Centrality, rapidity and transverse momentum dependence of J/ψ suppression in Pb-Pb collisions at √sNN= 2.76TeV

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Physics Letters B

www.elsevier.com/locate/physletb

Centrality, rapidity and transverse momentum dependence of J /ψ suppression in Pb–Pb collisions at √

s _NN = ² . 76 TeV

.ALICE Collaboration

^∗

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

Articlehistory:

Received3November2013

Receivedinrevisedform15May2014 Accepted20May2014

Availableonline27May2014 Editor:L.Rolandi

Keywords:

Relativisticheavyioncollisions Quarkgluonplasma Quarkonium J/ψsuppression Experimentalresults

The inclusiveJ/ψ nuclear modificationfactor(RAA) inPb–Pbcollisions at√_s

NN=².76 TeV has been measured byALICE as afunctionof centralityinthe e⁺e⁻ decaychannel atmid-rapidity(|^y|<0.8) and as a function ofcentrality, transverse momentum and rapidity in the μ⁺μ⁻ decay channel at forward-rapidity (2.5<y<4). TheJ/ψ yields measuredin Pb–Pbare suppressedcomparedto those in pp collisionsscaled bythenumber ofbinarycollisions.The RAAintegrated overacentralityrange correspondingto90%oftheinelasticPb–Pbcrosssectionis0.72±⁰.06(stat.)±⁰.10(syst.)atmid-rapidity and0.58±⁰.01(stat.)±⁰.09(syst.)atforward-rapidity.Atlowtransversemomentum,significantlylarger values of RAA are measured at forward-rapidity compared to measurements at lower energy. These features suggest thatacontribution tothe J/ψ yieldoriginates fromcharmquark (re)combination in thedeconfinedpartonicmedium.

©^{2014TheAuthors.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense} (http://creativecommons.org/licenses/by/3.0/).FundedbySCOAP³.

1. Introduction

The theory ofQuantum Chromodynamics (QCD) predicts that the hot anddense nuclear matter produced during the collision of ultra-relativisticheavy nucleibehaves asa deconfined Plasma ofQuarksandGluons(QGP). Thisphaseofmatter existsforonly a short time before the fireball cools down and the process of hadronization takes place. Heavy quarks are an important probe oftheQGPsincetheyareexpectedtobeproducedonlyduringthe initialstageofthecollisioninhard partonicinteractions, thusex- periencingtheentireevolutionofthesystem.Itwaspredictedthat inahotanddensedeconfinedmediumliketheQGP,boundstates of charm (c)and anti-charm (c)¯ quarks, i.e. charmonia, are sup- pressedduetothescreeningeffectsinducedbythehighdensityof colorcharges[1].Therelativeproductionprobabilitiesofcharmo- niumstateswithdifferentbindingenergiesmayprovideimportant informationonthepropertiesofthismediumand,inparticular,on itstemperature[2,3].Among thecharmoniumstates,the strongly boundJ/ψ isofparticularinterest.The J/ψ productionisacom- bination from prompt andnon-prompt sources. The prompt J/ψ yield consists of the sum of direct J/ψ (≈^{65%) and excited c}¯c states such as

χ

c andψ(2S) decaying into J/ψ+^{X (}≈^{35%) [4].}

Theseexcited stateshavea smallerbinding energythanthe J/ψ. Non-prompt J/ψ productionis directly relatedto beauty hadron productionwhoserelative contributionincreaseswiththe energy of the collision. Experimentally, J/ψ production was studied in

*

^Forcorrespondence,pleaseusee-mailaddress:[email protected].

heavy-ioncollisions attheSuperProton Synchrotron(SPS) andat the RelativisticHeavyIonCollider(RHIC),coveringa largeenergy range from about 20 to 200 GeV center-of-mass energy per nu- cleon pair (√

sNN). A suppression of the inclusive J/ψ yield in nucleus–nucleus(A–A)collisionswithrespecttotheonemeasured in proton–proton (pp) scaled by the number of binary nucleon–

nucleon collisions was observed. In the most central events, the suppressionisbeyondtheoneinduced bycoldnuclearmatteref- fects (CNM), such asshadowing andnuclear absorption, atboth SPS[5,6]andRHIC[7].AttheSPStheJ/ψsuppressioniscompat- ible withthemeltingoftheexcited stateswhereastheRHICdata suggest a small amount of suppression for the direct J/ψ [8,9].

Similar predictions on sequential suppression [3] were madefor thebottomoniumfamily,whichhasbecomeaccessibleattheLarge HadronCollider(LHC)energies. Thesequentialsuppressionofthe Υ (1S),Υ (2S)andΥ (3S)stateswasfirstobservedbytheCMSex- perimentinPb–Pbcollisionsat√

sNN=².76 TeV[10].

The first ALICE measurement of the inclusive J/ψ production incentralPb–Pbcollisionsat√

s_NN=².76 TeV atforward-rapidity has shown lesssuppression compared to PHENIX results in cen- tralAu–Aucollisionsat√

sNN=⁰.2 TeV[11].At√

sNN=².76 TeV, thecharmquarkdensityproducedinthecollisionsincreaseswith respectto SPSandRHICenergies [12].Thismayresultintheen- hancementoftheprobabilitytocreateJ/ψmesonsfrom(re)com- bination of charm quarks [13,14]. If the J/ψ mesons are fully suppressed in the QGP, their creation will take place at chem- ical freeze-out (near the phase boundary) as detailed in [13,15, 16].IfJ/ψ mesonssurviveintheQGP,productionmaytakeplace continuously during the QGP lifetime [14,17,18]. Because of the http://dx.doi.org/10.1016/j.physletb.2014.05.064

0370-2693/©^{2014TheAuthors.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense}(http://creativecommons.org/licenses/by/3.0/).Fundedby SCOAP³.

large increase of the c¯c cross-section towards LHC energy the (re)combination mechanismmaybecomedominantthere.Accord- ing to statistical[13] and partonic transport [17,18] models, this contributionleads to an increase ofthe R_AA atthe LHCwithre- spect to the one observed at RHIC. In particular, this scenario predicts an increase of the RAA from forward- to mid-rapidity, wherethedensityofcharm quarksis higher.Furthermore,inor- der to (re)combine, two charm quarks need to be close enough inphasespace,sothatlowtransversemomentumJ/ψproduction isexpected to be favored. The transverse momentum andrapid- itydependence of the J/ψ RAA are therefore crucialobservables to sharpen the interpretation of the results, providing a deeper insightonthebalancebetweenJ/ψ (re)combinationandsuppres- sion.

