J/ψ suppression at forward rapidity in Pb-Pb collisions at √sNN = 5.02 TeV

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Physics Letters B

www.elsevier.com/locate/physletb

J/ ψ suppression at forward rapidity 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:

Received29July2016

Receivedinrevisedform5December2016 Accepted26December2016

Availableonline10January2017 Editor:L.Rolandi

TheinclusiveJ/ψ productionhasbeenstudiedinPb–Pbandppcollisionsatthecentre-of-massenergy per nucleon pair √_s

NN=⁵.02 TeV, using the ALICE detector at the CERN LHC. The J/ψ meson is reconstructed, in the centre-of-mass rapidity interval 2.5<y<4 and in the transverse-momentum range pT<12 GeV/c,viaitsdecaytoamuonpair.InthisLetter,wepresent resultsontheinclusive J/ψ cross sectionin ppcollisions at √_s=⁵.02 TeV and onthe nuclear modification factor RAA. The latterispresentedasafunctionofthecentralityofthecollisionand,forcentralcollisions,asafunction of the transverse momentum pT ofthe J/ψ.The measured RAA values indicate asuppression of the J/ψ innuclear collisions and are thencomparedto ourprevious resultsobtained inPb–Pbcollisions at√_s

NN=².76 TeV.The ratioofthe RAA values atthe twoenergiesisalsocomputed andcompared tocalculationsofstatisticaland dynamicalmodels.Thenumericalvalueoftheratioforcentral events (0–10% centrality) is 1.17±⁰.04(stat)±⁰.20(syst). In central events, as a function of pT, a slight increaseofRAAwithcollisionenergyisvisibleintheregion2<pT<6 GeV/c.Theoreticalcalculations qualitativelydescribethemeasurements,withinuncertainties.

©^{2017TheAuthor.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense} (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP³.

1. Introduction

Whenheavynucleicollideatultrarelativisticenergies,astateof strongly-interacting matter is formed,characterised by hightem- perature anddensity, where quarks andgluons are not confined into hadrons (Quark–Gluon Plasma, QGP [1]). A detailed charac- terisation of the QGP is the object, since more than 25 years, of an intense research activity at the CERN/SPS [2] and at the BNL/RHIC [3–6] and CERN/LHC [7] ion colliders. Charmonia and bottomonia, which are bound states of charm–anticharm (cc) or bottom–antibottom (bb) quarks, respectively [8], are among the most sensitive probes of the characteristics of the QGP. A sup- pression of their yields in nucleus–nucleus (A–A) collisions with respecttoexpectationsfromproton–proton(pp)collisionswasex- perimentallyobserved.FortheJ/ψ meson,thegroundcc statewith quantumnumbersJ^PC=^1−−,asuppressionwasfoundattheSPS, inPb–Pb andIn–Ininteractions atthecentre-of-mass energyper nucleonpair√

sNN=¹⁷.2 GeV[9,10],RHIC,inAu–Auinteractions at√

s_NN=^{200 GeV[11,12],andfinally attheLHC,in Pb–Pbcol-} lisions at √

s_NN=².76 TeV [13,14]. Early theoretical calculations predictedJ/ψ suppressionto be induced by thescreening ofthe colourforce ina deconfined medium andto become stronger as theQGPtemperatureincreases[15,16].Inacomplementarywayto

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

thisstaticapproach,J/ψ suppressioncanalsobeseenastheresult of dynamical interactions with the surrounding partons [17–19].

TheLHCresults,integratedovertransversemomentum(p_T)down to pT=^{0,show a}suppression ofthe J/ψ, quantifiedthrough the ratio betweenitsyields in Pb–Pbandthose inpp, normalised to thenumberofnucleon–nucleoncollisionsinPb–Pb(nuclearmodi- ficationfactor, RAA).However,theobservedsuppressionissmaller thanatSPSandRHIC[20,21],inspiteofthehigherinitialtemper- atureoftheQGPformedattheLHC[22].Theeffectisparticularly evident for head-on(central) collisions.In orderto explain these observations, theoretical models require a contribution from J/ψ regeneration viaarecombinationmechanism[23,24]betweenthe c and c quarks,during thedeconfinedphaseand/oratthehadro- nisation of the system, which occurs when its temperature falls below the critical value Tc∼^{155 MeV [25].The strength ofthis} regeneration effectincreaseswiththeinitial numberofproduced cc pairs relativeto thetotalnumberofquarks and,therefore,in- creaseswiththecollisionenergy,explainingthereducedsuppres- sionattheLHC.Sincethebulkofcharm–quark productionoccurs at small momenta, recombination should be more important for low-pTJ/ψ,asobservedintheLHCresults[21].

An important test of the suppression and regeneration pic- tureof J/ψ productionattheLHC canbe obtainedby comparing the centrality and p_T dependence of the J/ψ R_AA, measured at

√sNN=².76 TeV,tothatobtainedat√

sNN=⁵.02 TeV,thehighest energyavailable up tonow innuclearcollisions. The suppression http://dx.doi.org/10.1016/j.physletb.2016.12.064

0370-2693/©^{2017TheAuthor.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense}(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP³.

effectsrelated tocolour screening shouldbecome stronger when increasingthecollisionenergy,duetothehigherQGPtemperature, andalsotherecombinationeffectsshouldbecomestronger,dueto theexpectedincrease ofthecc productioncrosssection. Thetwo effectsactinoppositedirectionsandthecomparisonoftheRAA at thedifferentenergies canprovideinsightsintheevolutionofthe relativecontributionofthetwoprocesses.

InthisLetter,wepresentthefirstresultsontheJ/ψ R_AA mea- sured by the ALICECollaboration in Pb–Pb collisions at √

s_NN= 5.02 TeV and the integrated and pT differential J/ψ production crosssection inpp collisions atthe same energy. In both Pb–Pb andpp collisions, the J/ψ is reconstructed via its dimuon decay channel at forwardrapidity, 2.5<y<4 and for p_T<12 GeV/c.

