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Nuclear Physics A NuclearPhysicsA00(2016)1–4 www.elsevier.com/locate/procedia + Charge-dependent anisotropic flow in Cu Au collisions Takafumi Niida for the STAR Collaboration1 WayneStateUniversity,666W.Hancock,Detroit,MI48201 6 1 0 2 n a Abstract J √ 5 We present the first measurements of charge-dependent directed flow in Cu+Au collisions at s = 200 GeV. The NN directedflowhasbeenmeasuredasfunctionsofthetransversemomentumandpseudorapiditywiththeSTARdetector. ] The results show a small but finite difference between positively and negatively charged particles. The difference is x e qualitativelyexplainedbythepatron-hadron-string-dynamics(PHSD)modelincludingtheeffectoftheelectricfield, - butmuchsmallerthanthemodelcalculation,whichindicatesonlyasmallfractionofallfinalquarksarecreatedwithin l c thelifetimeoftheinitialelectricfield. Higher-orderazimuthalanisotropicflowisalsopresenteduptothefourth-order u forunidentifiedchargedparticlesanduptothethird-orderforidentifiedchargedparticles(π,K,andp).Forunidentified n particles,theresultsarereasonablydescribedbytheevent-by-eventviscoushydrodynamicmodelwithη/s=0.08-0.16. [ ThetrendsobservedforidentifiedparticlesinCu+Aucollisionsaresimilartothoseobservedinsymmetric(Au+Au) collisions. 1 v Keywords: flow,asymmetricheavy-ioncollisions,initialelectricfield 7 1 0 1. Introduction 1 0 At the early stage of a non-central heavy-ion collision, a strong magnetic field perpendicular to the 1. reactionplaneiscreated.InasymmetricCu+Aucollisions,duetothedifferenceinthenumberofspectators, 0 notonlythemagneticfieldbutalsoastrongelectricfieldwouldexistpointingalongthereactionplanefrom 6 the Au-nucleus to Cu-nucleus. The lifetime of the electric field might be short, of the order of a fraction 1 of a fm/c. The quarks and antiquarks that have been already produced at this time would experience the : v Coulombforce,whichresultsinachargedependenceofparticledirectedflow[1,2].Thus,themeasurement i of the charge-dependent directed flow in Cu+Au collisions provides an opportunity to test different quark X (charge) production scenarios, e.g. two-wave quark production [3, 4], and shed light on the (anti-)quark r a productionmechanisminheavy-ioncollisions. Understandingthetimeevolutionofthequarkdensitiesin heavy-ioncollisionsisalsoveryimportantfordetailedtheoreticalpredictionsoftheChiralMagneticEffect andChiralMagneticWave,forwhichvariousexperimentsareactivelysearching. Intheseproceedings,the √ charge-dependentdirectedflowinCu+Aucollisionsat s =200GeVmeasuredwiththeSTARdetector NN ispresented. Resultsofhigher-orderflowarealsopresented. 1AlistofmembersoftheSTARCollaborationandacknowledgementscanbefoundattheendofthisissue. 2 /NuclearPhysicsA00(2016)1–4 2. Analysismethod Azimuthalanisotropiesweremeasuredwiththeeventplanemethoddefinedbelow: v =(cid:104)cosn(φ−Ψ )(cid:105)/Res{Ψ }, (1) n n n where φ is azimuthal angle of particles and (cid:104)(cid:105) means average over all particles in the events of the same centralitybins. TheΨ denotesnth-ordereventplane. Thefirst-ordereventplanewasreconstructedwiththe n ZeroDegreeCalorimeter(ZDC).TheZDCmeasuresspectatorneutronsandthuswouldminimizenon-flow effects such as those from the momentum conservation. For higher harmonics measurements, the event planes were reconstructed from charged tracks (0.15<p <2 GeV/c) reconstructed in the Time Projection T Chamber(TPC)andtheEndcapElectro-MagneticCalorimeter(EEMC).IncaseofusingtheTPC,charged tracksweredividedintotwosubevents(-1<η<-0.4and0.4<η<1)andv ofchargedparticlesofinterestwas n measuredwithanη-gapof0.4usingtheeventplanemethod(e.g. particlesofinterestaretakenfrom0<η<1 