Jet-like correlations with direct-photon and neutral-pion triggers at s NN =200 GeV

Contents lists available atScienceDirect

Physics

Letters

B

www.elsevier.com/locate/physletb

Jet-like

correlations

with

direct-photon

and

neutral-pion

triggers

at

√

s

_{N N}

=

200 GeV

STAR

Collaboration

L. Adamczyk

a

,

J.K. Adkins

t

,

G. Agakishiev

r

,

M.M. Aggarwal

af

,

Z. Ahammed

aw

,

I. Alekseev

p

,

D.M. Anderson

aq

,

A. Aparin

r

,

D. Arkhipkin

c

,

E.C. Aschenauer

c

,

M.U. Ashraf

at

,

A. Attri

af

,

G.S. Averichev

r

,

X. Bai

g

,

V. Bairathi

ac

,

R. Bellwied

as

,

A. Bhasin

q

,

A.K. Bhati

af

,

P. Bhattarai

ar

,

J. Bielcik

j

,

J. Bielcikova

k

,

L.C. Bland

c

,

I.G. Bordyuzhin

p

,

J. Bouchet

s

,

J.D. Brandenburg

ak

,

A.V. Brandin

ab

,

I. Bunzarov

r

,

J. Butterworth

ak

,

H. Caines

ba

,

M. Calderón de la Barca Sánchez

e

,

J.M. Campbell

ad

,

D. Cebra

e

,

I. Chakaberia

c

,

P. Chaloupka

j

,

Z. Chang

aq

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A. Chatterjee

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S. Chattopadhyay

aw

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X. Chen

w

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J.H. Chen

an

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at

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M. Cherney

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W. Christie

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H.J. Crawford

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D. McDonald

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Y.X. Pan

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B. Pawlik

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M. Przybycien

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*

Correspondingauthor.

E-mailaddress:nihar@rcf.rhic.bnl.gov(N.R. Sahoo). http://dx.doi.org/10.1016/j.physletb.2016.07.046

S. Ramachandran

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R.L. Ray

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J.B. Roberts

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a_{AGHUniversityofScienceandTechnology,FPACS,Cracow30-059,Poland} b_{ArgonneNationalLaboratory,Argonne,IL 60439,UnitedStates} c_{BrookhavenNationalLaboratory,Upton,NY 11973,UnitedStates} d_{UniversityofCalifornia,Berkeley,CA 94720,UnitedStates} e_{UniversityofCalifornia,Davis,CA 95616,UnitedStates} f_{UniversityofCalifornia,LosAngeles,CA 90095,UnitedStates} g_{CentralChinaNormalUniversity,Wuhan,Hubei430079,China} h_{UniversityofIllinoisatChicago,Chicago,IL 60607,UnitedStates} i_{CreightonUniversity,Omaha,NE 68178,UnitedStates}

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

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

n_{IndianInstituteofTechnology,Mumbai400076,India} o_{IndianaUniversity,Bloomington,IN 47408,UnitedStates}

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

r_{JointInstituteforNuclearResearch,Dubna 141980,Russia} s_{KentStateUniversity,Kent,OH 44242,UnitedStates}

t_{UniversityofKentucky,Lexington,KY 40506-0055,UnitedStates}

u_{KoreaInstituteofScienceandTechnologyInformation,Daejeon305-701,RepublicofKorea} v_{LamarUniversity,Beaumont,TX 77710,UnitedStates}

w_{InstituteofModernPhysics,ChineseAcademyofSciences,Lanzhou,Gansu730000,China} x_{LawrenceBerkeleyNationalLaboratory,Berkeley,CA 94720,UnitedStates}

y_{LehighUniversity,Bethlehem,PA 18015,UnitedStates} z_{Max-Planck-InstitutfurPhysik,Munich80805,Germany} aa_{MichiganStateUniversity,EastLansing,MI 48824,UnitedStates} ab_{NationalResearchNuclearUniversity MEPhI,Moscow115409,Russia} ac_{NationalInstituteofScienceEducationandResearch,Bhubaneswar751005,India} ad_{OhioStateUniversity,Columbus,OH 43210,UnitedStates}

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

ag_{PennsylvaniaStateUniversity,UniversityPark,PA 16802,UnitedStates} ah_{InstituteofHighEnergyPhysics,Protvino142281,Russia}

ai_{PurdueUniversity,WestLafayette,IN 47907,UnitedStates} aj_{PusanNationalUniversity,Pusan46241,RepublicofKorea} ak_{RiceUniversity,Houston,TX 77251,UnitedStates}

al_{UniversityofScienceandTechnologyofChina,Hefei,Anhui230026,China} am_{ShandongUniversity,Jinan,Shandong250100,China}

an_{ShanghaiInstituteofAppliedPhysics,ChineseAcademyofSciences,Shanghai201800,China} ao_{StateUniversityof NewYork,StonyBrook,NY11794,UnitedStates}

au_{UnitedStatesNavalAcademy,Annapolis,MD 21402,UnitedStates} av_{ValparaisoUniversity,Valparaiso,IN 46383,UnitedStates} aw_{VariableEnergyCyclotronCentre,Kolkata700064,India} ax_{WarsawUniversityofTechnology,Warsaw00-661,Poland} ay_{WayneStateUniversity,Detroit,MI 48201,UnitedStates}

