Nuclear Physics A NuclearPhysicsA00(2016)1–4 www.elsevier.com/locate/procedia Nuclear Modification Factor of D0 Meson in Au+Au √ = Collisions at s 200 GeV NN Guannan Xie (for the STAR Collaboration)1 6 1 LawrenceBerkeleyNationalLaboratory,Berkeley,CA94720,USA 0 UniversityofScienceandTechnologyofChina,Hefei,230026,China 2 n a J 4 Abstract ] x Heavy-flavorquarksaredominantlyproducedininitialhardscatteringprocessesandexperiencethewholeevolutionof e thesysteminheavy-ioncollisionsatRHICenergies. Thustheyaresuggestedtobeanexcellentprobetothemedium - l properties through their interaction with the medium. In this proceedings, we report our first measurement of D0 c productionviatopologicalreconstructionusingSTAR’srecentlyinstalledHeavyFlavorTracker(HFT).Wealsoreport u √ our new measurement of Nuclear Modification Factor (R ) of D0 mesons in central Au+Au collisions at s = n AA NN 200GeVasafunctionoftransversemomentum(p ). Newresultsconfirmthestrongsuppressionathigh p witha [ T T muchimprovedprecision,andshowthattheR athigh p arecomparablewithlighthadrons(π)andwithDmeson AA T 1 measurements at the LHC. Furthermore, several theoretical calculations are compared to our data, and with charm v diffusioncoefficient2πTD ∼2-12canreproduceboththeD0R andv datainAu+AucollisionsatRHIC. S AA 2 5 9 Keywords: Quark-gluonplasma,Nuclearmodificationfactor,HeavyFlavorTracker 6 0 1. Introduction 0 . 1 Themassofcharmquarkissignificantlylargerthanthoseoflightquarks,Λ ,andtheQGPtempera- QCD 0 tureatRHICenergies(m >> m ,Λ ,T ). Thereforecharmquarksaredominantlyproduced 6 c u,d,s QCD QGP(RHIC) intheearlystageofthecollisioninhardscatteringprocessesatRHIC.Theyexperiencethewholeevolution 1 : of the system and offer unique information for the study of hot and dense strongly-coupled Quark-Gluon v Plasma(sQGP)matter. i X Charmproductionhasbeensystematicallymeasuredin p+p(p)collisionsinvariousexperiments. Due r tothelargequarkmass,charmproductionin p+ pcollisionsisexpectedtobecalculablewithagoodpre- a cisioninperturbativeQCD.Figure1(left)showsthecharmdifferentialcross-sectionatmidrapidityversus √ transverse momentum in p + p collisions at s = 200 GeV-7 TeV [1, 2, 3, 4]. Experimental data are compared with Fixed-Order Next-to-Leading-Log (FONLL) pQCD calculations shown as grey bands [5]. Within uncertainties, FONLL pQCD calculations describe the data over a broad range of collision ener- gies. The precision of the experimental data allows to constrain the theoretical uncertainty in the pQCD calculations. 1AlistofmembersoftheSTARCollaborationandacknowledgmentscanbefoundattheendofthisissue. 