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Physics Letters B
www.elsevier.com/locate/physletb
Inclusive J/ ψ production in Xe–Xe collisions at √
s NN = 5 . 44 TeV
.ALICE Collaboration
a r t i c l e i n f o a b s t ra c t
Articlehistory:
Received23May2018
Receivedinrevisedform26July2018 Accepted23August2018
Availableonline31August2018 Editor:W.-D.Schlatter
InclusiveJ/ψ productionisstudied inXe–Xe interactionsatacentre-of-massenergypernucleonpair of √s
NN=5.44 TeV,using the ALICE detectorat the CERNLHC. The J/ψ meson is reconstructed via its decay into a muon pair, in the centre-of-mass rapidity interval 2.5<y<4 and down to zero transverse momentum.InthisLetter,thenuclearmodificationfactors RAAfor inclusiveJ/ψ,measured inthecentralityrange0–90%aswellasinthecentralityintervals0–20%and20–90%arepresented.The RAAvaluesarecomparedtopreviouslypublishedresultsforPb–Pb collisionsat√s
NN=5.02 TeVandto thecalculationofatransportmodel.AgoodagreementisfoundbetweenXe–Xe andPb–Pb resultsas wellasbetweendataandthemodel.
©2018Organisationeuropéennepourlarecherchenucléaire.PublishedbyElsevierB.V.Thisisanopen accessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3.
Thestudyoftheproductionofquarkoniumstatesplaysanim- portantroleinthecharacterizationofthepropertiesoftheQuark- Gluon Plasma (QGP) [1]. This state of matter, where quarks and gluonsarenot confinedinto hadrons, canbe produced inheavy- ion collisions at ultrarelativistic energies. Quarkonia are bound statesofheavyquark-antiquarkpairs(charmonia,cc andbottomo- nia,bb)and their productionrate issignificantly affected by the QGP. In particular, the color force responsible for the binding of heavyquarks isexpectedtobe screenedintheQGP, leading toa suppressionofquarkoniumproductionwhichcanberelatedtothe initialtemperatureofthesystem [2,3].Inaddition,atveryhighen- ergies,suchasthoseavailableattheLHC,theabundantproduction ofcharm-anticharmpairsleadstoarecombinationprocess,which mayoccurbothintheQGPphaseorwhenthesystemcoolsdown andhadronsare formed out of the free quarksand gluons [4,5].
The study of the interplay between suppression and recombina- tionprocessesoffersthepossibilityofa quantitativeinvestigation oftheexistence of colorlessbound states ofheavy quarks inthe QGP.
An extended set of results was obtainedfor the J/ψ, a char- moniumstatewithquantumnumbers JPC=1−−,atLHCenergies (√
sNN=2.76 and 5.02 TeV)in Pb–Pb collisions [6–12]. Compar- isonof these results to theoretical models [13–17] and to lower energydata [18,19] favorsthepicturedescribed above.The study ofthecollision ofnucleilighter than Pbmaygive additionalim- portantinformationontherelativecontributionofsuppressionand recombinationmechanisms.
AstepinthisdirectionisperformedinthisLetter,wherefirst resultsonJ/ψproductionatLHCenergiesinXe–Xe,acollisionsys-
E-mailaddress:alice-publications@cern.ch.
tem (AXe=129) lighter than Pb–Pb (APb=208), are presented.