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

bb_{NationalChengKungUniversity,Tainan70101,Taiwan}

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Articlehistory: Received7April2016 Accepted18July2016 Availableonline22July2016 Editor:V.Metag

Azimuthal correlations of charged hadrons with direct-photon (

γ

dir) and neutral-pion (

π

0) trigger

par-ticles are analyzed in central Au+Au and minimum-bias p+p collisions

at

√sN N=200 GeV in the

STAR experiment. The charged-hadron per-trigger yields at mid-rapidity from central Au+Au collisions are compared with p+p collisions

to

quantify the suppression in Au+Au collisions. The suppression of the away-side associated-particle yields per

γ

dir trigger is independent of the transverse momentum

of the trigger particle (ptrig

T ), whereas the suppression is smaller at low transverse momentum of the

associated charged hadrons (passoc

T ). Within uncertainty, similar levels of suppression are observed for

γ

dir and

π

0 triggers as a function of zT (≡passocT /p

trig

T ). The results are compared with

energy-loss-inspired theoretical model predictions. Our studies support previous conclusions that the lost energy reappears predominantly at low transverse momentum, regardless of the trigger energy.

©2016 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP3_.

1. Introduction

Overthepastdecade,experimentsattheRelativisticHeavyIon Collider (RHIC) at BNL have studied the hot and dense medium createdinheavy-ioncollisions.Thesuppressionofhigh-transverse momentum(pT)inclusivehadrons[1–3],indicativeofjet

quench-ing, corroborates the conclusion that the medium created is

opaqueto coloredenergetic partons[4–7].Thisphenomenon can beunderstoodasaresultofthemedium-inducedradiativeenergy loss of a hard-scattered parton as it traverses the Quark Gluon Plasma(QGP)createdinheavy-ioncollisions[8,9].Theangular cor-relationofchargedhadronswithrespecttoa direct-photon(

γ

dir) trigger was proposed as a promising probe to study the mech-anisms of parton energy loss [10]. The presence of a “trigger” particle,havingpT greaterthansomeselectedvalue,servesaspart oftheselectioncriteriatoanalyzetheeventforahardscattering. Directphotonsareproducedduringtheearlystageofthecollision, throughleading-orderpQCDprocessessuchasquark–gluon Comp-ton scattering (qg

→

q

γ

) and quark–antiquark pair annihilation (qq

¯

→

g

γ

).Intheseprocessesthetransverseenergyofthetrigger photonapproximates theinitial pT oftheoutgoing recoilparton, beforetherecoiling(“away-side”)partonlikelylosesenergywhile traversingthemediumandfragmentsintoajet.Thejet-likeyields associatedwithatriggerparticleareestimatedbyintegratingthe correlatedyieldsofchargedhadronsoverazimuthaldistancefrom thetrigger particle(

φ

). Any suppressionof thecharged-hadron per-triggeryieldsintheaway-sidejetsincentralAu

+

Aucollisions isthenquantifiedbycontrastingtotheper-triggeryieldsmeasured in p

+

p collisions, via theratio ofintegratedyields, IA A [11,12] (defined inEq. (8)). When requiring a hadron trigger (suchas a

π

0_{),the p}

T oftherecoilingparton(andhencetheaway-sidejet) isnotaswell approximatedby thetransverseenergyofthe trig-ger.For example, the PYTHIA Monte Carlo simulator [13] shows that, in p

+

p collisions at

√

sN N

=

200 GeV, a

π

0 trigger with

pT

>

12 GeV

/

c carries, on average, only 80

±

5% ofthe original scatteredparton’s pT. Thispercentage fromPYTHIAis consistent withthevaluesextractedfromthisanalysis,asdescribedbelow.

Despitethiscomplication,itiscompellingtocomparethe sup-pressionfor

γ

dir triggerswiththatfor

π

0 triggersbecauseofthe expecteddifferences ingeometrical biases atRHIC energies [14].

