Energy dependence of J/ψ production in Au + Au collisions at s NN =39,62.4 and 200GeV

Contents lists available atScienceDirect

Physics

Letters

B

www.elsevier.com/locate/physletb

Energy

dependence

of

J

/ψ

production

in

Au

+

Au

collisions

at

√

s

_{N N}

=

39

,

62

.

4 and 200

GeV

STAR

Collaboration

L. Adamczyk

a

,

J.K. Adkins

s

,

G. Agakishiev

q

,

M.M. Aggarwal

ae

,

Z. Ahammed

ax

,

N.N. Ajitanand

an

,

I. Alekseev

o

,

z

,

D.M. Anderson

ap

,

R. Aoyama

at

,

A. Aparin

q

,

D. Arkhipkin

c

,

E.C. Aschenauer

c

,

M.U. Ashraf

as

,

A. Attri

ae

,

G.S. Averichev

q

,

X. Bai

g

,

V. Bairathi

aa

,

A. Behera

an

,

R. Bellwied

ar

,

A. Bhasin

p

,

A.K. Bhati

ae

,

P. Bhattarai

aq

,

J. Bielcik

j

,

J. Bielcikova

k

,

L.C. Bland

c

,

I.G. Bordyuzhin

o

,

J. Bouchet

r

,

J.D. Brandenburg

aj

,

A.V. Brandin

z

,

D. Brown

w

,

I. Bunzarov

q

,

J. Butterworth

aj

,

H. Caines

bb

,

M. Calderón de la Barca Sánchez

e

,

J.M. Campbell

ac

,

D. Cebra

e

,

I. Chakaberia

c

,

P. Chaloupka

j

,

Z. Chang

ap

,

N. Chankova-Bunzarova

q

,

A. Chatterjee

ax

,

S. Chattopadhyay

ax

,

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ak

,

J.H. Chen

am

,

X. Chen

u

,

J. Cheng

as

,

M. Cherney

i

,

W. Christie

c

,

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v

,

H.J. Crawford

d

,

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g

,

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i

,

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c

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q

,

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al

,

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ag

,

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c

,

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af

,

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v

,

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t

,

J.E. Draper

e

,

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f

,

J.C. Dunlop

c

,

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q

,

N. Elsey

az

,

J. Engelage

d

,

G. Eppley

aj

,

R. Esha

f

,

S. Esumi

at

,

O. Evdokimov

h

,

J. Ewigleben

w

,

O. Eyser

c

,

R. Fatemi

s

,

S. Fazio

c

,

P. Federic

k

,

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j

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q

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au

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B. Mohanty

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M.M. Mondal

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D.A. Morozov

ag

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M.K. Mustafa

v

,

Md. Nasim

f

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T.K. Nayak

ax

,

J.M. Nelson

d

,

M. Nie

am

,

G. Nigmatkulov

z

,

T. Niida

az

,

L.V. Nogach

ag

,

T. Nonaka

at

,

S.B. Nurushev

ag

,

G. Odyniec

v

,

A. Ogawa

c

,

K. Oh

ai

,

V.A. Okorokov

z

,

D. Olvitt Jr.

ao

,

B.S. Page

c

,

R. Pak

c

,

Y. Pandit

h

,

Y. Panebratsev

q

,

B. Pawlik

ad

,

H. Pei

g

,

C. Perkins

d

,

P. Pile

c

,

J. Pluta

ay

,

K. Poniatowska

ay

,

J. Porter

v

,

M. Posik

ao

,

N.K. Pruthi

ae

,

M. Przybycien

a

,

J. Putschke

az

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H. Qiu

ah

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A. Quintero

ao

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S. Ramachandran

s

,

R.L. Ray

aq

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R. Reed

w

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M.J. Rehbein

i

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H.G. Ritter

v

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J.B. Roberts

aj

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O.V. Rogachevskiy

q

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J.L. Romero

e

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J.D. Roth

i

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L. Ruan

c

,

http://dx.doi.org/10.1016/j.physletb.2017.04.078

J. Rusnak

k

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O. Rusnakova

j

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N.R. Sahoo

ap

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m

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S. Salur

v

,

J. Sandweiss

bb

,

M. Saur

k

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J. Schambach

aq

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A.M. Schmah

v

,

W.B. Schmidke

c

,

N. Schmitz

x

,

B.R. Schweid

an

,

J. Seger

i

,

M. Sergeeva

f

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P. Seyboth

x

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N. Shah

am

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E. Shahaliev

q

,

P.V. Shanmuganathan

w

,

M. Shao

ak

,

A. Sharma

p

,

M.K. Sharma

p

,

W.Q. Shen

am

,

S.S. Shi

g

,

Z. Shi

v

,

Q.Y. Shou

am

,

E.P. Sichtermann

v

,

R. Sikora

a

,

M. Simko

k

,

S. Singha

r

,

M.J. Skoby

n

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N. Smirnov

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D. Smirnov

c

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W. Solyst

n

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L. Song

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P. Sorensen

c

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H.M. Spinka

b

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B. Srivastava

ah

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aw

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M. Strikhanov

z

,

B. Stringfellow

ah