Data were collected by the ALICE Collaboration atthe centre-of- mass energy per nucleon pair √
sNN=5.44 TeV, during a short run carriedoutatthe endof2017.Dueto thelimitedintegrated luminosity,Lint∼0.34 μb−1,thestatisticaluncertaintiesaresignif- icantlylargerthanthoseofthePb–Pb results [10],butnevertheless allowameaningfulcomparisonbetweenthetwosystems,interms ofthenuclearmodificationfactorRAA.Thisquantityisobtainedas theratio betweentheproductionyields innucleus–nucleuscolli- sionsandthecorrespondingproton–proton(pp)crosssection,nor- malizedtothenuclearthicknessfunctionTAA[20].ValuesofRAA smaller(larger)thanunityindicatesuppression(enhancement)ef- fectsfor the particleunder study.The results shownin thisLet- ter correspond to the centre-of-mass rapidity range 2.5< y<4, areintegratedovertransversemomentum(pT)andwereobtained by studying the J/ψ→
μ
+μ
− decay channel. The nuclear mod- ification factor is studied as a function of the centrality of the collision [21], expressed as a percentage of the hadronic Xe–Xe cross section. Theresults correspond toinclusive J/ψ production, whichis thesumofa prompt component(directlyproduced J/ψ andfeed-downfromothercharmoniumstates)andanon-prompt component,duetothedecayofparticlescontainingabquark.ALICEistheLHCexperimentdedicatedtothestudyofnuclear collisions,andisdescribedindetailinRefs. [22,23].Themainde- tectorusedinthisanalysisisa muonspectrometer [24], covering thepseudorapidityrange−4<
η
<−2.5.1 Itincludestrackingand trigger chambers, and reconstructs muons with pT larger than a1 IntheALICEreferenceframe,themuonspectrometercoversanegativeηrange andconsequentlyanegativeyrange.Wehavechosentopresentourresultswitha positiveynotation.
https://doi.org/10.1016/j.physletb.2018.08.047
0370-2693/©2018Organisationeuropéennepourlarecherchenucléaire.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3.
Fig. 1.Fitstoinvariantmassdistributionsofopposite-signdimuons,for0–90%Xe–Xe collisions.Intheleftpanel,theresultofafittotherawinvariantmassspectrumis shown,whileintherightpanelthefittothesamedistributionaftersubtractionofthemixed-eventbackgroundispresented.Thefitcurvesshowninbluerepresentthesum ofthesignalandbackgroundshapes,whiletheredlinescorrespondtotheJ/ψsignalandthebluedashedonestothebackground(seetextfordetails).(Forinterpretation ofthecolorsinthefigure(s),thereaderisreferredtothewebversionofthisarticle.)
giventhreshold,whichis setatthe triggerlevel. Inaddition,the V0 [25], aset ofscintillator detectorscovering 2.8<
η
<5.1 and−3.7<
η
<−1.7,isusedtodefinetheminimumbias(MB)inter- actiontriggervia acoincidenceofsignalsatpositive andnegativeη
values.The V0is alsousedforthe centralityestimate via afit ofthedistributionofthetotalsignalamplitudesintheframework oftheGlauber model [21].Thereconstructionoftheprimarycol- lision vertexis carried out in the two layers ofthe Silicon Pixel Detector(SPD),theinnermostpartoftheInnerTrackingSystemof the experiment [26], covering |η
|<2 and|η
|<1.4 respectively.Finally, rejection of non-hadronic Xe–Xe collisions is performed using the Zero Degree Calorimeters (ZDC) [27], which identifies electromagnetic interactions, while the V0 detects beam-gas col- lisionsoccurringoutsidethenominalinteractionpointregion.
ThedataanalyzedinthisLetteraretakenwithatriggerformed by the coincidence of the MB trigger signal and of at least one muontriggeredinthemuonspectrometer, witha pT=0.5 GeV/c threshold.Thedefinitionofthe triggerislessrestrictive thanthe oneusuallyadoptedforPb–Pb datataking(1GeV/c thresholdand twodetected muons),dueto themuch smallerinstantaneous lu- minosityforXe–Xe collisions. Standard selection criteria [10] are thenappliedtosucheventsandtothemuoncandidates.Inpartic- ular,it isrequired(i) that twoopposite-sign tracksreconstructed inthetrackingchambersofthemuonspectrometerarematchedto tracksegmentsinthetriggersystem,(ii)thatbothmuonsbelong- ingtothepair(dimuon)have−4<
η
μ<−2.5,and(iii)thattheir transverseposition Rabs attheendofthehadronabsorberofthe muonspectrometer satisfiesthecondition 17.6<Rabs<89.5 cm.Finally,thereconstructeddimuonshouldlayinthefiducialrapid- ityregionofthemuonspectrometer,2.5<y<4.