In thisLetter, we present results on the nuclear modification factorforinclusiveJ/ψinPb–Pbcollisionsat√

sNN=².76 TeV asa functionofcollisioncentrality,transversemomentumandrapidity.

Complementarytoourresults,J/ψsuppressionatlargetransverse momentum in Pb–Pb collisions, was reported previously by AT- LAS[19]andCMS[20].

2. Experimentalapparatusanddatasample

ALICEisageneralpurposeheavy-ionexperiment.Adetailedde- scriptionof the experimental apparatus can be found in [21]. It consistsof a central barrel covering the pseudo-rapidity interval

|

η

_|<0.9 andamuonspectrometercovering−⁴<

η

<−².5.¹ J/ψ production is measured in both rapidity ranges: at mid-rapidity in the dielectron decay channel and at forward-rapidity in the dimuondecaychannel.Inboth casestheJ/ψ transverse momen- tum(pT)coverageextendsdowntozero.

Atmid-rapidity,thedetectorsusedfortheJ/ψanalysisarethe Inner Tracking System(ITS) [22] and the Time Projection Cham- ber (TPC) [23].The ITSis composed of six concentriccylindrical layers of silicon detectors withradii ranging from 3.9 to 43 cm withrespecttothebeamaxis.Itsmainpurposeistoprovidethe reconstruction of the primary interaction vertex as well as sec- ondarydecayverticesofheavy flavoredparticles. Inaddition,the twoinnermostlayerscanprovideaninputatlevelzero(L0)tothe triggersystem.TheTPC,withan activevolumeextendingfrom85 to247 cmintheradialdirection,isthemaintrackingdetectorof thecentralbarrelandalso providesparticleidentificationvia the measurement ofthe specific energy loss (dE/dx) in the detector gas.

At forward-rapidity, the J/ψ analysis is carried out using the muonspectrometer[24].Thespectrometerconsistsofateninter- action length front absorber, filtering the muons in front of five tracking stations made of two planes of cathode pad chambers each. The third station is located inside a dipole magnet with a 3 Tmfieldintegral.ThespectrometeriscompletedbyaMuonTrig- gersystem(MTR) madeoftwo stations,each equippedwithtwo planesofresistiveplatechambers.Thetriggerchambersareplaced behind a1.2 m thick iron wall to stop secondaryhadrons escap- ing from the front absorber and low momentum muons coming mainlyfrom

π

andKdecays.Throughoutitsfulllength,aconical absorbermadeoftungsten,leadandsteelprotectsthemuonspec- trometeragainst secondaryparticles generated bythe interaction withthebeampipeofprimaryparticlesproducedatlarge

η

.

Additionalforwarddetectors,theVZERO[25]andtheZeroDe- gree Calorimeters (ZDC) [26], are used for triggering and event

1 IntheALICEreferenceframe,themuonspectrometercoversanegativeηrange andconsequentlyanegativeyrange.Wehavechosentopresentourresultswitha positiveynotation.

characterization.TheVZEROdetectoriscomposedoftwoscintilla- torarrays,32channelseach,placedonbothsidesoftheInteraction Point(IP).Itcovers2.8≤

η

≤⁵.1 (VZERO-A)and−³.7≤

η

≤ −¹.7 (VZERO-C). The ZDCare located ata distance of 114 mon both sidesoftheIPandcandetectspectatorneutronsandprotons.

TheresultspresentedinthisLetterarebasedondatacollected duringthe2010and2011LHCPb–Pbrunsforthedielectronanal- ysis andon data collected in the 2011run forthe dimuon one.

Forward-rapidityresultsinthedimuonchannelfromthe2010data set,basedonanintegratedluminosityabout25timessmallerthan the 2011 data set, have been published previously in [11]. The minimumbias(MB)triggerforthe2011datasetisdefinedbythe coincidenceofsignalsinthetwoVZEROarrayssynchronizedwith thepassageoftwo crossingPbbunches.Inthe2010dataset,the MB triggerhadan additionalrequirementon hitsintheITS. The twoMBtriggerdefinitions,however,leadtoverysimilartriggeref- ficiencies,whicharelargerthan95%forinelasticPb–Pbcollisions.

Electromagnetic interactions are rejected atthe level one trigger (L1)byapplyingacutontheminimumenergydepositedbyspec- tator neutrons in the ZDC. Beam induced background is further reducedattheofflinelevelbyapplyingtimingcutsonthesignals fromtheVZEROandZDCdetectors.

At mid-rapidity, the 2010 data sample used in the electron analysisconsistsof15millioneventscollectedwiththeMBtrigger, corresponding to an integratedluminosity of2.1 μb⁻¹.The 2011 event sample was enriched with central and semi-central Pb–Pb collisionsby usingthresholdsontheVZEROmultiplicityattheL0 trigger.Theinspectedintegratedluminosityamountsto25.6 μb⁻¹, out ofwhichweanalyzed20million central(0%–10%ofthecen- tralitydistribution) and20million semi-central(10%–50%)events.

Thesummed2010and2011datasetscorrespondtoan integrated luminosity of Lint=27.7±0.4(stat.)⁺₋²₁^._.²₈(syst.

σ

Pb–Pb) μb⁻¹. At forward-rapidity, the2011data sample ismadeofabout17mil- lion

μμ

MB triggers.The

μμ

MB trigger is definedas the occur- rence of the MB condition in coincidence with the detection in the MTR of two opposite-sign muons tracks. The MTR is capa- bleof (i) deliveringL0 triggerdecisionsat 40 MHzbasedon the detection of one or two muon trigger tracks, (ii) computing an approximate value ofthe transverse momentum of muon trigger tracks(p^trig_T )and(iii)applyingathreshold² onthep^trig_T .A1 GeV/c threshold,appliedonbothmuons, waschosentocollectthisdata sample. A scaling factor F_norm is used to obtain the number of equivalent MB events fromthe numberof

μμ

MB ones. It isde- fined as the ratio, in a MB data sample, of the number of MB eventsdividedbythenumberofeventsfulfillingthe

μμ

MB trig- ger condition. Its value, averaged over the entire data sample, is Fnorm=³⁰.56±⁰.01(stat.)±¹.10(syst.).Theintegratedluminosity used inthis analysisis thereforeLint=^NμμMB×^Fnorm/

σ

Pb–Pb= 68.8±⁰.9(stat.)±².5(syst.F_norm)⁺₋⁵₄^._.⁵₅(syst.