While the

π

0 _{trigger is likely to have been produced near the}

surface ofthe medium, the

γ

dir trigger doesnot sufferthe same bias, since the photon mean free path is much larger than the size ofthe medium. Comparing

γ

dir- and

π

0-triggered yields of-fersfurtheropportunitiestoexplorethegeometricbiasesandtheir interplaywithpartonenergyloss.Anext-to-leadingorder pertur-bativeQCDcalculation[15]suggeststhatproductionofhadronsat different zT is also affectedby differentgeometric biases, where

zT

≡

passocT

/

p trig

T (passocT and p trig

T are transversemomentaof asso-ciatedandtriggeredparticles,respectively)representstheratioof thetransversemomentumcarriedby achargedhadron inthe re-coiljettothatofthetriggerparticle.Thehigh-zT hadronsinajet recoiling from a

γ

dir preferentially originate from a parton scat-teringneartheaway-sidesurfaceofthemedium,sincescatterings deeperinthemedium willresultinastrongerdegradationofthe high-momentumcomponentsofthejet.Thehigh-zT hadronsina jetrecoilingfroma

π

0 _{preferentiallyemergefromscatterings}

tan-gentialto the surface fromthe alreadybiasedsurface-dominated triggerjets(which isconsistent withobservationsin [16]). These two mechanisms turn out to lead to the same level of suppres-sion[15].Onlyatlow zT doesthefullsamplingofthevolumeby

γ

dir triggersshow apredicteddifferencefromthatofthe surface-dominated

π

0_triggers.

Anadditionaleffectatlow zT maybetheredistributionofthe parton’slostenergywithinthelow-momentumjetfragments[17], which is not included in the calculation [15] described above. This was studied by the PHENIX Collaboration, which found an enhancement at low zT andlarge angles, fordirect photon trig-gers with ptrig_T in the range of 5–9 GeV

/

c at

√

sN N

=

200 GeV in themost central Au

+

Au collisions [11].Enhancements dueto thismechanismwouldbeexpectedinhadronsrecoilingfromother triggersaswell,suchas

π

0 _{orjets.ApreviousSTARmeasurement}

ofhadronsassociated withareconstructed jet has shownan en-hancementfor pT

<

2 GeV

/

c,fortwoclassesofjetswithbroadly separatedenergyscales,inwhichtheenhancementatlow pT bal-ancesthesuppressionathigh pT [18].

away-Fig. 1. (Coloronline.)Theazimuthalcorrelationfunctionsofchargedhadronspertriggerforπ0

rich(opencircles)andγrich(filledcircles)triggers,measuredincentral(0–12%)

Au+Aucollisionsandminimum-biasp+p collisions.Panels(a)and(c)areforAu+Auand p

+

p collisions,respectively,in1.2<passoc

T <3 GeV/c andpanels(b)and(d)

arethatfor3<passoc

T <5 GeV/c.Thedashedcurvesindicatethebackground(chargedhadronsnotcorrelatedwiththejet),shownonlyforπrich0 triggers,andthearrows

indicatetherangeoverwhichtheaway-sideisintegrated(greenandvioletcoloredarrowsrepresent

|φ −

π| ≤0.6 and

|φ −

π| ≤1.4 respectively).

sidemainly comesfromgluonjets[21].Thisisincontrasttothe away-side ofa

γ

dir trigger, which mainly comes fromquark jets, since at leading order a photon does not couple with a gluon. Thusit isexpectedthat,onaverage,theaway-side parton associ-atedwitha

π

0 _{suffersmoreenergylossthanthatofa}

_γ

dir dueto theadditionalcolorfactorfromgluons.Bycomparingthe suppres-sion ofaway-side associated hadrons for

γ

dir triggers to that for

π

0 _{triggers, onecangain informationaboutboththepath-length}

andthecolor-factordependenceofpartonenergyloss.

Thismanuscriptisorganized asfollows.The detectorsetup of theSTARexperimentisdiscussedinSec.2.Thetransverse shower-shape analysis used to discriminate between

π

0 _and

_γ

dir, and the procedures to extract the charged-hadron spectra,associated with

π

0 _and

_γ

dir triggers,arediscussedinSec. 3.Theper-trigger charged-hadronyieldsarepresentedasafunctionofzT,inSec.4. The dependences of the suppression of these yields in central Au

+

Au collisions relative to those in minimum-bias p

+

p

col-lisions on both the trigger energy and the associated transverse momentumare discussed,withcomparisonsto theoreticalmodel predictions.Finally,inSec.5,ourobservationsaresummarized.

2. Experimentalsetup

Thedatawere takenby theSolenoidal TrackeratRHIC (STAR) experimentin2011and2009 forAu

+

Auand p

+

p collisions at

√

sN N

=

200 GeV, respectively. Using the Barrel Electromagnetic Calorimeter(BEMC) [22] to selecteventscontaining a high-pT

γ

or

π

0_{,theSTARexperimentcollected anintegratedluminosity of}

2

.

8 nb−1ofAu

+

Aucollisionsand23 pb−1ofp

+

p collisions.STAR provides2

π

azimuthalcoverageandwidepseudo-rapidity(

η

) cov-erage. The Time Projection Chamber (TPC) is the main charged-particletrackingdetector[23],providingtrackinformationforthe chargedhadronswith

|

η

|

<

1

.

0. Thecentrality selection is deter-mined from the charged-particle multiplicity in the TPC within

|

η

|

<

0

.