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T. Sugiura

at

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M. Sumbera

k

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B. Summa

af

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X. Sun

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Y. Sun

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X.M. Sun

g

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B. Surrow

ao

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D.N. Svirida

o

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A.H. Tang

c

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ak

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T. Tarnowsky

y

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ba

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J. Thäder

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ar

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D. Tlusty

aj

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T. Todoroki

c

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q

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S. Trentalange

f

,

R.E. Tribble

ap

,

P. Tribedy

c

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j

,

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f

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T. Ullrich

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,

D.G. Underwood

b

,

I. Upsal

ac

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G. Van Buren

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∗

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Z. Zhang

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X.P. Zhang

as

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J.B. Zhang

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S. Zhang

ak

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J. Zhang

u

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Y. Zhang

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J. Zhang

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S. Zhang

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J. Zhao

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C. Zhong

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L. Zhou

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C. Zhou

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X. Zhu

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Z. Zhu

al

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M. Zyzak

l

a_{AGHUniversityofScienceandTechnology,FPACS,Cracow30-059,Poland} b_{ArgonneNationalLaboratory,Argonne,Illinois 60439}

c_{BrookhavenNationalLaboratory,Upton,NewYork 11973} d_{UniversityofCalifornia,Berkeley,California 94720} e_{UniversityofCalifornia,Davis,California 95616} f_{UniversityofCalifornia,LosAngeles,California 90095} g_{CentralChinaNormalUniversity,Wuhan,Hubei430079} h_{UniversityofIllinoisatChicago,Chicago,Illinois 60607} i_{CreightonUniversity,Omaha,Nebraska 68178}

j_{CzechTechnicalUniversityinPrague,FNSPE,Prague,11519,CzechRepublic} k_{NuclearPhysicsInstituteASCR,25068Prague,CzechRepublic}

l_{FrankfurtInstituteforAdvancedStudiesFIAS,Frankfurt60438,Germany} m_{InstituteofPhysics,Bhubaneswar751005,India}

n_{IndianaUniversity,Bloomington,Indiana 47408}

o_{AlikhanovInstituteforTheoreticalandExperimentalPhysics,Moscow117218,Russia} p_{UniversityofJammu,Jammu180001,India}

q_{JointInstituteforNuclearResearch,Dubna,141980,Russia} r_{KentStateUniversity,Kent,Ohio 44242}

s_{UniversityofKentucky,Lexington,Kentucky,40506-0055} t_{LamarUniversity,PhysicsDepartment,Beaumont,Texas 77710}

u_{InstituteofModernPhysics,ChineseAcademyofSciences,Lanzhou,Gansu730000} v_{LawrenceBerkeleyNationalLaboratory,Berkeley,California 94720}

w_{LehighUniversity,Bethlehem,Pennsylvania18015} x_{Max-Planck-InstitutfurPhysik,Munich80805,Germany} y_{MichiganStateUniversity,EastLansing,Michiagan 48824} z_{NationalResearchNuclearUniversityMEPhI,Moscow115409,Russia}

aa_{NationalInstituteofScienceEducationandResearch,Bhubaneswar751005,India} ab_{NationalChengKungUniversity,Tainan70101}

ac_{OhioStateUniversity,Columbus,Ohio 43210} ad_{InstituteofNuclearPhysicsPAN,Cracow31-342,Poland} ae_{PanjabUniversity,Chandigarh160014,India}

af_{PennsylvaniaStateUniversity,UniversityPark,Pennsylyania 16802} ag_{InstituteofHighEnergyPhysics,Protvino142281,Russia} ah_{PurdueUniversity,WestLafayette,Indiana 47907} ai_{PusanNationalUniversity,Pusan46241,RepublicofKorea} aj_{RiceUniversity,Houston,Texas 77251}

ak_{UniversityofScienceandTechnologyofChina,Hefei,Anhui230026} al_{ShandongUniversity,Jinan,Shandong250100}

am_{ShanghaiInstituteofAppliedPhysics,ChineseAcademyofSciences,Shanghai201800} an_{StateUniversityOfNewYork,StonyBrook,NewYork11794}

ao_{TempleUniversity,Philadelphia,Pennsylvania 19122} ap_{TexasA&MUniversity,CollegeStation,Texas 77843} aq_{UniversityofTexas,Austin,Texas 78712} ar_{UniversityofHouston,Houston,Texas 77204} as_{TsinghuaUniversity,Beijing100084} at_{UniversityofTsukuba,Tsukuba,Ibaraki,Japan}

au_{SouthernConnecticutStateUniversity,NewHaven,Connecticut06515} av_{UnitedStatesNavalAcademy,Annapolis,Maryland 21402}

ax_{VariableEnergyCyclotronCentre,Kolkata700064,India} ay_{WarsawUniversityofTechnology,Warsaw00-661,Poland} az_{WayneStateUniversity,Detroit,Michigan 48201}

ba_{WorldLaboratoryforCosmologyandParticlePhysics(WLCAPP),Cairo11571,Egypt} bb_{YaleUniversity,NewHaven,Connecticut 06520}

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Articlehistory: Received26July2016

Receivedinrevisedform11April2017 Accepted20April2017

Availableonline10May2017 Editor:M.Doser

Theinclusive J/ψ transversemomentumspectraandnuclearmodificationfactorsarereportedat mid-rapidity(|y|<1.0)inAu+Aucollisionsat√sN N=39,62.4and200GeVtakenbytheSTARexperiment.

