Jet-like correlations with neutral pion triggers in pp and central Pb–Pb collisions at 2.76 TeV

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Physics Letters B

www.elsevier.com/locate/physletb

Jet-like correlations with neutral pion triggers in pp and central Pb–Pb collisions at 2.76 TeV

.ALICE Collaboration

a r t i c l e i n f o a b s t ra c t

Articlehistory:

Received29August2016

Receivedinrevisedform10October2016 Accepted18October2016

Availableonline24October2016 Editor:L.Rolandi

We presentmeasurementsoftwo-particlecorrelationswithneutralpiontrigger particlesoftransverse momenta8<p^trig_T <16 GeV/c andassociated chargedparticlesof0.5<p^assoc_T <10 GeV/c versusthe azimuthalangledifferenceϕatmidrapidityinppandcentralPb–Pbcollisionsat√_s

NN=².76 TeV with ALICE.Thenewmeasurementsexploitassociatedchargedhadronsdownto0.5 GeV/c,whichsignificantly extendsourpreviousmeasurementthatonlyusedchargedhadronsabove3 GeV/c.Aftersubtractingthe contributionsofthe flowbackground,v2to v5,theper-triggeryieldsare extractedfor |ϕ_|<0.7 on thenearand for|ϕ₋π_|<1.1 ontheawayside.Theratioofper-triggeryieldsinPb–Pbtothosein ppcollisions,IAA,ismeasuredonthenearandawaysideforthe0–10% mostcentral Pb–Pbcollisions.

On theawayside,theper-triggeryieldsinPb–Pbarestronglysuppressedtothelevelof IAA≈⁰.6 for p^assoc_T >3 GeV/c,whilewithdecreasing momentaanenhancementdevelopsreachingabout 5 at low p^assoc_T .Onthenearside,anenhancementofIAAbetween1.2 atthehighestto1.8 atthelowestp^assoc_T is observed. Thedata are comparedto parton-energy-loss predictionsof the JEWELand AMPT event generators,aswellastoaperturbativeQCDcalculationwithmedium-modifiedfragmentationfunctions.

Allcalculationsqualitativelydescribetheaway-sidesuppressionathighp^assoc_T .OnlyAMPTcapturesthe enhancementatlow p^assoc_T ,bothonthenearandaway side.However, italsounderpredicts IAA above 5 GeV/c,inparticularonthenear-side.

©^{2016TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense} (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP³.

1. Introduction

Strongly interacting matter consisting of deconfined quarks andgluons, the quark–gluon plasma (QGP), is produced in high- energyheavy-ion (HI)collisions attheRelativisticHeavy IonCol- lider (RHIC)[1–4] andatthe LargeHadronCollider (LHC)[5–13].

Among others, jet quenching [14,15],the phenomenon that high transversemomentum (pT)partonssufferenergylossbymedium- induced gluonradiation [16,17]andcollisions withmedium con- stituents[18,19],iswidelyconsidered asstrongevidenceforQGP formation.Jet quenchinghasbeen observedatRHIC [20–37] and attheLHC[5–7,38–51]viameasurementsofinclusivehadronand jet productionathigh pT,di-hadron angularcorrelations anddi- jetenergyimbalance,andviathemodificationofjetfragmentation functions.

Inparticular,measurementsusingtwo-particleangularcorrela- tions betweentrigger (high-pT) particles andassociated particles have been extensively used to search for remnants of the radi- atedenergy andthemedium responseto thehigh-pT parton. By varying the transverse momentum for trigger (p^trig_T ) and associ-

E-mailaddress:alice-publications@cern.ch.

ated (p^assoc_T ) particles one can probe different momentum scales to study theinterplay of softandhard processes.At RHIC, for a relatively low momentum range of p^trig_T and p^assoc_T below about 4 GeV/c,two-particle azimuthal anglecorrelationswerefound to be broadened andexhibiting a double-shoulder structure on the away side[29,32].Thesestructureswere originallydescribed em- ployingavarietyofdifferentmechanisms,like ˇCerenkovgluonra- diation[52],largeanglegluonradiation[53,54],Machconeshock- wave [55], and jets deflected by the medium [56]. Later it was understood that azimuthal correlations spanning a long-range in pseudorapidity (

η

) are affected not only by the second (v2) but alsohigher-orderflowharmonics (vn,n≥^{3),whichoriginatefrom} anisotropicpressuregradientswithrespecttotheinitial-statesym- metry planes[57,58].Takingintoaccountthesehigherharmonics can accountformostofthe observedstructuresinthemeasured two-particle angular correlations. Thus, possible jet-medium ef- fects atlow pT need to be studied after takinginto account the anisotropicflowbackgroundincludinghigherharmonics.