ThenuclearmodificationfactorRAAforthecollisionsystemun- derstudyisdefined,forthecentralityintervali,as
RiAA
=
NiJ/ψ
BRJ/ψ→μ+μ−NiMBA
ε
iTAAiσ
Jpp/ψ,
(1)where NJi/ψ is the numberof detected J/ψ in the i-th centrality interval, BRJ/ψ→μ+μ− =(5.96±0.03)% is the branching ratio of thedimuondecaychannel [28], NiMB isthenumberofMB events corresponding tothe analyzedtriggered eventsample, A
ε
i isthe product ofthe detector acceptancetimes the reconstruction effi- ciency, TAAi is the average nuclear thickness function [29], andσ
Jpp/ψ istheinclusiveJ/ψ crosssectionforppcollisions,atthesame energy andin the same kinematic range asthe Xe–Xe data. Re- sults are given forthe centrality interval 0–90% andfor the two sub-intervals0–20%and20–90%.Exceptfor thedetermination of
σ
Jpp/ψ,the other quantitiesen- teringthedefinitionofRAA areevaluatedfollowingthesamepro- cedureusedfortheanalysisofthePb–Pb datasampleanddetailed inRef. [10].The extraction of NJ/ψ is performed with two different ap- proaches.Inthefirst,therawopposite-signdimuoninvariantmass distribution isfittedwitha superpositionofresonanceandback- groundshapes [30], theformerbeingtuned toMonteCarlo(MC) simulations and the latter corresponding to empirical functions.
Inthe second,thebackgroundisestimatedvia amixed-eventin- variant mass distribution, obtained from the collected sample of muon-triggered eventsandsubtractedfromtherawspectrum [9].
The resultingdistribution is then fitted withthe sum of a reso- nance shape and a continuum function accountingfor the small residual backgroundcomponent. Duetothe low statisticalsignif- icance of the present data sample, the widthof the J/ψ meson, which is usually kept asa free parameter in the invariant mass fits, is fixed to
σ
J/ψ=70 MeV/c2, corresponding to the value of this quantity obtained in previous analyses [10,31,32]. For each of the two approaches, several fits were performed varying the fit mass range, the signal and background shapes and the J/ψ widthby ±1 MeV/c2.Theobtainedvalue forthecentralityinter- val0–90% isNJ/ψ=241±47(stat.)±26(syst.),wherethecentral value andthestatisticaluncertaintycorrespond tothe averageof the fit results and to the average of the corresponding statisti- cal uncertainties, respectively. The systematic uncertainty is ob- tained as the root mean square of the distribution of the NJ/ψ values obtained with the various fits. The corresponding values for the 0–20% and 20–90% centrality sub-intervals are NJ/ψ = 175±42(stat.)±23(syst.)andNJ/ψ=77±20(stat.)±7(syst.),re- spectively.Fig.1showsasanexampletheresultsoftwofitstothe0–90%
Xe–Xedimuoninvariantmassdistribution,correspondingtofitting therawspectrum(leftpanel)orthemixed-eventbackgroundsub- tractedmassdistribution(rightpanel).
The product of the acceptance times the reconstruction effi- ciency A
ε
for J/ψ is evaluated via a MC simulation, based on the GEANT3 transport model [33], which takes into account thealignmentofthemuonspectrometerdetectorsandtheirefficiency.
The input pT and y distributions forthe J/ψ acceptancecalcula- tion cannot be tuned directly to data, dueto the low integrated luminosity of the data sample. It is therefore assumed that the shapeoftheyandpTdistributionsissimilarfordifferentcollision systemsincentralityintervalscorrespondingto thesameaverage number of participant nucleons, weighted by the corresponding number of nucleon–nucleon collisions, Npartw . The weighting is introducedtotakeintoaccountthattheJ/ψ productioncrosssec- tionis proportional to the numberof nucleon–nucleon collisions and that therefore the average Npart in wide centrality bins is systematicallyshifted towards highervalues.Followingthisargu- ment,thedifferentialdistributionsmeasuredinPb–Pb collisionsat
√sNN=5.02 TeV[10] forthe20–40%centralityrangeareusedas input distribution for the MC calculation, since NwpartPbPb,20−40%
isequal,within ∼2%,toNwpartXeXe,0−90%,estimatedviaaGlauber MCcalculation.The systematicuncertaintyontheJ/ψ acceptance value due to the choice of the J/ψ rapidity and transverse mo- mentumdistributionsamountsto2%andisevaluatedbychoosing alternative input shapes corresponding to other Pb–Pb centrality ranges.