Asuppressionof J/ψproduction,withrespecttotheproductioninp+p scaledbythenumberofbinary nucleon–nucleoncollisions,isobservedincentralAu₊Aucollisionsatthesethreeenergies.Nosignificant energydependenceofnuclearmodificationfactorsisfoundwithinuncertainties.Themeasurednuclear modificationfactorscanbedescribedbymodelcalculationsthattakeintoaccountbothsuppressionof direct J/ψ productionduetothe colorscreeningeffectand J/ψ regenerationfromrecombinationof uncorrelatedcharm–anticharmquarkpairs.

©2017TheAuthor.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3_.

1. Introduction

TheRelativisticHeavy IonCollider(RHIC) wasbuiltto investi-gate strongly interacting matter at high temperature and energy densityinthelaboratorythroughhigh-energyheavy-ioncollisions. Atextremelyhightemperaturesandbaryondensities,a transition fromthehadronicphase ofmatter to anewdeconfined partonic

phase, the Quark–Gluon Plasma (QGP), is predicted by Quantum

Chromodynamics(QCD)

[1–8]

.Ithasbeenproposedthatthecolor potentialinquarkoniacouldbe screenedbyquarksandgluonsin theQGP

[9]

.Quarkoniaareboundstatesofcharm–anticharm(c

¯

c)

orbottom–antibottom(bb)

¯

quarkpairs.Asaconsequence, quarko-niumproductioncrosssectionsinheavy-ion collisionsdivided by the corresponding number of binary nucleon–nucleon collisions,

Ncoll, areexpected tobe suppressed compared tothose in p

+

p

collisionsifQGPisformedinheavy-ioncollisions.

The J

/ψ

is the most abundantly produced quarkonium state

accessibleto experiments.Over the past twentyyears, J

/ψ

sup-pression in hot and dense media has been a topic of growing

interest. Various measurements of J

/ψ

production in heavy ion collisions havebeenperformed indifferentcollision systemsand atdifferentenergies,andindeedasuppressionof J

/ψ

production has been observed [10–13]. A similar centrality dependent sup-pressionwasfoundatSPS(S

+

U

√

sN N

=

19.4GeV

[14]

,Pb

+

Pb

√

sN N

=

17.2 GeV[15] andIn

+

In

√

sN N

=

17.2 GeV [12]) and

atRHIC(Au

+

Au

√

sN N

=

200GeV

[16,17]

)formid-rapidity,even

thoughthetemperatureandenergydensityreachedinthese stud-iesaresignificantlydifferent

[18]

.Furthermore,astronger suppres-sionatforwardrapidity(1

.

2

<

|

y

|

<

2

.

2)comparedtomid-rapidity (

|

y

|

<

0

.

35) was observed at RHIC [16]. These observations indi-catethateffectsotherthancolorscreeningareimportantfor J

/ψ

production.Amongtheseeffects, J

/ψ

productionfromthe recom-bination of cc

¯

[19,20], together with color screening effect, play importantrolesinexplainingthesimilarsuppressions atSPS and RHIC

[21]

.Withthehigher temperatureanddensityatRHIC,the increasedcontributiondueto regenerationfromthelargercharm

quark density could compensate for the enhanced suppression.

Thiscould also explain a stronger suppression atforward rapid-ityatRHIC wherethecharmquark densityislower comparedto mid-rapidity [20–23]. In addition to the color screening and re-generationeffects, thereare alsomodifications fromcold nuclear

*

Correspondingauthor.

E-mailaddress:wangmei@rcf.rhic.bnl.gov(W. Zha).

matter(CNM)effects,suchasnuclearpartondistributionfunction modification

[24]

,energylossby thecollidingnuclei

[25]

,Cronin effect [26], and other final state effects, such as nuclear absorp-tion

[27]

anddissociationbyco-movers

[28]

.Thesuppressiondue totheseeffectshasbeensystematicallystudiedexperimentallyvia

p

+

A collisions

[29–39]

.However,theextrapolationfromp

+

A to A

+

Aisstillmodeldependent.

The nuclearmodification factorof J

/ψ

productioninPb

+

Pb collisions at

√

sN N

=

2

.