In this article, we presentmeasurements of two-particle cor- relations with neutral pions (

π

⁰) of transverse momenta 8<

p^trig_T <16 GeV/c astriggerandchargedhadronsof0.5<p^assoc_T <

10 GeV/c asassociated particles versus the azimuthal angle dif-

http://dx.doi.org/10.1016/j.physletb.2016.10.048

0370-2693/©^{2016TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense}(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP³.

ference

ϕ

at midrapidity in pp andcentral Pb–Pb collisionsat

√sNN=².76 TeV withALICE[59]attheLHC.Theneutralpionsare identified in the di-photon decay channel using a shower-shape and invariant-mass based identification technique of energy de- positsreconstructedwiththeElectromagneticCalorimeter(EMCal).

The newmeasurement exploits associatedhadrons reconstructed withtheInner Tracking System (ITS)and TimeProjection Cham- ber (TPC) down to 0.5 GeV/c, and hence significantly extends ourpreviousmeasurement[40],whichonlyusedchargedhadrons above 3 GeV/c, to low p^assoc_T . Furthermore,using

π

⁰ as a refer- enceavoidsadmixturesfromchangingparticlecompositionofthe triggerparticle,andhenceshould simplifycomparisons withcal- culations.Aftersubtracting thedominantbackground,induced by theanisotropicflowharmonicsv2 to v5,theper-triggeryieldsare extractedfor|

ϕ

_|<0.7 on thenear andfor|

ϕ

₋

π

_|<1.1 on theawayside.Theper-triggeryieldmodificationfactor,IAA,quan- tified as the ratio of per-trigger yields in Pb–Pb to those in pp collisions, ismeasured on thenear andaway side forthe 0–10%

mostcentral Pb–Pb collisions. The dataare compared to parton- energy-lossmodelpredictionsusingtheJEWEL[60]andAMPT[61]

eventgenerators,aswell asto aperturbative QCD (pQCD) calcu- lation[62] withmedium-modifiedfragmentationfunctions.Previ- ouslyatRHIC,

π

⁰-hadroncorrelationswerealsomeasuredtostudy IAA and jet fragmentation [35,37]. Compared to these measure- ments,we lower thethresholdforassociated chargedhadronsto 0.5 GeV/candsubstracttheharmonicflowcontributionsuptothe fifth order. Besides providingaccess to medium properties, mea- surementsof

π

⁰-hadron correlationsdetermine the mostimpor- tantbackgroundcontribution ofdirectphoton–hadron correlation measurements[36,37].

Thearticle is organized asfollows.Section 2briefly describes theexperimentalsetupanddatasetsused.Section3discussesthe neutral pion identification technique, the

π

⁰-hadron correlation and I_AA measurements. Section 4presents thedata andcompar- isonwithmodelcalculations.Section5providesasummary.

2. Experimentalsetupanddatasets

Adetaileddescription oftheALICEdetectorsystemsandtheir performance can be found in [59,63].The detectors used forthe presentanalysisare briefly described here. Thesearethe ITSand theTPCfor chargedparticletracking, theEMCal forneutralpion reconstruction, and the forward scintillator arrays (V0) and two Zero Degree Calorimeters (ZDC) for online triggering as well as eventselectionandcharacterization.

