Long-range pseudorapidity dihadron correlations in d plus Au collisions at root S-NN=200 GeV

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DOI: 10.1016/j.physletb.2015.05.075

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Physics

Letters

B

www.elsevier.com/locate/physletb

Long-range

pseudorapidity

dihadron

correlations

in

d

+

Au collisions

at

√

s

_NN

=

200 GeV

STAR

Collaboration

L. Adamczyk

a

,

J.K. Adkins

u

,

G. Agakishiev

s

,

M.M. Aggarwal

af

,

Z. Ahammed

aw

,

I. Alekseev

q

,

J. Alford

t

,

A. Aparin

s

,

D. Arkhipkin

c

,

E.C. Aschenauer

c

,

G.S. Averichev

s

,

A. Banerjee

aw

,

R. Bellwied

as

,

A. Bhasin

r

,

A.K. Bhati

af

,

P. Bhattarai

ar

,

J. Bielcik

k

,

J. Bielcikova

l

,

L.C. Bland

c

,

I.G. Bordyuzhin

q

,

J. Bouchet

t

,

A.V. Brandin

ab

,

I. Bunzarov

s

,

T.P. Burton

c

,

J. Butterworth

al

,

H. Caines

ba

,

M. Calder’on de la Barca S’anchez

e

,

J.M. Campbell

ad

,

D. Cebra

e

,

M.C. Cervantes

aq

,

I. Chakaberia

c

,

P. Chaloupka

k

,

Z. Chang

aq

,

S. Chattopadhyay

aw

,

J.H. Chen

ao

,

X. Chen

w

,

J. Cheng

at

,

M. Cherney

j

,

W. Christie

c

,

M.J.M. Codrington

ar

,

G. Contin

x

,

H.J. Crawford

d

,

S. Das

n

,

L.C. De Silva

j

,

R.R. Debbe

c

,

T.G. Dedovich

s

,

J. Deng

an

,

A.A. Derevschikov

ah

,

B. di Ruzza

c

,

L. Didenko

c

,

C. Dilks

ag

,

X. Dong

x

,

J.L. Drachenberg

av

,

J.E. Draper

e

,

C.M. Du

w

,

L.E. Dunkelberger

f

,

J.C. Dunlop

c

,

L.G. Efimov

s

,

J. Engelage

d

,

G. Eppley

al

,

R. Esha

f

,

O. Evdokimov

i

,

O. Eyser

c

,

R. Fatemi

u

,

S. Fazio

c

,

P. Federic

l

,

J. Fedorisin

s

,

Feng

h

,

P. Filip

s

,

Y. Fisyak

c

,

C.E. Flores

e

,

L. Fulek

a

,

C.A. Gagliardi

aq

,

D. Garand

ai

,

F. Geurts

al

,

A. Gibson

av

,

M. Girard

ax

,

L. Greiner

x

,

D. Grosnick

av

,

D.S. Gunarathne

ap

,

Y. Guo

am

,

S. Gupta

r

,

A. Gupta

r

,

W. Guryn

c

,

A. Hamad

t

,

A. Hamed

aq

,

R. Haque

ac

,

J.W. Harris

ba

,

L. He

ai

,

S. Heppelmann

ag

,

A. Hirsch

ai

,

G.W. Hoffmann

ar

,

D.J. Hofman

i

,

S. Horvat

ba

,

H.Z. Huang

f

,

X. Huang

at

,

B. Huang

i

,

P. Huck

h

,

T.J. Humanic

ad

,

G. Igo

f

,

W.W. Jacobs

p

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H. Jang

v

,

K. Jiang

am

,

E.G. Judd

d

,

S. Kabana

t

,

D. Kalinkin

q

,

K. Kang

at

,

K. Kauder

i

,

H.W. Ke

c

,

D. Keane

t

,

A. Kechechyan

s

,

Z.H. Khan

i

,

D.P. Kikola

ax

,

I. Kisel

m

,

A. Kisiel

ax

,

D.D. Koetke

av

,

T. Kollegger

m

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L.K. Kosarzewski

ax

,

L. Kotchenda

ab

,

A.F. Kraishan

ap

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P. Kravtsov

ab

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K. Krueger

b

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I. Kulakov

m

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L. Kumar

af

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R.A. Kycia

ae

,

M.A.C. Lamont

c

,

J.M. Landgraf

c

,

K.D. Landry

f

,

J. Lauret

c

,

A. Lebedev

c

,

R. Lednicky

s

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J.H. Lee

c

,

X. Li

ap

,

X. Li

c

,

W. Li

ao

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Z.M. Li

h

,

Y. Li

at

,

C. Li

am

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M.A. Lisa

ad

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F. Liu

h

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T. Ljubicic

c

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W.J. Llope

ay

,

M. Lomnitz

t

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R.S. Longacre

c

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X. Luo

h

,

L. Ma

ao

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R. Ma

c

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G.L. Ma

ao

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Y.G. Ma

ao

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N. Magdy

az

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R. Majka

ba

,

A. Manion

x

,

S. Margetis

t

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C. Markert

ar

,

H. Masui

x

,

H.S. Matis

x

,

D. McDonald

as

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K. Meehan

e

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N.G. Minaev

ah

,

S. Mioduszewski

aq

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B. Mohanty

ac

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M.M. Mondal

aq

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D.A. Morozov

ah

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M.K. Mustafa

x

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B.K. Nandi

o

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Md. Nasim

f

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T.K. Nayak

aw

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G. Nigmatkulov

ab

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L.V. Nogach

ah

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S.Y. Noh

v

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J. Novak

aa

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S.B. Nurushev

ah

,

G. Odyniec

x

,

A. Ogawa

c

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K. Oh

aj

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V. Okorokov

ab

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D.L. Olvitt Jr.