5.TheBEMCisasamplingcalorimeter,andeach calorime-termoduleconsistsofalead-scintillator structureandan embed-dedwirechamber, theBarrelShower MaximumDetector(BSMD). TheBSMDissituatedapproximatelyfiveradiationlengthsfromthe front face of the BEMC. BEMC towers (each covering 0.05 units in

η

and

φ

) provideameasurementoftheenergyof electromag-neticclusters,whereastheBSMD,duetoitshighgranularity(0.007

unitsin

η

and

φ

),provideshighspatialresolutionforthecenterof a cluster andthe transversedevelopment of theshower. Electro-magneticclustersareconstructedfromtheresponseofoneortwo towers, depending on the location ofthe centroidas determined by theBSMD.Thetransverseextentoftheshower isusedto dis-tinguishbetween

γ

dir showersanddecayphotonsfrom

π

0.Details ofthe

π

0

_/

_γ

_{discriminationarediscussedinthenextsection.}

3. Analysisdetails

Events having a transverse energy in a BEMC cluster ET

>

8 GeV, with

|

η

|

≤

0

.

9, are selected for this analysis. In order to distinguish a

π

0_{,which athigh p}

T predominately decaysto two photonswithasmallopeningangle,fromasingle-photoncluster, a transverse shower-shapeanalysis isperformed. In thismethod, the overall BEMC cluster energy (Ecluster), the individual BSMD stripenergies(ei),andthedistancesofthestrips(ri)fromthe cen-teroftheclusterareusedtoconstructthe“TransverseShower Pro-file” (TSP). The TSPis definedas, TSP

=

Ecluster/



ieir1i.5 [12,24]. The

π

0

rich (nearly pure sample of

π

0) and

γ

rich (enhanced frac-tion of

γ

dir) samples are selected by requiring TSP

<

0

.

08 and 0

.

2

<

TSP

<

0

.

6, respectively, in both p

+

p and Au

+

Au colli-sions. The

π

0

rich sample isestimatedtobe

∼

95% pure

π

0, deter-mined fromstudiesofsimulated

π

0 _and

_γ

dir embedded intoreal data.The

φ

azimuthalcorrelationsareconstructedwith charged-hadrontrackswithin1

.

2 GeV

/

c

<

passoc_T

<

ptrig_T and

|

η

|

<

1

.

0.Both trigger samples are selected with 12

<

ptrig_T

<

20 GeV

/

c (or 8

<

ptrig_T

<

20 GeV

/

c for the studyof the

γ

dir ptrigT dependence) and

|

η

|

<

0

.

9. There is an additional requirement that no track with momentum greaterthan3 GeV/c is pointingtothe triggertower. This track-rejection cut prevents significant contamination of the measuredBEMCenergyofthetriggerparticle.ThepT thresholdof thetrack-rejectioncutwasvariedbetween1and4 GeV/c,asapart ofthesystematicstudies,andthevariationsshowednosignificant differenceintheaway-sidecharged-hadronyields.

The correlation functionsrepresent the number of associated charged hadrons (Nassoc) per trigger particle,

(

1

/

Ntrig)(dNassoc/

oftheazimuthalcorrelationfunctionsfor

γ

rich- and

π

rich0 -triggered associated chargedhadrons, for different passoc_T ranges, is shown for the 12% most central Au

+

Au and minimum-bias p

+

p

col-lisions. In the lower passoc_T bins, the uncorrelated background (shownin Fig. 1as dashed curves) ishigher than that inhigher

passoc_T bins, especiallyinAu

+

Aucollisions, whereasin p

+

p

col-lisions, this uncorrelated background is small in all passoc_T bins. On the near-side (

φ

∼

0) the

π

0

rich-triggered correlated yields arelargerthan thosefor

γ

rich triggers,asexpected. Thenon-zero near-side

γ

rich-triggered yields are dueto the backgroundin the

γ

richtriggersampleandareusedtodeterminetheamountof back-ground,asfurtherdiscussedbelow.Inthehigherpassoc

T range,itis alsoobserved that the away-side (

φ

∼

π

)

γ

rich-triggered yields aresmaller thanthose ofthe

π

0

rich triggers, whichcanbe under-stood since the

π

0 _{triggers originate from the fragmentation of}

partonsgenerallyhaving a higherenergythan the corresponding direct-photontriggers.

Thebackgroundsubtractionandthepair-acceptancecorrection (in

φ

)havebeenperformedusingamixed-eventtechnique (see

e.g.[16])foreachzT bin.Eventmixingisperformedamongevents havingsimilarvertexpositionandcentralityclass.In Au

+

Au col-lisions, the background (i.e. what is not correlated with the jet) may still contain azimuthal correlations due to flow. The distri-butions of background pairs for different zT bins are therefore modulatedwiththesecond Fourier(elliptic flow)coefficient (v2)

oftheparticleazimuthaldistributionmeasuredwithrespecttothe eventplane. It is given by B

[

1

+

2



vtrig₂



vassoc₂

cos

(

2

φ)

]