The tracking detectors are located inside a large solenoidal magnetprovidingahomogeneousfieldstrengthof0.5 T,andnom- inallyprovide reconstructed tracks within |

η

_|<0.9 over the full azimuth.TheITSconsistsofsixlayersofsilicondetectors.Thetwo inner layers are the SiliconPixel Detector (SPD),the two middle layers the Silicon DriftDetector (SDD), and two outer layers the SiliconStripDetector (SSD). The TPCprovides trackingandparti- cleidentificationbymeasuring the curvatureofthetracks inthe magneticfield andthe specificenergy lossdE/dx.The combined informationoftheITSandTPCallows one todetermine themo- mentaofchargedparticlesintheregionof0.15 to100 GeV/cwith aresolutionof1 to10%,respectively.TheEMCalisaPb-scintillator sampling calorimeter used primarily to measure the energy de- posit (cluster)inducedbyelectrons,positronsandphotons.Itcon- sistsof 10 active supermodules witha total of 11520 individual cells,eachcoveringanangularregionof

ϕ

_×

η

₌0.014×⁰.014, andspans intotal 100 degrees in azimuthand |

η

_|<0.7. Its en- ergyresolutioncanbeparameterizedas σE

E =

A²+^B_E² +^C_E²2 with A=¹.68,B=¹¹.27 andC=⁴.84 forthedepositedenergyEgiven

inGeV [64].The V0detectors, whichare primarily usedfortrig- gering,event selection andeventcharacterization, consist oftwo arraysof32 scintillatortileseach,coveringthefullazimuthwithin 2.8<

η

<5.1 (V0-A)and−³.7<

η

<−¹.7 (V0-C).Inaddition,two neutronZDCs,locatedat+114 m (ZNA) and−114 m (ZNC)from the interactionpoint, areused foreventselectionin Pb–Pb colli- sions.

The data used for the present analysis were collected during the 2011 LHC data taking periods with pp and Pb–Pb collisions atthecentre-of-massenergypernucleon–nucleonpairof√

s_NN= 2.76 TeV.In thecaseofppcollisions, theanalyzeddatawere se- lectedbytheEMCallevel-0triggerrequiringasingleshowerwith an energy larger than 3.0 GeV, in additionto the minimum bias triggercondition (a hit ineitherV0-A,V0-C,orSPD). Inthecase ofPb–Pb collisions,thedatawereselectedbyanonlinetriggerde- signedtoselectcentralcollisions.Thetriggerwasselectingevents based onthe sumofamplitudesintegrated inone LHC clockcy- cle (25 ns)onlineintheforwardV0detectorsaboveafixedthresh- old. Offline when one can integrate the signal over severalclock cycles the trigger was found to be 100% efficient for 0–8% and about80% for8–10% mostcentralPb–Pb collisions.Theinefficiency inthe8–10%rangewasestimatedtoleadtoanegligibledifference ofless than 1% inthe measured per-triggeryield. Forthe offline analysis0–10% centralcollisions wereused asexplainedin detail inRef.[65].Inboth,theppandPb–Pb analyses,onlyeventswith areconstructedvertexin|^zvtx|<10 cm withrespecttothenomi- nalinteractionvertexpositionalongthebeamdirectionwereused.

Afterallselection criteria,about440 Keventsinpp (correspond- ingto0.5/nb)and5.2 M (correspondingto0.6/μb)inPb–Pb were keptforfurtheranalysis.

Neutralpionsin|

η

_|<0.7 areidentifiedintheEMCalusingthe so called“cluster splitting”method, which aims to reconstructa high pT

π

⁰ (above 6 GeV/c) by first capturingboth decaypho- tons in a single, so called “merged” cluster, which then is split into two clusters, as further explained below. Clusters are ob- tainedby groupingall neighboringcells, whosecalibratedenergy is above 50 (150) MeV, starting from a seed cell with at least 100 (300)MeVforpp (Pb–Pb)data.Anon-linearitycorrection,de- rived from electron test beam data, ofabout 7% at 0.5 GeV and negligibleabove3 GeV,isappliedtothereconstructedclusteren- ergy.Clustersfromneutralparticlesareidentifiedbyrequiringthat thedistancebetweentheextrapolatedtrackpositions ontheEM- Calsurface andtheclusterfulfillstheconditions

η

>0.025 and

ϕ

>0.03 forpp,and

η

>0.03 and

ϕ

>0.035 forPb–Pb data.