whenusingthesubeventfromthebackwardangle). Theresultsfrombothsubeventsareconsistentandthe averageoftwomeasurementswasusedasfinalresults. TheeventplaneresolutionRes{Ψ }wasestimated n bythreesubeventsmethod[5]. Systematicuncertaintieswereestimatedbyvaryingeventz-vertexandtrack quality cuts. The effect of the event plane determination was also taken into account in the systematic uncertainty. Forhigher-orderv ,thescalarproductmethod[6]wasalsotestedjustasacross-check. n 3. Results Cu+Au 200 GeV 20-30% 30-40% 40-50% 50-60% n syst. uncert. evev10.04 wY .Aru.t> Z0DC-SMD Au-going 0.04 STAR Preliminary 0.04 pnoesgiatitvivee 0.04 1 |h |<1 syst. uncert. from EP PHSD+EF 10-40% |h |<2 positive 0.02 p + 0.02 negative 0.02 0.02 p - 0 0 0 0 0.0040 1 2 0.00340 1 2 0.00340 1 2 0.00340 1 2 3 PD HvSD+EF p 10-p40% [G (xe0V.1)/c] p [GeV/c] p [GeV/c] p [GeV/c] 0.002 PHS1D p 10-40% T 0.002 T 0.002 T 0.002 T v1 0 0 0 0 D- 0.002 - 0.002 - 0.002 - 0.002 - 0.004 - 0.004 - 0.004 - 0.004 0 1 2 30 1 2 30 1 2 30 1 2 3 p [GeV/c] p [GeV/c] p [GeV/c] p [GeV/c] T T T T Fig.1.ve1venofpositiveandnegativeparticlesasafunctionofpT infourcentralitybinsandthedifferencebetweenbothcharges,∆v1. ThePHSDmodelcalculationswithandwithouttheinitialelectricfield(EF)[2]arecompared.Themodelcalculationof∆v1withthe EFisscaledby0.1.Seethetextforthedefinitionofpositivedirectionofv1(Ψ1).Thisplothasbeenupdatedsincethepresentation2. Figure1showsv ofpositive(h+)andnegative(h−)chargedparticlesasafunctionof p infourcen- 1 T tralitybins2,wherev ismeasuredwithrespecttothespectatorplaneinAu-goingsideandthesignofthe 1 ΨAuisdefinedtobepositive. Inasymmetriccollisions,themagnitudeofv isnolongersymmetricoverthe 1 1 pseudorapidityunlikesymmetriccollisions,thereforetheevencomponentofv ismeasuredinthisanalysis. 1 Thev at p <1GeV/chasnegativevalueandpositiveatthehigher p , whichmeansmorelow(high) p 1 T T T 2Notethatthefiguresintheseproceedingshavebeenupdatedsincethepresentationtoaccountforasoftwareissueinthecalculation oftheeventplaneresolution.Thefinalresultsonv1arequantitativelyclosetothosepresentedattheconferenceandhadnoimpacton thephysicsconclusions.Thev2andv3inperipheralcollisionsbecomelargerafterthiscorrection. /NuclearPhysicsA00(2016)1–4 3 particles are emitted to the direction of Cu (Au) spectator. Bottom panels of Fig. 1 show the difference betweenbothcharges,∆v =vh+ −vh−. In20-40%centrality,the∆v seemstobenegativein p <2GeV/c, 1 1 1 1 T which is qualitatively consistent with the expectation from the initial electric field (EF), i.e. more posi- tivelychargedparticleswouldmovetothedirectionoftheEFandnegativelychargedparticlesmovetothe oppositeside. Theparton-hadron-string-dynamics(PHSD)model calculations with and without the effect of the EF [2] are compared to the data, where the ∆v for the cal- 1 culationincludingtheeffectoftheEFisscaledby0.1. en Cwu.r+.tA ZuD 2C0-0S MGDeV Au-going Themodelassumesthatallelectricchargesareaffected evv1 0.005 1<pT<2 GeV/c, Y A1u<0 bytheEF,resultinginalargeseparationofv between 1 positiveandnegativechargesasshownintheupperleft 0 panelofFig.1. The∆v issmallerthanthemodelpre- 1 diction,whichindicatesthattheelectricchargesexist- - 0.005 10-40% ingwithinthelifetimeoftheEF(∼0.25fm/c)aremuch hh+- ssyysstt.. uunncceerrtt.. smallerthanthatofallquarkscreatedinthecollisions. - 0.01 syst. uncert. from EP Figure 2 shows v and ∆v as a function of η - 1 -0.5 0 0.5 1 1 1 0.002 h in 10-40% centrality2, where p is integrated over 1<p <2GeVandthesignofΨAuTisdefinedtobeneg- v1 0.001 T 1 D ative (opposite to Fig. 1). The v charge separation is 0 1 clearlyseenin|η|<1and∆v increaseswithη,although STAR Preliminary 1 -0.001 -1 -0.5 0 0.5 1 themagnitudeofv1alsochangeswithη. h Figure 3 shows v , v , and v of