, where B representsthelevel ofbackgroundpairs andisdetermined as-sumingapplicabilityofthe“Zero-Yieldat1radian”(ZYA1)method, avariationon the“Zero-YieldatMinimum”(ZYAM) method[25]. The



vtrig₂

(



v₂assoc

) istheaveragevalueofthesecond-order flow coefficient [26] of the trigger (associated) particle at the mean

ptrig_T (passoc_T )ineachzT bin.Theflowterminthebackground sub-tractiononlyhasasignificanteffectforAu

+

AucollisionsatlowzT, andthehigherorderflowcomponentsareignoredastheir magni-tudesaresmallinthemostcentralAu

+

Aucollisions.In p

+

p

col-lisions,B isdeterminedassumingaflat(uncorrelated)background. The trigger-associated charged-hadron yields are determined from the azimuthal correlation functions, per trigger particle (

π

0

richand

γ

richsamples),per

φ

,bothonthenearside(

φ

∼

0) andthe away side (

φ

∼

π

). In this analysis, the near-side and away-sideyieldsareextractedbyintegratingthecorrelation func-tions, for given zT bins, over

|φ|

≤

1

.

4 and

|φ −

π

|

≤

1

.

4, respectively.Therawnear-sideandaway-side associated charged-hadronyields arecorrected fortheassociated-particleefficiencies

determined by embedding simulated charged hadrons into real

events.Theaveragetrackingefficienciesforchargedhadrons(with

passoc_T

>

1

.

2 GeV

/

c) aredeterminedvia detectorsimulations tobe around70% and90% forcentralAu

+

Auandminimum-bias p

+

p

collisions,respectively.The

π

0_{-triggeredyieldsarecalculatedfrom}

the

π

0

rich-triggered correlation functions, with no further correc-tion forthe contamination inthe trigger sample, becauseof the highpurityinthe

π

0

richsample.

Away-side charged-hadron yields for

γ

dir triggers are deter-minedbyassumingzeronear-sideyieldfor

γ

dirtriggers,andusing thefollowingexpression

Yγdir+h

=

Y_γaway rich+h

−

RY away π0 rich+h 1

−

R

.

(1) Here Y_γaway rich+h (Y away π0 rich+h

) represents the away-side yield of

γ

rich (

π

0

rich),andR isgivenby

R

=

Ynear γrich+h Ynear π0 rich+h

,

(2)

the ratio of the near-side yield in the

γ

rich-triggered correlation function to the near-side yield in the

π

0

rich-triggered correlation function.Thismeans

1

−

R

=

N

γdir

Nγrich

,

(3)

whereNγdir _(Nγrich_{) isthenumberof}

_γ

dir (

γ

rich) triggers.The val-uesof1

−

R,representingthefractionsofsignalinthe

γ

richtrigger

sample, are found to be 40% and 70% for p

+

p and the

cen-tralAu

+

Aucollisions,respectively.Usingthistechnique,almostall sourcesofbackground(includingphotonsfromasymmetrichadron decaysandfragmentationphotons)canberemoved,assumingthat theircorrelationsaresimilartothosefor

π

0 _{triggers.This}

assump-tionwas testedusingPYTHIAsimulations,withdecayphotonsas thetriggerparticles,anditwasfoundtobevalidtowithinatleast 15%(thestatisticalprecisionofthePYTHIAstudy).

Systematicuncertainties includetheeffectsoftrack-quality se-lectioncriteria,neutral-clusterselectioncriteria,

π

0

_/

_γ

discrimina-tion(TSP)cutsforthe

π

0

richand

γ

richsamples,thesizeoftheZYA1 normalization region, the v2 uncertainty range, and the

yield-integrationwindows.All ofthesesources ofuncertaintyare eval-uated foreachdata pointindividually.Forgroupsofsources that arenot independent,such asdifferentyield-extractionconditions, themaximumdeviationamongthedifferentconditionsistakenas thecontributiontothesystematicerror.Thesystematic uncertain-tiesfromsourcesthatareconsideredtobeindependentareadded inquadrature.The

π

0

_/

_γ

_{discrimination uncertaintydominatesin}

most zT bins, varying between10and25%. Thetrack-quality se-lection criteria typically contributes a 5–10% uncertainty. In the lowest zT bin in Au

+

Au collisions for

π

0 triggers, the yield ex-traction uncertainty dominateswith asmuch as 50% uncertainty in the near-side yield. The variation ofthe pT threshold for the track-rejectioncutfortheneutral-towertriggerselectiontypically hasanegligibleeffect.

4. Resultsanddiscussion

In this measurement, both

π

0 _and

_γ

dir triggers are required to be within a range of 12

<

ptrig_T

<

20 GeV

/

c, or 8

<

ptrig_T

<

20 GeV

/

c for the study of the ptrig_T dependence. In contrast to a

γ

dir trigger, a

π

0 trigger carries a fraction of the initial par-ton energy of the hard-scattered parton. In this case, the zT for a trigger

+

associated-particle pair is only a loose approximation of the fractional parton energy carried by the jet constituent. Theintegratedaway-sideandnear-sidecharged-hadronyieldsper

π

0 _{trigger, D}

₍

_z

T

)

,areplottedasafunctionofzT,bothforAu

+

Au (0–12% centrality) and p

+

p collisions, in Fig. 2. Yields of the away-side associated chargedhadrons are suppressed, in Au

+

Au relative to p

+

p, at all zT except in the low zT region. On the otherhand,nosuppressionisobservedonthenear-sideinAu

+

Au, relativetop

+

p collisions,duetothesurfacebiasimposedby trig-geringonahigh-pT

π

0.

