Effect of event selection on jetlike correlation measurement in d + Au collisions at √sNN = 200 GeV

Contents lists available atScienceDirect

Physics

Letters

B

www.elsevier.com/locate/physletb

Effect

of

event

selection

on

jetlike

correlation

measurement

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

ax

,

I. Alekseev

q

,

J. Alford

t

,

A. Aparin

s

,

D. Arkhipkin

c

,

E.C. Aschenauer

c

,

G.S. Averichev

s

,

A. Banerjee

ax

,

R. Bellwied

at

,

A. Bhasin

r

,

A.K. Bhati

af

,

P. Bhattarai

as

,

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

bb

,

M. Calder’on de la Barca S’anchez

e

,

J.M.

Campbell

ad

,

D. Cebra

e

,

M.C. Cervantes

ar

,

I. Chakaberia

c

,

P. Chaloupka

k

,

Z. Chang

ar

,

S. Chattopadhyay

ax

,

J.H. Chen

ao

,

J. Cheng

au

,

M. Cherney

j

,

W. Christie

c

,

M.J.M. Codrington

as

,

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

,

R. Derradi de Souza

g

,

B. di Ruzza

c

,

L. Didenko

c

,

C. Dilks

ag

,

X. Dong

x

,

J.L. Drachenberg

aw

,

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

,

C.A. Gagliardi

ar

,

D. Garand

ai

,

F. Geurts

al

,

A. Gibson

aw

,

M. Girard

ay

,

L. Greiner

x

,

D. Grosnick

aw

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D.S. Gunarathne

aq

,

Y. Guo

am

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S. Gupta

r

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A. Gupta

r

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W. Guryn

c

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A. Hamad

t

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A. Hamed

ar

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R. Haque

ac

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J.W. Harris

bb

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L. He

ai

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S. Heppelmann

ag

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A. Hirsch

ai

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G.W. Hoffmann

as

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D.J. Hofman

i

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S. Horvat

bb

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H.Z. Huang

f

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X. Huang

au

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B. Huang

i

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P. Huck

h

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T.J. Humanic

ad

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G. Igo

f

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W.W. Jacobs

p

,

H. Jang

v

,

E.G. Judd

d

,

S. Kabana

ap

,

D. Kalinkin

q

,

K. Kang

au

,

K. Kauder

i

,

H.W. Ke

c

,

D. Keane

t

,

A. Kechechyan

s

,

Z.H. Khan

i

,

D.P. Kikola

ay

,

I. Kisel

m

,

A. Kisiel

ay

,

S.R. Klein

x

,

D.D. Koetke

aw

,

T. Kollegger

m

,

L.K. Kosarzewski

ay

,

L. Kotchenda

ab

,

A.F. Kraishan

aq

,

P. Kravtsov

ab

,

K. Krueger

b

,

I. Kulakov

m

,

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

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A. Lebedev

c

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R. Lednicky

s

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J.H. Lee

c

,

X. Li

aq

,

C. Li

am

,

X. Li

c

,

W. Li

ao

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Z.M. Li

h

,

Y. Li

au

,

M.A. Lisa

ad

,

F. Liu

h

,

T. Ljubicic

c

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W.J. Llope

az

,

M. Lomnitz

t

,

R.S. Longacre

c

,

X. Luo

h

,

L. Ma

ao

,

G.L. Ma

ao

,

Y.G. Ma

ao

,

R. Ma

c

,

N. Magdy

ba

,

R. Majka

bb

,

A. Manion

x

,

S. Margetis

t

,

C. Markert

as

,

H. Masui

x

,

H.S. Matis

x

,

D. McDonald

at

,

N.G. Minaev

ah

,

S. Mioduszewski

ar

,

B. Mohanty

ac

,

M.M. Mondal

ar

,

D.A. Morozov

ah

,

M.K. Mustafa

x

,

B.K. Nandi

o

,

Md. Nasim

f

,

T.K. Nayak

ax

,

G. Nigmatkulov

ab

,

L.V. Nogach

ah

,

S.Y. Noh

v

,

J. Novak

aa

,

S.B. Nurushev

ah

,

G. Odyniec

x

,

A. Ogawa

c

,

K. Oh

aj

,

V. Okorokov

ab

,

D.L. Olvitt Jr.