ThemeasurementsrefertoinclusiveJ/ψ production,thatincludes bothpromptJ/ψ (directJ/ψ andfeed-downfromhigher-massres- onances) and non-prompt J/ψ (from decay of beauty hadrons).

ThenuclearmodificationfactorisobtainedbynormalisingtheJ/ψ yield in Pb–Pb collisions to the product of the nuclear overlap function times the corresponding J/ψ cross section measured in pp, atthe sameenergy andin the samekinematic window. The resultson R_AA are presentedasa function of the J/ψ p_T andof thecentralityofthecollision.

2. Experimentalapparatusanddatasample

TheALICEdetectordesignandperformance areextensivelyde- scribedin[26] and[27].Theanalysispresentedhereis basedon the detection ofmuons in the forward muon spectrometer [28], whichcoversthepseudo-rapidityrange−4<

η

<−2.5.¹ Inaddi- tion, the Silicon PixelDetector (SPD) [29] is used to reconstruct the primary vertex. The V0 detectors [30] provide a minimum- bias(MB) triggerandareused todetermine thecentralityofthe collision,while the T0 Cherenkovcounters [31] are used forthe luminositydeterminationinppcollisions. Finally,theZeroDegree Calorimeters(ZDC)areusedtorejectelectromagneticPb–Pbinter- actions [32]. Abrief descriptionof thesedetectors isgiven here- after.

Themuonspectrometercontainsafrontabsorber,madeofcar- bon, concrete and steel, placed between 0.9 and 5 m from the Interaction Point (IP), which filters out hadrons, thus decreasing the occupancy in the downstream tracking system. The latter is composed of five stations, each one consisting of two planes of CathodePadChambers (CPC).The thirdtrackingstation isplaced inside the gap of a dipole magnet with a 3 T m field integral.

Twotrigger stations, each one equipped with two planes of Re- sistive Plate Chambers (RPC), are located behind a 7.2 interac- tionlength iron wall, which absorbs secondary hadronsescaping the front absorber and low-momentum muons. The muon trig- gersystemdeliverssingle-muonanddimuontriggerswithapro- grammable transverse-momentum threshold. Finally, throughout itsentirelength,aconicalabsorberaroundthebeampipe(θ <2^◦) madeof tungsten, lead andsteel shields the muon spectrometer againstsecondaryparticlesproducedbytheinteractionoflarge-

η

primaryparticlesinthebeampipe.

Theprimary vertexis reconstructedusinghitpairs inthetwo cylindricallayers ofthe SPD[26,29], whichhave average radiiof 3.9and7.6cm,andcoverthepseudo-rapidityintervals|

η

_|<2 and

|

η

_|<1.4,respectively.

The two V0detectors [30],with 32 scintillator tiles each, are placedoneach sideoftheIP,coveringthepseudo-rapidityranges

1 IntheALICEreferenceframe,themuonspectrometercoversanegativeηrange andconsequentlyanegativeyrange.Wehavechosentopresentourresultswitha positiveynotation.

2.8<

η

<5.1 and−³.7<

η

<−¹.7.Thecoincidenceofthesignals from the two hodoscopes defines the MB trigger. Beam-induced backgroundisreducedbyapplyingtimingcutsonthesignalsfrom the V0s and ZDCs. The latter are positioned along the beam di- rection at ±^{112.5 m from the IP. Finally, the T0 detectors [31],} madeof two arraysof quartz Cherenkovcounters, are placed on bothsidesoftheIP,coveringthepseudo-rapidityintervals−³.3<

η

<−^{3 and4}.6<

η

<4.9.

In Pb–Pb collisions, the centrality determination is based on a Glauber fit ofthe total V0signal amplitude distribution asde- scribed in [33,34].A selection corresponding to the mostcentral 90% of the hadronic cross section was applied; for these events theMBtriggerisfullyefficient.

ForbothPb–Pb andpp datataking, thetriggercondition used inthe analysisis a

μμ

-MBtrigger formed by thecoincidence of the MB trigger and an unlike-sign (US) dimuon trigger. The lat- terhasatriggerprobability foreach ofthetwomuon candidates that increaseswiththe muon p_T,is 50%at1.0 GeV/c (0.5GeV/c) in Pb–Pb (pp) collisions, andsaturates at pT≈².5 GeV/c, where it reaches a value of about 98%. Like-sign dimuon triggers were also collected, mainly for background normalisation purposes in thePb–Pbanalysis.

The data samples used in this analysis correspond to an in- tegrated luminosity L^Pb–Pb_int ≈ ^{225 μb−1 for Pb–Pb and Lpp}_int≈ 106 nb⁻¹ forppcollisions.

3. Dataanalysis

Theanalysisprocedurewasverysimilarforthetwodatasam- ples describedinthisLetter.Inthefollowingparagraphs, thePb–

Pbanalysisisfirstpresented,followedbythedescriptionofthepp one.

The J/ψ candidates were formed by combining pairs of US tracks reconstructed in the geometrical acceptance of the muon spectrometer using the trackingalgorithm described in [28].The same single-muon and dimuon selection criteria as in previous analyses[21]wereapplied,andtracksinthetrackingsystemwere required to match a track segment in the muon trigger system (triggertracklet).

The J/ψ rawyields were determined fromthe invariant mass distributionofUSdimuonsusingtwomethods.Inthefirstone,the USdimuoninvariant massdistributions werefittedwiththesum of a signal and a background function. In the second approach, the background, estimated using an event-mixing technique and normalisedusingthelike-signdimuondistributions[21],wassub- tracted and the resulting spectra were fitted with the sum of a signalfunctionanda(small)residualbackgroundcomponent.

