Phi-meson Production in Heavy-Ion Collisions at RHIC

6002 guA 8 1v9108060/ex-lcu:nviXraS-LBlyth1fortheSTARCollaboration

1

RelativisticNuclearCollisionsGroup,LawrenceBerkeleyNationalLaboratory,1CyclotronRoad,MS70R319,Berkeley,CA,94720E-mail:slblyth@lbl.gov

Abstract.Wepresentthefirstellipticflow,v2,measurementfortheφAu+Aucollisionsat

√

-mesonin

φ-mesonProductioninHeavy-IonCollisionsatRHIC2

Theφ-mesoncanalsoprovideinformationonparticleproductionmechanismssinceitisameson,butatthesametimeisalmostasheavyasaΛbaryon.Comparingtheφ-mesonnuclearmodificationfactor,RCP,withotheridentifiedhadronscanhelptodifferentiatebetweenmass-typeandmeson/baryon-typedependenciesofparticleproduction.Mediumeffectsonparticleproductionmaybestudiedthroughcomparisonoftheφ-mesontransversemomentum,pT,distributionswithmodelexpectations.

AnalyseddataarefromtheAu+AuRunIV200GeVdataset.Forthev2andspectralanalysis∼13.5millionminimumbiasevents(0-80%)wereanalysed,while∼4millioncentraltriggeredeventswereusedtoobtainpTspectrain(0-5%)centralcollisions.Theφ-mesonwasreconstructedthroughitsdecaytotwochargedkaons(branchingratio=49.1%)whichwereidentifiedthroughtheirionisationenergylossintheSTARTimeProjectionChamber(TPC).Toextracttherawyieldofφ-mesons,aninvariantmass,minvdistributionwasconstructedusingallcombinationsofpositivelyandnegativelychargedkaonsfromeachevent.ThecombinatorialbackgroundfromuncorrelatedK+K−pairswasestimatedusingevent-mixing,scaledtothesame-eventdistributionandthensubtracted.TheresultingminvdistributionwasfittedwithaBreit-Wignerdistribution(todescribetheshapeoftheφmasspeak)plusastraightline(fortheresidualbackground).2.Results

Thefirstmeasurementoftheφ-mesonellipticflow,v2,at

√

φ-mesonProductioninHeavy-IonCollisionsatRHIC3

KS0(opensquares)andΛ(opencircles)[6]aswellasthemultistrangebaryonsΞandΩ[7](notshownhere).ForpT<2GeV/c,theφ-mesonv2seemsconsistentwithahydrodynamic-typemass-orderingwhileatintermediatepT,(2moreconsistentwiththev2(pT)oftheKS0

andthescalingforNQ=2,thantheΛ(NQ=3).Thereforetheφv2seemstobedeterminedbyitsmesonpropertiesratherthanitsmass.Thisisfurtherevidenceoftheparticlespecies-typedependenceofv2observedatintermediatepTatRHIC,andcanbedescribedintermsofparticleproductionthroughquarkrecombinationorcoalescence[8,9,10].

Thenuclearmodificationfactor,RCP,canprovideinsightonthesystemcreatedincentralcomparedtoperipheralcollisionssinceitdescribestheyieldofφ-mesonsproducedincentralcomparedtoperipheralcollisions,scaledbythenumberofbinarycollisions(calculatedusingaGlaubermodel)andisgivenbyRCP=dN/dpT/󰀁Nbin󰀂|cent

φ-mesonProductioninHeavy-IonCollisionsatRHIC

101 dN2/dpdy (GeV/c)-2T2πp110101010101010-14

STAR PreliminarySTAR Preliminary10.8N(Ω) / N(φ)0.60.40.200Ω/φ Reco. TTΩ/φ Reco. sumΩ/φ (STAR)-2-3-4-5-6-7Tφ (STAR) 0-12%Ω + Ω (STAR)Reco. sum Reco. TT24p (GeV/c)T0624p (GeV/c)T6Figure3.LeftpanelshowstheφandΩpTspectraforthemostcentralcollisionscomparedtorecombinationmodelexpectationsfrom[12].Thesumofall(thermal-thermal(TT)recombination)contributionstothemodelisshownasthesolid(dashed)lines.TherightpanelshowstheΩ/φratioasafunctionofpTcomparedtotherecombinationmodelexpectations.

productionfromfragmentationprocessesinthismodelisverysmall.Theseexpectationsseemtomatchthetrendofthedata.TheratiooftheΩ/φyieldsisshownintherightpanelofFig.3.Inthiscase,themodelpredictsamonotonicincreaseintheΩ/φratiountilpT∼8GeV/c.AlthoughthedatadeviatefromthemodelpredictionsathigherpT,theirtrendcanbedescribeduptopT∼4GeV/c.3.SummaryandConclusions

Insummary,thelargev2oftheφatlowpT,despiteitslowcross-sectionforinteractionsinthehadronicphaseaswellasthefactthatthev2(pT)scaleswithNQ=2,arestrongindicationsofpartoniccollectivityanddeconfinementofthemediumcreatedinAu+AucollisionsatRHIC.Thegoodagreementoftheφ-mesonRCP(pT)withRCPvaluesofothermesonsratherthantheheavierbaryons,providesadditionalconfirmationofthemeson/baryon-typedependenceofthisobservableatintermediatepT.Thescalingofthev2andRCPvaluesoftheφ-mesonwithNQ=2isexpectedfromrecombinationandcoalescencemodelsofparticleproduction.Inaddition,thetrendofthecentralΩ/φratiocanbedescribeduptopT∼4GeV/cbyamodelbasedonrecombinationofthermalstrangequarksinthemedium.

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