aq

,

B.S. Page

p

,

Y.X. Pan

f

,

Y. Pandit

i

,

Y. Panebratsev

s

,

T. Pawlak

ay

,

B. Pawlik

ae

,

H. Pei

h

,

C. Perkins

d

,

P. Pile

c

,

M. Planinic

bc

,

J. Pluta

ay

,

N. Poljak

bc

,

K. Poniatowska

ay

,

J. Porter

x

,

A.M. Poskanzer

x

,

N.K. Pruthi

af

,

M. Przybycien

a

,

J. Putschke

az

,

H. Qiu

x

,

A. Quintero

t

,

S. Ramachandran

u

,

R. Raniwala

ak

,

S. Raniwala

ak

,

R.L. Ray

as

,

H.G. Ritter

x

,

J.B. Roberts

al

,

O.V. Rogachevskiy

s

,

J.L. Romero

e

,

A. Roy

ax

,

L. Ruan

c

,

J. Rusnak

l

,

*

Correspondingauthor.

E-mailaddress:yil@purdue.edu(L. Yi). http://dx.doi.org/10.1016/j.physletb.2015.02.068

O. Rusnakova

k

,

N.R. Sahoo

ar

,

P.K. Sahu

n

,

I. Sakrejda

x

,

S. Salur

x

,

A. Sandacz

ay

,

J. Sandweiss

bb

,

A. Sarkar

o

,

J. Schambach

as

,

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

,

B. Sharma

af

,

M.K. Sharma

r

,

W.Q. Shen

ao

,

S.S. Shi

x

,

Q.Y. Shou

ao

,

E.P. Sichtermann

x

,

M. Simko

l

,

M.J. Skoby

p

,

D. Smirnov

c

,

N. Smirnov

bb

,

D. Solanki

ak

,

L. Song

at

,

P. Sorensen

c

,

H.M. Spinka

b

,

B. Srivastava

ai

,

T.D.S. Stanislaus

aw

,

R. Stock

m

,

M. Strikhanov

ab

,

B. Stringfellow

ai

,

M. Sumbera

l

,

B.J. Summa

ag

,

Z. Sun

w

,

Y. Sun

am

,

X. Sun

x

,

X.M. Sun

h

,

B. Surrow

aq

,

D.N. Svirida

q

,

M.A. Szelezniak

x

,

J. Takahashi

g

,

Z. Tang

am

,

A.H. Tang

c

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T. Tarnowsky

aa

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A.N. Tawfik

ba

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J.H. Thomas

x

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A.R. Timmins

at

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D. Tlusty

l

,

M. Tokarev

s

,

S. Trentalange

f

,

R.E. Tribble

ar

,

P. Tribedy

ax

,

S.K. Tripathy

n

,

B.A. Trzeciak

k

,

O.D. Tsai

f

,

J. Turnau

ae

,

T. Ullrich

c

,

D.G. Underwood

b

,

I. Upsal

ad

,

G. Van Buren

c

,

G. van Nieuwenhuizen

y

,

M. Vandenbroucke

aq

,

R. Varma

o

,

G.M.S. Vasconcelos

g

,

A.N. Vasiliev

ah

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R. Vertesi

l

,

F. Videbaek

c

,

Y.P. Viyogi

ax

,

S. Vokal

s

,

S.A. Voloshin

az

,

A. Vossen

p

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J.S. Wang

w

,

Y. Wang

h

,

F. Wang

ai

,

Y. Wang

au

,

G. Wang

f

,

H. Wang

c

,

J.C. Webb

c

,

G. Webb

c

,

L. Wen

f

,

G.D. Westfall

aa

,

H. Wieman

x

,

S.W. Wissink

p

,

R. Witt

av

,

Y.F. Wu

h

,

Z. Xiao

au

,

W. Xie

ai

,

K. Xin

al

,

Q.H. Xu

an

,

H. Xu

w

,

N. Xu

x

,

Y.F. Xu

ao

,

Z. Xu

c

,

W. Yan

au

,

Y. Yang

w

,

Q. Yang

am

,

Y. Yang

h

,

C. Yang

am

,

S. Yang

am

,

Z. Ye

i

,

P. Yepes

al

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L. Yi

ai

,

∗

_,

_{K. Yip}

c

_,

_{I.-K. Yoo}

aj

_,

_{N. Yu}

h

_,

_{H. Zbroszczyk}

ay

_,

_{W. Zha}

am

_,

_{J.B. Zhang}

h

_,

_{X.P. Zhang}

au

_,

S. Zhang

ao

,

Z. Zhang

ao

,

Y. Zhang

am

,

J.L. Zhang

an

,

F. Zhao

f

,

J. Zhao

h

,

C. Zhong

ao

,

X. Zhu

au

,

Y. Zoulkarneeva

s

,