Variousshapeswere consideredforthesignalandbackground contributions. For the J/ψ signal either an extended Crystall Ball (CB2)functionorapseudo-Gaussianwithamass-dependentwidth wereused[35].Thenon-Gaussiantailsofthesignalfunctionswere fixed either (i) to the values obtainedin MonteCarlo (MC) sim- ulations, where simulated J/ψ→

μ

⁺

μ

⁻ are embedded into real eventsto accountfor theeffectofthe detectoroccupancy, or(ii) to the values obtainedin a high-statisticspp collision sample at

√s=^{13 TeV, collected undersimilardetector}conditions.The tail parameters exhibit a dependence on the pTand rapidity of the J/ψ andamild dependenceonthecentralityofthecollision.The smallcontributionofthe ψ(2S)signal was takenintoaccount in the fits, its mass and widthbeing tied to those ofthe J/ψ [36].

Forthebackground,whentheUSdimuonmassspectrumwas fit- ted, a variable-width-Gaussian with a mass-dependent width or the ratio of a 2nd to a 3rd order polynomial were used. When considering the US dimuon distributions after subtraction of the background obtained with the event-mixing procedure, a small

Fig. 1.(Colouronline.) InvariantmassdistributionsofUSdimuonswith2.5<y<4 andpT<12 GeV/c.Thetop(bottom)rowshowsthedistributionbefore(after)background subtractionwiththeevent-mixingtechnique.Theleftpanelscorrespondtothemostcentralevents(0–10%)whiletherightpanelstoaperipheral(70–80%)centralityrange.

Thefitcurvesshowninbluerepresentthesumofthesignalandbackgroundshapes,whiletheredlinescorrespondtotheJ/ψsignalandthegreyonestothebackground.

dimuoncontinuum component isstill presentandwas fitted us- ingthesumoftwoexponentials.Severalfittingsub-ranges,within the interval 2<mμμ<5 GeV/c², were used for both signal ex- tractionprocedures.

Fig. 1showsexamplesoffitstotheUSdimuoninvariantmass distributions withand without backgroundsubtraction usingthe event-mixing technique, fordifferent selectionsin centrality. The raw J/ψ yield in each centrality or pT interval was determined as the average of the results obtained with the two fitting ap- proaches,the variousparameterisations of signal andbackground andthedifferentfittingranges,whilethecorrespondingsystematic uncertainties weredefinedasthe RMSoftheseresults.Afurther contributiontothesystematicuncertaintywasestimatedbyusing adifferent setofresonancetails obtainedusingintheMC simu- lationadifferentparticletransportmodel(GEANT4[37]insteadof GEANT3[38]).ThetotalnumberofJ/ψ,integratedovercentrality, pTandy,isNJ/ψ=².77±⁰.02(stat)±⁰.05(syst)·^{105.Thesystem-} aticuncertaintyrangesfrom1.6%to2.8%asafunctionofcentrality andfrom1.2%to3.1%asafunctionofpT.

Thenuclear modificationfactor,asa functionofthecentrality class i of thecollision andforthe J/ψ transverse-momentum in- tervalpT,iscalculatedas

Rⁱ_AA

(

p_T

) =

^N

i J/ψ

(

pT

)

BR_J/ψ→μ+μ⁻N_MBⁱ A

ε

ⁱ

(

pT

)

T_AAⁱ

σ

_J^pp_/ψ

(

pT

) ,

(1)

where Nⁱ_J/ψ(p_T)isthe numberofextractedJ/ψ in agivencen- tralityandpTrange,BR_J/ψ→μ⁺μ⁻=⁵.96±⁰.03% isthebranching ratio of the dimuon decay channel [39], Nⁱ_MB is the number of equivalent minimum-biasevents, A

ε

ⁱ(pT) is theproduct ofthe detector acceptance times the reconstruction efficiency, ^T_AA^{i is} theaverageofthenuclearoverlapfunction,and

σ

_J^pp_/ψ(pT)isthe inclusiveJ/ψ crosssectionforppcollisionsatthesameenergyand inthesamekinematicrangeasthePb–Pbdata.

The A

ε

valuesweredeterminedfromMCsimulations,withthe generated pTand y distributions for the J/ψ adjusted on data, and separately tuned for each centrality class using an iterative approach.Unpolarised J/ψ productionwas assumed[21].Forthe

tracking chambers, the time-dependent status of each electronic channel during the datatakingperiod was takenintoaccount as well asthemisalignment ofthedetectionelements. Theefficien- ciesofthemuontriggerchambersweredeterminedfromdataand were then applied inthe simulations. Finally, thedependence of the efficiency on thedetector occupancy was taken into account by embedding MC-generated J/ψ into real minimum-bias Pb–Pb events.

For J/ψ produced within 2.5< y<4 and pT<12 GeV/c, in 0–90%mostcentralcollisions,the A

ε

valueis0.136±⁰.007(syst). A relative decrease of the efficiencyby 14% was observed when going from peripheral to central collisions. As a function of pT, A

ε

has a minimum value of about0.12 at pT≈¹.5 GeV/c, and then steadilyincreases up to about 0.4at the upper end of the considered range.The followingsourcesofsystematicuncertainty on A

ε

were considered.A firstcontribution of2% dueto thein- put MC pTand y distributions was estimated by (i) varying the input shapesthat were tuned ondatawithin their statisticalun- certaintiesand(ii)takingintoaccounttheeffectofpossible pT−^y correlationsbycomparing,asafunctionofcentrality,the A