σ

Pb–Pb)μb⁻¹assuming aninelasticPb–Pbcross-section

σ

Pb–Pb=⁷.7±⁰.1⁺₋⁰₀^._.⁶₅b[26].

The centrality determination is based on a fit to the VZERO amplitude distribution asdescribed in[27]. Thefit, basedon the Glauber model,allows forthe extractionof collision-relatedvari- ablessuchastheaverageofnumberofparticipantnucleons^Npart andtheaverageofthenuclearoverlapfunction^TAA^{per central-} ityclass.NumericalvaluesaregiveninTable 1.Boththe electron and muon analyses were carried out on an eventsample corre- sponding to the most central 90% of the inelastic Pb–Pb cross- section. In this centrality range the efficiency of the MB trigger is100% andthecontamination fromelectromagneticprocesses is negligible.

2 Thethresholdisdefinedas p^trig_T forwhichthetriggerprobabilityis50%and doesnotleadtoasharpcutinpT.

Table 1

Theaverageofnumberofparticipatingnucleons^{Npartandtheaveragevalueof} thenuclearoverlapfunctionTAAwiththeirassociatedsystematicuncertaintyfor thecentralityclasses,expressedinpercentagesofthenuclearcross-section[27], usedintheseanalyses.

Centrality ^Npart ^TAA^(mb−1)

0%–10% 356.0±³.6 23.44±⁰.76

10%–20% 260.1±³.8 14.39±⁰.45

20%–30% 185.8±³.3 8.70±⁰.27

30%–40% 128.5±².9 5.00±⁰.18

40%–50% 84.7±2.4 2.68±0.12

50%–60% 52.4±1.6 1.317±0.071

60%–70% 29.77±⁰.98 0.591±⁰.036

70%–80% 15.27±⁰.55 0.243±⁰.016

80%–90% 7.49±⁰.22 0.0983±⁰.0076

0%–20% 308.1±³.7 18.91±⁰.61

10%–40% 191.5±3.3 9.36±0.30

40%–90% 37.9±1.2 0.985±0.051

0%–90% 124.4±2.2 6.27±0.21

3. Dataanalysis

J/ψcandidatesareformedbycombiningpairsofopposite-sign (OS)electronandmuontracksreconstructedinthecentral barrel andinthemuonspectrometer,respectively.

Electron candidates are selected by cutting on the quality of tracks reconstructed in the ITS andthe TPC. The selection crite- riaare very similar tothose used in the previous analysisof pp collisionsat√

s=^{7 TeV[24] usingatighterselectiononelectron} identification.Ahit inoneofthetwoinnermostlayers oftheITS is required. This rejects a large fraction of background resulting fromphoton conversions in thedetector material.The tracks are required to have at least 70 out of a maximum of 159 clusters intheTPCandto passaquality cut basedonthe

χ

² of theTPC trackfitdivided by thenumberof clustersattachedtothe track.

Electronidentificationisdone usingtheTPC, requiringthe dE/dx signal to be compatible with the electron expectation within a band of (−².0;+³.0)

σ

or (−¹.5;+³.0)

σ

for the 2010 or 2011 data, respectively, where

σ

denotes the resolution of the dE/dx measurement.Duetoalower dE/dxresolutionforthe2011data, for|

η

_|<0.5 amore restrictiveelectron selection, (−⁰.9;+³.0)

σ

, isapplied.The electron/hadronseparationisfurther improvedby rejecting tracks which are compatible with the pion expectation within3.5

σ

andwiththeprotonexpectationwithin3.5

σ

or4.0

σ

in2010or2011data,respectively.SincetheelectronsfromaJ/ψ decayhaveamomentumof1.5 GeV/c inthemotherparticlerest frame,acutofp_T>0.85 GeV/conthecandidatetracksisapplied torejectthecombinatorialbackgroundfromlowmomentumelec- trons.Finally,toensuregoodtrackingandparticleidentificationin theTPC,onlycandidateswithin|

η

_|<0.8 areselected.

Muontracksarereconstructedinthemuonspectrometerasde- tailed in [24] for pp collisions. This procedure remains basically unchangedfor Pb–Pb collisions. However, to cope withthe large background incentral events, some selection criteriawere tight- ened compared to the pp analysis. The search area for finding clustersassociated to tracks is reducedby a factorof nine, both muon candidates have to match a track segment in the trigger chambers and the track pseudo-rapidity has to be in the range

−⁴<

η

<−².5. A further cut on thetrack transverse coordinate atthe endof the front absorber (R_abs) is applied (17.6≤^Rabs≤ 89.5 cm) toensurethatmuonsemittedatsmallangles,i.e.those that have crossed a significant fractionof the thick beamshield, are rejected. Finally, to remove events very close to the edge of thespectrometeracceptance,onlymuonpairsintherapidityrange 2.5<y<4 areaccepted.

In the e⁺e⁻ decay channel, the J/ψ yields are extracted by countingthenumberofentriesintheinvariantmassrange2.92<

me⁺e⁻ <3.16 GeV/c² after subtracting the combinatorial back- ground. Due to the radiative decay channel and the energy loss ofthe electronsinthedetectormaterial viabremsstrahlung, only

≈^{68% oftheJ}/ψ arereconstructedwiththemassinthecounting massinterval.Thebackgroundshapeisobtainedusingthemixed- event(ME) technique.Uncorrelatedlepton pairs arecreatedfrom different Pb–Pb events that have similar global properties such as centrality,primary vertexposition andeventplane angle. The backgroundshapefromMEisscaledtomatchthesame-event(SE) invariantmassdistributionintheranges1.5<m_e+e⁻<2.5 GeV/c² and 3.2<m_e+e⁻ <4.2 GeV/c². These mass ranges were chosen such that they are close to the signal region and have equal number of bins on each side of the signal region. The lower (1.5 GeV/c²) andupper(2.5 GeV/c²)limitsofthefirstmassrange are chosen in order to avoid sensitivity to correlated low-mass dielectronpairsandJ/ψbremsstrahlungtail,respectively.Thesec- ond massrangeis limitedby theupperlimitofthesignal region (3.2 GeV/c²) andextends to4.2 GeV/c² to matchinsizethe first massinterval.Heretheinfluenceoftheψ(2S)onthebackground matching procedure is neglected since the ψ(2S) dileptonyields areexpectedtoberoughly60timessmallerthanJ/ψ yields(esti- mation basedonLHCbmeasurementsofJ/ψ [28]andψ(2S)[29]

cross-sectionsinpp collisionsat7 TeV).Agoodmatchingbetween the SEandME distributionsisobservedover abroadmassrange outsidetheJ/ψ massregion,asvisibleinthetoppanelsofFig. 1.