ap

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B.S. Page

p

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Y.X. Pan

f

,

Y. Pandit

i

,

Y. Panebratsev

s

,

T. Pawlak

ax

,

B. Pawlik

ae

,

H. Pei

h

,

C. Perkins

d

,

A. Peterson

ad

,

P. Pile

c

,

M. Planinic

bb

,

J. Pluta

ax

,

N. Poljak

bb

,

K. Poniatowska

ax

,

J. Porter

x

,

M. Posik

ap

,

A.M. Poskanzer

x

,

N.K. Pruthi

af

,

J. Putschke

ay

,

H. Qiu

x

,

A. Quintero

t

,

S. Ramachandran

u

,

R. Raniwala

ak

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S. Raniwala

ak

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R.L. Ray

ar

,

H.G. Ritter

x

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J.B. Roberts

al

,

O.V. Rogachevskiy

s

,

J.L. Romero

e

,

A. Roy

aw

,

L. Ruan

c

,

*

Correspondingauthor.

E-mailaddress:[email protected](L. Yi).

http://dx.doi.org/10.1016/j.physletb.2015.05.075

0370-2693/©2015TheAuthors.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP3_.

266 STAR Collaboration / Physics Letters B 747 (2015) 265–271

J. Rusnak

l

,

O. Rusnakova

k

,

N.R. Sahoo

aq

,

P.K. Sahu

n

,

I. Sakrejda

x

,

S. Salur

x

,

A. Sandacz

ax

,

J. Sandweiss

ba

,

A. Sarkar

o

,

J. Schambach

ar

,

R.P. Scharenberg

ai

,

A.M. Schmah

x

,

W.B. Schmidke

c

,

N. Schmitz

z

,

J. Seger

j

,

P. Seyboth

z

,

N. Shah

f

,

E. Shahaliev

s

,

P.V. Shanmuganathan

t

,

M. Shao

am

,

M.K. Sharma

r

,

B. Sharma

af

,

W.Q. Shen

ao

,

S.S. Shi

x

,

Q.Y. Shou

ao

,

E.P. Sichtermann

x

,

R. Sikora

a

,

M. Simko

l

,

M.J. Skoby

p

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N. Smirnov

ba

,

D. Smirnov

c

,

D. Solanki

ak

,

L. Song

as

,

P. Sorensen

c

,

H.M. Spinka

b

,

B. Srivastava

ai

,

T.D.S. Stanislaus

av

,

R. Stock

m

,

M. Strikhanov

ab

,

B. Stringfellow

ai

,

M. Sumbera

l

,

B.J. Summa

ag

,

Y. Sun

am

,

Z. Sun

w

,

X.M. Sun

h

,

X. Sun

x

,

B. Surrow

ap

,

D.N. Svirida

q

,

M.A. Szelezniak

x

,

J. Takahashi

g

,

A.H. Tang

c

,

Z. Tang

am

,

T. Tarnowsky

aa

,

A.N. Tawfik

az

,

J.H. Thomas

x

,

A.R. Timmins

as

,

D. Tlusty

l

,

M. Tokarev

s

,

S. Trentalange

f

,

R.E. Tribble

aq

,

P. Tribedy

aw

,

S.K. Tripathy

n

,

B.A. Trzeciak

k

,

O.D. Tsai

f

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T. Ullrich

c

,

D.G. Underwood

b

,

I. Upsal

ad

,

G. Van Buren

c

,

G. van Nieuwenhuizen

y

,

M. Vandenbroucke

ap

,

R. Varma

o

,

A.N. Vasiliev

ah

,

R. Vertesi

l

,

F. Videbaek

c

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Y.P. Viyogi

aw

,

S. Vokal

s

,

S.A. Voloshin

ay

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A. Vossen

p

,

Y. Wang

h

,

F. Wang

ai

,

H. Wang

c

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J.S. Wang

w

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G. Wang

f

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Y. Wang

at

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J.C. Webb

c

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G. Webb

c

,

L. Wen

f

,

G.D. Westfall

aa

,

H. Wieman

x

,

S.W. Wissink

p

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au

,

Y.F. Wu

h

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Z. Xiao

at

,

W. Xie

ai

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K. Xin

al

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Z. Xu

c

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Q.H. Xu

an

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N. Xu

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Y.F. Xu

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C. Yang

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P. Yepes

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∗

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I.-K. Yoo

aj

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N. Yu

h

,

H. Zbroszczyk

ax

,

W. Zha

am

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J.B. Zhang

h

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at

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an

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h

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C. Zhong

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L. Zhou

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Y. Zoulkarneeva

s

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M. Zyzak

m

a_{AGHUniversityofScienceandTechnology,Cracow30-059,Poland} b_{ArgonneNationalLaboratory,Argonne,IL 60439,USA}