ε

val- ueswiththecorrespondingresultofa 2-Dacceptancecalculation in classes of pT and y. A second contribution comes from the trackingefficiencyanditwas estimatedby comparingthesingle- muon tracking efficiency values obtained, in MC and data, with aprocedurethatexploitstheredundancyofthetracking-chamber information[21].A3%systematicuncertaintyonthedimuontrack- ingefficiencyisobtainedandisapproximatelyconstantasafunc- tion of centrality and kinematics. The systematic uncertainty on thedimuontriggerefficiencyrepresentsthethirdcontributionand it has two origins: the intrinsic efficiencies of the muon trigger chambersandtheresponse ofthetriggeralgorithm.Thefirstone wasdeterminedfromtheuncertaintiesonthetriggerchamberef- ficiencies measured from data andapplied to simulations and it amountsto1.5%.Thesecondonewasestimatedbycomparingthe p_T dependence,at thesingle-muon level,ofthe trigger response functionbetweendataandMCanditvariesbetween0.2%and4.6%

asafunction of pT.Combiningthe twosources,a systematicun- certainty rangingfrom1.5% to4.8%asa functionoftheJ/ψ pT is obtained. Finally, thereis a 1% contributionrelatedto the choice

Table 1

Summaryofsystematicuncertainties,inpercentage,onRAAandd²σ_J^pp_/ψ/dydpT.ValuesmarkedwithanasteriskcorrespondtocorrelateduncertaintiesasafunctionofpT (secondandfifthcolumn)orcentrality(thirdcolumn).ThereisnocorrelationbetweentheuncertaintiesrelatedtotheanalysisofthePb–Pbandoftheppsample.The contentsofthe“ppreference”rowcorrespondtothequadraticsumofthecontributionsindicatedford²σ_J^pp_/ψ/dydpT,excludingonlytheBRuncertaintywhichcancelsout whenformingtheRAA.

Source RAA d²σJ^pp/ψ/dydpT

0–90%

pT<12 GeV/c

vspT (0–20%)

vscentrality (pT<8 GeV/c)

pT<12 GeV/c vspT

Signal extr. 1.8 1.2–3.1 1.6–2.8 3 1.5–9.3

MC input 2 2 2^∗ 2 0.7–1.5

Tracking eff. 3 3 3^∗ 1 1

Trigger eff. 3.6 1.5–4.8 3.6^∗ 1.8 1.5–1.8

Matching eff. 1 1 1^∗ 1 1

F(L^pp_int) 0.5 0.5^∗ 0.5^∗ (2.1) (2.1^∗)

BR – – – 0.5 0.5^∗

TAA 3.2 3.2^∗ 3.1–7.6

Centrality 0 0.1^∗ 0–6.6

pp reference 5.0 3–10

2.1^∗(L^pp_int) 4.9^∗

ofthe

χ

² cut used indefining thematching betweentherecon- structedtracksandthetriggertracklets.

ThenormalisationfactortothenumberofequivalentMBevents wasobtainedasNⁱ_MB=^Fi·^Nμμ-MB,where Nμμ-MBisthenumber of

μμ

-MB triggered events, and Fⁱ is the inverse of the proba- bilityofhaving adimuon triggerin a MBevent inthecentrality rangei.The Fⁱ values were calculated withtwo differentmeth- ods,by applying thedimuon triggercondition inthe analysison minimum-biasevents,orfromtherelativecountingrateofthetwo triggers[40].Theobtainedvalue, inthe0–90% centralityclass,is F=¹¹.84±⁰.06,where the uncertainty is dominated by a sys- tematiccontribution correspondingto thedifferencebetweenthe resultsobtained with the two approaches. As a function of cen- trality, Fⁱ=^F·ⁱ,where ⁱ isthefractionof theinelasticcross sectionofagivencentralityclasswithrespecttothewhole0–90%

centralityrange(e.g.0.1/0.9for0–10%centralityandsoon).

The values for ^T_AA^{i and for the average number of partici-} pantnucleons^Nipart^{wereobtainedviaaGlauber}calculation[33, 34,41]. The systematicuncertainty is3.2% forthe 0–90% central- ity range and was obtained by varying within uncertainties the densityparametersofthePbnucleusandthenucleon–nucleonin- elasticcrosssection[34,41].

Finally,the effects of the uncertainty on the value of the V0 signalamplitudecorrespondingto90%ofthehadronicPb–Pbcross sectionwereestimatedbyvaryingsuchavalueby±⁰.5%[33]and redefiningcorrespondingly thecentralityintervals.The systematic effecton RAA ranges from0.1% to6.6%fromcentraltoperipheral collisions.

TheJ/ψ cross-sectionvaluesinpp collisionsat√

s=⁵.02 TeV, bothintegratedandpTdifferential,wereobtainedwithananalysis proceduresimilartotheonedescribedinthepreviousparagraphs for Pb–Pb. In particular, the same criteria for single-muon and dimuonselectionwereadopted.

Thesignal extraction was then performedby fittingthe spec- trawiththesumofasignalandabackgroundcontribution,using shapessimilarto thoseadoptedforthePb–Pb analysis.The back- groundsubtractionvia theevent-mixing technique wasnot used, asthe signal-over-background ratiois largerby afactor ∼^40,in thepT-integratedspectra,withrespecttocentralPb–Pbcollisions, making the influence of the backgroundestimate much lessim- portantinthedeterminationoftheuncertaintyonN^pp_J/ψ.Thevalue N^pp_J/ψ=⁸⁶⁴⁹±¹²³(stat)±²⁹⁷(syst)isobtained,withthesystem- aticuncertaintydeterminedasforthePb–Pbanalysis.

ThedeterminationofA

ε

ppwascarriedoutviaMCsimulations.

