Chirality imbalance could occur in local domains inside the hot nuclear matter formed in high-energy heavy-ion collisions. In the presence of a strong magnetic field, this chirality imbalance will induce an electric charge separation along the magnetic field direction due to the chiral magnetic effect (CME) [1]. Previous azimuthal-angle correlation measurements [2] with unidentified charged particles have manifested charge separation signals consistent with the predictions of the CME. But the magnitudes of the background contributions have not been evaluated. In this poster, we present the correlation results with identified particles (protons, pions and kaons) using STAR data of 39 GeV Au+Au collisions. The results will be compared with those from Au+Au at √s NN = 200 GeV, as well as the published results of unidentified particles at √s NN = 39 GeV. Identified particle results for p-π and K-π behave as expected: Opposite sign signal is above the same sign signal, and the the signal is smaller for more central collisions. Cannot claim a charge-separation signal in p-π or K-π because the normalized signal + background value is compatible with background at this level of precision. Study of physics background, e.g. flow, are underway for the charged pairs. *e-mail: [email protected] Knudsen 5-137, 475 Portola Plaza, Los Angeles, CA 90095-1547 Yiwen Huang* (UCLA) for the STAR collaboration Search for Chiral Magnetic Effect with Identified Particles in Au+Au Collisions at √s NN = 39 GeV from RHIC/STAR Chiral Magnetic Effect (CME) Local chirally imbalanced domains may be created in heavy-ion collisions on an event-by-event basis. A chiral system bears a non-zero axial chemical potential, μ 5 . In non-central collisions, a strong magnetic field (B ~ 10 17 -10 18 Gauss) will be produced. An electric current will be induced in chirally imbalanced domains along the B field: Chiral Magnetic Effect (CME). Charge separation fluctuations for all charged particles have been experimentally observed. For identified particles: Is there an indication of CME? dN ± dφ / 1+2a ± sin(φ ± - RP ) γ = hcos(φ ↵ + φ β - 2 RP )i =[hv 1,↵ v 1,β i + B in ] - [ha ↵ a β i + B out ] Motivation Background Contribution: suppressed to a level close to the magnitude of v2 Directed flow: baseline unrelated to the magnetic field P-even quantity: still sensitive to separation effect, i.e., different for “same sign ” and “oppo sign ” Identified Particle Analysis References Acknowledgements Thanks to the STAR Collaboration for the opportunity of this project. Thanks to Professor Huan Zhong Huang and Dr. Gang Wang for mentorship, support and guidance. Thanks to Liwen Wen for help on code and data. Flow Background Experimental Approach “Three-point- correlation” observable “γ” to avoid zero value yield by direct measurement of “a”. Observable: γ correlator Summary Event Plane Reconstruction Solenoidal T racker At RHIC (STAR) Data of Au+Au at 39 GeV 200 GeV were collected by STAR detector in RHIC run 2010. Particle Identification: Time Projection Chamber(TPC) dE/dx is used for proton/pion/kaon identification; Time of Flight (TOF) detector provides mass information used for particle identification. Shifting method is used to make corrections to the event plane. Baseline:κ estimated to be 1.2- 1.4 [3] for three methods[4][5][6]. Indication of signal if higher than baseline: κ that forces ΔH = 0 κ might be different for different particle pairs: different possible implications Correlation signal is contaminated with the background contributions due to collective motion of the collision system (ilow) • elliptic ilow coupled with transverse momentum conservation (TMC), local charge conservation (LCC) and also decay of the clusters. H: background-subtracted correlator, CME contributions Inconclusive to claim a charge-separation signal in proton- pion or in kaon-pion because signal + background is consistent with background at this level of precision. [1]D. Kharzeev, Phys. Lett. B 633 (2006) 260. [2]L. Adamczyk et al., Phys. Rev. Lett. 113 (2014) 052302. [3]F. Wen, L. Wen and G. Wang. arXiv:1608.03205 [nucl-th]. [4]B. Zhang, C.M. Ko, B.-A. Li and Z.-W. Lin, Phys. Rev. C 61, 067901 (2000). [5]B. alver et al. [PHOBOS Collaboration], Phys. Rev. C 83, 024913 (2011); [6]B.B Back et al. [PHOBOS Collaboration], Phys. Rev. C 72, 051901(R) (2005). Cuts VertexZ (cm) (-40,40) Eta (-1,1) Dca (cm) Kaon <1 Pion <2 Proton <1 Tof β >0 Ylocal* (-1.8,1.8) Mass 2 ( GeV 2 /c 4 ) Proton (0.8, 1) Pion (0.01, 0.1) Kaon (0.2, 0.35) p T ( GeV/c) Proton > 0.4 Pion > 0.15 Kaon > 0.2 P ( GeV/c) Proton <2 Pion < 1.6 Kaon < 1.6 nSigma of Proton (-2, 2) nSigma of Kaon (-2, 2) nSigma of Pion (-2, 2) Identiiied Particles Event Particles Opposite sign signal is above the same sign signal, and the the signal is smaller for more central collisions. Proton-Pion *Ylocal is the distance between a TOF hit and the TPC track projection. Kaon-Pion
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Correlation signal is contaminated with the background contributions due to collective motion of the collision system (flow) • decay of the clusters will contribute to the charge separation
signal • elliptic flow coupled with transverse momentum conservation
(TMC) and local charge conservation (LCC)H: background-subtracted correlator, CME contributions
Chiralityimbalancecouldoccurinlocaldomainsinsidethehotnuclearmatterformedinhigh-energyheavy-ioncollisions.Inthepresenceofastrongmagneticfield,thischiralityimbalancewillinduceanelectricchargeseparationalongthemagneticfielddirectionduetothechiralmagneticeffect(CME)[1].Previousazimuthal-anglecorrelationmeasurements[2]withunidentifiedchargedparticleshavemanifestedchargeseparationsignalsconsistentwiththepredictionsoftheCME.Butthemagnitudesofthebackgroundcontributionshavenotbeenevaluated.Inthisposter,wepresentthecorrelationresultswithidentifiedparticles(protons,pionsandkaons)usingSTARdataof39GeVAu+Aucollisions.TheresultswillbecomparedwiththosefromAu+Auat√sNN=200GeV,aswellasthepublishedresultsofunidentifiedparticlesat√sNN=39GeV.