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_{SUBATECH,Nantes44307,France}

aq_{TempleUniversity,Philadelphia,PA 19122,USA} ar_{TexasA&MUniversity,CollegeStation,TX 77843,USA} as_{UniversityofTexas,Austin,TX 78712,USA} at_{UniversityofHouston,Houston,TX 77204,USA} au_{TsinghuaUniversity,Beijing100084,China}

aw_{ValparaisoUniversity,Valparaiso,IN 46383,USA} ax_{VariableEnergyCyclotronCentre,Kolkata700064,India} ay_{WarsawUniversityofTechnology,Warsaw00-661,Poland} az_{WayneStateUniversity,Detroit,MI 48201,USA}

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

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

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Articlehistory:

Received29December2014

Receivedinrevisedform24February2015 Accepted26February2015

Availableonline3March2015 Editor: V.Metag

Dihadroncorrelations areanalyzedin√sNN=200 GeV d+Au collisionsclassifiedbyforwardcharged particlemultiplicityandzero-degreeneutralenergyintheAu-beamdirection.Itisfoundthatthejetlike correlated yield increaseswith the event multiplicity. Aftertaking intoaccount thisdependence, the non-jetcontributionontheawaysideisminimal,leavinglittleroomforaback-to-backridgeinthese collisions.

©2015TheAuthors.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3_.

Hightransversemomentum(pT)particleyieldmeasuredatthe RelativisticHeavyIonCollider(RHIC)wasfoundtobestrongly sup-pressedinrelativisticheavy-ioncollisionscomparedtoelementary proton–protoncollisions [1–4]. It was concluded that the strong high-pT suppressionwas duetofinal-stateeffects inthehotand densequark–gluonplasmacreatedinthosecollisions[1–4]. Instru-mentalto thisconclusionwas the control experimentofproton– nucleus,or deuteron–gold(d

+

Au)collisions asrealized atRHIC, that excluded cold nuclear effects as the possible primary cause forthesuppression[1–4].Theobservationsofthelong-range pseu-dorapidityseparation(



η

) dihadroncorrelationsatsmallrelative azimuth(

φ

)incontrolexperiments p

+

p and p

+

Pb[5–7] colli-sionsattheLargeHadronCollider(LHC)werethereforesurprising, becausethe observedlong-rangecorrelations were similarto the novellong-rangecorrelationfirstdiscoveredinheavy-ioncollisions atRHIC[8–11],calledthe“ridge”.Theheavy-ionridgewas primar-ilyattributedtocollective anisotropicflow [12].Collectiveflowis not normally expected forsmall collision systems where the di-hadroncorrelationsaredominatedbyjetcorrelations.Toreduceor removejet contributions, dihadroncorrelation inlow-multiplicity collisions was subtracted fromthat in high-multiplicitycollisions inpreviousexperiments[6,7,13].Applyingsuchasubtraction pro-cedure revealed a back-to-backridge at

φ

∼

π

, along withthe ridge at

φ

∼

0 in p

+

Pb at

√

sNN

=

5

.

02 TeV [6,7]. Using the same subtraction technique, PHENIX also observed a (near- and away-side)double ridge in d

+

Au collisions at

√

sNN

=

200 GeV within

|

η

|

<

0

.

7 [13].Asobservedinlargersystems,thedouble ridgeisreminiscentofanon-jet ellipticflow contribution[14,15]. Otherphysicsmechanismshavehoweveralsobeenproposed,such asthe colorglass condensate where two-gluon densities are en-hancedatsmall

φ

overawiderangeof



η

[16–18],orquantum initial anisotropy from the space momentum uncertainty princi-ple[19].