Since no appreciable dependence of the tracking efficiency as a function of the hadronic multiplicity can be seen in pp, a pure MC (i.e.,withoutembedding)was used. The input p_T and y dis-

tributions wereobtainedfromthemeasuredonesviaan iterative procedure,andunpolarisedJ/ψ productionwasassumed[42].The obtainedvalueis A

ε

pp=⁰.243±⁰.007(syst),withthesystematic uncertainties onthetracking,triggerandmatchingefficiencycal- culatedasinthePb–Pbanalysis.Becauseofthelimitedppstatis- tics,thesystematicuncertaintyontheMCinputswasnotobtained througha2-Dacceptancecalculation,asdoneinthePb–Pbanaly- sis,butitwasdeterminedcomparingtheA

ε

valuesobtainedusing J/ψ p_T (y)distributionsevaluatedinvariousy (p_T)intervalsinpp collisionsat√

s=^{7 TeV[43].}

The integrated luminosity was calculated as L^pp_int=(N^ppμμ-MB· F^pp)/

σ

_ref^pp,where

σ

_ref^ppisareference-triggercrosssectionmeasured ina vander Meerscan,following theproceduredetailedin[44], and F^pp is the ratio of the reference-trigger probability to the

μμ

-MB trigger probability.The corresponding numerical value is L^pp_int=¹⁰⁶.3±².2(syst) nb⁻¹, where the quoted uncertainty re- flectsthevanderMeerscanuncertainty.

Finally,theinclusiveJ/ψ crosssectioninppcollisions at√ s= 5.02 TeV wasobtainedas

d²

σ

_J^pp_/ψ

dydpT

=

^N

pp J/ψ

(

p_T

)

BR_J/ψ→μ+μ⁻L^pp_intA

ε

_pp

(

p_T

)

p_T

y

.

(2) Table 1 summarises the systematicuncertainties on the mea- surementofthenuclearmodificationfactorsandd²

σ

_J^pp_/ψ/dydp_T.

TheRAAvaluespresentedinthefollowingrefertoinclusiveJ/ψ production, i.e. include both prompt and non-prompt J/ψ. Since beauty-hadrondecaysoccur outsidetheQGP, thenon-promptJ/ψ RAA is related to the nuclear modification of the beauty-hadron p_T distributions. The difference betweenthe R_AA of prompt and inclusive J/ψ can be estimated as in [21], using the fraction FB ofnon-prompttoinclusiveJ/ψ inppcollisionsandassumingtwo extreme casesfor the R^non-prompt_AA ofnon-prompt J/ψ, namely no medium effects on b-quarks (R^non-prompt_AA =^{1) or their complete} suppression(R^non-prompt_AA =^{0). F}B wasobtainedbyaninterpolation oftheLHCbmeasurementsinppcollisionsat√

s=².76 and7 TeV [43,45,46].ThequantitativeeffectontheinclusiveJ/ψ RAAispro- videdinthefollowingalongwiththeresults.

4. Results

The p_T-differential inclusive J/ψ cross section in pp collisions at √

s=⁵.02 TeV, in the region 2.5<y<4, isshown in Fig. 2.

The cross section value, integratedover the interval 2.5<y<4, p_T<12 GeV/c is

σ

_J^pp_/ψ=⁵.61±⁰.08(stat)±⁰.28(syst)μb. These results are used as a reference in the determination of the nu- clearmodificationfactorforPb–Pbcollisions.Boththedifferential

Fig. 2.(Colouronline.) Thedifferentialcrosssectiond²σ_J^pp_/ψ/dydpTforinclusiveJ/ψ productioninppcollisionsat√

s=5.02 TeV.Theerrorbarsrepresentthestatistical uncertainties,theboxesaroundthepointstheuncorrelatedsystematicuncertain- ties.Theuncertaintyontheluminositymeasurementrepresentsacorrelatedglobal uncertainty.

Fig. 3.(Colouronline.) Thenuclearmodificationfactor for inclusiveJ/ψ produc- tion,asafunctionofcentrality,at √

sNN=5.02 TeV,comparedtopublished re- sultsat√

sNN=2.76 TeV[20].Theerrorbarsrepresentstatisticaluncertainties,the boxesaroundthepointsuncorrelatedsystematicuncertainties,whilethecentrality- correlatedglobaluncertaintiesareshownasafilledboxaroundRAA=^1.Thewidths ofthecentralityclassesusedintheJ/ψanalysisat√_s

NN=⁵.02 TeV are2%from0 to12%,then3%upto30%and5%formoreperipheralcollisions.

andintegrated pp cross section valuesare consistent withthose obtained via an interpolation [45,47] of the measured values at

√s=².76 and 7 TeV [48,49], which were used for the deter- mination of the nuclear modification factorin p–Pb collisions at

√s_NN=⁵.02 TeV[40,47,50].

ThenuclearmodificationfactorforinclusiveJ/ψ productionin Pb–Pb collisions at√

s_NN=⁵.02 TeV, integratedover thecentral- ityrange0–90%,andforthe interval 2.5<y<4, p_T<12 GeV/c isR_AA(p_T<12 GeV/c)=⁰.65±⁰.01(stat)±⁰.05(syst),showinga significantsuppression oftheJ/ψ withrespecttopp collisions at thesame energy.When restrictingthe p_T rangeto 8GeV/c, cor- respondingtotheintervalcoveredinthe√

s_NN=².76 TeV results, one obtains R_AA(p_T <8 GeV/c)=⁰.66±⁰.01(stat)±⁰.05(syst). The ratio between the latter value and the corresponding one at √

s_NN = ².76 TeV, R_AA(p_T < 8 GeV/c)= ⁰.58±⁰.01(stat)± 0.09(syst)[20],is1.13±⁰.02(stat)±⁰.18(syst).Whencalculating theratio,the quoted uncertainties onthe two valuesare consid- eredasuncorrelated,exceptforthe^TAAcontribution.