c_{BrookhavenNationalLaboratory,Upton,NY 11973,USA} d_{UniversityofCalifornia,Berkeley,CA 94720,USA} e_{UniversityofCalifornia,Davis,CA 95616,USA} f_{UniversityofCalifornia,LosAngeles,CA 90095,USA} g_{UniversidadeEstadualdeCampinas,SaoPaulo13131,Brazil} h_{CentralChinaNormalUniversity(HZNU),Wuhan430079,China} i_{UniversityofIllinoisatChicago,Chicago,IL 60607,USA} j_{CreightonUniversity,Omaha,NE 68178,USA}

k_{CzechTechnicalUniversityinPrague,FNSPE,Prague,11519,CzechRepublic} l_{NuclearPhysicsInstituteASCR,25068ˇRež/Prague,CzechRepublic} m_{FrankfurtInstituteforAdvancedStudiesFIAS,Frankfurt60438,Germany} n_{InstituteofPhysics,Bhubaneswar751005,India}

o_{IndianInstituteofTechnology,Mumbai400076,India} p_{IndianaUniversity,Bloomington,IN 47408,USA}

q_{AlikhanovInstituteforTheoreticalandExperimentalPhysics,Moscow117218,Russia} r_{UniversityofJammu,Jammu180001,India}

s_{JointInstituteforNuclearResearch,Dubna,141980,Russia} t_{KentStateUniversity,Kent,OH 44242,USA}

u_{UniversityofKentucky,Lexington,KY 40506-0055,USA}

v_{KoreaInstituteofScienceandTechnologyInformation,Daejeon305-701,RepublicofKorea} w_{InstituteofModernPhysics,Lanzhou730000,China}

x_{LawrenceBerkeleyNationalLaboratory,Berkeley,CA 94720,USA} y_{MassachusettsInstituteofTechnology,Cambridge,MA 02139-4307,USA} z

Max-Planck-InstitutfurPhysik,Munich80805,Germany

aa_{MichiganStateUniversity,EastLansing,MI 48824,USA} ab_{MoscowEngineeringPhysicsInstitute,Moscow115409,Russia}

ac_{NationalInstituteofScienceEducationandResearch,Bhubaneswar751005,India} ad_{OhioStateUniversity,Columbus,OH 43210,USA}

ae_{InstituteofNuclearPhysicsPAN,Cracow31-342,Poland} af_{PanjabUniversity,Chandigarh160014,India}

ag_{PennsylvaniaStateUniversity,UniversityPark,PA 16802,USA} ah_{InstituteofHighEnergyPhysics,Protvino142281,Russia} ai_{PurdueUniversity,WestLafayette,IN 47907,USA} aj_{PusanNationalUniversity,Pusan609735,RepublicofKorea} ak_{UniversityofRajasthan,Jaipur302004,India}

al_{RiceUniversity,Houston,TX 77251,USA}

am_{UniversityofScienceandTechnologyofChina,Hefei230026,China} an_{ShandongUniversity,Jinan,Shandong250100,China}

ao_{ShanghaiInstituteofAppliedPhysics,Shanghai201800,China} ap_{TempleUniversity,Philadelphia,PA 19122,USA}

aq_{TexasA&MUniversity,CollegeStation,TX 77843,USA} ar_{UniversityofTexas,Austin,TX 78712,USA} as_{UniversityofHouston,Houston,TX 77204,USA} at_{TsinghuaUniversity,Beijing100084,China}

au_{UnitedStatesNavalAcademy,Annapolis,MD 21402,USA} av_{ValparaisoUniversity,Valparaiso,IN 46383,USA}

aw_{VariableEnergyCyclotronCentre,Kolkata700064,India} ax_{WarsawUniversityofTechnology,Warsaw00-661,Poland} ay_{WayneStateUniversity,Detroit,MI 48201,USA}

az_{WorldLaboratoryforCosmologyandParticlePhysics(WLCAPP),Cairo11571,Egypt} ba_{YaleUniversity,NewHaven,CT 06520,USA}

bb_{UniversityofZagreb,Zagreb,HR-10002,Croatia}

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Articlehistory:

Received1March2015

Receivedinrevisedform19May2015 Accepted29May2015

Availableonline3June2015 Editor: V.Metag

Dihadronangularcorrelationsind+Au collisionsat√sNN=200 GeV arereportedasafunctionofthe

measured zero-degreecalorimeter neutralenergy andthe forward charged hadron multiplicityinthe Au-beamdirection.Afinitecorrelatedyieldisobservedatlargerelativepseudorapidity(

η

)onthenear side(i.e. relativeazimuthφ∼0).Thiscorrelated yieldasafunctionof

η

appearstoscalewiththe dominant,primarilyjet-related,away-side(φ∼

π

)yield.TheFouriercoefficientsoftheφcorrelation,

Vn= cos nφ,haveastrong

η

dependence.Inaddition,itisfoundthatV1isapproximatelyinversely

proportionaltothemid-rapidityeventmultiplicity,whileV2isindependentofitwithsimilarmagnitude

intheforward(d-going)andbackward(Au-going)directions.