76 TeV has been measured at the LHC [40–42]. In comparison with results from RHIC in Au

+

Au col-lisions at

√

sN N

=

200 GeV, the J

/ψ

production is significantly

lesssuppressed, whichsuggests significantlymore recombination contribution at LHC energies. The measurement of J

/ψ

produc-tionatforwardrapidity (1

.

2

<

|

y

|

<

2

.

2) inAu

+

Aucollisionsby the PHENIXexperiment at

√

sN N

=

39 and62.4GeVindicates a

similar suppression levelasthat at

√

sN N

=

200GeV

[43]

.

Mea-surementsof J

/ψ

invariantyieldsatdifferentcollisionenergiesat RHICindifferentcentralitiesatmid-rapiditycanshednewlighton theinterplayofthemechanismsfor J

/ψ

productionandmedium properties.

Inthisletter,wefurtherstudythecollisionenergydependence of J

/ψ

production andtest thehypothesis ofthetwo competing mechanismsofcolorscreeningandregeneration.Wepresent mea-surements of the J

/ψ

production at mid-rapidity (

|

y

|

<

1) with theSTARexperimentinAu

+

Aucollisionsat

√

sN N

=

39,62.4

us-ingdatacollectedinyear2010andat

√

sN N

=

200GeVusingthe

combineddatainyear2010

[17]

and2011andstudythenuclear modificationfactorsattheseenergies.

2. Experimentandanalysis

The STAR experimentis a large-acceptance multi-purpose de-tector which covers full azimuth in the pseudorapidity interval

Table I

Summaryofcentrality bins,averagenumber ofparticipants Npart,number of

binarycollisionsNcoll,andnuclearoverlapfunctionTA AfromMCGlauber

sim-ulationofAu+Auat√sN N=39,62and200GeV.Theerrorsindicateuncertainties

fromtheMCGlaubercalculations.

√_s

N N(GeV) Centrality (%) Npart Ncoll TA A (fm−2)

39 0–20 273±6 629±26 187±5 20–40 137±11 245±26 71±7 40–60 59±10 79±17 23±5 0–60 156±8 316±22 93±6 62 0–20 276±5 664±25 187±5 20–40 139±10 258±27 71±7 40–60 60±10 82±18 23±5 0–60 157±9 332±23 93±6 200 0–20 280±6 785±29 187±5 20–40 142±11 300±31 71±7 40–60 62±10 95±21 23±5 0–60 161±9 393±27 93±6

[47],theTime-of-Flight(TOF)detector

[48]

,andtheBarrel Electro-magneticCalorimeter(BEMC)

[49]

.TheTPCprovidestrackingand particle identification via the ionization energy loss (



dE

/

dx



) of charge particles. The TOF [48] measures the velocity ofparticles, whichgreatlyimprovedelectronidentificationatlowmomentum. Thisdetector,combinedwiththeTPC

[47]

,clearly identifies elec-tronsbyrejectinghadronsinthelowandintermediatemomentum range(p

<

1

.

5GeV

/

c).TheBEMC

[49]

,alead-scintillator calorime-ter,isusedtoimprovetheelectronidentificationathigh momen-tum(p

>

1

.

5 GeV

/

c).Theelectronidentificationmethodissimilar toRefs.[17,50].

Collision centrality was determined from the uncorrected

chargedparticlemultiplicitydN

/

d

η

within

|

η

|

<

0

.

5 usingaMonte Carlo(MC) Glaubermodel

[51]

.ThedependenceofdN

/

d

η

onthe collision vertex position Vz and the beam luminosity has been

included to take acceptance andefficiency changes on the

mea-sureddN

/

d

η

intoaccount.Foreachcollisioncentrality,anaverage nuclear overlap function,



TA A



, average number of participants,



Npart



, andaverage numberofbinary collisions,



Ncoll



,were

re-lated to an observed multiplicity range. Centrality definitions in Au

+

Aucollisions for

√

sN N

=

39,62.4and200GeVare

summa-rizedin

Table I

.

Thedaughtertracksofthe J

/ψ

candidatesarerequiredtohave atleast25out ofthe45possibleTPChits,andadistanceof clos-est approach (DCA) from the primary vertex of less than 3 cm. Lowmomentum(p

<

1

.

5 GeV

/

c)electronandpositroncandidates are separated from hadronsby selecting on the inverse velocity,

|

1

/β

−

1

|

<

0

.

03, where

β

is the velocity measured in the TOF normalizedbythespeedoflight.Thecut valueisdetermined us-ing athree standarddeviationwindow. At highmomentum(p

>

1

.

5 GeV

/

c),a cutontheratioofmomentumtoenergydepositedin towersfromBEMC(0

.

3

<

pc

/

E

<

1

.

5)isusedtosuppresshadrons. The electronandpositron candidates arethen identified by their specific energyloss (



dE

/

dx



) in theTPC. More than 15 TPChits are requiredto calculate



dE

/

dx



.The normalized



dE

/

dx



is de-finedasfollows:

n

σ

e

=

ln

(



dE

/

dx



m

/



dE

/

dx



th_e

)

RdE/dx

(1)

where



dE

/

dx



m_and

_

_dE

_/

_dx

_

th_{representmeasuredandtheoretical}

values,respectively,and RdE/dxistheexperimentalln

(

dE

/

dx

)

res-olution.The n

σ

e cut forelectronidentificationis

−

1

.