Concerningthereconstructionefficiency,itslightlydependson thecollisioncentrality,duetothedetectoroccupancyinthemuon spectrometer. The effect was evaluated in the analysis of Pb–Pb events [10] byembeddingthesimulatedJ/ψ signalintorealevents corresponding to various centralities. For this analysis, starting fromthePb–Pb results,thedecreasein A
ε
XeXe,0−90% withrespect toasimulationcontainingonlyJ/ψ isestimatedtobe4.2%(values for0–20%and20–90%centralityrangesare5.5%and1.6%,respec- tively).Thesystematicuncertaintyonthereconstructionefficiency isevaluatedfollowingtheprocedureusedinRef. [10],leadingtoa 3.6%effect.Theresultingvaluefortheproductofacceptancetimesrecon- structionefficiencyfor J/ψ productionin 0–90% Xe–Xe collisions is A
ε
XeXe,0−90%=0.228±0.009(syst.),withanegligiblestatistical uncertainty.The normalizationfactor NMB is evaluated by multiplying the numberof opposite-sign dimuon triggers by a factor Fnorm, cor- responding to the inverse of the probability of having a trig- geredmuon in a MB event. Thisquantity is computedfrom the eventtrigger inputinformation andthe level-0triggermask. The procedure and the evaluation of the systematic uncertainty are described in Ref. [10]. The obtained value is Fnorm=2.428± 0.001(stat.)±0.024(syst.).
The reference cross section for the calculation of RAA is ob- tainedstartingfromthemeasuredvalueoftheinclusiveJ/ψ cross sectioninppcollisionsat√
s=5.02 TeV [10].Thisquantityisthen correctedtoaccountforthedifferentcentre-of-massenergyofthe Xe–Xe data, using an interpolation of available ALICEpp results at √
s=2.76, 5.02, 7, 8 and 13 TeV [32]. The obtained value is
σ
Jpp/ψ=5.99±0.09(stat.)±0.30(syst.)μb−1,wherethesystematic uncertaintycontains asmallterm(0.4%)relatedto theinterpola- tionprocedure,calculatedasthemaximumspreadbetweenresults obtainedwithvariousinterpolatingfunctions [34].Thenuclear thickness function TAA is evaluated forthe var- ious centrality intervalsvia a Glauber model calculation, and its uncertaintyisestimatedbyvarying withinuncertainties theden- sity parameters of the Xe nucleus [29,35]. For 0–90% centrality itsvalue amountsto TAA=3.25±0.25 mb−1,while for0–20%
and20–90% one obtains TAA=9.90±0.62 mb−1 and TAA= 1.35±0.14 mb−1,respectively.
Finally,a systematicuncertainty on the definitionof the cen- tralityintervalsisevaluatedbyvarying thevalueoftheV0signal amplitudecorresponding to90% centralityby±0.5%andrecalcu- latingcorrespondinglythecentralityintervals.
Table 1
Summaryofsystematicuncertaintiesonthecalculationofthenu- clearmodificationfactors.Thetrackingefficiencytermincludesa 1%contributionduetothechoiceoftheχ2cutofthematching betweentheinformationoftrackingandtriggerdetectors.Allthe uncertaintiesarecorrelatedamongthevariouscentralityranges, exceptthoseonthesignalextraction,TAAandthedefinitionof thecentralityintervals.