Thedifferenceindihadroncorrelationsbetweenhigh- and low-multiplicityeventswouldbe attributabletonon-jetphysicsif jet-likecorrelationsareidenticalinthesetwoeventclasses.However, sincejetparticleproductioncontributestotheoverallmultiplicity, theselection ofhigh-multiplicityeventsmaydemanda relatively large number of jet-correlated particles. In fact, such differences

have been observed previously by the STAR experiment in

two-particlecorrelationsin p

+

p andvariousmultiplicity d

+

Au col-lisions[20,21].Moststudiestodatehaveattemptedto remove/re-ducethesimpleauto-correlationsbetweenjet productionand en-hancedmultiplicity by selecting events via multiplicity measure-mentsatlarge



η

fromthejet.STAR,withitspseudorapidityand azimuthal coverage larger than typicaljet sizes, iswell suited to

investigatethedetailsofdihadronjetlikecorrelationsandpossible effectsfromeventselection.

The data reported here were taken during the d

+

Au run in 2003bytheSTARexperiment[21,22].ThedetailsoftheSTAR ex-periment can be found in Ref. [23]. Minimum-bias (MB) d

+

Au events were triggered by coincidence of signals from the Zero Degree Calorimeters (ZDC)

|

η

|

>

6

.

5 [24] and the Beam–Beam Counters (BBC) [23]. Charged particle tracks were reconstructed in the TimeProjection Chamber (TPC) [25] andthe forward TPC

(FTPC) [26]. The primary vertex was determined from

recon-structed tracks inthe TPC. In this analysisevents were required to havea primaryvertex position

|

zvtx

|

<

50 cm fromthe center ofTPC. Particle tracks usedin thecorrelation analysiswere from theTPC(

|

η

|

<

1),andrequiredtohaveatleast25outofthe max-imumpossibleof45hitsandadistanceofclosestapproachtothe primaryvertexwithin3 cm.

Twoquantitieswere usedtoselectd

+

Au events:thecharged particle multiplicity within

−

3

.

8

<

η

<

−

2

.

8 measured by the FTPCinthe Au-beam direction(FTPC-Au) [21,22] andtheneutral energy (attenuatedADC signal)measured by theZDC inthe Au-beamdirection (ZDC-Au).These measures are referred to,in this article,generallyas“eventactivity.” Whilepositive butweak cor-relationswere observedbetweenthesemeasures,thesameevent fractionpercentagedefinedbythesemeasures,e.g. eventswiththe 0–20% highest FTPC-Au multiplicities or ZDC-Au energies, corre-spondtosignificantlydifferentd

+

Au eventsamples.

The two particles in pairs used in dihadron correlations are customarily calledtrigger and associatedparticle [3]. The trigger particle istypically chosen athigh pT andall other particlesare used as associated particles. In this analysis pair density distri-butions _N1

trig

d2_N

dηdφ aremeasuredin relativeazimuthal angle

φ

andpseudorapiditydistance



η

andare normalizedbythe num-ber oftriggerparticles.The correlation dataarecorrected forthe associated particle tracking efficiency of 85%

±

5%

(

syst

.)

[21,22], which does not vary from low to high event activity in d

+

Au collisions. Here, high (low) event activity refers to event classes selected by high (low) FTPC-Au multiplicities or ZDC-Au neutral energies. The detector non-uniformity in

φ

and acceptance in

Table 1

Gaussian+pedestal Y_√jetlike 2π σexp

 −(η)2

2σ2



+C fitresultstonear-sidecorrelatedyield densitiesind+Au collisions.Thepercentilesindicatefractionsofselectedevents, 40–100%beinglow-activityand0–20%high-activity.Firsterrorsarestatistical,and secondsystematic(duetoZYAM).Anadditional5%efficiencyuncertaintyappliesto YjetlikeandC . Event selection χ2_{/ndf σ(×10}−3_{) Y} jetlike(×10−4) C(×10−4) FTPC 40–100% 19/25 336±7±1 461±11±5 19±5±9 20–40% 18/25 362±8±3 546±15+₋714 24±7+ 20 −11 0–20% 19/25 382±10±9 596±19+₋1511 70±8±12 ZDC 40–100% 19/25 352±7₋+62 501±11±1 22±5+ 14 −8 20–40% 26/25 372±9±7 580±18±17 43±8±12 0–20% 17/25 376±10±3 568±20±17 59±9+₋2714

neutral energy (measured by ZDC-Au). The mixed-event correla-tionsarenormalizedto100%at



η

=

0.