5

<

n

σ

e

<

2.

The combinationof thesecuts enablesthe identification of elec-tronsandpositronsovera widemomentumrange

[17]

.The

elec-tron sample purity integratedover themeasured momentum

re-gion is over 90%. Our measurement of J

/ψ

covers the rapidity range

|

y

|

<

1 duetotheSTARacceptanceanddecaykinematics.

The J

/ψ

signalisextractedbysubtracting combinatorial back-groundreconstructedfromtheunlike-signmixed-eventsspectrum. Thelike-signdistributionscanbeusedasnormalizationreferences forthemixed-eventsmethod.Thelike-signandmixed-events dis-tributionsareobtainedasfollows:

1) Like-sign:Electrons(orpositrons)ofthesamechargesignare pairedwithinthesameevent.

2) Mixed-events:Eventsarecategorizedaccordingtotheposition alongthebeamlineoftheprimaryvertexandcentralityofthe event.Electronsfromoneeventarepairedwithpositronsfrom other randomevents froman eventpool withsimilar global features such as collision centrality and vertex position. The vertexpositionisdividedinto20binsandtheeventcentrality into 10 bins to ensure that the mixing is done using tracks fromsimilarconditions.

The invariantmassdistribution ofe+e− pairsbeforeandafter thecombinatorialbackgroundsubtractionin0–60%centralAu

+

Au collisions areshownin

Fig. 1

for

√

sN N

=

39,62.4,and200GeV.

The mixed-event background is normalized to the like-sign dis-tribution in amass rangeof 2

.

0–4

.

0 GeV

/

c2 _{andthe normalized} shapesshowcloseagreement.Thenormalizationtechniqueis de-scribedinRef.[52].Themassdistributionofe+e− isfittedbythe

J

/ψ

signal shape obtained from MC simulation, which includes the resolution of the TPC, bremsstrahlung of the daughter elec-tronsinthedetectorandinternalradiationof J

/ψ

,combinedwith a straight line for residual background. The residual background mainly comesfromthe correlatedopen charm decaysandDrell– Yanprocesses.The raw J

/ψ

signal isobtainedfrombincounting in themass range 2

.

7–3

.

2 GeV

/

c2 aftercombinatorial and resid-ualbackgroundsubtraction.Thefractionof J

/ψ

countsoutsideof

the mass window was determined from the J

/ψ

MC simulated

signal shape andwas foundto be

∼

9%. Thiswas used tocorrect the numberof J

/ψ

counts. Signal-to-backgroundratios forthese threeenergiesareobservedtobe0.62,0.39,and0.04,respectively for the transverse momentum (pT) interval 0–3 GeV

/

c (39 and

62 GeV)and0–5GeV

/

c (200GeV).The J

/ψ

invariantyieldis

de-finedas BrJ/ψ→e+e− d2_N 2

π

pTdpTdy

=

1 2

π

pT



pT



y N_J/ψ→e+_e− A



NE V T (2)

where NJ/ψ→e+e− is the uncorrected number of reconstructed J

/ψ

, NE V T is thenumberofeventsinthe relevantAu

+

Au

cen-trality selection, A



is the detector’s geometric acceptance times its efficiency(about0.05

∼

0.12depending on pT,centralityand

collisionenergy),and



pT and



y arethebinwidthinpT andy,

respectively.Acceptanceandefficiencycorrections(TPCandBEMC

related) are estimated by MC simulations with GEANT3 package

[53].Some oftheefficiencycorrectionssuchasthose correspond-ing totheTOFanddE

/

dx relatedcutsare extracteddirectlyfrom data [52]. The acceptance and efficiency correction procedure is similartoRefs.[17,50].

Fig. 1. Thee+e− invariant massdistributionof J/ψ candidates(blackopencircles),like-signcombinatorialbackground(bluedashedline),mixedeventcombinatorial background(red solidline),and J/ψ candidateswith mixedevent backgroundsubtracted(blacksolidcircles) inAu+Aucollisionsat √sN N = 39(a),62.4(b),and

200 GeV (c)forcentrality0–60%.The J/ψsignalshapefromaMCsimulationiscombinedwithalinearresidualbackgroundandisfittedtothecombinatorialbackground subtracteddata(blacksolidline).(Forinterpretationofthereferencestocolorinthisfigurelegend,thereaderisreferredtothewebversionofthisarticle.)

thecontributionsfromthedifferentsources.Therangesinthe ta-ble are corresponding to the pT, centrality and collision energy

dependence of uncertainties. The uncertainties are partially cor-relatedamong the pT andcentrality intervals. The total

system-atic uncertainties in the integrated pT range are 20%, 11%, and

10% at

√

sN N

=

39,62.4, and200 GeV, respectively. At

√

sN N

=

39 GeV,thelargesystematicuncertaintyontheparticle identifica-tionBEMCrelatedcutsisduetothelargeuncertaintiesassociated tothecutsthemselves.Thenormalizationuncertaintyon the nu-clearmodificationfactorincludestheuncertaintyfrom



TA A



and

thestatisticalandsystematicuncertaintyofthe J

/ψ

crosssection inp

+

p.Thecentralityandtransversemomentumdependenceof thetotalsystematicuncertaintiesarereflectedintheresultsshown inSection3.