2 /NuclearPhysicsA00(2016)1–4 AtRHICenergies,charmquarksareproducedmostlyviainitialhardscatterings. Thishasbeenproved by charm total cross sections measured from different collision systems. Figure 1 right panel shows that, charmtotalcrosssectionfollowsanumber-of-binary-collision(N )scaling[1,5,6,7,9]. bin ThemodificationofthecharmedmesonproductionisquantifiedwithaNuclearmodificationfactorR . AA R iscalculatedastheratiobetweentheD0invariantyieldinAu+Aucollisionstothe p+pdatascaledby AA N . OurpreviousresultshowsthattheD0R asafunctionof p issignificantlydifferentfromunity[7]. bin AA T 400 d+Au S = 200 GeV 350 (D0+e) NN Sys. error 300 NLO err. p+p NNdb) (/dy|σµccy=0122505000 (D0+Dr*u)n12 Au+Au (D0) 100 run9 FONLL in p+p STARPreliminary 50 STARPreliminary FONLL err. 0 1 10 102 103 number of binary collisions N bin √ Fig.1. (Left)Charmquarkpairproductioncrosssectionvs. pT atmid-rapidityin p+p(p)collisionsat s = 200GeV-7TeV. FONLLpQCDcalculationsareshownasshadedbands[5].(Right)Charmcrosssectionatmid-rapidityfromp+ptocentralAu+Au collisionsfromSTAR. 2. ExperimentandAnalysis TheSTARexperimentisalarge-acceptancemulti-purposedetectorwhichcoversfullazimuthandpseu- dorapidityof|η|<1. Inthisanalysis,thedataweretakenbytheSTARexperimentusingthenewlyinstalled Heavy Flavor Tracker (HFT) in the year 2014 RHIC run. The HFT is a high resolution silicon detec- tor, which consists of three subsystems: two conventional strip detectors and one pixel detector using the state-of-the-artMonolithicActivePixelSensors(MAPS)technology[10]. TheHFTisdesignedtogreatly improveopenheavyflavorhadronmeasurementsbythetopologicalreconstructionofsecondarydecayver- tices. The data sample used in this analysis is ∼780M minimum bias events taken in Au+Au collisions at √ s = 200 GeV with the HFT. Events are required to have reconstructed collision position within 6 cm NN along the beam direction from the detector center. D0 and D0 are reconstructed in the hadronic K∓π± channel, withabranchingratioof∼ 3.9%andalifetimeofcτ ∼ 123µm. Kaonsandpionsareidentified viaacombinationoftheenergylossdE/dxmeasuredbytheTimeProjectionChamberandβmeasuredby the Time-Of-Flight detector [8]. Secondary vertices are reconstructed as the middle point at the Distance oftheClosestApproach(DCA)betweenthetrajectoriesoftwodaughterparticles. WiththeHFT,several topologicalcutsareusedtogreatlyreducethecombinationalbackground. Rectangulartopologicalcutsare optimized in each D0 p bins using the Toolkit for Multivariate Data Analysis (TMVA) package for best T significanceofD0signal. Figure2showstheD0invariantmassdistributionsintwo p bins. Comparedwithourformerpublished T result,the D0 signalsignificancescaledtothesamenumberofeventshasbeenimprovedbyafactorof4. The remaining combinatorial background is estimated with like-sign and mixed-event methods. The TPC efficiency is calculated by embedding simulated tracks into real event background. The HFT related and topologycutefficienciesarefrom D0 decaysimulationbasedontheHFTmatchingtoTPCratioandtrack pointingresolutiondirectlyfromdata. Theresultsfor p <2GeV/carebeingfinalized, andweshowthe T resultsbelowforD0 p >2GeV/c. T /NuclearPhysicsA00(2016)1–4 3 220 30 Au+Au 200GeV, 0 80% MB Au+Au 200GeV, 0 80% MB 200 3]0 1.0 < pT < 2.0 GeV/c 180 5.0 < pT < 8.0 GeV/c ×21) [V/c 2205 uluiknnellii,kk esea,, msmaeixm eeedv t eevvtt 2)MeV/c114600 #DS0/ =S 4+1B7= 1±4 2.49 uluiknnellii,kk esea,, msmaeixm eeedv t eevvtt unts/(10 Me 1105 #DS0/ =S 8+0B3=82 2±. 