Fig. 3showstheaway-side D

(

zT

)

for

γ

dir triggers,asextracted fromEq.(1),asafunctionofzT forcentralAu

+

Auand minimum-bias p

+

p collisions.The

π

0_{-triggered away-side charged-hadron}

yieldscannotbedirectlycomparedtothoseof

γ

dir triggers,asthe

π

0 _{trigger isa fragment of a higherenergy parton. Onecan}

Fig. 2. (Coloronline.)ThezT dependenceofπ0-h±away-side(a)andnear-side(b)

associatedcharged-hadronyieldspertriggerforAu+Auat0–12% centrality(filled symbols)and p

+

p (opensymbols)collisionsat √sN N=200 GeV.Verticallines

representthestatisticalerrors,andtheverticalextentoftheboxesrepresents sys-tematicuncertainties.

Fig. 3. (Coloronline.)ThezT dependenceofγdir-h±away-sideassociated

charged-hadronyieldspertriggerforAu+Auat0–12% centrality(filleddiamonds)andp+p (opendiamonds)collisions.Verticallinesrepresentstatisticalerrors,andthe verti-calextentoftheboxesrepresentssystematicuncertainties.



passoc_T

ptrig_T

=

0

.

17

±

0

.

04

.

(4) From that, thefraction ofenergy carriedby the

π

0 _{trigger, with} ptrig_T

=

12–20 GeV

/

c,isestimatedtobe

ptrig_T

p_Tjet-charged

=

85

±

3%

,

(5)

wherep_Tjet-charged isequaltotheptrig_T plusthetotal pT carriedby the near-sideassociated chargedhadrons. Thisis consistent with whatisobtainedwhenapplyingthesameanalysison

π

0_-triggered

charged-hadroncorrelationsfromaPYTHIAsimulation.InPYTHIA, the neutral associated energy can also be accounted for, giving usanestimate ofthefractional energycarriedby the

π

0 _trigger,

whenaccountingforallassociatedparticles(chargedandneutral), ptrig_T

p_Tjet

=

80

±

5%

.

(6)

Fig. 4. (Coloronline.)ThezT=passocT /p

γdir

T dependencesofγdir-h± away-side

as-sociatedcharged-hadronyieldspertriggerfor p

+

p (opencircles)collisionsand thatofzcorrT =p

assoc

T /p

jet

T dependenceoftheπ0-h± away-sideassociated

charged-hadronyields(opendiamonds)areshown.Verticallinesrepresentstatisticalerrors bars,andtheverticalextentoftheboxesrepresentssystematicuncertainties.

Applying thisratioas acorrection factor to the zT valuesof the away-side D

(

zT

)

for

π

0 triggers in p

+

p collisions resultsinthe

D

(

zcorr T

)

distribution,where zcorr_T

=

p assoc T p_Tjet

.

(7)

Since zcorr_T represents thefractional momentumofthejet carried bytheassociatedparticles,itis(totheextentthatthepγ_T isagood approximation oftheinitial pT oftherecoilparton)equivalent to thezT measuredwhenusing

γ

dirtriggers.D

(

zcorrT

)

isdirectly com-pared to the fragmentation function measured via direct-photon triggersin

Fig. 4

andshowsreasonableagreement.

In order to quantify the medium modification for

γ

dir- and

π

0_{-triggered recoil jet production as a function of z}

T, the ratio, definedas IA A

=

D

(

zT

)

Au Au D

(

zT

)

pp

,

(8)

of the per-triggerconditional yields in Au

+

Au tothose in p

+

p

collisions is calculated. In the absence of medium modifications,

IA A isexpected to beequal to unity. Fig. 5 showstheaway-side mediummodificationfactorfor

π

0 _triggers(Iπ0

A A)and

γ

dir triggers (Iγdir

A A), as a function of zT. Iπ

0

A A and I

γdir

A A show similar suppres-sionwithinuncertainties.AtlowzT (0

.

1

<

zT

<

0

.

2),bothIπ

0

A A and

Iγdir

A A showanindicationoflesssuppressionthanathigherzT.This observationisnotsignificantinthezT-dependenceofIA A because the uncertainties in the lowest zT bin are large. However, when

IA A is plottedvs. passocT (shown ina later figure), the conclusion is supported with somewhat more significance. At high zT, both

Iπ_{A A}0 andIγdir

A A showafactor

∼

3–5 suppression.