Dihadroncorrelations, aftercombinatorialbackground subtrac-tion,areoftenusedtostudycorrelationsoriginatingfromjets[3]. However,othercorrelationsthanjetsarealsopresent,suchas res-onancedecays.Thepartsofthedihadroncorrelationsusedforthe jet studyaretherefore referred toas“jetlike”correlationsin this Letter. In orderto obtain jetlike correlationsin d

+

Au collisions, auniformcombinatorialbackgroundissubtracted.Thebackground normalizationisestimatedbytheZero-Yield-At-Minimum(ZYAM) assumption[8,27].Afterthecorrelatedyield distributionisfolded intotherangeof0

< φ <

π

,ZYAMistakenasthelowestyield averageover a

φ

window of

π

/

8 radianwidth. TheZYAM sys-tematic uncertainty is estimated by the yields at the ZYAM

φ

location averaged over ranges of width of

π

/

16 and 3

π

/

16

ra-dians. We also fit the

φ

correlations by two Gaussians (with

centroids fixed at 0 and

π

) plus a pedestal. The fitted pedestal is consistentwith ZYAMwithin the statisticaland systematic er-rorsbecausethenear- andaway-side peaksarewell separatedin

d

+

Au collisions.

Fig. 1(a) and1(b) show the correlated yield densities per ra-dianper unit ofpseudorapidity asa functionof



η

forboththe near-side (

|φ|

<

π

/

3) and away-side (

|φ −

π

|

<

π

/

3) ranges in (a) low and (b) highFTPC-Au multiplicity collisions. Both the trigger and associated particle pT ranges are 1

<

pT

<

3 GeV

/c.

The ZYAM background estimate is done for individual



η

bins

separately.The statisticalerrors ofthedata points include point-to-point statistical errors from the ZYAM values, since each



η

binhasitsownZYAMvalue.Thenear-sideyieldsexhibitGaussian peaksandtheaway-sideyieldsare approximatelyuniformin



η

. AGaussian

+

pedestal function _√Yjetlike

2π σexp



−

(η)2 2σ2



+

C fits tothe near-sidedataaresuperimposedinFig. 1(a,b)assolidcurves,and thefit parametersare listedin Table 1.The Gaussian area Yjetlike

measuresthenear-sidejetlikecorrelatedyieldperradian.Thefits indicatearatio

α

=

Y_jetlikehigh

/Y

low

jetlike

=

1

.

29

±

0

.

05

(

stat

.)

±

0

.

02

(

syst

.)

of jetlike yields in high to low FTPC-Au multiplicity collisions. For ZDC-Au event selection, the jetlike ratio parameter is

α

=

1

.

13

±

0

.

05

(

stat

.)

±

0

.

03

(

syst

.)

. The

α

parameter for events se-lectedbyFTPC-Aumultiplicityisfurtherfromunitycomparedto

α

foreventsselectedbyZDC-Auenergy.Theratiosoftheaway-side correlated yields are 1

.

32

±

0

.

02

(

stat

.)

±

0

.

01

(

syst

.)

for FTPC-Au multiplicityand1

.

22

±

0

.

02

(

stat

.)

±

0

.

01

(

syst

.)

forZDC-Auenergy selectedeventsrespectively.Thecorrelatedyieldratiosaresimilar (within 2 standard deviations) betweenthe near and away side, consistentwithback-to-backjetcorrelations.Inaddition,the near-sideGaussianpeakiswiderinhigh- thaninlow-activitycollisions. A similar broadeningof jetlike peak was previously observed in

d

+

Au collisionscomparedwiththatinp

+

p collisions[21]. In previous studies, dihadron correlations in low-multiplicity events are subtracted from high-multiplicity events.The residual

correlation isoften attributedto non-jet origins assuming jetlike correlationsareequalinhigh- andlow-multiplicitycollisions[13]. ThedifferencesbetweenhighandlowFTPC-Aumultiplicityevents from ourdata are shownin Fig. 1(c). A constant fitto the near-andaway-sidedifferencegivesa

χ

2

_/

_ndf

₌

₅₀

_/

_{9 and6}

_.