This is a clear sign that the contribution of correlated pairs to the OS mass spectrum is small withrespect to the uncorrelated background or has a similar shape. The bottom panels of Fig. 1 show thebackground-subtractedinvariantmassspectracompared to theJ/ψ signal shapefromaMonte-Carlo (MC)simulation.The bremsstrahlungtail fromtheelectron energylossin thedetector material and theJ/ψ radiative decaychannel (J/ψ →^e+e−

γ

) is welldescribedintheMC.AsshowninFig. 1,itispossibletostudy theJ/ψproductioninthreecentralityintervals(0%–10%,10%–40%, 40%–90%),withasignal-to-backgroundratio(S/B),evaluatedinthe range 2.92<m_e+e⁻<3.16 GeV/c², increasing from 0.02 to 0.25 fromcentraltoperipheralcollisions.

Inthe

μ

⁺

μ

⁻ decaychannel,theJ/ψ rawyieldisextractedin eachcentralityandkinematicintervalbyusingtwodifferentmeth- ods.Inthefirstapproach,theOSdimuoninvariantmassdistribu- tionisfittedwiththesumofanextendedCrystalBall(CB2)func- tionto describethesignal,andaVariableWidthGaussian(VWG) function for the background.The CB2 function extends the stan- dardCrystalBall(Gaussianpluspower-lawtailatlowmasses[30]) by an additional power-law tail athigh masses withparameters independent ofthelow massones. The VWGfunction isa Gaus- sian functionwithafourthparametertoallow linearvariationof the widthwith the invariant mass of the dimuon pair. The J/ψ signal isclearly visibleinallcentrality, pT or yintervalsevenbe- fore any background subtraction, as can be observed in the top panels of Fig. 2, where examples of invariant mass spectra fits in selected pT intervals are shown. The signal-to-background ra- tio, evaluated within 3 standard deviations with respect to the J/ψ polemass, variesfrom 0.16atlow pT up to 1.2at high pT. The corresponding values incentralityand y intervals are inthe range0.16–6.5 fromcentral toperipheralcollisions and0.19–0.59 from low to high rapidity. In all cases, the significance is larger than 10. In the second approach, the combinatorial background was subtracted usingan event-mixingtechnique.The background shape obtained from ME was normalized to the data through a combinationofthemeasuredlike-signmuonpairsfromSE.Fig. 2 (bottompanels) showsthe resultingmassdistributionfittedwith the sum of a CB2 andan exponential which accounts for resid-

Fig. 1.(Coloronline.)Top panels:invariantmassdistributionsforopposite-sign(OS)andmixed-events(ME)electronpairs.Bottompanels:OSinvariantmassspectra,after thesubtractionoftheMEdistributions,withacomparisontotheMonteCarlosignal(solidlines)superimposed.TheMCsignalisscaledtomatchtheintegraloftheOS distributionwithinthemasscountingwindow.Fromlefttoright,distributionscorrespondtothecentralityranges0%–10%,10%–40%and40%–90%,respectively.Thepanels forthe0%–10%and10%–40%centralityrangesareobtainedfromthe2011data,whiletheonesforthe40%–90%centralityrangeareobtainedfromthe2010data.Thetwo verticaldashedlinesshownineachpanelindicatethemassintervalusedforsignalcounting.

Fig. 2.(Coloronline.)Top panels:fitsofthedimuoninvariantmassspectrainselectedpTintervals.Bottompanels:idemaftersubtractionofthecombinatorialbackground withtheeventmixingtechnique.Distributionscorrespondtothecentralityclass0%–90%and2.5<y<4.

ualcorrelatedbackground.Inbothapproaches,thepositionofthe peak ofthe CB2 function (mJ/ψ), as well asits width(

σ

J/ψ), are freeparametersofthefit.Theirvalues,obtainedbyfittingthein- variant mass spectrum integrated over pT, y and centrality, are mJ/ψ=³.103±⁰.001 GeV/c² (shiftedup by 0.2%withrespectto thePDG mass [31]) and

σ

J/ψ=⁰.071±⁰.001 GeV/c². More de-

tails aboutthefittingprocedures andthe differentparameters of thesignalandbackgroundlineshapesarediscussedinSection4.

The measured number of J/ψ (N_Jⁱ_/ψ) in a centralityclass i is normalized tothe corresponding number ofMB events falling in thecentralityclass(Nⁱ_events) andfurthercorrectedforthebranch- ingratio(BR)ofthedileptondecaychannel,theacceptance Aand

Table 2

InclusiveJ/ψproductioncross-sectionsatmid-rapidityusedintheinterpolationprocedure.TheJ/ψareassumedunpolarizedandthesystematicuncertaintiesdonotinclude thecontributionfromunknownpolarization.

Experiment Collision energy√

s Rapidity range Blldσ/dyaty=0 stat. syst. Reference

(TeV) (nb) (nb) (nb)

PHENIX 0.2 |y|<0.35 44.30 1.40 6.80 [38]

CDF 1.96 |y|<0.6 201.6 1.0 ¹⁷₋₁₆^.8_.3 [39]

ALICE 2.76 |y|<0.9 255.8 58.7 45.9 [40]

ALICE 7 |y|<0.9 409.9 36.8 59.4 [24,41]

Interpolation 2.76 |^y|<0.8 252.6 16.4 25.8 this work

theefficiency

ⁱ ofthedetector.Inthe

μ

⁺

μ

⁻ analysis, N_eventsⁱ is computedby multiplying the number of

μμ

MB triggered events bytheFnormfactor(describedinSection2)scaledbythewidthof thecentralityclass i.The inclusiveJ/ψ yieldforthemeasured pT andy rangesisthengivenby:

Y_Jⁱ_/ψ

=

^N

iJ/ψ

BR_J/ψ→l+l⁻Nⁱ_eventsA

×

ⁱ

.