Charged hadronsreconstructed withthe ITSandTPCareselected by a hybridapproach designedto compensate localinefficiencies in the ITS. Twodistinct track classesare accepted in the hybrid approach [63]: (i) trackscontaining atleast threehitsin theITS, includingatleastonehitintheSPD,withmomentumdetermined without the primary vertex constraint, and (ii) tracks containing lessthanthreehitsintheITSornohit intheSPD,withthepri- mary vertex included in the momentum determination. Class (i) contains 90% and class (ii) 10% of all accepted tracks, indepen- dentofpT.TrackcandidatesarefurtherrequiredtohaveaDistance ofClosestApproach (DCA)totheprimary vertexlessthan2.4 cm inthe planetransverse tothebeam, andlessthan3.0 cm inthe beamdirection.Acceptedtracksarerequiredtobein|

η

|<0.8 and pT>0.5 GeV/c.Correctionsforthedetectorresponseareobtained fromMonteCarlo (MC)detectorsimulations,reproducingthesame conditionsasduring datataking. Ingeneral, weusePYTHIA6[66]

for pp and HIJING [67] for Pb–Pb collisions as eventgenerators, andGEANT3[68]forparticletransportthroughthedetector.

Fig. 1.Clustershowershape (leftpanel)andinvariantmass (rightpanel)distributionsfor8<E<16 GeV andNLM=^{2 comparedbetween}reconstructedπ⁰candidatesin dataandclustersoriginatingfromπ⁰inHIJINGfor0–10% Pb–Pb collisions.Thedistributionsareshownafterapplyingtheenergy-dependentselectionsonσ_long² andMγ γ.

3. Dataanalysis

Neutral pions are detected in the two photon decay channel

π

⁰_→

γ γ

measuredintheEMCalusing M_π0

=

2E₁E₂

(

1

−

^cos

θ

12

) ,

(1)

where Mπ⁰ is the reconstructed

π

⁰ mass, E₁ and E₂ are the measured energies of two photons, and θ12 is the opening an- gle between the photons measured in the laboratory frame. The opening angle decreases with increasing

π

⁰ momentum due to the larger Lorentz boost. When the energy of the

π

⁰ is larger than5–6 GeV, thedecayphotonsarecloseenough thatthe elec- tromagnetic showersthey induce start to overlap in neighboring calorimetercellsoftheEMCal.

Above 9 GeV more than half of the

π

⁰ deposit their energy in a single merged cluster. Below 15 GeV merged clusters from

π

⁰ mostly havetwolocalmaxima (NLM=^{2), whilewithincreas-} ingenergytheshowersfurthermerge,leading tomergedclusters from

π

⁰withmainlyonelocalmaximum (NLM=1)above25 GeV.

Merged clusters can be identified based on their shower shape, characterized by the larger principal component squared of the cluster two-dimensionalarea in

η

andφ,

σ

_long² [69].To discrimi- natetwo-photonmergedclustersfromsingle-photonclusters,

σ

_long²

isgenerallyrequiredtobegreaterthan0.3.Fromdetectorsimula- tionswededucedatighterselection,requiringλ_min<

σ

_long² < λmax, wherethe minimumandmaximum ranges areparameterized by exp(a+^{b E})+^c+^dE+^e/E asafunctionofcluster energyE (in GeV).Forλmin,weusea=².135,b= −⁰.245,c=^d=^e=^0,while forλmax the valuesdepend on thenumber oflocal minima,and are a=⁰.066,b= −⁰.020, c= −⁰.096, d=⁰.001, ande=⁹.91 for NLM=^1,anda=⁰.353,b= −⁰.0264,c= −⁰.524,d=⁰.006, ande=²¹.9 for N_LM=^{2.Within 8}<p_T<16 GeV/c, the range forneutralpionsconsideredinthisanalysis,morethan80%ofthe clustershavetwolocalmaxima.

Themerged clusteris subsequentlysplitinto twosub-clusters bygroupingneighboringcellsinto3×^{3 clusterscenteredaround} thetwo highestcells (seeds)ofthe mergedcluster.Cells thatare neighbor ofboth seeds aresplit basedon thefractionof seedto cluster energy. To select

π

⁰ candidates, we use a 3

σ

-wide win- dow,^M −³

σ

<Mγ γ <^M +³

σ

,wherethe average (^{M) and} thewidth (

σ

)ofthemassdistributionobtainedfromGaussianfits depend on the energy of the cluster (in GeV), andare each pa- rameterizedasa+^{bE.Thevaluesfora andb are obtainedfrom} detectorsimulations forNLM=^{1 and 2,}respectively, andarethe same for pp and Pb–Pb data. In the p_T range relevant for the

analysis, the parametersfor ^{Mare a}=⁰.044 and b=⁰.005 for N_LM=^1,anda=⁰.115,b=⁰.001 for N_LM=^2,whilefor