₄

_/

_9,

respec-tively,whileaGaussian fittothenearsidegives

χ

2

_/

_ndf

₌

₂

_.

₃

_/

_8.

These differences resemble jetlike correlation features, consistent with a Gaussian peak on the near side and a uniform distribu-tionontheawayside.Theythereforesuggestthatthedifferenceis likelyofjetlikeorigin.

Asafirstattemptto“address”thejetlikecorrelatedyield differ-ence,thejetlikeratioparameter

α

isappliedasascalingfactorto thelow-activitydatabeforeitissubtractedfromthehigh-activity data. Thisprocedure assumesthat the away-side correlated yield scaleswiththenear-sideone,whichisbasedonmomentum con-servation arguments. The resulting subtracted data are shown in Fig. 1(d).Theshapeofthenear-sidedifferenceistheresultof sub-tracting a narrowGaussian froma wide one ofequal area offset by a pedestal. On the away side, once the low-activity data are scaled up,the correlatedyields areconsistent betweenhigh- and low-activity collisions as shown by the open circles in Fig. 1(d). This suggests that the away-side difference between high- and low-activityeventsmaybeprimarily duetoadifferenceinjetlike correlations.

As seeninTable 1,thefitpedestal valuesofC alsoshows de-pendence on event activity. Finite correlated yields above ZYAM existonthenearsideatlarge



η

,wherethenear-sidejet contri-bution shouldbe minimal.Thislarge



η

correlation datawillbe studiedelsewhere[28].

Toinvestigatefurthertheinfluenceofeventselectiononjetlike correlations, Fig. 2(a)showsYjetlike asa functionoftheevent

ac-tivity,representedbytheuncorrected chargedhadronmultiplicity

dN/d

η

at midrapidity, in events selected according to the FTPC-Au multiplicity (solid squares) andZDC-Au neutralenergy (open squares), respectively. Five event samples are selected by each

measure, corresponding to 60–100%, 40–60%, 20–40%, 10–20%,

and0–10%events.Thesystematicuncertainties areobtainedfrom Gaussian fits to the



η

correlations, as in Fig. 1, varied by the ZYAM systematic uncertainties. Fig. 2(a) shows that the near-side jetlike correlated yield has a smooth linear dependence on event activity. Qualitatively similar behavior is also observed at the LHC [29]. Such a dependence is not observed in the HIJING [30] simulationof d

+

Au collisions at RHICas illustrated by the curve in Fig. 2(a).The HIJING calculations are scaled down such that thelowestmultiplicity binmatchestherealdata.The multi-plicity dependenceof the jetlike yield is clearly differentfor the HIJINGsimulations.

The jetlike ratio

α

parameter can quantify the effect of the eventselection onjetlikecorrelations. Fig. 2(b)showsthe pT de-pendence of the

α

parameter. The systematic uncertainties are given by ZYAM uncertainties as in Fig. 2(a). Two sets of data points are shown: one (solid circles) has the trigger pT fixed to 0

.

5

<

p(_Tt)

<

1 GeV

/c and

shows the

α

parameter as a func-tion of the associated particle p_T(a) with bin of 0

.

5 GeV

/c.

This trigger pT range is similar to 0

.

5

<

p(Tt)

<

0

.

75 GeV

/c used

by PHENIX [13]. The

α

parameter islarger thanunity andrelatively insensitive to p(_Ta) forthisparticular p(_Tt) choice.The other setof points(solidtriangles)shows

α

asfunctionofp(_Tt)withafixedp(_Ta)

of 0

.

5

<

p(_Ta)

<

1 GeV

/c.

In thiscase the

α

parameter decreases with p(_Tt).

preferen-Fig. 1. Thedihadroncorrelatedyieldnormalizedperradianperunitofpseudorapidityasfunctionofηind+Au collisionsonthenear(|φ|<π/3,solidcircles)andaway side(|φ −π|<π/3,opencircles).Shownarethe(a)lowand(b)highFTPC-Auactivitydata,andthehigh-activitydataaftersubtractingthe(c)unscaledand(d)scaled low-activitydata.Triggerandassociatedparticleshave1<pT<3 GeV/c and|η|<1.TheGaussian+pedestalfittothenearsideissuperimposedasthesolidcurves.Error

barsarestatisticalandboxesindicatethesystematicuncertainties.