(1)

The acceptancetimes efficiency product (A×

ε

) isdefined as the ratio between the number of reconstructed J/ψ divided by thenumberofgeneratedonesinthe kinematicrangeunderstudy.

In the e⁺e⁻ decay channel, A×

ε

is calculated fromMC simu- lations. These MC events are a superposition ofPb–Pb collisions generated withan appropriate HIJING [32] tune reproducing the measured charged particle density [33] and J/ψ generated from parametrized pT and y distributions (details in Section 4). The J/ψ dielectron decays are performed using PHOTOS [34,35]. The particles are then transported through a simulation of the AL- ICE detector using GEANT3.21 [36]. The geometrical acceptance is about 34%. The estimated integrated A×

ε

for the J/ψ emit- tedin|^y|<0.8 amountto0.080,0.085and0.093forthe0%–10%, 10%–40%and40%–90% centralityclassesinthe2010datasample and0.026and0.028forthe0%–10%and10%–40%centralityclasses inthe2011one, respectively.The largedifference inefficiencyis mainlydue toa lower efficiencyofthe two innermostITSlayers andstrongerparticleidentificationcutsusedforthe2011dataset.

Inthe

μ

⁺

μ

⁻decaychannel,A×

ε

wascomputedusinganembed- ding technique whereMC J/ψ particlesare injected intothe raw dataofrealeventsandthenreconstructed.TheMCJ/ψ pT and y parametrizationis takenfrom actual measurements.PYTHIA [37]

takes care of the J/ψ decays and the daughter particles signals inthe detector,givenby GEANT3.21, are then addedto the real Pb–Pbevents.When studiedasafunctionofcentrality, A×

ε

de- creasesby7.9%,from0.127inthe80%–90%centralityclassto0.117 inthe0%–10% one. Thecentralityintegrated A×

ε

(0%–90%cen- tralityclass)is0.120 witha negligiblestatisticaluncertainty. The geometricalacceptanceisabout18%.Thequantity A×

ε

showsa non-monotonicdependenceonp_T,startingatapproximately0.124 at zero transverse momentum, reaching a minimum of 0.103 at 1.5 GeV/c andthenlinearlyincreasingupto0.264at8 GeV/c.The rapidity dependenceof A×

ε

reflects thegeometrical acceptance ofthemuon pairswithamaximum of0.189centered at y=³.3 decreasingtowardstheedgesoftheacceptanceto0.033(0.060)at y=².5 (y=⁴.0).

Finally,thenuclearmodificationfactoriscalculatedastheratio between the corrected J/ψ yield in Pb–Pb collisions Y_J^Pb–Pb_/ψ and theJ/ψcross-sectioninpp collisionsscaledbythenuclearoverlap function:

RAA

=

^Y

Pb–Pb J/ψ

^TAA

× σ

_J^pp_/ψ

^.

⁽²⁾

In the dielectron analysis, the pp reference was obtained by interpolatingtheinclusiveJ/ψ cross-sectionsatmid-rapiditymea-

suredby PHENIX[8]at√

s=⁰.2 TeV,CDF[39] at√

s=¹.96 TeV andALICE[40,24,41]at√

s=².76and 7 TeV. Allthedatapoints usedinthisprocedurearelistedinTable 2.Theinterpolationwas done by fitting the data points with several functions assuming a linear,an exponential,apower laworapolynomial √

s depen- dence. Thevalue oftheinterpolatedppreferenceatmid-rapidity, d

σ

_J^pp_/ψ/dy=⁴.25±⁰.28(stat.)±⁰.43(syst.) μb, is consistent with theonemeasuredbyALICE[40],butthetotaluncertaintyistwice smaller,beingdrivenmainlyby theCDFresult. Thestatisticalun- certainty was obtainedfromthe fittingprocedure,while thesys- tematiconewasobtainedbychangingthefitfunctionandbyshift- ingthedatapointswithintheirexperimentalsystematicuncertain- ties. Forthe dimuonanalysis, thepp referenceandits associated uncertaintiesareextractedfromtheALICEmeasurement[40].

4. Systematicuncertainties

The main sources ofsystematic uncertainties forthe J/ψ R_AA evaluationare thetrackingefficiency,thesignal extractionproce- dure,theparameterizationoftheJ/ψkinematicdistributionsused as input for the MC simulations, the uncertainty on the nuclear overlapfunction andtheuncertaintyonthe J/ψ pp cross-section at√

s=².76 TeV.Otheranalysis-dependentsourcesaredetailedin thefollowing.Thesystematicuncertaintieshavebeenevaluatedas afunctionofcentralityand, forthedimuonanalysis,ofpTand y.

AnoverviewofsystematicuncertaintiesisgiveninTable 3.

In the dielectronanalysis, thesystematic uncertainties on the signalextractionareestimatedbyvaryingthemassregionusedto countthesignal,themassintervalusedformatchingtheMEback- groundandtheOSdistribution.Inallthesecases,thebackground- subtracted OS distribution is compared to the MC signal shape andthe normalized

χ

² obtainedisalways closeto 1.Thisshows that,afterbackgroundsubtraction,thenumberofcorrelatedpairs not relatedto J/ψ decaysissmallanddoesnotinducea sizeable systematicuncertainty.Thecentralitydependentsystematicuncer- taintyonthesignalextraction,takenastheRMSofthedistribution ofthenumberofJ/ψobtainedfromalltheperformedtests,ranges from7%to9%and4%to6%for2010and2011data,respectively.

The systematicuncertainties dueto trackreconstructionandpar- ticle identification are evaluated by varying all the analysiscuts.

For each cut variation, the number of J/ψ signal counts is cor- rectedwiththecorresponding A×

ε

.The RMSofthisquantity is found tovaryinthe range6–9%and4–5%inthe2010and2011 data, respectively. Since the signal extraction procedure must be used for every cut variation, the systematic uncertainties due to analysis cuts and signal extraction cannot be truly disentangled.

Thus, aglobalsystematicuncertaintyisintroducedastheRMSof thedistribution ofcorrectedresultswhenvaryingboth signalex- tractionparametersandcutvalues.Thesesystematicuncertainties range between 8% and11% depending on the centrality interval.

The central valuefor thecorrected J/ψ yieldis chosen tobe the meanofthedistributionobtainedfromalltheperformedtests.