3. Results

The J

/ψ

invariant yields as afunction of pT in Au

+

Au

col-lisions at

√

sN N

=

39, 62.4, and200 GeVfor differentcentrality

bins are shown in Fig. 2. As expected, the J

/ψ

invariant yields arelarger inAu

+

Aucollisions atlarger center-of-massenergies. Resultsfromthecurrentmeasurements(year2011)arecompared with the published results from data taken in 2010, they show

close agreement with each other. These two measurements are

combined together to cumulate more statistics for the nuclear modificationfactorsinthispaper.

Table II

The contributions of systematic uncertainty sources for √sN N = 39, 62.4 and

200 GeV.Theuncertaintiesarepartiallycorrelatedamongthe pT and centrality

intervals.

Systematic uncertainty source 39 GeV 62.4 GeV 200 GeV TPC tracking cuts (%) 8 7 6 BEMC related cuts (%) 17–25 3–5 1–2 TOF related cuts (%) 2 2 2 Yield extraction (%) 6–12 2–7 5–11 Total (%) 19–29 10–12 8–12

Ncoll(%) 4–22 4–22 4–22 TA A(%) 3–22 3–22 3–22

σppJ/ψ (%) 12 7 14

Nuclear modification factors (RC P, RA A) are used to quantify

thesuppressionof J

/ψ

production.RC P isaratioofthe J

/ψ

yield

in central collisions to peripheral collisions (centrality: 40–60%) anddefinedasfollows:

RC P

=

dN/dy Ncoll

(

central

)

dN/dy Ncoll

(

peripheral

)

(3)

where



Ncoll



and dN

/

dy are the average number of nucleon–

nucleoncollisionsand J

/ψ

yieldinagivencentrality,respectively.

dN

/

dy is obtained from the integration of the J

/ψ

pT

spec-trum. The extrapolationof the pT spectrum to the full coverage

Fig. 2. J/ψ invariantyieldsinAu+Aucollisionsat√sN N =39,62.4and200GeVasafunctionofpT fordifferentcentralities.Theerrorbarsrepresentthestatistical

uncertainties.Theboxesrepresentthesystematicuncertainties.TheSTARpublishedresultsarefromRefs.[50]and[17].

Fig. 3. J/ψRC Presults(withrespectto40–60%peripheralcollisions)forAu+Auas

afunctionofNpart.Theerrorbarsrepresentthestatisticaluncertainties.Theboxes

representthesystematicuncertaintiescombinedwithuncertaintiesfromNcollin

differentcentralitybins.

dN dpT

=

a

×

pT

(

1

+

b2_p2 T

)

n (4) dN dpT

=

l

×

pT

×

exp− mT h

_,

_mT

=



p2_T

+

m2_J/ψ (5)

where a, b, n, h and l are free parameters. The fit results from Eq. (4) have been assigned as central value, andthe differences (

<

2%) between these two functional fits have been taken as a sourceof systematicuncertainty. Notethat RC P reflectsonly

rel-ativesuppression–ifthemodificationof J

/ψ

yieldincentraland peripheralbinsisthesame,RC P isequalto 1.TheRC P,asa

func-tion ofthe average number of participantnucleons (



Npart



), for

Au

+

Aucollisions at

√

sN N

=

39,62.4 and200GeV, are shown

in

Fig. 3

.Notethat theperipheralbinselection is40–60%central Au

+

Aucollisions forthesethreeenergies. Thesystematic uncer-taintiesfor RC P aremainlyfrom



Ncoll



andyield extraction.

Sys-tematicuncertainties originatingfromTPC,BEMCandTOFrelated cuts,arenegligibleormostlycancel.Asuppressionisobservedin centralAu

+

Aucollisionsat

√

sN N

=

62.4GeV,whichissimilarto

thatat

√

sN N

=

200GeV.

RA A isobtainedfromcomparing J

/ψ

productioninA

+

A

col-lisionstop

+

p collisions,definedasfollows:

RA A

=

1



TA A



d2NA A

/

dpTdy d2

_σ

pp

/

dpTdy (6)

where d2NA A

/

dpTdy is the J

/ψ

yield in A

+

A collisions and d2

σ

pp

/

dpTdy is the J

/ψ

cross section in p

+

p collisions. The

nuclear overlap function with impact parameter b is defined as

TA A

(

b

)

=



TA

(

s

)

TA

(

s

−

b

)

d2s, where TA

(

s

)

is theprobability per

unit transverse areaof anucleon beinglocatedinthe targetflux tube.TheuncertaintiesfromTA A areestimatedbyvaryingthe

ra-dius andskin depth of the nuclei in the Glauber calculations. If there areno hotorcold nuclearmatter effects,thevalue of RA A

shouldbeunity.