Fig. 3 shows the centrality dependence of R_AA at √ s_NN = 5.02 TeV. The results are compared to the values obtained at

√s_NN =².76 TeV [20], and correspond to the same transverse-

Fig. 4.(Colouronline.) Comparisonofthecentralitydependence(with10%width centralityclasses)oftheinclusiveJ/ψ RAAfor0.3<pT<8 GeV/cwiththeoreti- calmodels[17–19,52–55].ThemodelcalculationsdonotincludethepTcut(except forTM1),whichwasanywayfoundtohaveanegligibleimpact,sincetheyonlyin- cludehadronicJ/ψproduction.Theerrorbarsrepresentthestatisticaluncertainties, theboxesaroundthedatapointstheuncorrelatedsystematicuncertainties,while thecentrality-correlatedglobaluncertaintyisshownasafilledboxaroundRAA=1.

Thebracketsshowninthethreemostperipheralcentralityintervalsrepresentthe rangeofvariationofthehadronicJ/ψ RAAunderextremehypothesisonthephoto- productioncontaminationontheinclusiveRAA.

momentumrange,pT<8 GeV/c.Thecentralitydependence,char- acterised by an increasing suppression with centrality up to N_part∼^{100,followedby an} approximatelyconstant R_AA value, is similar atthe twoenergies.Asystematicdifferenceby about15%

is visiblewhencomparingthe twosets ofresults,evenifthe ef- fect iswithin thetotaluncertaintyofthemeasurements.The RAA of promptJ/ψ wouldbe about10% higher if R^non-prompt_AA =^{0 and} about 5% (1%) smaller if R^non-prompt_AA =^{1 for central} (peripheral) collisions.

An excessofvery-low pT J/ψ,compared totheyieldexpected assumingasmoothevolutionoftheJ/ψ hadro-productionandnu- clear modification factorwas observed in peripheral Pb–Pb colli- sions at√

s_NN=².76 TeV [51]. This excess might originate from the photo-production of J/ψ andcould influence the R_AA in pe- ripheral collisions. To quantify the expected difference between the hadronicJ/ψ RAA andthe measured values the method de- scribed in[21] was adopted.The hadronicJ/ψ RAA,for 0<pT<

8 GeV/c,is estimatedtobe about 34%,17% and9% smaller than the measured values in the 80–90%, 70–80% and 60–70% cen- trality classes, respectively. The variation decreases to about 9%, 4% and2%, respectively, when considering the RAA forJ/ψ with 0.3<pT<8 GeV/c, due to the remaining small contribution of photo-produced J/ψ.Fig. 4 showsRAA asafunction ofcentrality, for0.3<p_T<8 GeV/c.

ComparingtheresultsofFig. 3andFig. 4,alesspronouncedin- creaseof RAA forperipheraleventscanindeedbeseenwhensuch aselectionisintroduced.Thesameextremehypothesesasin[21]

were made to define upper and lower limits, represented with bracketsonFig. 4.Thus,theselectionofJ/ψ with p_T>0.3 GeV/c makestheresultsmoresuitable foracomparisonwiththeoretical modelsthatonlyincludehadronicJ/ψ production.

We start by comparing the results to a calculation based on a statistical model approach [52], where J/ψ are created, like all other hadrons, only atchemical freeze-out accordingto their statistical weights. In this model, the nucleon–nucleon cc pro- duction cross section is extrapolated from LHCb pp measure- mentsat√

s=^{7 TeV[56]using}FONLL calculations[57],obtaining d

σ

_cc/dy=⁰.45 mb inthe y rangecovered bythedata.Then,the nuclearmodificationofthepartondistributionfunctions(shadow- ing) is accounted for via the EPS09 NLO parameterisation [58].

Fig. 5.(Colouronline.) TheratiooftheinclusiveJ/ψRAAfor0.3<pT<8 GeV/cbe- tween√

sNN=⁵.02 and2.76TeV,comparedtotheoreticalmodels[17–19,52–55], shownasafunctionofcentrality.Themodelcalculations donotinclude the pT cut(exceptforTM1),whichwasanywayfoundtohaveanegligibleimpact,since theyonlyinclude hadronicJ/ψ production.Theerrorbarsrepresentthe statisti- caluncertaintiesandtheboxesaroundthedatapointstheuncorrelatedsystematic uncertainties.Thecentrality-correlatedglobaluncertaintyisshownasafilledbox aroundr=^{1 andisobtainedasthequadraticsumofthe}correspondingglobalun- certaintiesat√

sNN=².76 and5.02 TeV.

The corresponding 17% uncertainty on the extrapolated d

σ

cc/dy plusshadowingisusedwhencalculatingtheuncertaintybandsfor thismodel.Theresultsarealsocomparedtothecalculationsofa transport model(TM1) [18,54,55]based on a thermalrate equa- tion, which includes continuous dissociation and regeneration of theJ/ψ both intheQGPandinthehadronicphase.Theinclusive cc cross section is taken as d

σ

_cc/dy=⁰.57 mb, consistent with FONLL calculations,while the J/ψ productioncross section value inN–Ncollisions isd

σ

J/ψ/dy=³.14 μb.Theresultsofthismodel areshownasa bandincludinga variationoftheshadowingcon- tribution between 10% and 25% and a 5% uncertainty on the cc crosssection.Theresultsarethencomparedtothecalculationsof asecondtransportmodel(TM2)[19],whichimplementsahydro- dynamicdescriptionofthemediumevolution.Theinputnucleon–

nucleon cross sections for cc and J/ψ are taken as d

σ

cc/dy= 0.82 mb, corresponding to theupper limit ofFONLL calculations, andd

σ

J/ψ/dy=³.5 μb.Alsoforthismodeltheband corresponds tothechoice ofeithernoshadowing, orashadowingeffect esti- matedwiththeEPS09NLO parameterisation.Finally,thedataare comparedtoa‘co-mover’model[17,53],wheretheJ/ψ aredisso- ciated via interactions withthe partons/hadrons producedin the same rapidity range, using an effective interaction cross section

σ

^co-J/ψ=⁰.65 mb,basedoncalculationsthatdescribedlower en- ergyexperimentalresults.Regenerationeffectsareincluded,based ond

σ

cc/dy valuesrangingfrom0.45to0.7mb,whichcorrespond totheuncertainty bandshownforthe model.Shadowingeffects, calculatedwithintheGlauber–Gribovtheory[59],areincludedand are consistent with EKS98/nDSg predictions [60,61]. Finally, the contributionofnon-promptproductionistakenintoaccountinthe transportmodelsTM1andTM2,whileitisnot consideredinthe othercalculations.