Inthedimuonanalysis,thesystematicuncertaintyonthesignal extraction is estimatedby fitting the invariant mass distribution

Table 3

SystematicuncertaintiesenteringtheRAAcalculation.ThetypeI(II)standsforcorrelated(uncorrelated)uncertaintieswithinagivensetofdatapoints.Theuncorrelated systematicuncertainties(typeII)aregivenasarange.

Channel μ⁺μ⁻ e⁺e⁻

Centrality pTory Centrality

value (%) type value (%) type value (%) type

signal extraction 1–3 II 1–5 II 8–11 II

tracking efficiency 11 and 0–1 I and II 1 and 8–14 I and II

trigger efficiency 2 and 0–1 I and II 1 and 2–4 I and II n/a

input MC parameterization 3 I 0–8 II 5 I

matching efficiency 1 I 1 II n/a

centrality limits 0–5 II 0–1 I 0–3 II

TAA 3–8 II 3 I 3–5 II

σ_J^pp_/ψ 9 I 6 and 5–6 I and II 12 I

Fnorm 4 I 4 I n/a

withandwithoutbackground subtractionandby varyingthe pa- rametersthatdefinethepowerlawshapesatlowandhighmasses oftheCB2signalfunction.Thesefitparametersarenotconstrained by the data and cannot be let free during the fitting procedure.

Theyhavebeenfixedtodifferentvaluesextractedeitherfromsim- ulationsorfrompp data, wherethesignal-to-backgroundratiois morefavorable. Fits corresponding to thevarious choices forthe CB2tailsare performed,keepingthebackgroundparametersfree, andvarying the invariant mass rangeused forthe fits. The raw J/ψyieldisdeterminedastheaverageoftheresultsobtainedwith theaboveprocedureandthecorrespondingsystematicuncertainty is defined as the RMS of the deviations fromthe average. As a functionofcentrality(pT or y)thesystematicuncertaintyforthe signalextractionvariesfrom1%to3%(1%to5%).Thesinglemuon trackingandtriggerefficiencies, _trk and _trg,are estimatedwith theembedded J/ψ simulation.Thecentralitydependenceofthese quantitiesisweak,since thedecreaseinmostcentralcollisionsis about1%and3.5%for

trk and

trg respectively.A11% systematic uncertaintyon the tracking efficiencyis estimated by comparing itsdeterminationbased onrealdataandona MCapproach.This estimationreliesonacalculationofthetrackingefficiencyineach stationusingthedetectorredundancy(twoindependentdetection planesper station).Thesingle trackefficiency,definedasproduct ofthe stationefficiencies, is calculated fortracks fromreal data andfromsimulation.Thesingletrackefficienciesaretheninjected inpure J/ψ simulationsandthedifferenceusedastheJ/ψ track- ingsystematicuncertainty.ThepTandydependenceoftheformer uncertaintyleads toabintobinuncorrelatedcomponentranging between8%and14%.ThesystematicuncertaintyontheJ/ψ A×

ε

correctionsrelatedtothetriggerefficiencyis2%,mostly givenby theuncertaintyontheintrinsicefficiencyofthetriggerchambers.

Thesystematicuncertaintyrelatedtotheresponsefunctionofthe trigger is always below 1% except in the lowest J/ψ pT interval where a value of 3% was estimated. As a function of centrality, thesystematicuncertainty ofthe trackingor thetrigger efficien- ciesis1%inthemostcentralcollisionsandbecomesnegligiblefor peripheral collisions. The uncertainty on the matching efficiency betweentracksreconstructedinthetrackingandtriggerchambers amountsto1%.Itiscorrelated asafunction ofthe centralityand uncorrelatedasafunctionofpT andy.

The A×

ε

calculationdependson the J/ψ pT and y distribu- tionsusedasaninputtotheMC,andsystematiceffectsoriginating from different parameterizations of these distributions must be takenintoaccount.Inthee⁺e⁻ analysis,theJ/ψ (pT, y) parame- terizationis basedonan interpolation oftheRHIC, CDFandLHC datainpp andpp collisions¯ [42]correctedusingnuclearshadow- ingcalculations[43].Thesystematicuncertaintywas evaluatedby varyingthe slope ofthe pT shape ina wide rangesuch that the averagep_Tchangesbetween1.5 GeV/cand3.0 GeV/c.TheJ/ψ p_T

spectrainA–Acollisions measuredbyPHENIX[9]atmid-rapidity andALICEatforwardrapidity [40]atall availablecentralities, to- gether withtheir uncertainties, are well covered inthe envelope determined by the considered variations. A centrality correlated systematic uncertainty of 5% was obtained following this proce- dure. In the

μ

⁺

μ

⁻ analysis, the MC J/ψ parameterizations are based onthe pT and y distributions measured fordifferentcen- tralityclasses.The pT–ycorrelationobservedbyLHCbinpp colli- sions[28] wasalsoincludedinthesystematicstudy.Acorrelated variation in A×

ε

of3% was observed as a function of central- ity.The pT(y)dependenceofthissystematicuncertaintybringsa maximumcontributionof1%(8%)oneachpoint.

Furthersourcesofsystematicuncertaintiesaffectingthenuclear modification factor are the uncertaintyon the limits of thecen- trality classes [27], on the nuclear overlap function and on the J/ψcross-sectioninppcollisionsat√

s=².76 TeV.Anuncertainty onthenormalizationfactorFnorm,accountingforrunbyrun fluc- tuations onthisquantity, isalsoadded inthemuon analysis. All numericalvaluescanbefoundinTable 3.

When computing the A×

ε

factor, we assumed that J/ψ are produced unpolarized and no systematic uncertainty is assigned to a possible polarization. In pp collisions, mid-rapidity (pT>

10 GeV/c) and forward-rapidity (pT>2 GeV/c) measurements havebeen done at√

s=^{7 TeV andindeed show that J}/ψ polar- ization is compatiblewithzero [44–46]. In Pb–Pb collisions, J/ψ mesonsproducedfromcharmquarksinthemediumareexpected tobeunpolarized.

