>20 GeV/c.Thehorizontalsolidlinerepresentsthe statisticaluncertaintyofthemeasurementwhiletheyellowfilledbandshowsthe systematicuncertainty.Theresultiscomparedtotheoreticalcalculationswithand withoutnuclearmodificationofthePDFs [21,22,53–55].Allmodelcalculationsin- corporatePDFsornPDFsdeterminedbyconsideringtheisospinofthePb-nucleus.

nPDFsetsare characterisedby theirdifferentapproximationsand by differentinput dataincluded inthecalculations(see Ref. [22]

and referencestherein for details).Only the mostrecent EPPS16 parameterisation includesLHCjet, W^± andZ⁰ data,although the W^± and Z⁰ dataprovide only weak constraints on nPDFs at the currentperturbativeorderofthecalculation(NLO) [21,56].Ingen- eral,thenPDFshavelargeruncertaintiescomparedtothefreepro- ton PDFs,sincethey are lessconstrainedfromdata.CT14+^EPS09 andCT14+^{EPPS16 estimatescombineCT14andEPS09orEPPS16} uncertainties,whereasnCTEQ15doesaglobalstudyoftheproton andnuclearmeasurementuncertaintiesincludedinthefit.EPPS16 allows much morefreedom for theflavour dependenceofnPDFs than othercurrentanalyses, whichresults inlargeruncertainties.

AllpQCDcalculationsshowninFig.2thatusenPDFsdescribethe measurementwell.

The rapidity dependence of the Z⁰-boson invariant yields di- videdby^TAA^{isshownin Fig.3(a).Theresultsarecompared to} pQCD calculations usingthe CT14 [53] PDFset both with(green filled box) and without (blue hatched box) the EPPS16 [22] pa- rameterisation of the nPDFs. In both cases, the Pb-isospin effect ismodelled bycombiningtheprotonandneutronPDFsornPDFs.

EPPS16 decreasestheyields butdoesnothaveastrong influence ontherapiditydependenceofthecalculation.Thecalculationsthat utilise vacuumPDFs overestimate data in the two rapidity inter- vals,whereasthosethatutilisenPDFsareingoodagreementwith data.

In this analysis, the ratio RAA utilises a theoretically calcu- latedreferencecrosssectionforppcollisions [20],whichis

σ

pp= 11.92±⁰.43 pb. The value of RAA forthe 0–90% centralityclass isdetermined tobe 0.77±⁰.10(stat.)±⁰.06(syst.),deviatingby 2.1

σ

fromunity.The pQCD calculation usingCT14 [53] and con- sideringonlytheisospineffects,finds R^CT14_AA =¹.052±⁰.038.The modification of the PDFs in nuclei results in a net reduction of theyields,andconsequently inRAA valueslowerthanunity,with R^CT14_AA ^+EPPS16=⁰.845±⁰.068,in agreementwithdata.The rapid-

Fig. 3.Invariantyieldofμ⁺μ⁻fromZ⁰in2.5<y<4.0 dividedby^TAA^{(a)andnuclear}modificationfactor(b)asafunctionofrapidityforPb–Pbcollisionsat√_s

NN= 5.02 TeV,consideringmuonswith−⁴.0<η<−².5 andpT>20 GeV/c.Theverticalerrorbarsarestatisticalonly.Thehorizontalerrorbarsdisplaythemeasurementbin width,whiletheboxesrepresentthesystematicuncertainties.Thefilledblackboxinpanel(b),locatedat RAA=1,showsthenormalisationuncertainty.Theresultsare comparedtotheoreticalcalculationswithandwithoutnuclearmodificationofthePDFs.ThefilledblueboxesshowthecalculationusingtheCT14PDF,whilethegreen stippledboxesshowthecalculationusingCT14PDFwithEPPS16nPDF [22,53].AllmodelcalculationsincorporatePDFsornPDFsthataccountfortheisospinofthePb nucleus.