The data are described by the various calculations, the latter havingrather largeuncertainties, due tothe choice ofthe corre- sponding input parameters, and in particular of d

σ

_cc/dy. It can be notedthat for mostcalculationsa better description is found whenconsideringtheirupperlimit.Fortransportmodelsthiscor- respondsto a minimum contributionor even absenceof nuclear shadowing, which can be clearly considered as an extreme as- sumptionfor primary J/ψ, considering the J/ψ measurements in p–Pbcollisions[47,50].

Fig. 6.(Colouronline.) The pT dependenceofthe inclusiveJ/ψ RAA at √ sNN= 5.02 TeV,comparedtothecorrespondingresultat√

sNN=2.76 TeV[20]andtothe calculationofatransportmodel[18,54,55](TM1),inthecentralityinterval0–20%.

The pTdependence ofrisalso shownforbothdataandtheory.Theerrorbars representstatisticaluncertainties,theboxesaroundthepointsuncorrelatedsystem- aticuncertainties,whilepT-correlatedglobaluncertaintiesareshownasafilledbox aroundRAA=1.

Acorrelationbetweentheparametersofthemodelsispresent whencomparingtheir calculationsfor√

sNN=².76 and5.02TeV.

Therefore, the theoretical uncertainties can be reducedby form- ing the ratio r=^RAA(5.02 TeV)/R_AA(2.76 TeV). Concerning data, the uncertainties on ^TAA ^{cancel. In Fig. 5 the centrality de-} pendence of r, calculated for 0.3<pT<8 GeV/c, is shown and compared tomodels. Forprompt J/ψ the ratior wouldbe about 2% (1–2%) higher if beauty hadrons were fully (not) suppressed by the medium. The transport model of Ref. [18,54,55] (TM1) showsadecreaseofrwithincreasing centrality,duetothelarger suppression effects at high energy, followed by an increase, re- lated to the effect of regeneration, which acts in the opposite directionand becomes dominantfor central collisions. The other transport model (TM2)[19] also exhibits an increase forcentral collisions, while for peripheral collisions the behaviour is differ- ent. In the co-mover model [17,53], no structure is visible as a functionofcentrality,andthecalculationfavours r-valuesslightly belowunity,implying thatinthismodeltheincrease ofthesup- pression effects withenergy maybe dominant over theregener- ation effects forall centralities. Finally,the statisticalmodel [52]

shows a continuous increase of r with centrality, dominated by the increase inthe cc crosssection with energy.The uncertainty bands shown in Fig. 5 correspond to variations of about 5% in thecc crosssectionat√

s_NN=⁵.02 TeV,plusa10%relativevaria- tion of the shadowingcontribution between thetwo energies in the case of TM1. The data are, within uncertainties, compatible withthetheoretical models,andshow noclearcentralitydepen- dence. The ratiofor central collisions and 0.3<pT<8 GeV/c is r^0–10%=¹.17±⁰.04(stat)±⁰.20(syst).

Finally,the studyofthe p_T dependenceof R_AA has proven to be a sensitive test of the presence ofa regeneration component which,incalculations,leadstoanincreaseatlow pT.Fig. 6shows, forthe centralityinterval 0–20%, RAA asa function oftransverse momentum, compared to the corresponding results obtained at

√s_NN=².76 TeV, andto a theoretical modelcalculation. The re- gion pT<0.3 GeV/c was not excluded, because the contribution ofJ/ψ photo-productionisnegligiblewithrespecttothehadronic oneforcentralevents[51].Inthesamefigurethe pT dependence ofrisalsoshown.AhintforanincreaseofRAA with√

sNN isvis- ibleintheregion2<p_T<6 GeV/c,whilether-ratioisconsistent

withunityelsewhere.Thisfeatureisqualitativelydescribedbythe theoreticalmodel(TM1)alsoshowninthefigure.ThepromptJ/ψ RAAisexpectedtobe7%larger(2%smaller)forpT<1 GeV/c and 30%larger(55%smaller)for10<p_T<12 GeV/c whenthebeauty contribution is fully (not) suppressed. Assuming that R^non-prompt_AA does not vary significantly between the two collision energies, the ratio r appears to be less sensitive to the non-prompt J/ψ contribution.The effectis negligible forthe caseoffull suppres- sion of beauty hadrons, while it varies from no increase at low transverse momentum up to a maximum increase of about 15%

for5<pT<6 GeV/c ifnosuppressionisassumed. Thetransport modelofRef.[18,54,55](TM1)fairlydescribestheoverallshapeof theR_AA p_Tdependence.

5. Conclusion

We reportedthe ALICEmeasurement ofinclusiveJ/ψ produc- tion in pp and Pb–Pb collisions at √

sNN=⁵.02 TeV at the LHC.