Fig. 2. (a)Thenear-sidejetlikecorrelatedyieldobtainedfromGaussianfitas in Fig. 1asfunctionoftheuncorrecteddN/dηatmidrapiditymeasuredintheTPC. Twoeventselectionsareused:FTPC-Aumultiplicity(filledsquares)andZDC-Au en-ergy(opensquares).ThecurveistheresultfromaHIJINGcalculation.(b)Theratio ofthecorrelatedyieldsinhighoverlowFTPC-Aumultiplicityeventsasafunction ofp(_Ta)(p(_Tt))wherep(_Tt)(p(_Ta))isfixed.Errorbarsarestatisticalandcapsshowthe systematicuncertainties.

tiallyselectjetseitheroflarger energyorhappeningto fragment intomoreparticles.However, suchan auto-correlationbiasisnot observedintheHIJINGmodelimplementationasclearlyshownin Fig. 2(a).Event-activity dependentsampling ofjet energies could alsobecaused byother physics origins;forexample,there could bepositivecorrelationsbetweenparticleproductionfromjetsand

fromunderlyingevents.The dependenceofjetlike correlationsat midrapidity on forward event activity could be driven by such mechanisms as initial-statekT effects or final-statejet

modifica-tions by possible medium formation [3,4] in the small d

+

Au

collisionsystem.

The PHENIX experimentreported a double-ridge difference in

the dihadron

φ

correlations between high- and low-activity

eventsin the acceptancerange 0

.

48

<

|

η

|

<

0

.

7 with event ac-tivity definedby total charge in theBBC at

−

3

.

9

<

η

<

−

3[13]. Fig. 3(a)showstheSTARdataanalyzedina similaracceptanceof 0

.

5

<

|

η

|

<

0

.

7 for highandlow-activity eventsdefinedby the FTPC-Au whichhassimilar

η

coverage asPHENIX’sBBC. The sys-tematic uncertainties shownby the histograms are the quadratic sumofthoseduetoefficiencyandZYAM,aswellastheZYAM sta-tisticalerror,becauseitiscommonforall

φ

bins.Thecorrelated yields are larger in high- than in low-activity collisions on both thenearandawayside aspreviously discussed.Thedifference of therawassociatedyield(i.e.noZYAMsubtraction)inhigh-activity eventsminus thejetlike correlatedyield(i.e.withZYAM subtrac-tion) in low-activity events is shown in Fig. 3(b) by the open points. Thesystematicuncertainties are thequadraticsumof the statisticalandsystematicuncertaintiesonZYAMofthelow-activity data. The additional 5% efficiency uncertainty is not shown be-causeitisanoverallscalenotaffectingtheshapeofthedihadron correlation, therefore not affectingthe physics conclusions. Back-to-back double ridges are apparent and are qualitatively consis-tentwiththePHENIXobservation[13].However,thedouble-ridge structure is largely due to the residual jetlike correlation differ-enceasdemonstratedby our dataabove.Interpreting the double ridgesassolelydueto non-jetcontributions inhigh-activity data isthereforepremature.

Again,toaccountforthejetlikecorrelationdifference,onemay multiply theZYAM-subtracted low-activitydata bythe jetlike ra-tio

α

parameter before subtraction. Fig. 3(b)shows, asthe solid points, the raw associated particle yield (i.e. no ZYAM subtrac-tion) in the high FTPC-Au multiplicity data after subtracting the

Fig. 3. (a)Thedihadroncorrelatedyieldnormalizedperradianperunitof pseudo-rapidityasafunctionofφind+Au collisionsatlow(40–100%,opencircles)and high(0–20%,closedcircles)FTPC-Aumultiplicities.Triggerandassociatedparticles are1<pT<3 GeV/c within0.5<|η|<0.7.ZYAMpositionsareindicatedwith

arrows.(b)TherawassociatedyieldathighFTPC-Aumultiplicity minusthe un-scaled(opencircles)andscaled(closedcircles)ZYAM-subtractedcorrelatedyields atlowFTPC-Aumultiplicityversusφ.Errorbarsarestatisticalandboxesindicate thesystematicuncertainties.

propagatedtotalerrorfromZYAMaswellasthefiterroron

α

.The near-sidedifferenceisnon-zero abovetheunderlyingevent base-lineforthe



η

rangeused.Thisisbecause thissimple

α

scaling doesnotaccountfortheobservedbroadeningofthenear-side jet-likepeakfromlow- tohigh-activitycollisions,althoughthejetlike yielddifferencehasbeentakencareof.Thiscausesasignificantly larger difference in the intermediate range of 0

.