positive charged 2 3 4 particles as a function of p using the event plane methodandscalarproductmTethod. Bothmethodsare Fig.2. v1 ofpositiveandnegativeparticlesand∆v1 asa functionofηin10-40%centrality. Thisplothasbeenup- in a good agreement. Calculations from an event-by- datedsincethepresentation2. event viscous hydrodynamic model [7] are compared tothedataofv andv . Themodelresultsusingη/s=0.08andη/s=0.16qualitativelyagreewiththedata 2 3 in 0-5% and 20-30% centrality bins. The centrality dependence of v are similar to the results in Au+Au n collisions[8,9,10]. Therewasnosignificantdifferencebetweenpositiveandnegativechargedparticlesfor higher-orderflows. v2 0-5% v2 20-30% v2 40-50% 0.2 STAR Preliminary 0.2 0.2 v30.01.0510Bohz/esk= 01v.n0{28} pT2 [Ge vV30/c.031].051 0 1 p2T [Ge vV30/c.301].051 0 syst. 1uncert. frpo2mT [EGPeV/c3] even v1 00..0046 Cwu.r+.tπKA Z++u+D+ π2KC-0--0S MGDeV Au-going, ΨA1u>0 10-40% 0.1 h /s=0.16 0.1 0.1 p+p syst. uncert. from EP (π) 0.05 0.05 0.05 0.02 0 0 0 v4 0 1 pT2 [Ge vV4/c3] 0 1 p2T [GeV/c3] 0 Cu+A1u s =p2 T2 0[G0 eGVe/Vc3] 0 0.1 0.1 NN positive particles STAR Preliminary 0.05 0.05 EP-method −0.020 1 2 3 p [GeV/c] 0 0 Scalar product T 0 1 2 3 0 1 2 3 p [GeV/c] p [GeV/c] T T Fig. 4. v1 of π±, K±, and p+p¯ as a function of pT in Fceign.tr3a.litvy2b,ivn3s,manedasvu4readswaifthuntchteioenveonftppTlanine0m-5e%tho,d20a-n3d0%th,easncdala4r0p-5ro0d%-1p0re-s4e0n%tatcioenn2tr.ality. Thisplothasbeenupdatedsincethe uctmethod. Calculationsfromtheevent-by-eventviscoushydrodynamic model[7]arecompared. Thisplothasbeenupdatedsincethepresenta- tion2. 4 /NuclearPhysicsA00(2016)1–4 Identified particle v are also measured combining the TPC dE/dx and the time-of-flight information n fromTime-Of-Flightdetector. Thev ofcharge-combinedπ±,K±,and p+p¯ arepresentedinFig.4andthe 1 v andv ofπ+(π−),K+(K−),and p(p¯)fordifferentcentralitybinsarepresentedinFig.3. Thesametrends 2 3 observedinsymmetriccollisions, suchasthemassorderingatlow p (<2GeV/c)andthebaryon-meson T splittingatintermediate p ,areobserved. T }2 0 }-52% 10 }-220% 20 }-320% 30-40% Ψ Ψ Ψ Ψ {v2 0.2 STAR Prelimina{vry20.2 {v20.2 {v20.2 0.1 0.1 0.1 0.1 0 0 0 0 }Ψ30.150 1 p 2[G0e }Ψ-V53%/03c.1]5 0 1 p 2[G10e }Ψ-V230/30c%.1]5 0 1 p2 [G20e-3V0/3c%] 00Cu+A11u sp = 22[ G20e0V G/33ecV] { T { T { T NNT 3 3 3 v 0.1 v 0.1 v 0.1 π+ π- K+ K- 0.05 0.05 0.05 p p syst. uncert. 0 0 0 π+ 0 1 2 3 0 1 2 3 0 1 2 3 K+ p p [GeV/c] p [GeV/c] p [GeV/c] T T T Fig.5.v2andv3ofπ,K,andp(p¯)asafunctionofpT fordifferentcentralitybins.Thisplothasbeenupdatedsincethepresentation2. 4. Conclusions √ Charge-dependentanisotropicflowinCu+Aucollisionsat s =200GeVhasbeenmeasuredwiththe NN STARdetector. Chargedifferenceofv isclearlyobserved,whichisconsistentwiththeeffectoftheinitial 1 electricfield.Themagnitudeof∆v ismuchsmallerthanthePHSDmodelpredictions,likelyindicatingthat 1 onlyasmallfractionofallfinalstatequarksarecreatedatthetimewhentheelectricfieldisstrong. These resultscouldshedlightonthetimeevolutionofquarkproductioninheavy-ioncollisions.Higher-orderflow v havebeenalsopresented;theyexhibitsimilartrendsobservedinsymmetriccollisions. n References [1] Y.Hirono,M.Hongo,T.Hirano,Phys.Rev.C90(2014)021903. [2] V.Voronyuk,V.D.Toneev,S.A.Sergei,W.Cassing,Phys.Rev.C90(2014)064903. [3] S.Pratt,PoSCPOD2013(2013)023. [4] B.S.A,P.Danielewicz,S.Pratt,Phys.Rev.Lett.85(2000)2689. [5] A.M.Poskanzer,S.A.Voloshin,Phys.Rev.C58(1998)1671. [6] C.Adleretal.(STARCollaboration),Phys.Rev.C66(2002)034904. [7] P.Boz˙ek,Phys.Lett.B717(2012)287–290. [8] J.Adamsetal.(STARCollaboration),Phys.Rev.C72(2005)014904. [9] J.Adamsetal.(STARCollaboration),Phys.Rev.C88(2013)014904. [10] A.Adareetal.(PHENIXCollaboration),Phys.Rev.Lett.107(2011)252301.