A systematic difference by about15% isvisible when comparing the R_AA measured at √

s_NN=⁵.02 TeV to the one obtained at

√sNN=².76 TeV, even if such an effect is within the total un- certainty of themeasurements. When removing very-low pT J/ψ (p_T<0.3 GeV/c), the R_AA showsa lesspronounced increase for peripheralevents,whichcanbeascribedtotheremovalofalarge fractionof electromagnetic J/ψ production [51]. Theseresults, as well as those on the ratio of the nuclear modification factors between √

sNN=⁵.02 and 2.76 TeV, are described by theoreti- cal calculations, and closerto their upper limits. The pT depen- dence of R_AA exhibits an increase at low p_T, a feature that in the model which is compared to the data is related to an im- portantcontributionofregenerated J/ψ.A hintforan increase of RAA between√

sNN=².76 and 5.02 TeV is visible in the region 2<p_T<6 GeV/c,whiletheresultsareconsistentelsewhere.The resultspresentedinthispaperconfirmthatalsoatthehighesten- ergiesreachedtodayattheLHC,dataonJ/ψ productionsupporta picturewhereacombinationofsuppressionandregenerationtakes placeintheQGP,thetwomechanismsbeingdominantathighand low pT,respectively.

Acknowledgements

The ALICE Collaboration would like to thank all its engineers andtechnicians fortheir invaluablecontributionstotheconstruc- tion of the experiment and the CERN accelerator teams for the outstanding performance of the LHC complex. The ALICECollab- oration gratefully acknowledges the resources and support pro- vided by all Grid centres and the Worldwide LHC Computing Grid(WLCG)Collaboration.The ALICECollaborationacknowledges the following funding agencies fortheir support in building and running the ALICE detector: A. I. Alikhanyan National Science Laboratory (Yerevan Physics Institute) Foundation (ANSL), State Committee of Science and World Federation of Scientists (WFS), Armenia; Austrian Academy of Sciences and Nationalstiftung für Forschung, Technologie und Entwicklung, Austria; Conselho Na- cional de Desenvolvimento Científico e Tecnológico (CNPq), Uni- versidadeFederal doRioGrande doSul(UFRGS), Financiadorade Estudos e Projetos (Finep) and Fundação de Amparo à Pesquisa doEstado de São Paulo (FAPESP),Brazil; Ministryof Science and Technology of the People’s Republic of China (MOST), National Natural Science Foundation of China (NSFC) and Ministry of Ed- ucation of China (MOE), China; Ministry of Science, Education and Sport and Croatian Science Foundation, Croatia; Ministry of Education, Youth and Sports of the Czech Republic, Czech Re- public; The Danish Council for Independent Research – Natural Sciences, the CarlsbergFoundation andDanish NationalResearch

Foundation (DNRF), Denmark; Helsinki Institute of Physics (HIP), Finland; Commissariat à l’Énergie Atomique et aux Énergies Al- ternatives (CEA) and Institut National de Physique Nucléaire et de Physique des Particules (IN2P3) and Centre National de la Recherche Scientifique(CNRS),France;Bundesministeriumfür Bil- dung, Wissenschaft, Forschung und Technologie (BMBF) and GSI Helmholtzzentrum für Schwerionenforschung GmbH, Germany;

Ministry of Education, Research and Religious Affairs, Greece;

National Research, Development and Innovation Office, Hungary;

Department of Atomic Energy, Government of India (DAE) and Council of Scientific and Industrial Research (CSIR), New Delhi, India; IndonesianInstitute of Science, Indonesia; Centro Fermi – MuseoStorico dellaFisica eCentroStudi e RicercheEnricoFermi andIstitutoNazionalediFisicaNucleare (INFN),Italy;Institutefor Innovative Science and Technology, Nagasaki Institute of Applied Science (IIST), Japan Society for the Promotion of Science (JSPS), KAKENHIandJapaneseMinistryofEducation,Culture, Sports,Sci- enceand Technology (MEXT),Japan; Consejo Nacionalde Ciencia y Tecnología (CONACYT), through Fondo de Cooperación Interna- cional enCiencia yTecnología(FONCICYT)andDirección General de Asuntos del Personal Academico (DGAPA), Mexico; Nationaal instituut voor subatomaire fysica (Nikhef), Netherlands; The Re- search Council of Norway, Norway; Commission on Science and Technology forSustainableDevelopmentintheSouth(COMSATS), Pakistan;PontificiaUniversidadCatólicadelPerú,Peru;Ministryof ScienceandHigherEducationandNationalScienceCentre,Poland;

KoreaInstituteofScienceandTechnologyInformationandNational ResearchFoundationofKorea(NRF),RepublicofKorea;Ministryof Education andScientific Research,Institute ofAtomicPhysicsand Romanian National Agency for Science, Technology and Innova- tion,Romania;JointInstituteforNuclearResearch(JINR),Ministry of Education andScience of theRussian Federation andNational Research Centre Kurchatov Institute, Russia; Ministry of Educa- tion,Science,ResearchandSportoftheSlovakRepublic,Slovakia;

National ResearchFoundation ofSouth Africa, South Africa; Cen- tro de Aplicaciones Tecnológicas y Desarrollo Nuclear (CEADEN), Cubaenergía,Cuba; Ministerio deCienciaeInnovación andCentro de Investigaciones Energéticas, Medioambientales y Tecnológicas (CIEMAT),Spain;SwedishResearchCouncil(VR) andKnut&Alice WallenbergFoundation(KAW),Sweden;EuropeanOrganizationfor Nuclear Research, Switzerland; National Science and Technology Development Agency (NSDTA), Suranaree University of Technol- ogy (SUT) andOffice of theHigher EducationCommission under NRU projectofThailand,Thailand;TurkishAtomicEnergy Agency (TAEK),Turkey;NationalAcademyofSciencesofUkraine,Ukraine;

ScienceandTechnologyFacilitiesCouncil(STFC),UnitedKingdom;

NationalScienceFoundationoftheUnitedStatesofAmerica(NSF) andUnitedStatesDepartmentofEnergy,OfficeofNuclear Physics (DOENP),UnitedStatesofAmerica.

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