: baseline unrelated to the magnetic fieldBIN - BOUT: background contribution, suppressed to a level close to the magnitude of v2P-even quantity: still sensitive to separation effect, i.e., different for “same sign” and “oppo sign”
Identifiedparticleresultsforp-πandK-πbehaveasexpected:Oppositesignsignalisabovethesamesignsignal,andthethesignalissmallerformorecentralcollisions.Cannotclaimacharge-separationsignalinp-πorK-πbecausethenormalizedsignal+backgroundvalueiscompatiblewithbackgroundatthislevelofprecision.Studyofphysicsbackground,e.g.flow,areunderwayforthechargedpairs.
*e-mail: [email protected] Knudsen 5-137, 475 Portola Plaza, Los Angeles, CA 90095-1547
Yiwen Huang* (UCLA) for the STAR collaboration
Search for Chiral Magnetic Effect with Identified Particles in Au+Au Collisions
at √sNN = 39 GeV from RHIC/STAR
Chiral Magnetic Effect (CME)
Localchirallyimbalanceddomainsmaybecreatedinheavy-ioncollisionsonanevent-by-eventbasis.Achiralsystembearsanon-zeroaxialchemicalpotential,μ5.Innon-centralcollisions,astrongmagneticfield(B~1017-1018Gauss)willbeproduced.AnelectriccurrentwillbeinducedinchirallyimbalanceddomainsalongtheBfield:ChiralMagneticEffect(CME).Chargeseparationfluctuationsforallchargedparticleshavebeenexperimentallyobserved.Foridentifiedparticles:IsthereanindicationofCME?
dN±d�
/ 1 + 2a±sin(�± � RP )
� = hcos(�↵
+ �
�
� 2 RP
)i= [hv1,↵v1,�i+B
in
]� [ha↵
a
�
i+B
out
]
Motivation
Background Contribution: suppressed to a level close to the magnitude of v2
Directed flow: baseline unrelated to the magnetic field
P-even quantity: still sensitive to separation effect, i.e., different for “same sign” and “oppo sign”
Identified Particle Analysis
References
AcknowledgementsThankstotheSTARCollaborationfortheopportunityofthisproject.ThankstoProfessorHuanZhongHuangandDr.GangWangformentorship,supportandguidance.ThankstoLiwenWenforhelponcodeanddata.
Flow BackgroundExperimental Approach
“Three-point-correlation” observable “γ” to avoid zero value yield by direct measurement of “a”.
Observable: γ correlator
Summary
Event Plane Reconstruction
Solenoidal Tracker At RHIC (STAR)
DataofAu+Auat39GeV200GeVwerecollectedbySTARdetectorinRHICrun2010.ParticleIdentification:TimeProjectionChamber(TPC)dE/dxisusedforproton/pion/kaonidentification;TimeofFlight(TOF)detectorprovidesmassinformationusedforparticleidentification.
Shiftingmethodisusedtomakecorrectionstotheeventplane.
Baseline:κestimatedtobe1.2-1.4[3]forthreemethods[4][5][6].Indicationofsignalifhigherthanbaseline:κthatforcesΔH=0κmightbedifferentfordifferentparticlepairs:differentpossibleimplications
Correlationsignaliscontaminatedwiththebackgroundcontributionsduetocollectivemotionofthecollisionsystem(ilow)• ellipticilowcoupledwithtransversemomentumconservation(TMC),localchargeconservation(LCC)andalsodecayoftheclusters.
H:background-subtractedcorrelator,CMEcontributions
Inconclusivetoclaimacharge-separationsignalinproton-pionorinkaon-pionbecausesignal+backgroundisconsistentwithbackgroundatthislevelofprecision.
[1]D.Kharzeev,Phys.Lett.B633(2006)260.[2]L.Adamczyketal.,Phys.Rev.Lett.113(2014)052302.[3]F.Wen,L.WenandG.Wang.arXiv:1608.03205[nucl-th].[4]B.Zhang,C.M.Ko,B.-A.LiandZ.-W.Lin,Phys.Rev.C61,067901(2000).[5]B.alveretal.[PHOBOSCollaboration],Phys.Rev.C83,024913(2011);[6]B.BBacketal.[PHOBOSCollaboration],Phys.Rev.C72,051901(R)(2005).
CutsVertexZ(cm) (-40,40)
Eta (-1,1)
Dca(cm)Kaon <1Pion <2
Proton <1
Tofβ >0
Ylocal* (-1.8,1.8)
Mass2(GeV2/c4)
Proton (0.8,1)
Pion (0.01,0.1)
Kaon (0.2,0.35)
pT(GeV/c)
Proton >0.4
Pion >0.15
Kaon >0.2
P(GeV/c)
Proton <2
Pion <1.6
Kaon <1.6
nSigmaofProton (-2,2)
nSigmaofKaon (-2,2)
nSigmaofPion (-2,2)
IdentiiiedParticles
Event
Particles
Oppositesignsignalisabovethesamesignsignal,andthethesignalissmallerformorecentralcollisions.
Proton-Pion
*YlocalisthedistancebetweenaTOFhitandtheTPCtrackprojection.
Kaon-Pion
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