5

<

|

η

|

<

0

.

7. When



η

range closer to zero is used, e.g.

|

η

|

<

0

.

3, the jet-like difference is dipped (below the baseline) on the near side after

α

scaling. This is shown by the negative solid data points at



η

∼

0 in Fig. 1(d). Barring from the difference caused by thebroadening,thereisafinitepedestal valuefromthenear-side Gaussian

+

pedestal fitthat increaseswitheventactivityas afore-mentioned.Thispedestaldifferenceremains inthenear-sidepeak inFig. 3(b).

After the jetlike contribution is removed by the scaled sub-traction, theaway-side difference issignificantly diminished. The resultsare similar using theZDC-Au eventactivity. This suggests thatanypossiblecontributionfromnon-jetlikelong-range correla-tions,such astheback-to-backridge,issmall.Althoughitdoesa betterjobofremovingjetlikecontributionsthanasimple subtrac-tionoflow-activityfromhigh-activitydata,thescaled subtraction maynotcompletelyremovethejetlikecontributions.Thisissofor tworeasons.One,the away-sidejetlike yieldina given pT range maynot strictly scale withthe near-sideone between high- and low-activitycollisions,dependingonthedetailsofdijetproduction andfragmentation. Two, the jetlike correlation shapes,being dif-ferent on the near side, can also be different on the away side, e.g. due toincreasing kT broadening(or acoplanarity)withevent activity.

Insummary, dihadroncorrelations are measured at midrapid-ityusingthe STARTPC asfunction oftheforward rapidity event activity in d

+

Au collisions at

√

sNN

=

200 GeV. The event ac-tivityisclassifiedbythemeasured FTPC-Auforwardcharged par-ticle multiplicity or the ZDC-Au zero-degree neutral energy. The correlatedyields areextractedbysubtracting theestimated back-groundusingZYAM.It isfoundthat thecorrelated yieldislarger

inhigh- thaninlow-activitycollisions andthe



η

-dependenceof theobservedyielddifferenceresemblesjetlikefeatures,suggesting ajetlikeorigin.Therecouldbemultiplereasonsforthedifference, rangingfromsimpleauto-correlationbiasestophysicaldifferences between high- and low-activity d

+

Au collisions. The away-side correlation difference is significantly diminished after scaling the low-activity data by the ratio of the near-side jetlike correlated yields. Ourdatademonstratethat thedihadroncorrelation differ-ence betweenhigh- and low-activity events at RHIC is primarily due to jets. Ind

+

Au collisions at RHIC such event-selection ef-fectsonjetlikecorrelationsmustbeaddressedbeforeinvestigating possiblenon-jetcorrelationssuchasanisotropicflow.

Acknowledgements

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

NERSC Center atLBNLandtheOpen ScienceGrid consortiumfor providingresourcesandsupport.Thisworkwas supportedinpart bytheOfficesofNPandHEPwithintheU.S. DOEOfficeofScience, the U.S. NSF,the Sloan Foundation,the DFG clusterof excellence ‘Origin and Structure of the Universe’ of Germany, CNRS/IN2P3,

STFC and EPSRC of the United Kingdom, FAPESP CNPq of Brazil,

Ministry of Ed. and Sci. of the Russian Federation, NNSFC, CAS,

MoST, andMoE of China, GA and MSMT of the Czech Republic,

FOM and NWO of the Netherlands, DAE, DST, and CSIRof India,

PolishMinistryofSci.andHigherEd.,Korea ResearchFoundation, MinistryofSci.,Ed. andSportsoftheRep.ofCroatia,Russian Min-istryofSci.andTech.,andRosAtomofRussia.

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