To obtain RA A at

√

sN N

=

39 and62.4 GeV, we haveto

de-rivethe J

/ψ

crosssectioninp

+

p collisionsbecausethereareno measurementsavailableforthe p

+

p referencesatSTARforthese twoenergies.Thereareseveralp

+

p measurementsfromfixed tar-get p

+

A experiments

[54–56]

andfromIntersectingStorageRing (ISR) colliderexperiments [57,58] nearthese two energies. How-ever,the pT shapesfromRef.[57] andRef.[58] at

√

s

=

63GeV are inconsistent witheach other and the crosssection measure-ments at

√

s

=

39 GeV are comparableto (or even larger than) that at

√

s

=

63 GeV. Therefore, we use the cross section de-rived in Ref. [59] as our p

+

p reference baselines for

√

sN N

=

39 and62.4GeV. In Ref. [59],the world-wide experimental data on J

/ψ

cross sections and kinematicdistributions in p

+

p and p

+

A collisions at

√

s

=

6.8–7000 GeV are examined in a sys-tematicway.Theauthorsexplorethe

√

s dependenceofthe inclu-sivecrosssection,rapidityandtransversemomentumdistributions phenomenologicallyanddevelopastrategyfortheinterpolationof the J

/ψ

crosssection andkinematics atRHIC energies. This ap-proach isfound todescribe theworld-wide J

/ψ

datareasonably well. With this strategy, the predicted J

/ψ

cross section times

branching ratio at

√

s

=

39 and 62.4 GeV in mid-rapidity are

Br

(

J

/ψ

→

e+e−

)

d

σ

/

dy

|

_|y|<1.0

=

9

.

0

±

0

.

6 and 17

.

6

±

2

.

1 nb, re-spectively.

Withthederived p

+

p referencesfor

√

s

=

39and62.4GeV, andthemeasured p

+

p baselineat

√

s

=

200GeV

[50,60]

,we ob-taintheRA A of J

/ψ

forpT

>

0 asafunctionof



Npart



inAu

+

Au

collisionsat

√

sN N

=

39,62.4,and200GeV,asshownin

Fig. 4

(a).

The pT-differential J

/ψ

RA A isshownin

Fig. 4

(b).Thebarsand

boxes on the data points representthe statistical andsystematic uncertainties, respectively. The shaded and hatched bands indi-catetheuncertaintieson thebaseline J

/ψ

crosssection in p

+

p

collisions

[50,59,60]

and



TA A



,respectively.Thebandsonthe

Fig. 4. Theresultsof J/ψ RA A asafunctionofNpart(a)and pT (b)inAu+Aucollisionsat√sN N =39,62.4and200GeV.Theerrorbarsrepresentthestatistical

uncertainties.Theboxesrepresentthesystematicuncertainties.The shadedand hatchedbandsindicate theuncertaintiesonthe baseline J/ψ crosssection inp+p collisions[50,59,60]andTA A,respectively.TheALICEpointsarefrom[61].Theratiooffeed-down J/ψ fromhigherchamoniumstatestoinclusive J/ψ isfrom[63].The

STARhigh-pT(3<pT<10GeV/c)results,representedasopencircles,arefrom[50].

ψ(

2S

)

and

χ

c meltingandnomodificationofthe J

/ψ

yield

[63]

arealsoincluded forcomparison.Suppression of J

/ψ

production is observed in Au

+

Au collisions from

√

sN N

=

39 to 200 GeV

withrespecttotheproductionin p

+

p scaledby



TA A



.For RA A

asa function of



Npart



,no significant energydependence is

ob-served within uncertainties from

√

sN N

=

17.2 to 200 GeV. For

the J

/ψ

RA A as a function of pT, significant suppression is

ob-served atlow pT (pT

<

2 GeV

/

c) from

√

sN N

=

39to 200 GeV.

Themodificationof J

/ψ

productionisconsistent within the sys-tematicuncertainties forthese collision energies. The ALICE [61]

pointsarealsoshownforcomparison.Asshowninthefigure,the ALICERA A resultsarehigherthanthemeasurementsatRHICand

SPSandshow a differenttrendasafunction of pT.

Fig. 5

shows

thecomparisonof RA A betweenmid-rapidityfromSTARand

for-ward rapidity from PHENIX from

√

sN N

=

39 to 200 GeV. The

suppression of J

/ψ

shows no significant rapidity dependenceat

√

sN N

=

39nor62.4GeVwithinuncertainties.