©2015TheAuthors.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3.

Relativistic heavy-ion collisions are used to study quantum chromodynamics (QCD) at high energy densities at the Rela-tivistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC) [1–5]. Final-state particle emission in such collisions is anisotropic, quantitatively consistent withhydrodynamic flow re-sultingfromthe initial-stateoverlapgeometry[6,7]. Two-particle correlationsare widelyusedto measureanisotropic flowand jet-like correlations[8]. A near-side long-range correlation (at small relativeazimuth

φ

andlargerelativepseudorapidity



η

),called the “ridge,” has been observed after elliptic flow subtraction in central heavy-ion collisions at RHIC and the LHC [9–14]. It is attributedprimarily to triangularflow, resulting froma hydrody-namicresponsetoinitialgeometryfluctuations[15,16].

Asreference, p

+

p, p

+

A andd

+

Au collisionsareoftenused to compare with heavy-ion collisions. Hydrodynamics is not ex-pectedtodescribethesesmall-systemcollisions.However, alarge



η

ridge has beenobserved in high-multiplicity p

+

p [17] and p

+

Pb [18–21] collisions atthe LHC aftera uniformbackground subtraction.The similarityto theheavy-ion ridgeis suggestiveof ahydrodynamic descriptionofitsorigin,inconflictwithearly ex-pectations.Indeed,hydrodynamiccalculationswithevent-by-event fluctuations can describe the observed ridge and attribute it to ellipticflow [22,23].Other physics mechanisms are alsopossible, suchasthecolorglasscondensatewherethetwo-gluondensityis enhancedatsmall

φ

overawiderangeof



η

[24–26],or quan-tuminitialanisotropy[27].

Furthermore, a back-to-back ridge is revealed by subtracting dihadron correlations in low-multiplicity p

+

Pb from those in high-multiplicity collisions at the LHC [19–21]. A similar double ridge isobserved in d

+

Au collisions at RHIC by PHENIX within 0.48

<

|

η

|

<

0.70 using the same subtraction technique [28]. A recentSTARanalysishaschallengedtheassumptionofthis sub-traction procedure that jet-like correlations are equal in high-and low-multiplicity events [29]. It was shown that the double ridge at these small-to-moderate



η

has a significant contribu-tion from residual jet-like correlations despite performing event selectionsvia forward multiplicities [29].A recent PHENIX study oflarge



η

correlations,withoutrelying onthesubtraction tech-nique,suggestsalong-rangecorrelation consistent with hydrody-namic anisotropic flow [30]. In order to further understand the underlyingphysicsmechanism, hereinthisLetter,wepresentour resultson long-range(large



η

) correlationsind

+

Au collisions at

√

sNN

=

200 GeV as afunctionof



η

andthe event

multiplic-ity.Thelarge acceptanceoftheSTARdetectoris particularlywell suitedforsuchananalysisoverawiderrangein



η

.

Thedataweretakenduringthed

+

Au runin2003bytheSTAR experiment[31,32].ThedetailsoftheSTARdetectorcanbefound inRef.[33].Minimum-biasd

+

Au eventswere triggeredby coin-cidence of signals fromthe Zero DegreeCalorimeters (ZDC) [34]

andtheBeam–Beam Counters(BBC) [33].Particle trackswere re-constructed in the Time Projection Chamber (TPC) [35] and the forwardTPC(FTPC)[36].Theprimaryvertexwasdeterminedfrom reconstructedtracks.Inthisanalysis,eventswererequiredtohave aprimaryvertexposition

|

zvtx

|

<

50 cm fromtheTPCcenteralong

the beam axis. TPC(FTPC) tracks were required to have at least 25(5)out ofthemaximumpossible45(10) hitsanda distanceof closestapproachtotheprimaryvertexwithin3 cm.

Three measurements were used to selectd

+

Au events: neu-tral energy by the ZDC and charged particle multiplicity within

−

3.8

<

η

<

−

2.8 bytheFTPC[31,32],bothintheAu-beam direc-tion,andchargedparticlemultiplicitywithin

|

η

|

<

1 byTPC.Weak but positive correlations were observed between these measure-ments;thesameeventfractiondefinedby thesemeasures corre-spondedtosignificantlydifferentd

+

Au eventsamples[29].Inthis workwe study0–20%high-activityand40–100%low-activity col-lisionsaccordingtoeachmeasure.