Fig. 4.Invariantyieldofμ⁺μ⁻fromZ⁰in2.5<y<4.0 dividedby^{TAA(a)andnuclear}modificationfactor(b)asafunctionofcentrality(representedby^NpartNcoll)for Pb–Pbcollisionsat√

sNN=5.02 TeV,consideringmuonswith−4.0<η<−2.5 andpT>20 GeV/c.Theverticalerrorbarsarestatisticalonly,whiletheboxesrepresentthe systematicuncertainties.Thefilledblackboxinpanel(b),locatedatRAA=1,showsthenormalisationuncertainty.Theresultsarecomparedtotheoreticalcalculationswith centrality-dependentnPDFsthataccountfortheisospinofthePbnucleus [53,54,57].

ity dependence of RAA is presented in Fig. 3(b). The values are smallerthanunity,withaslightrapiditydependence.Thedataare well-describedbycalculationsincludingnPDFs(greenfilledboxes), whilethecalculationsincludingonlyisospineffects(bluehatched boxes)tendtooverestimatethemeasuredvalues.

Z⁰-boson production is studied as a function of the collision centrality,expressedintermsof^NpartN_collasshowninFig.4.The valueofR_AAiscompatiblewithunityinperipheralcollisions,with R_AA(20–90%)=⁰.96±⁰.19(stat.)±⁰.04(syst.)±⁰.06(corr. syst.), whileit is2.6

σ

smallerthan unityinthecentral collisions, with RAA (0–20%)=⁰.67±⁰.11(stat.)±⁰.03(syst.) ±⁰.04(corr. syst.).

The value for 0–20% central collisions deviatesfrom the predic- tionsusingvacuumPDFs(R^CT14_AA )by3

σ

.Thedataarecomparedto calculationsincludingacentrality-dependentnuclearmodification ofthePDFs [57],whichdescribethedatawithinuncertainties.

5. Conclusion

We havereportedthe first measurement ofZ⁰-boson produc- tionatforwardrapiditiesinPb–Pbcollisions at√

sNN=⁵.02TeV.

Theinvariant yields divided by theaveragenuclear overlapfunc- tionareevaluatedasafunctionofrapidityandaveragenumberof participantnucleons weightedby thenumberofbinary nucleon–

nucleon collisions. Thecorresponding valuesof thenuclear mod- ification factor are estimated by dividing the measured yields in Pb–Pbcollisionsbytheexpectedcross-sectioninppcollisionsesti- matedwithNLOpQCDcalculations.ThevalueofR_AAiscompatible with unityin the 20–90% centralityclass (withinlarge statistical uncertainty), whereas it is smaller than unity by 2.6 times the quadraticsumofthestatisticalandsystematicuncertaintiesinthe 0–20% most central collisions. The results are well-described by the calculationsthat include modifications of thePDFs innuclei.

In contrast, the calculations with vacuumPDFs overestimate the centrality-integrated RAAby2.3

σ

andRAA inthe0–20% mostcen- tralcollisionsby3

σ

.

Acknowledgements

The ALICE Collaboration would like to thank H. Paukkunen forproviding thepQCD calculationswithEPS09 andEPPS16,and F. LyonnetandA. KusinaforprovidingthepQCDcalculationswith nCTEQ15.

The ALICECollaboration would like to thank all its engineers andtechniciansfortheirinvaluablecontributionstotheconstruc- tion of the experiment and the CERN accelerator teams for the outstanding performance of the LHC complex. The ALICE Collab-

oration gratefully acknowledges the resources and support pro- videdbyallGridcentresandtheWorldwideLHCComputingGrid (WLCG) collaboration. The ALICECollaboration acknowledges the followingfundingagenciesfortheir support inbuildingandrun- ningtheALICEdetector:A.I. Alikhanyan NationalScienceLabora- tory(YerevanPhysicsInstitute)Foundation (ANSL),StateCommit- teeofScienceandWorldFederationofScientists(WFS), Armenia;