Source 0–90% 0–20% 20–90%
Signal extraction 11% 13% 8%
MC input 2% 2% 2%
Tracking efficiency 2% 2% 2%
Trigger efficiency 3% 3% 3%
Fnorm 1% 1% 1%
TAA 8% 6% 10%
Centrality 0% 0% 1%
pp reference 5% 5% 5%
Fig. 2.TheinclusiveJ/ψ nuclearmodificationfactorforXe–Xe collisionsat√s
NN= 5.44 TeV.TheresultsareplottedusingascentralityvariableNpartw ,obtainedby weighting,ineachcentralityinterval,theNpartdistributionwiththecorrespond- ingdistributionofthenumberofnucleon–nucleoncollisions.Theerrorbarsrep- resentthe statisticaluncertainties,theboxesaroundthe pointsthe uncorrelated systematicuncertainties.Correlateduncertaintiesareshownasafilledboxaround unity.The results arecomparedwith the samequantityfor Pb–Pb collisionsat
√s
NN=5.02 TeV [10] andtotheresultsofthecalculationofatransportmodel [13, 14].ForPb–Pb,theweightingofNpartwiththenumberofnucleon–nucleoncolli- sionswasnotperformed,sinceitleadstoanegligibleeffectwhenthecentrality intervalsarenarrow.
Table 1 showsa summary of the systematicuncertainties for the RAA measurement for the three analyzed centrality ranges.
ThemaincontributionscomefromtheestimateofTAAandfrom thesignal extraction.The formerisdominatedby theuncertainty onthe surfacethicknessofthe Xenucleus. Thelatter,beingesti- matedinadata-drivenwayasdetailedabove,maysufferfromthe statistical limitations of the data sample. The quoted values can thereforebeconsideredtobeaconservativeestimate.
The pT-integratednuclearmodificationfactorforinclusiveJ/ψ production in Xe–Xe collisions at √
sNN =5.44 TeV, measured in 2.5< y <4 and in the 0–90% centrality range, is RAA= 0.54±0.11(stat.)±0.08(syst.). Thisvalue can becompared with the corresponding one for Pb–Pb collisions at √
sNN=5.02 TeV, RPbPbAA =0.65±0.01(stat.)±0.04(syst.)[10].Theirratioamountsto 0.84±0.16(stat.)±0.13(syst.),showingthatthetwovaluesagree within about 0.8
σ
.Following the approach ofRef. [9], it can be shownthattheXe–Xe nuclearmodificationfactorforpromptJ/ψ could be up to 10% higher (lower) than the inclusive RAA if the non-promptJ/ψcomponentfromthedecaysofhadronscontaining abquarkisnot(completely)suppressed.InFig.2the RAA values for0–20%and20–90%Xe–Xe collisionsareplotted,andcomparedwith the centrality dependence of the nuclear modification fac- tor for Pb–Pb collisions [10]. The latter shows, after a decrease up to Npart∼100, a saturation at RAA∼0.65–0.7 towards more centralevents,andthetwoXe–Xe pointsarefoundtobeinagree- ment,withintheirlargeruncertainties,withthePb–Pb results.The Xe–Xe andPb–Pb resultsarealsocomparedwiththecalculationof atransportmodelbyDuandRapp [13,14].Aclosesimilarityofthe predictedsuppression patternsforPb–Pb and Xe–Xe is observed, whichfairlyreproducestheexperimentalresults.
In summary, we have measured inclusive J/ψ production in Xe–Xe collisionsat√
sNN=5.44 TeV.Resultsonthenuclearmod- ification factors were given for various centrality selections and comparedtocorresponding resultsforPb–Pb collisionsat√
sNN= 5.02 TeVandtoatheoreticalmodel.Withintheexperimentalun- certainties,agoodagreementisfoundbetweenthe RAAmeasured inthe two systems andwiththe calculation. Theseresults show thattherelativecontributionofsuppressionandregenerationpro- cessesissimilarforcollisionsproducingsimilar Npart valuesfrom differentcollisionsystems.