Theoretical modelpredictions, labeledasQin [27] andZOWW

[15,28],usingthesamekinematiccoveragefor

γ

dir triggered away-sidecharged-hadronyields,arecomparedtothedata.Inthemodel by Qin et al., the energy loss mechanism is incorporated into a thermalizedmediumforAu

+

Aucollisionswithimpactparameters of 0–2

.

4 fm byusing afull (3

+

1)-hydrodynamicevolutionmodel description.Althoughthismodelalsoincludesjet-mediumphotons (photons coming from the interaction of hard partons with the medium [29,30]) and fragmentation photons (photons radiating fromhardpartons[30]),bothofthesecontributetoIγdir

Fig. 5. (Coloronline.)TheIγdir

A A (redsquares)andIπ 0

A A(bluecircles)triggersare

plot-tedasafunctionofzT.ThepointsforIγdirA A areshiftedby

+

0.03 inzTforvisibility.

Theverticallinesrepresentstatisticalerrorandtheverticalextentoftheboxes rep-resents systematicerrors.Thecurvesrepresenttheoreticalmodelpredictions[15,17, 27,28].

ThecalculationbyZOWWalsoincorporatestheparameterized par-ton energy loss into a bulk-medium evolution [28]. It does not includefragmentationorjet-mediumphotons, andalsodescribes theexperimental measurementofIγdir

A A asafunctionof zT forthe topcentralAu

+

Aucollisions. Thecalculated Iπ_{A A}0 (alsoby ZOWW) shows a somewhat larger suppression than the Iγdir

A A at low zT. The difference at low zT between the IγA Adir andthe Iπ

0

A A (as cal-culated by ZOWW) is likely due to the color factor effect and thedifferences in averagepath lengths between

π

0 _{triggers and}

γ

dir triggers. The calculated difference in the suppression is ap-proximately 50% at zT

=

0

.

1. The data are not sensitive to this difference within the measured uncertainties. These models (Qin andZOWW) donot includearedistribution ofthelost energyto thelower pT jet fragments,in contrasttothe YaJEM model[17]. The YaJEM model is also shown in Fig. 5, although for a some-what lower trigger pT range of9–12 GeV/c. It predicts IγA Adir

=

1 atzT

=

0

.

2 (corresponding to passocT

∼

1

.

8 GeV

/

c) andrising well above1in thezT rangeof0.1–0.2 [17].Thisis calculatedwitha smallintegration window of

π

/

5 around

φ

=

π

. Althoughthis calculation has a different ptrig_T cut, such a large rise is not ob-servedinourdata.Incontrasttotheothercalculationsshown(Qin andZOWW),therisein Iγdir

A A atlow zT inYaJEM ispredominantly due to the redistribution of lost energy. In this picture, the

in-mediumshowerismodified bythemediumandasuppressionat

highzT resultsinanenhancementatlower zT.Theauthors com-pare the “medium-modified shower” picture to an “energy loss” picture,wheretheenergyiscarriedthroughthemediumbya sin-gleparton, andthelostenergywouldonlyshowup atextremely low energies andlarge angles.In such a picture,they argue that theriseinIγdir

A A atlow zT wouldbe moremodestand IγA Adir would remainlessthan1.

BecausePHENIXhasreportedanenhancementatlowzT (zT

<

0

.

4) in Iγdir

A A atlarge angles [11],it is interesting to compareour resultsoverthefullintegrationwindowof

|φ −

π

|

<

1

.

4 radians toan Iγdir

A A calculatedwithasmallerwindowof

|φ −

π

|

<

0

.

6 ra-dians in Fig. 6. Within our uncertainties, an enhancement effect is only seen in the lowest zT bin for

π

0 triggers. However, for thePHENIX measurement, zT

<

0

.

4 corresponds to lower pT for theassociated hadrons(



2 GeV/c),since the pγ_T was chosen in therange of5–9 GeV/c. In ouranalysis, associated hadronswith

pT

<

2 GeV

/

c areonlypresentatzT

<

0

.

2.Theapparent inconsis-tencybetweenSTARandPHENIX,wheninvestigatingtherecovery ofthelostenergyasafunctionofzT,indicatesthat passocT maybe themorepertinentvariable.The conclusionisthat the“modified

Fig. 6. (Coloronline.)TheratiosofD(zT)obtainedusinganintegrationwindowof

|φ −π| <1.4 radiansoverthatof

|φ −

π| <0.6 radiansforγdir−h±(leftpanel)

andπ0_−h±_{(rightpanel),areplottedasafunctionofzTforAu+Auat0–12%} cen-tralcollisions.Theverticallinesrepresentstatisticalerrorbarsandboxesrepresent systematicerrors.Sincethisisaratioofyieldsfortwooverlappingangular win-dows,muchoftheuncertaintiescancel;andonlythesurvivinguncertaintiesare shown.