Asshownin

Fig. 6

,theoreticalcalculations

[21]

withinitial sup-pressionand J

/ψ

regenerationdescribe thedatawithin1.6 stan-darddeviationforthesethreecollisionenergies.TheRA A resultsas

afunctionofcollisionenergyfor0–20% centralityare alsoshown in

Fig. 7

.Theoreticalcalculationsarealsoincludedforcomparison. Thecalculationsincludetwo components: directsuppression and

Fig. 5. J/ψ RA AresultsasafunctionofNpartinAu+Aucollisionsat√sN N=39,

62.4and200GeV.Theerrorbarsrepresentthestatisticaluncertainties.Theboxes representthesystematicuncertainties.Theshadedandhatchedbandsindicatethe uncertaintiesonthebaseline J/ψ crosssectioninp+p collisions[50,59,60]and

TA A,respectively.ThePHENIXresultsarefrom[43,62].

regeneration. The direct suppression represent the “anomalous” suppression ofprimordial J

/ψ

sdue toCNM andcolor screening effects.Accordingtothemodelcalculations

[21]

,theRA A isabout

Fig. 6. Theresultsof J/ψ RA A asafunctionofNpart,incomparisonwithmodelcalculations[21],forAu+Aucollisionsat√sN N =200(a),62.4(b)and39GeV(c),

respectively.Theerrorbarsrepresentthestatisticaluncertainties.Theboxesrepresentthesystematicuncertainties.Thedottedandhatchedbandsindicatetheuncertainties onthebaseline J/ψ crosssectioninp+p collisions[50,59,60]andTA A,respectively.Solidlinesare J/ψ modificationfactorsfrommodel[21];dash-dottedlineare

Fig. 7. TheresultsofJ/ψ RA Aasafunctionofcollisionenergyforcentrality0–20%,

incomparisonwith modelcalculations[21].The SPSresult(√sN N = 17.2GeV)

isfrom[10,15];theALICEpoint(√sN N =2.76TeV)isfrom[61].Theerrorbars

representthestatisticaluncertaintiesand theboxesrepresentthesystematic un-certainties.Theboxesincludethesystematicuncertainties,theuncertaintiesonthe baseline J/ψcrosssectioninp+p collisions[50,59,60]andtheuncertaintiesfrom

TA A.Solidlineisthetotal J/ψmodificationfactorsfrommodel;dash-dottedline

isthesuppressedprimordialproduction;dashed lineistheregeneration compo-nent.Thetheorycalculationsareonlydoneforthefivespecificenergypoints,and connectedbystraightlines.Note:sinceALICEdatashownosignificantcentrality dependence,itisappropriatetousetheavailable0–10%dataat2.76TeVinthis figure.

0.6forcentral Au

+

Aucollisions at

√

sN N

=

200GeVwithonly

CNM effects. The regeneration component is responsible for the contributionfromtherecombinationofcorrelatedoruncorrelated

c

¯

c pairs.The feed-downto J

/ψ

from

χ

c and

ψ

 hasbeen taken

intoaccountinthecalculations.Nosignificantenergydependence of RA A for 0–20%centralityis observedat

√

sN N

<

200GeV. As

thecollisionenergyincreasestheQGPtemperatureincreases,thus the J

/ψ

colorscreeningbecomesmoresignificant.However,inthe theoreticalcalculation[21],theregenerationcontributionincreases withcollisionenergyduetotheincreaseinthecharmpair produc-tion,andcompensatestheenhancedsuppressionarising fromthe highertemperature.ThehigherRA AatALICEmayindicatethatthe

surviving J

/ψ

saremainlycomingfromtherecombination contri-bution.Themodelcalculationdescribestheenergydependenceof

J

/ψ

productionfromSPStoLHC. 4. Summary

Insummary,we report onrecentSTARmeasurements of J

/ψ

production atmid-rapidity in Au

+

Au collisions at

√

sN N

=

39,

62.4and200GeV.Suppressionof J

/ψ

production,withrespectto theproductionin p

+

p scaledbythenumberofbinarynucleon– nucleoncollisions, isobservedatthesethreeenergies.No signifi-cantenergydependenceofthenuclearmodificationfactor(either

RA A or RC P) is found within uncertainties. Model calculations,

whichincludedirectsuppressionandregeneration,reasonably de-scribethecentralityandenergydependenceof J

/ψ

productionin high-energyheavyioncollisions.

Acknowledgements

We thank the RHIC Operations Group and RCF at BNL, the

NERSC Center at LBNL,the KISTI Center in Korea, and the Open ScienceGridconsortiumforprovidingresourcesandsupport.This workwassupportedinpartbytheOfficeofNuclearPhysicswithin theU.S.DOEOfficeofScience,theU.S.NSF,theMinistryof Educa-tionandScience oftheRussian Federation, NSFC,CAS, MoSTand

MoE of China, the NationalResearch Foundation ofKorea, NCKU (Taiwan), GAandMSMT ofthe Czech Republic, FIASofGermany, DAE,DST,andUGCofIndia,theNationalScienceCentreofPoland, National Research Foundation, the Ministryof Science, Education andSportsoftheRepublicofCroatia,andRosAtomofRussia. References

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