5. Results

In the e⁺e⁻ decay channel, the inclusive J/ψ R_AA was stud- iedasafunctionofthecollisioncentrality(0%–10%,10%–40%and 40%–90%) for pT>0 GeV/c and |^y|<0.8. In the

μ

⁺

μ

⁻ decay channel,theeventsamplecollectedinthe2011runwiththeded- icated

μμ

MB triggerallowsforthestudyoftheR_AA asafunction ofthecentralityofthecollisionsinnineintervals. Furthermore,a differential studyofthe RAA asa functionof transversemomen- tumorrapidityisalsofeasible.Dataareanalyzedinsevenintervals inthepT range0<pT<8 GeV/c rangeandsixintervalsinthe y range 2.5<y<4. The chosen binning matches the one adopted forthe√

s=².76 TeV ppresults,whichareusedasthereference fortheevaluationofthenuclearmodificationfactor.

Fig. 3showstheinclusiveJ/ψ RAAatmid- andforward-rapidity asafunctionofthenumberofparticipantnucleons^Npart^.Statis- ticaluncertaintiesareshownasverticalerrorbars,whiletheboxes representthevariousuncorrelatedsystematicuncertainties added in quadrature.The systematicuncertainties correlated bin by bin (typeII inTable 3)are summedin quadratureandreferred toas globalsyst. inthe legend.At forward-rapidity a clear suppression

Fig. 3.(Coloronline.) Centralitydependenceofthenuclearmodificationfactor,RAA, ofinclusiveJ/ψ productioninPb–Pbcollisionsat √

sNN=2.76 TeV,measuredat mid-rapidityandat forward-rapidity.The pointtopointuncorrelated systematic uncertainties(typeII)arerepresentedasboxesaroundthedatapoints,whilethe statisticalonesareshownasverticalbars.Globalcorrelatedsystematicuncertainties (typeI)arequoteddirectlyinthelegend.

is observed, independent of centrality for ^Npart>70. Although withlarger uncertainties, themid-rapidity RAA shows asuppres- sion of the J/ψ yield too. The centrality integrated RAA values are R^0%–90%_AA =⁰.72±⁰.06(stat.)±⁰.10(syst.)andR^0%–90%_AA =⁰.58± 0.01(stat.)±⁰.09(syst.)atmid- andforward-rapidity,respectively.

Thesystematicuncertaintieson both RAA valuesincludethecon- tributionarising from^TAA calculations.Thisamounts to3.4% of the computed ^TAA ^{value and is a correlated systematic uncer-} tainty common to the mid- and forward-rapidity measurements.

PHENIX mid- (|^y|<0.35) andforward-rapidity (1.2<|^y|<2.2) results on inclusive J/ψ RAA at √

sNN=⁰.2 TeV exhibit a much strongerdependenceonthecollision centralityandasuppression ofaboutafactorofthreelargerinthemostcentralcollisions[9].

The measured inclusive J/ψ RAA includes contributions from prompt and non-prompt J/ψ; the first one results from direct J/ψ production and feed-down from ψ(2S) and

χ

c, the second one arises from beautyhadron decays. Non-prompt J/ψ are dif- ferentwithrespecttothepromptones,sincetheirsuppressionor productionisinsensitivetocolorscreeningorregenerationmech- anisms. Beauty hadron decay mostly occurs outside the fireball, anda measurementofthenon-promptJ/ψ RAA isthereforecon- nectedto thebeauty quark in-mediumenergyloss (see [47] and referencestherein).At mid-rapidity,the contributionfrombeauty hadron feed-down to the inclusive J/ψ yield in pp collisions at

√s=^{7 TeV is}approximately15%[48].ThepromptJ/ψ R_AAcanbe evaluatedaccordingtoR^prompt_AA =(RAA−^Rnon-promptAA )/(1−^FB)where FB is the fraction ofnon-prompt J/ψ measured in pp collisions, andR^non-prompt_AA isthenuclearmodificationfactorofbeautyhadrons inPb–Pb collisions. Thus, the prompt J/ψ RAA at mid-rapidity is expectedtobe about7%smaller thantheinclusivemeasurement ifthe beauty productionscales with the numberof binary colli- sions (R^non-prompt_AA =^{1) andabout17% larger ifthebeautyis fully} suppressed (R^non-prompt_AA =^{0). At} forward-rapidity, the non-prompt J/ψfractionwasmeasuredbytheLHCbCollaborationtobeabout 11(7)%in pp collisionsat √

s=⁷(2.76) TeV inthe pT rangecov- eredbythisanalysis[28,49].Then,thedifferencebetweentheR_AA ofpromptJ/ψ andthe oneforinclusiveJ/ψ isexpectedtobe of about−^{6% and7% inthe two} aforementionedextremecases as- sumedforbeautyproduction.

Fig. 4.(Coloronline.) Toppanel:transversemomentumdependenceofthecentral- ityintegratedJ/ψ RAAmeasuredbyALICEinPb–Pbcollisionsat√

sNN=².76 TeV comparedtoCMS[20]resultsatthesame√

sNN.Bottompanel:transversemomen- tumdependenceoftheJ/ψ RAAmeasuredbyALICEinthe0%–20%mostcentral Pb–Pbcollisionsat√

sNN=2.76 TeV comparedtoPHENIX[9]resultsinthe0%–20%

mostcentralAu–Aucollisionsat√_s

NN=⁰.2 TeV.

In the top panel of Fig. 4, the J/ψ RAA atforward-rapidity is shown as a function of pT for the 0%–90% centrality integrated Pb–Pb collisions. It exhibits a decrease from 0.78 to 0.36, indi- cating that high pT J/ψ are more suppressed than low pT ones.

Furthermore,athighpTadirectcomparisonwithCMSresults[20]

atthe same√

sNN is possible,themain differencebeingthat the CMS measurement covers a slightly more central rapidity range (1.6<|^y|<2.4). In theoverlapping pT range a similar suppres- sion isfound. One should add herethat thetwo CMSpoints are not independent andcorrespondto differentintervalsof theJ/ψ pT (3<pT<30 GeV/c and 6.5<pT<30 GeV/c). In the bot- tom panelofFig. 4,theforward-rapidityJ/ψ RAA forthe0%–20%

mostcentral collisions isshown. The observed pT dependenceof the RAA formostcentralcollisions isveryclosetotheoneinthe 0%–90% centralityclass.Thisisindeedexpectedsincealmost70%

of the J/ψ yield iscontained inthe 0%–20% centrality class.Our dataarecomparedtoresultsobtainedbyPHENIXin0%–20%most central Au–Au collisions at √

s_NN=⁰.2 TeV, in the rapidity re- gion1.2<|^y|<2.2[9].AstrikingdifferencebetweentheJ/ψ R_AA patterns can be observed.In particular,in thelow p_T region the ALICE R_AA resultisa factorofup tofourhighercomparedtothe PHENIXone.Thisobservationisinqualitativeagreementwiththe