3360 Counts/(10 1168020000 Co 5STARPreliminary 40 STARPreliminary 20 10.7 1.75 1.8 1.85 1.9 1.95 2 2.05 2.1 10.7 1.75 1.8 1.85 1.9 1.95 2 2.05 2.1 MKπ (GeV/c2) MKπ (GeV/c2) Fig.2.InvariantmassspectraofKπpairsfor1<pT <2GeV/c(Left)and5<pT <8GeV/c(Right).Theblackpointsareunlike-sign pairswiththeD0signal.Theblueandredpointsshowthelike-signandmixed-eventbackgroundrespectively. 3. PhysicsResultsandDiscussion Although the total charm cross section follows N scaling, the p spectrum is significantly modified bin T in Au+Au collisions [7]. D0 invariant yield (left) and nuclear modification factor R (right) in the most √ AA centralAu+Aucollisionsat s =200GeVareshowninFigure3. ThenewD0invariantyieldsfromthe NN HFT(blue)areconsistentwithpublisheddata(red)withsignificantlyimprovedprecision[7]. π 22]dy) [(GeV/c)dppN/(2dTT1111100000−−−−−54321 STAR D0 @ Au+Au 200 GeV Au+ADDu00 22000 111004%/ 1[/12 0[/]20] RAA 1.521 Au+Au → D0 @ 200 GeV 0 10% pπDDD+ 00 0p0 22 1u0012n110%c04%e/ 1 rSAt1.TLAICRE 10−6 p+p D0+D* 0.5 10−7 STApR+p LPevry esclailmed biyn <aNbriny> STARPreliminary 10−80 1 2 3 4 5 6 7 8 00 2 4 6 8 pT(GeV/c) pT (GeV/c) √ Fig.3.D0mesoninvariantyields(Left)andnuclearmodificationfacto√rRAA(Right)inmostcentralAu+Aucollisionsat sNN=200 GeV.AlsoshownintheleftpanelareD0yieldsinp+pcollisionsat sNN=200GeVfittedwithaLevyfunction.Intherightpanel, verticalbarsondatapointsindicatestatisticaluncertainties,whilethebracketsaresystematicuncertaintiesinAu+Aucollisions.The greybandsareuncertaintiesfromp+pbaseline.RAAofDmesonsfromLHCandπfromRHICarealsoshown.Therearetwovertical boxesaroundunityfromlefttorightrelatedtoAu+AuNcollandp+pnormalizationuncertainties. Figure3(rightpanel)showstheR resultsfromthemostcentral(0-10%)Au+Aucollisions. Thenew AA resultsfromtheHFTareconsistentwiththepublishedonesinthemeasured p range. Furthermore, they T have highly improved precision in Au+Au collisions. The grey bands show uncertainties from the p+ p baselinefromourpreviousmeasurementbeforetheHFTinstallation[1],whichisexpectedtobeimproved aswellwiththeHFTdatatakenintheyear2015RHICrun. TheR showsastrongsuppressionathigh p AA T indicatingstrongcharmmediuminteractionsatthiskinematicregion. Attheintermediate p range(∼0.7-2 T GeV/c),thedatashowsanenhancementwhichcanbedescribedbymodelsincludingcoalescenceofcharm andlightquarks. WecompareourD0 R resultswiththoseofpionsatRHIC(redsquares)andD-mesonsatLHC(blue AA triangles)[1,7,11,12].AsshowninFigure3,athighp R (D0)isclosetoR (π),whichcanbeexplained T AA AA bytakingintoaccountthefactthatcharmenergylossisaninterplayofelasticandradiativeenergyloss[14]. The D-meson R at p > 2 GeV/c is also comparable between RHIC and LHC despite of a factor of 14 AA T differenceincollisionenergy. The left panel of Fig. 4 shows the R results from the most central (0-10%) Au+Au collisions com- AA paredwithvariousmodelcalculations. TheDukemodelusesaLangevinsimulationwithaninputdiffusion coefficientparameter-2πTD = 7,where D