Acknowledgements
The ALICE Collaboration would like to thank all its engineers andtechnicians fortheir invaluablecontributionstotheconstruc- tionoftheexperimentandtheCERNacceleratorteamsfortheout- standingperformanceoftheLHCcomplex.TheALICECollaboration gratefully acknowledges the resources and support provided by all Gridcentres andtheWorldwide LHCComputing Grid(WLCG) collaboration. The ALICE Collaboration acknowledges the follow- ingfundingagenciesfortheirsupportinbuildingandrunningthe ALICEdetector:A.I. AlikhanyanNationalScience Laboratory(Yere- vanPhysicsInstitute) Foundation(ANSL),State Committee ofSci- enceandWorldFederationofScientists(WFS),Armenia; Austrian AcademyofSciencesandNationalstiftungfürForschung,Technolo- gie und Entwicklung, Austria; Ministry of Communications and High Technologies, NationalNuclear Research Center, Azerbaijan;
Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Universidade Federal do Rio Grande do Sul (UFRGS), Fi- nanciadoradeEstudoseProjetos(Finep)andFundaçãodeAmparo à Pesquisa do Estado de São Paulo (FAPESP), Brazil; Ministry of Science & Technology of China (MSTC), National Natural Science Foundation of China (NSFC) and Ministry of Education of China (MOEC), China; Ministry of Science andEducation, Croatia; Min- istryofEducation,YouthandSportsoftheCzech Republic,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 desParticules (IN2P3)and Centre Nationalde laRecherche Scientifique(CNRS), France; Bundesministerium für Bildung, Wissenschaft, Forschung und Technologie (BMBF) and GSI Helmholtzzentrum für Schw- erionenforschung GmbH, Germany; General Secretariat for Re- search andTechnology, Ministryof Education,Research andReli- gions,Greece;NationalResearch,DevelopmentandInnovationOf- fice,Hungary;DepartmentofAtomicEnergy,GovernmentofIndia (DAE),DepartmentofScienceandTechnology,GovernmentofIndia (DST), University GrantsCommission, Governmentof India(UGC) andCouncilofScientific andIndustrial Research(CSIR),India;In- donesian Institute of Science, Indonesia; Centro Fermi – Museo StoricodellaFisicaeCentroStudieRicercheEnricoFermiandIsti- tutoNazionalediFisicaNucleare(INFN),Italy;InstituteforInnova- tiveScienceandTechnology,NagasakiInstitute ofAppliedScience (IIST),Japan SocietyforthePromotionofScience (JSPS)KAKENHI and Japanese Ministry of Education, Culture, Sports, Science and
Technology (MEXT), Japan; Consejo Nacional de Ciencia (CONA- CYT)yTecnología,throughFondodeCooperaciónInternacionalen Ciencia yTecnología(FONCICYT) andDirección Generalde Asun- tos delPersonalAcademico(DGAPA),Mexico;NederlandseOrgan- isatie voorWetenschappelijkOnderzoek (NWO),Netherlands; The ResearchCouncilofNorway,Norway;CommissiononScienceand Technology forSustainableDevelopmentintheSouth(COMSATS), Pakistan;PontificiaUniversidadCatólicadelPerú,Peru;Ministryof ScienceandHigherEducationandNationalScienceCentre,Poland;
KoreaInstituteofScienceandTechnologyInformationandNational ResearchFoundationofKorea(NRF),RepublicofKorea;Ministryof Education andScientific Research,Institute ofAtomicPhysicsand RomanianNationalAgencyforScience,TechnologyandInnovation, Romania; Joint Institute for Nuclear Research (JINR), Ministry of EducationandScienceoftheRussianFederationandNationalRe- search Centre Kurchatov Institute, Russia; Ministry of Education, Science, ResearchandSportof theSlovak Republic, Slovakia; Na- tionalResearchFoundationofSouthAfrica,SouthAfrica;Centrode AplicacionesTecnológicasyDesarrolloNuclear (CEADEN),Cubaen- ergía,CubaandCentrodeInvestigacionesEnergéticas,Medioambi- entales yTecnológicas(CIEMAT),Spain;SwedishResearchCouncil (VR)andKnut&AliceWallenbergFoundation(KAW),Sweden;Eu- ropean Organization for Nuclear Research, Switzerland; National Science andTechnology Development Agency (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 States ofAmerica (NSF) andUnited StatesDepartment of Energy,OfficeofNuclearPhysics(DOENP),UnitedStatesofAmer- ica.
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