AustrianAcademy ofSciences andNationalstiftung fürForschung, Technologie und Entwicklung, Austria; Ministry of Communica- tions and High Technologies, National Nuclear Research Center, Azerbaijan; Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Universidade Federal do Rio Grande do Sul (UFRGS), Financiadora de Estudos e Projetos (Finep) and Fun- dação de Amparo à Pesquisa do Estado de São Paulo (FAPESP), Brazil; Ministry of Science & Technology of China (MSTC), Na- tional Natural Science Foundation of China (NSFC) and Ministry of Education of China (MOEC), China; Ministry of Science, Edu- cationandSports andCroatian Science Foundation,Croatia; Min- istryofEducation,YouthandSportsoftheCzechRepublic, Czech Republic; The Danish Council for Independent Research – Natu- ral Sciences, the Carlsberg Foundation and Danish National Re- search Foundation (DNRF), Denmark;Helsinki Institute ofPhysics (HIP),Finland;Commissariatàl’EnergieAtomique(CEA)andInsti- tut Nationalde Physique Nucléaire etde Physique des Particules (IN2P3)andCentre National de laRecherche Scientifique (CNRS), France; Bundesministerium für Bildung, Wissenschaft, Forschung und Technologie (BMBF) and GSI Helmholtzzentrum für Schwe- rionenforschungGmbH,Germany;GeneralSecretariatforResearch and Technology, Ministry of Education, Research and Religions, Greece; National Research, Development and Innovation Office, Hungary; Department of Atomic Energy, Government of India (DAE),DepartmentofScienceandTechnology,GovernmentofIndia (DST),University Grants Commission, Governmentof India(UGC) andCouncil ofScientificandIndustrialResearch(CSIR), India;In- donesian Institute of Science, Indonesia; Centro Fermi – Museo StoricodellaFisicaeCentroStudieRicercheEnricoFermiandIsti- tutoNazionalediFisicaNucleare(INFN),Italy;InstituteforInnova- tiveScienceandTechnology,NagasakiInstituteofAppliedScience (IIST),Japan Societyforthe PromotionofScience(JSPS)KAKENHI andJapanese Ministry of Education, Culture, Sports, Science and Technology(MEXT),Japan;ConsejoNacionaldeCiencia(CONACYT) yTecnología,throughFondodeCooperaciónInternacionalenCien- cia y Tecnología (FONCICYT) and Dirección General de Asuntos delPersonalAcademico(DGAPA),Mexico;NederlandseOrganisatie voor Wetenschappelijk Onderzoek (NWO), Netherlands; The Re- search Council of Norway, Norway; Commission on Science and TechnologyforSustainableDevelopmentintheSouth(COMSATS), Pakistan;PontificiaUniversidadCatólicadelPerú,Peru;Ministryof ScienceandHigherEducationandNationalScienceCentre,Poland;

KoreaInstituteofScienceandTechnologyInformationandNational ResearchFoundationofKorea(NRF),RepublicofKorea;Ministryof EducationandScientificResearch,Institute ofAtomicPhysicsand RomanianNationalAgencyforScience,TechnologyandInnovation, Romania; Joint Institute for Nuclear Research (JINR), Ministry of EducationandScienceoftheRussianFederationandNationalRe- search Centre Kurchatov Institute, Russia; Ministry of Education, Science,Researchand SportoftheSlovak Republic, Slovakia; Na- tionalResearchFoundationofSouthAfrica,SouthAfrica;Centrode AplicacionesTecnológicasyDesarrolloNuclear(CEADEN),Cubaen- ergía,CubaandCentrodeInvestigacionesEnergéticas,Medioambi- entalesyTecnológicas(CIEMAT),Spain;SwedishResearchCouncil (VR)andKnut&AliceWallenbergFoundation(KAW),Sweden;Eu- ropean Organization for Nuclear Research, Switzerland; National Scienceand Technology DevelopmentAgency (NSDTA), Suranaree University of Technology (SUT) and Office of the Higher Educa-

tionCommissionunderNRUprojectofThailand,Thailand;Turkish Atomic Energy Agency (TAEK), Turkey; National Academy of Sci- encesofUkraine,Ukraine;ScienceandTechnologyFacilitiesCoun- cil (STFC), United Kingdom; National Science Foundation of the United Statesof America (NSF) andUnited StatesDepartment of Energy,OfficeofNuclearPhysics(DOENP),UnitedStatesofAmer- ica.

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