Fig. 5.(Coloronline.) TherapiditydependenceoftheJ/ψ RAAmeasuredinPb–Pb collisionsat √

sNN=².76 TeV.Bothmid- andforward-rapiditymeasurementsin- cludeacommoncorrelatedsystematicuncertaintyof3.4%duetoTAA.Themid- rapiditymeasurementcoverstherapidityrange|y|<0.8 andtheforward-rapidity oneisgiveninintervalsof0.25unitofrapidityfromy=2.5 toy=4.

calculations from[17,50]wherethe(re)combinationdominancein theJ/ψ productionleads to a decrease ofthe ^p2T ^{in A–Acolli-} sionswithrespect to pp collisions. Althoughatthe two energies the rapidity coverages are not the same andCNM effects might havea different size,our results point tothe presence ofa new contributiontotheJ/ψyieldatlow p_T.

Finally,thedependenceoftheJ/ψ RAA onrapidityisdisplayed inFig. 5 forthe 0%–90% centralityclass. At forward-rapidity, the J/ψ R_AAdecreasesbyabout40%from y=².5 to y=^4.Theresult fromtheelectronanalysisisconsistentwithaconstantorslightly increasingRAA towardsmid-rapidity.

6. Conclusions

The inclusive J/ψ nuclear modification factor has been mea- suredbyALICEasafunctionofcentrality, pT and y inPb–Pbcol- lisionsat√

sNN=².76 TeV,downto zero pT.Atforward-rapidity, R_AA showsa clear suppression ofthe J/ψ yield,with no signifi- cantdependenceoncentralityfor^Npart^{larger than70.Atmid-} rapidity,theJ/ψ RAAiscompatiblewithaconstantsuppressionas afunction ofcentrality. At forward-rapiditythe J/ψ R_AA exhibits astrong pT dependenceanddecreases bya factorof2 fromlow pT to high pT. This behavior strongly differs from that observed byPHENIXat√

s_NN=⁰.2 TeV.Thisresultsuggeststhatafraction oftheJ/ψyieldisproducedvia(re)combinationofcharmquarks.

Inaddition,theindicationofanon-zeroJ/ψ ellipticflowinPb–Pb collisions at√

s_NN=².76 TeV observedby ALICE[51] brings an- otherhintinfavorof(re)combinationscenarios.Preciseknowledge ofthecold nuclear effectsis necessaryforfurther understanding ofthe J/ψ behavior. The measurement ofthe J/ψ productionin p–Pbcollisions at theLHC [52,53] willallow one to sharpen the interpretationoftheseresults.

Acknowledgements

The ALICECollaboration would like to thank all its engineers andtechniciansfortheirinvaluablecontributionstotheconstruc- tion of the experiment and the CERN accelerator teams for the outstandingperformanceoftheLHCcomplex.

TheALICECollaboration gratefully acknowledgesthe resources andsupportprovided byallGrid centresandtheWorldwideLHC ComputingGrid(WLCG)Collaboration.

The ALICE Collaboration acknowledges the following funding agencies fortheir supportin buildingandrunning theALICEde- tector: StateCommitteeofScience,World FederationofScientists (WFS) andSwiss Fonds Kidagan, Armenia; Conselho Nacional de DesenvolvimentoCientífico eTecnológico (CNPq),Financiadorade EstudoseProjetos(FINEP),FundaçãodeAmparoàPesquisadoEs- tado de São Paulo (FAPESP);National NaturalScience Foundation of China (NSFC), the Chinese Ministry of Education (CMOE) and theMinistryofScienceandTechnologyofChina(MSTC);Ministry ofEducationandYouthoftheCzechRepublic;DanishNaturalSci- ence Research Council, the Carlsberg Foundation andthe Danish NationalResearchFoundation;TheEuropeanResearchCouncilun- der the European Community’s Seventh Framework Programme;

Helsinki Institute of Physics andthe Academy of Finland; French CNRS-IN2P3,the‘RegionPaysdeLoire’,‘RegionAlsace’,‘RegionAu- vergne’ andCEA,France;German BMBFandtheHelmholtzAsso- ciation;GeneralSecretariatforResearchandTechnology,Ministry ofDevelopment, Greece;HungarianOTKA andNationalOfficefor Research and Technology (NKTH); Department of Atomic Energy and Department of Science and Technology of the Government of India; Istituto Nazionale di Fisica Nucleare (INFN) and Cen- tro Fermi – MuseoStorico della Fisica e CentroStudi e Ricerche

“Enrico Fermi”, Italy; MEXT Grant-in-Aid for Specially Promoted Research, Japan; JointInstitute forNuclear Research, Dubna; Na- tionalResearchFoundationofKorea(NRF);CONACYT,DGAPA,Méx- ico,ALFA-ECandtheEPLANETProgram(EuropeanParticlePhysics LatinAmericanNetwork); StichtingvoorFundamenteelOnderzoek der Materie(FOM) andtheNederlandse Organisatievoor Weten- schappelijk Onderzoek (NWO), Netherlands; Research Council of Norway (NFR); Polish Ministry of Science and Higher Education;

NationalScienceCentre,Poland;MinistryofNationalEducation/In- stituteforAtomicPhysicsandCNCS-UEFISCDI–Romania;Ministry ofEducationandScience ofRussianFederation, RussianAcademy ofSciences,RussianFederalAgencyofAtomicEnergy,RussianFed- eralAgencyforScienceandInnovationsandTheRussianFounda- tionforBasicResearch;MinistryofEducationofSlovakia;Depart- mentofScienceandTechnology,SouthAfrica;CIEMAT,EELA,Min- isterio de Economía y Competitividad (MINECO) of Spain, Xunta deGalicia(Conselleríade Educación),CEADEN,Cubaenergía,Cuba, andIAEA(InternationalAtomicEnergyAgency);SwedishResearch Council (VR) and Knut & Alice Wallenberg Foundation (KAW);

Ukraine Ministry of Education andScience; United Kingdom Sci- ence and Technology Facilities Council (STFC); The United States DepartmentofEnergy,theUnitedStatesNationalScienceFounda- tion,theStateofTexas,andtheStateofOhio.

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