The pairs of particles used in dihadron correlations are cus-tomarily called the trigger and the associated particle. Two sets ofdihadroncorrelationsareanalyzed:TPC–TPCcorrelationswhere boththetriggerandassociatedparticlesarefromtheTPC(

|

η

|

<

1), andTPC–FTPC correlationswhere the triggerparticle isfrom the TPCbuttheassociatedparticleisfromeithertheFTPC-Au(

−

3.8

<

η

<

−

2.8) orFTPC-d (2.8

<

η

<

3.8).The pT rangesofthetrigger

and associated particles are both 1

<

pT

<

3 GeV/c. The

associ-atedparticleyields arenormalizedpertriggerparticle.Theyields arecorrectedfortheTPCandFTPCassociatedparticletracking effi-cienciesof85%

±

5% (syst.) and70%

±

5% (syst.),respectively,which donotdependontheeventactivityind

+

Au collisions[31,32].

The detectornon-uniformity in

φ

iscorrected by the event-mixingtechnique,whereatriggerparticlefromoneeventispaired withassociatedparticlesfromanotherevent.Themixedeventsare required to be within 1 cm in zvtx, with the same multiplicity

(by FTPC-Au or TPC) or similar energy (byZDC-Au). The mixed-eventcorrelationsarenormalizedto100%at



η

=

0 forTPC,and at

±

3.3 forFTPC-d andFTPC-Auassociatedparticles,respectively.

Twoanalysisapproachesaretaken.Oneistoanalyzethe corre-latedyields aftersubtractingauniformcombinatorialbackground. The background normalizationis estimatedby the Zero-Yield-At-Minimum (ZYAM) assumption [9,37]. ZYAMis taken asthe low-est yield averaged over a

φ

window of

π

/8 radian

width,

af-268 STAR Collaboration / Physics Letters B 747 (2015) 265–271

Fig. 1. Correlateddihadronyield,perradianperunitofpseudorapidity,asafunctionofφinthreerangesofηind+Au collisions.ShownarebothlowandhighZDC-Au activitydata.Boththetriggerandassociatedparticleshave1<pT<3 GeV/c.ThearrowsindicateZYAMnormalizationpositions.Theerrorbarsarestatisticalandhistograms

indicatethesystematicuncertainties.

Table 1

Near- (|φ|<π/3)andaway-side(|φ −π|<π/3)correlatedyieldsandZYAMbackgroundmagnitude,perradianperunitofpseudorapidity,atlargeηinlow- and high-activityd+Au collisions.Positive(negative)ηcorrespondstod(Au)-goingdirection.Boththetriggerandassociatedparticleshave1<pT<3 GeV/c.Allnumbershave

beenmultipliedby104_{.ErrorsarestatisticalexcepttheseconderrorofeachZYAMvaluewhichissystematicandappliesalsotothecorrespondingnear- andaway-side}

yields.Anadditional5%efficiencyuncertaintyapplies. Event activity Event selection 1.2<|η| <1.8 Event selection −4.5< η<−2 2< η<4.5

ZYAM near away ZYAM near away ZYAM near away

40–100% ZDC 1896±7+₋113 10±4 346±5 ZDC 978±2+ 1 −2 2±1 55±1 361±1+ 1 −2 1±1 38±1 0–20% 3043±11+₋1526 53±7 456±7 1776±4+ 2 −1 10±2 70±2 438±2+ 1 −2 1±1 31±1 40–100% FTPC 1324±7+2 −6 7±4 347±5 TPC 636±2+ 1 −2 6±1 59±1 309±2+ 1 −1 3±1 45±1 0–20% 3468±10+₋75 43±6 429±7 1899±3+ 2 −5 15±2 75±2 445±1+ 1 −3 2±1 27±1

ter the correlated yield distribution is folded into the range of

0

< φ <

π

.TheZYAMsystematicuncertaintyisestimatedbythe

yields averaged over windows of half and three half the width. Wealsofitthe

φ

correlationsby twoGaussians(withcentroids fixedat0and

π

) plusapedestal.Thefittedpedestalisconsistent withZYAMwithinthestatisticalandsystematicerrorsbecausethe near- andaway-sidepeaksarewell separatedind

+

Au collisions. Thesystematicuncertaintiesonthecorrelatedyields aretakenas the quadratic sum of the ZYAM and tracking efficiency system-atic uncertainties. The other approach is to analyze the Fourier coefficients of the

φ

correlation functions, Vn

= 

cos nφ



. No

backgroundsubtractionisrequired.Systematicuncertaintiesonthe Fouriercoefficients are estimated, by varying analysiscuts, to be lessthan 10%for V1 and V2,andsmaller thanthe statistical

er-rorsforV3.

Fig. 1showsthe ZYAM-subtractedcorrelated yields asa func-tionof

φ

inZDC-Aulow- andhigh-activityd

+

Au collisions.The TPC–TPC correlation at large



η

is shown in panel (a), whereas theTPC–FTPCcorrelationsareshowninpanels(b)and(c)for Au-andd-going directions,respectively. The ZYAM statisticalerroris includedaspartofthesystematicuncertaintydrawninFig. 1 be-causeit is commonto all

φ

bins.No difference is observed in TPC–TPC correlationsbetween positive andnegative



η

, so they are combined in Fig. 1(a). The away-side correlated yields are foundtobe largerinhigh- thanlow-activityd

+

Au collisionsfor TPCandFTPC-Au correlations. The opposite behavior isobserved fortheFTPC-d correlations,Fig. 1(c).