isheavyquarkspacialdiffusioncoefficientandTismedium S S 4 /NuclearPhysicsA00(2016)1–4 1.8 Au+Au 200GeV, 0-10% 0.3 D0 2014 Au+Au 200GeV, 0-80% Non-flow est. 1.6 D02010/11 0.25 D0 TAMU TAMU 11..24 SDUukBeATECH 0.2 SDUukBeATECH RAA 1 p+puncert. v20.15 0.8 0.1 0.6 0.05 0.4 0.2 0 STAR Preliminary STAR Preliminary 00 1 2 3 4 5 6 7 8 0.050 1 2 3 4 5 6 7 Transverse Momentum p (GeV/c) Transverse Momentum p (GeV/c) T T √ Fig.4.D0mesonRAA(left)andv2(right)vs.pTinAu+Aucollisionsat sNN=200GeVcomparingwithvariousmodelcalculations. temperature,whichwastunedtotheLHCD-mesonR data[13,14]. TheTAMUcalculationusesanon- AA perturbativeapproachandthefullT-matrixcalculationwithinternalenergypotential,whichpredicts2πTD S tobe∼2-10[14]. TheSUBATECHgroupusesthepQCDcalculationwithHardThermalLooptechnique whichindicatesthe2πTD ∼2-4[14].ThesethreemodelscandescribeourR datapointsreasonablewell. S AA Inthemeantime,wealsocompareourfirstmeasurementof D0 v tomodelcalculations[10]. TheTAMU 2 andSUBATECHcalculationscandescribethemeasuredD0v aswell,whilethespecificDUKEcalculation 2 with2πTD =7seemstounderestimatetheD0v .Ourdatafavormodelswithfinitecharmflow.Tofurther S 2 constrainourunderstandingofthemediumdiffusioncoefficient,itwillbebeneficialtosystematicallystudy eachingredientindifferentmodelcalculations. 4. SummaryandOutlook √ Wereportthefirstmeasurementof D0 R inthemostcentralAu+Aucollisionsat s =200GeV AA NN usingtheSTARnewlyinstalledHFTdetector. Newresultsconfirmthestrongsuppressionathigh p witha T muchimprovedprecision.Theoreticalmodelswithcharmdiffusioncoefficient2πTD ∼2-12canreproduce S simultaneouslyboththeD0R andv datainAu+AucollisionsatRHIC. AA 2 Inyear2015,STARhascollectedhighstatisticsdatasetswiththeHFTin p+pand p+Aucollisionsat √ s =200GeV.Analysesofthesedatasetswillimproveourp+pbaselinefortheR estimationandhelp NN AA understand cold nuclear matter effects. In addition, STAR has requested to collect 2 billion MB Au+Au eventsintheyear2016runwhichwillallowmoreprecisedeterminationofthesQGPtransportproperties. Acknowledgement WeexpressgratitudetoRNCgroupatLBNLandHEPGatUSTCfortheirsupport. FromUSTC,the authorissupportedinpartbytheNSFCunderGrantNo. 11375172andMOSTunderNo. 2014CB845400. References [1] L.Adamczyketal.(STARCollaboration),Phys.Rev.D86,072013(2012). [2] Z.Ye,QM14(STARCollaboration),Nucl.Rhys.A931,520(2014). [3] D.Acostaetal.(CDFCollaboration),Phys.Rev.L91,241804(2003). [4] B.Abelevetal.(ALICECollaboration),Jour.ofHighEnergyPhys.01,128(2012). [5] M.Cacciarietal.,Phys.Rev.L95,122001(2005). [6] L.Adamczyketal.(STARCollaboration),Phys.Rev.L94,62301(2005). [7] L.Adamczyketal.(STARCollaboration),Phys.Rev.L113,142301(2014). [8] Ackermannetal.Nucl.Instrum.Meth.A499624-632(2003). [9] R.Vogt,Eur.Phys.J.ST155,213(2008). [10] G.Contin,M.Lomnitz,QM15(STARCollaboration) [11] L.Adamczyketal.(STARCollaboration),Phys.Lett.B655,104-113(2007) [12] B.Abelevetal.(ALICECollaboration),arXiv:1509.06888[nucl-ex] [13] S.Caoetal.,Phys.Rev.C92,024907(2015) [14] A.Andronicetal.,arXiv:1506.03981[nucl-ex]