Fig. 7. (Coloronline.)ThevaluesofIγdir_{A A} areplottedasafunctionofptrig_T (leftpanel) andpassoc

T (rightpanel).Theverticallineandshadedboxesrepresent statisticaland

systematicerrors,respectively.Thecurvesrepresentmodelpredictions[15,27,28].

fragmentationfunction”(constructed fromthein-mediumjet-like yields asa functionof zT) is notuniversal. In particular,the lost energy isnot recovered at a fixed range of zT, butperhaps ata given range of passoc_T . The conclusion that the lost energy is re-covered atlarger anglesonly for pT

<

2 GeV

/

c,regardlessof the triggerenergy,isconsistentwiththeconclusionoftheSTARpaper onjet-hadroncorrelations[18].

The earlier measurements [12] at low trigger energy (8

<

ptrig_T

<

16 GeV

/

c)showthesamelevelofsuppression(factor 3–5) via the medium modification factor (Iπ_{A A}0 and Iγdir

A A) down to

zT

∼

0

.

3. This suggeststhat IA A doesnot depend on the trigger energyatmidtohighzT for

γ

dir and

π

0-triggeredaway-sidejets withtrigger pT rangingfrom8to20 GeV/c.Thisisfurther inves-tigatedin

Fig. 7

with

γ

dir triggers,sincethephotontriggerenergy closely approximates the initial outgoing parton energy. The left panel shows Iγdir

A A as a function of p trig

T , for 0

.

3

<

zT

<

0

.

4. The per-triggernuclearmodification factorof

γ

dir-triggeredaway-side charged-hadronyieldsisindependentofthetriggerenergyofthe

γ

dir withinour25% systematicuncertainty.Thisindicates thatthe away-side partonenergylossisnotsensitive totheinitial parton energyinthisrangeof8–20 GeV/c,asmeasuredwithourlevelof precision. The ZOWWcalculation alsopredicts Iγdir

A A asafunction of ptrig_T tobe approximatelyflat inthisrange.In therightpanel, the valuesof Iγdir

A A are plottedasfunctionof passocT . Itshowsthat thelow-passoc

thoseathighpassoc_T .Bothmodelpredictionsshown[15,27],which donotincludetheredistribution oflostenergy,areinagreement withthedata.

5. Summary

In summary, in order to understand the medium

modifica-tion of partons in the QGP, away-side charged-hadron yields for

γ

dir and

π

0 triggersincentral(0–12%)Au

+

Aucollisionsare com-paredwiththoseinminimum-bias p

+

p collisions.Both Iπ_{A A}0 and

Iγdir

A A show similar levelsof suppression,withtheexpected differ-ences due to the color-factor effect and the path-length depen-dencenotmanifestingthemselves withinexperimental uncertain-ties. At low zT and low passocT ,the data are consistent with less suppressionthanathigher passoc

T .Thesuppressionshowslittle dif-ferenceforintegrationwindowsof

±

0.6vs.

±

1.4 radiansaround

φ

=

π

, withan enhancement atlarge angles observedonly for

zT

<

0

.

2 (passocT

<

2

.

4 GeV

/

c) for

π

0 triggers.There isno trigger-energydependence observed inthe suppression of

γ

dir-triggered yields, suggestinglittle dependenceforenergy loss onthe initial parton energy, in the range of ptrig_T

=

8–20 GeV

/

c. The data are consistent with model calculations[15,27,28], in which the sup-pressioniscausedbypartonenergylossinathermalizedmedium. Thesecalculationsdo not includeredistribution of energywithin the shower. The very large Iγdir

A A at low zT predicted by models of in-medium shower modification (including energy redistribu-tion) [17] is not observed for ptrig_T

>

12 GeV

/

c. This is in con-trasttothe PHENIXresult[11],wherethe Iγdir

A A exceedsunity, for

ptrig_T 5–9 GeV

/

c.However, itisnotclearthat theredistribution of lost energy would scale withthe jet energy. In fact, our studies support previous conclusions that the lost energyreappears pre-dominantly at low pT (approximately pT

<

2 GeV

/

c), regardless ofthetrigger pT.Thisleadstothe importantconclusion thatthe modified fragmentation function is not universal (i.e. it doesnot havethesamezT dependenceforalltrigger pT).

Acknowledgments

We thankX.N. WangandG.-Y Qin forprovidingtheir model predictionsandhelpfuldiscussion.WethanktheRHICOperations

Group andRCFatBNL,theNERSC CenteratLBNL,theKISTI Cen-terinKorea,andtheOpen ScienceGridconsortiumforproviding resourcesandsupport.Thisworkwassupportedinpartbythe Of-fice of Nuclear Physicswithin the U.S.DOE OfficeofScience, the U.S.NSF,theMinistryofEducationandScienceoftheRussian Fed-eration,NSFC,CAS,MOST andMOE ofChina,theNationalResearch Foundation ofKorea, NCKU(Taiwan),GA andMSMTofthe Czech Republic, FIAS ofGermany, DAE, DST, and UGC of India,the Na-tionalScienceCentreofPoland,NationalResearchFoundation,the MinistryofScience,EducationandSportsoftheRepublicof Croa-tia,andRosAtomofRussia.

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