On the near side, the correlated yields are consistent with zerointhelow-activityeventsand,inFTPC-d,inthehigh-activity eventsaswell. (Note thatthe yieldvalue cannot be negative be-causeoftheZYAMassumption.) Incontrast,inTPCandFTPC-Au, finitecorrelatedyieldsareobservedinhigh-activityevents.A simi-larresultwasobservedbyPHENIX[30].InFig. 1,theeventactivity isdeterminedby ZDC-Au.Foreventactivitydeterminedby FTPC-AuorTPCmultiplicity,thedataarequalitativelysimilar.InTable 1,

the correlated yields integratedover thenear side (

|φ|

<

π

/3)

andthe away side(

|φ −

π

|

<

π

/3),

normalizedby the integra-tion range, are tabulated together withthe ZYAM magnitudefor low- andhigh-activityeventsdeterminedbythevariousmeasures. Fortriggerparticles inour pT rangeof1

<

pT

<

3 GeV/c,the

away-side correlationind

+

Au collisions isexpectedtobe domi-nated byjet-like correlations[38].Inspecting thenear-side corre-lationamplitudeatlarge



η

,anypossiblenon-jet,e.g. anisotropic flow,contributionsontheawaysideshouldbeorderofmagnitude smaller. Perhapsthe observed away-side dependence on ZDC-Au eventactivityarisesfromacorrelationbetweenjetproductionand the forward beam remnants. Or, the underlying physics may be more complex;forexample theopposite away-side trends inthe Au- and d-going directions may arise from different underlying partondistributionsinhigh- andlow-activitycollisions.Thefinite correlatedyieldonthenearsideis,ontheother hand,rather sur-prising becausejet-like contributions shouldbe minimal atthese large



η

distances.Hijingsimulation[38] ofd

+

Au collisions in-dicatesthatjetcorrelationswithinourpT rangeafterZYAM

back-groundsubtractionisconsistentwithzeroat

|

η

|

>

1.5.

To study the



η

dependence of the correlated yields in the TPC andFTPC, the correlation data are divided intomultiple



η

bins. In Fig. 2(a), the near- and away-side correlated yields are shown as a function of



η

. To avoid auto-correlations, we have used ZDC-Auforeventselectionsforboth theTPCandFTPC cor-relation data.UnlikeinFig. 1, theZYAMstatisticalerrors are de-pendent of



η

and aretherefore includedinthe statisticalerror bars ofthe data points. The away-side correlation shape, notice-ablyconcavedforTPC,ispresumablydeterminedbytheunderlying parton-partonscatteringkinematics.Onthenearside,finite corre-latedyields are observedatlarge



η

ontheAu-going sidein all bins,whiletheyieldsareconsistentwithzeroonthed-goingside. As aforementioned,similar resultshavebeenpreviously observed in heavy-ion [9–14], p

+

p [17], and p

+

Pb collisions [18–21]. There, the trigger and associated particles were taken from the

Fig. 2. Theηdependenceof(a)thenear- (|φ|<π/3)andaway-side(|φ −π|< π/3)correlatedyields,and(b)theratioofthenear- toaway-sidecorrelatedyields ind+Au collisions.Positive(negative)ηcorrespondstod(Au)-goingdirection.Only highZDC-Auactivitydataareshown.Theerrorbarsarestatisticalandhistograms indicatethesystematicuncertainties(forη>2 in(b)thelowerboundfalls out-sidetheplot).Thedashedcurvein(b)isalinearfittotheη<−1 datapoints.

same

η

region. As a result, the correlated yields were approxi-matelyuniformin



η

[39],andwere dubbedthe “ridge.” Inthe threegroups of correlation datain Fig. 2(a),the trigger particles comefromtheTPC,buttheassociatedparticlescomefrom differ-ent

η

regions. Significant differencesin pair kinematics resultin thesteps at



η

= ±

2 eventhoughtheir



η

gapsaresimilar. De-spitethis,forsimplicity,werefertothelarge



η

correlatedyields inourdataalsoasthe“ridge.”

Inordertoelucidatetheformationmechanismoftheridge,we studyin Fig. 2(b) the ratio of the near- to away-side correlated yields.BecausetheZYAMvalueiscommonforthenearandaway side,its statisticalerrorisincluded aspartofthe systematic un-certainty; this part of the systematic uncertaintyis uncorrelated between



η

bins. While thelarge peak at



η

∼

0 isdueto the near-side jet, the ratio at



η

<

−

1 is rather insensitive to



η

, whether the correlations are from TPC or FTPC-Au. A linear fit (dashed-linein Fig. 2(b)) tothose datapoints at



η

<

−

1 yields aslope parameter of

−

0.023

±

0.019+₋0_0..020₀₁₀ with

χ

2

_/ndf

₌

_2.6/3,

indicatingthat theratio isconsistent withaconstant within one standarddeviation. Therather constantratioisremarkable, given thenearlyorderofmagnitudedifference intheaway-side jet-like correlatedyields across



η

= −

2 dueto thevastly differentpair kinematics.Sincetheaway-sidecorrelatedyieldsaredominatedby jets[38], thefinite,



η

-independentratio at



η

<

−

1 may sug-gestaconnectionbetweenthenear-sideridgeandjetproduction, even though any possible jet contribution to the near-side ridge at

|

η

|

>

1 shouldbe minimal. Ontheother hand,thenear-side ridgedoesnot seem toscale withtheZYAM value,which repre-sentsthe underlying background. A linear fit to the ratio of the near-sidecorrelatedyieldoverZYAMinthesame



η

<

−

1 region givesaslopeparameterof6.5

±

1.6+₋3_2..7₁

×

10−3_{,significantly}

devi-atingfromzero.

Thecorrelatedyields discussedabovearesubjecttotheZYAM backgroundsubtraction. Anotherwayto quantify theridge is via Fouriercoefficientsoftheazimuthal correlationfunctionswithout

Fig. 3. TheηdependenceofthesecondharmonicFouriercoefficient,V2,inlow

andhighZDC-Auactivityd+Au collisions.Theerrorbarsarestatistical.Systematic uncertaintiesare10%andareshownbythe histograms,forclarity,onlyforthe high-activitydata.

backgroundsubtraction.Fig. 3showsthesecond harmonicFourier coefficient (V2) asa function of



η

forboth highandlow

ZDC-Auenergycollisions.The V2 valuesareapproximatelythesamein

high- andlow-activity collisions atlarge



η

.Both decrease with increasing

|

η

|

fromthe small



η

, jet dominated,region to the large



η

, ridge, region by nearly one order of magnitude. The



η

behaviorofV2,ameasureofmodulationrelativetothe

aver-age,isqualitativelyconsistentwiththe



η

-dependentratioofthe near-side correlated yield over ZYAM. One motivation to analyze correlation data using Fourier coefficients is their independence of a ZYAM subtractionprocedure. One way for V2 to develop is

throughfinal-stateinteractionswhich,ifprevalentenough,maybe describedintermsofhydrodynamic flow.IfV2 isstrictlyofa

hy-drodynamicellipticfloworigin,thedatawouldimplyadecreasing collective effect at backward/forward rapidities that is somehow independentoftheactivityleveloftheevents.

To gain further insights, the multiplicity dependencies of the first, second and third Fourier coefficients V1, V2 and V3 are

showninFig. 4.Three



η

rangesarepresentedforFTPC-Au,TPC, andFTPC-d correlations, respectively.Results byboth theZDC-Au andFTPC-Au eventselectionsare shown, plottedasafunction of thecorresponding measuredchargedparticlepseudorapidity den-sityatmid-rapiditydNch

/

d

η

.TheabsolutevalueoftheV1

param-eter ineach



η

rangevariesapproximately as

(

dNch

/

d

η

)

−1 (see

thesuperimposedfitsinFig. 4(a)).Thisisconsistentwithjet con-tributionsand/orglobalstatisticalmomentumconservation.Onthe other hand,the V2 parameter ineach



η

rangeisapproximately

independentofdNch

/

d

η

overtheentiremeasured range(see the

fittedconstantinFig. 4(b)).SimilarbehaviorofV2 isalsoobserved

inp

+

Pb collisionsattheLHC[13,40,41].Fig. 4showsthattheV3

valuesaresmallandmostly consistentwithzero,exceptforTPC– TPCcorrelationatthelowestmultiplicity.

Ind

+

Au collisions,dihadroncorrelationsaredominatedbyjets, even at large



η

, where the away-side jet contributes [38].The behaviorof V1 suggeststhatthe jetcontributionto Vn isdiluted

bythemultiplicity.ThesimilarV2valuesand



η

dependenciesin

differentmultiplicity collisionsare,therefore,rathersurprising.In order to accommodate a hydrodynamic contribution, there must be a coincidental compensation of the reduced jet contribution withincreasingmultiplicity,overtheentiremeasuredmultiplicity range,byanemerging,non-jetcontribution,suchasellipticflow.

Whether or not a finite correlated yield appears on the near side depends on the interplay between V1 and V2 (higher

or-dertermsarenegligible).AlthoughtheV2 parametersaresimilar,

thesignificantlymorenegative V1inlow- versushigh-multiplicity

ap-270 STAR Collaboration / Physics Letters B 747 (2015) 265–271

Fig. 4. Fouriercoefficients(a) V1,(b)V2, and(c)V3 versusthe measured

mid-rapiditychargedparticledNch/dη. EventactivityselectionsbybothZDC-Auand

FTPC-Auareshown.TriggerparticlesarefromTPC,andassociatedparticlesfrom TPC(triangles),FTPC-Au(circles),andFTPC-d (squares),respectively.Systematic un-certaintiesareestimatedtobe10%onV1andV2,andsmallerthanstatisticalerrors

forV3.Errorsshownarethequadraticsumofstatisticalandsystematicerrors.The

dashedcurvesaretoguidetheeye.

plies also to the TPC–FTPC correlation comparison between the Au- and d-going directions.The V2 values are rathersimilar for

FTPC-d (forwardrapidity)andFTPC-Au(backwardrapidity) corre-lations,butthemorenegative V1 ford-goingdirectioneliminates

thenear-side V2 peak.Iftherelevantphysics ind

+

Au collisions

isgovernedby hydrodynamics,thenitmaynot carry significance whetherornotthereexistsafinitenear-sidelong-rangecorrelated yield, which wouldbe a simple manifestation of the relative V1

andV2 strengths.

OurV2 dataarequalitativelyconsistentwiththatfromPHENIX

[30].WhilePHENIXfocused onthe pT dependence,we studythe

Fouriercoefficientsasafunctionof



η

affordedbythelargeSTAR acceptance, as well as the event multiplicity. Hydrodynamic ef-fects, ifthey exist ind

+

Au collisions,should naively differover themeasuredmultiplicityrangeandbetweenAu- andd-going di-rections.However, the V2 parametersare approximatelyconstant

over multiplicity,andquantitatively similar betweenthe Au- and d-goingdirections.Ontheotherhand,thecorrelationcomparisons betweenlow- andhigh-activitydatarevealdifferenttrendsforthe Au- andd-goingdirections.Thehigh- andlow-activitydifferencein theFTPC-AucorrelationinFig. 1(b)mayresembleellipticflow,but thatintheFTPC-d correlationinFig. 1(c)isfarfromanellipticflow shape.Incombination, thesedatasuggestthatthefinitevaluesof Vn cannot be exclusively explained by hydrodynamic anisotropic

flowind

+

Au collisionsatRHIC.

In summary, dihadron angular correlations are reported for d

+

Au collisions at

√

sNN

=

200 GeV as a function of the event

activityfromtheSTARexperiment.The eventactivityisclassified by the measured zero-degreeneutralenergyin ZDC, thecharged hadronmultiplicityinFTPC,bothintheAu-goingdirection,orthe multiplicity in TPC. In a recent paper we have shown that the short-rangejet-likecorrelatedyieldincreaseswiththeevent activ-ity[29].Inthispaperwefocusonlong-rangecorrelationsatlarge

|

η

|

,where jet-like contributions are minimal on the near side, although theaway side isstill dominatedby jet production.Two approachesaretaken,onetoextractthecorrelatedyields abovea uniformbackgroundestimatedbytheZYAMmethod,andtheother to calculate the Fourier coefficients, Vn

= 

cos nφ



, of the

di-hadron

φ

correlations.Thefollowingpointsareobserved:(i) The away-sidecorrelatedyieldsarelargerinhigh- thaninlow-activity collisions intheTPCandFTPC-Au,butlowerinFTPC-d;(ii) Finite near-sidecorrelatedyieldsareobservedatlarge



η

abovethe es-timated ZYAMbackground in high-activity collisions in both the TPCandFTPC-Au (referredtoasthe“ridge”);(iii) Theridge yield appears toscalewiththeaway-sidecorrelatedyield atthe corre-sponding



η

<

−

1,whichisdominatedbytheaway-sidejet;(iv) The V2 coefficientdecreaseswithincreasing

|

η

|

,butremains

fi-niteatbothforwardandbackwardrapidities(

|

η

|

≈

3)with sim-ilar magnitude;(v) The V1 coefficient is approximatelyinversely

proportionaltotheeventmultiplicity,buttheV2appearstobe

in-dependentofit.Whilehydrodynamicellipticflowisnotexcluded withacoincidentalcompensationofjetdilutionbyincreasingflow contributionwithmultiplicityandanunexpectedequalityof ellip-ticflowbetweenforwardandbackwardrapidities,thedatasuggest thatthereexistsalong-rangepair-wisecorrelationind

+

Au colli-sionsthatiscorrelatedwithdijetproduction.

Acknowledgements

We thank the RHIC Operations Group and RCF at BNL, the NERSC Center at LBNL and the Open Science Grid consortium forproviding resources andsupport.Thiswork was supported in part by the Offices of NP and HEP within the U.S. DOE Office of Science, the U.S. NSF, the Sloan Foundation, the DFG cluster of excellence ‘Origin and Structure of the Universe’ of Germany, CNRS/IN2P3,STFCandEPSRCoftheUnitedKingdom,FAPESPCNPq ofBrazil,Ministry ofEducation andScience oftheRussian Feder-ation,NNSFC,CAS,MoST,andMoEofChina,GAandMSMTofthe Czech Republic,FOMandNWO oftheNetherlands,DAE,DST,and CSIRofIndia,PolishMinistryofScienceandHigherEducation, Ko-reaResearchFoundation,MinistryofScience,EducationandSports of theRepublicOfCroatia,Russian MinistryofScience and Tech-nology,andRosAtomofRussia.

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