σ

they are a=⁰.012 andb=^{0 for N}LM=^{1, anda}=⁰.009, b=⁰.001 for NLM=^{2.Fig. 1 showsacomparisonof}

σ

_long² and Mγ γ distri- butions for clusterswith 8<E<16 GeV and N_LM=^{2 between} reconstructed

π

⁰ candidatesindataandclustersoriginatingfrom

π

⁰ inHIJINGfor0–10% Pb–Pb collisions.Sincetheinvariantmass distribution is obtained by splitting individual clusters, there is nocombinatorialbackgroundbyconstruction.However,thereisof coursecontamination inthesignalregionforexamplefromdecay photons,whichneedstobeestimatedfromMonteCarlo.

Ascommonlydone[70],theassociatedyieldpertriggerparticle

Y

( ϕ , η ) =

¹ N_trig

d²N_assoc d

ϕ

d

η ⁼

S

( η , ϕ )

M

( η , ϕ )

⁽²⁾

isdefinedasthenumberofassociatedparticlesinintervalsofaz- imuthal angledifference

ϕ

=

ϕ

trig−

ϕ

assoc and pseudo-rapidity difference

η

=

η

trig−

η

assoc relative to the number of trigger particles. The trigger acceptance is |

η

_|<0.7, while the associ- ated particle acceptance is |

η

_|<0.8. The acceptance corrected yieldcanbeobtainedfromtheratiooftwo-particlecorrelationsof same SandmixedeventsM.Thesignaldistribution S(

η

,

ϕ

)= 1/Ntrigd²Nsame/d

η

d

ϕ

istheassociatedyield pertrigger parti- cleforparticlepairsfromthesameevent.Thebackgrounddistri- butionM(

η

,

ϕ

)=

α

d²N_mixed/d

η

d

ϕ

correctsforpairaccep- tanceandpairefficiency.Itisconstructedbycorrelatingthetrigger particles in one event with the associated particles from other events within similar multiplicity and z-vertex positionintervals.

The factor

α

is chosen to normalizethe background distribution such that it is unity for pairs where both particles go into ap- proximately the samedirection (i.e.

ϕ

_≈0,

η

_≈0).To account for different pair acceptance andpair efficiency as a function of zvtx,the yield is constructed foreach zvtx interval, and thefinal per-trigger yield is obtainedby calculatingthe weighted average of the zvtx intervals. The final results are integrated over

η

and providedasone-dimensionaldistribution,C(

ϕ

)= _N¹_trig^dN_d^assoc_ϕ ^,for 8<p^trig_T <16 GeV/c andvarious p^assoc_T intervalsbetween0.5 and 10 GeV/c.

Corrections for the detector response, which include

π

⁰ re- construction efficiency and purity, charged-particle tracking effi- ciency andcontamination fromsecondaryparticles, aswell as p_T resolution areobtainedfromdetectorsimulations. The

π

⁰ recon- struction efficiency, whichis between 0.2 and 0.3 depending on pT and collision system, leads to only a smallcorrection on the measured correlations ofabout2%, since the per-triggeryield by

definition is largely insensitive to the inefficiency of finding the trigger particle. The

π

⁰ purity, which in the momentum range ofthe measurement isabout 90% in pp and85% in Pb–Pb colli- sions,affects themeasured correlations by 1%. The p_T resolution ofreconstructed

π

⁰ estimatedfromdetectorsimulationsisabout 5% and10% forpp andPb–Pb collisions, respectively, slightlyin- creasingwith pT.Thecharged-particletrackingefficiencyisabout 75–85% dependingon p_T andcollisionsystem. Thecontamination bysecondaryparticlesfromparticle–materialinteractions,conver- sions,andweak-decay productsoflong-livedparticlesisbetween 4–8%. Both the tracking inefficiency and contamination, are cor- rectedforinthemeasured correlationsinintervalsof p^assoc_T .The trigger- and associated-particlepair pT resolutions lead to acor- rectionoflessthan2.5%.

Toobtain thejet-relatedcontribution fromthe measured per- triggeryields,oneusuallysubtractsnon-jetrelatedsourcesofpar- ticleproduction,

J

( ϕ ) =

^C

( ϕ ) −

^B

( ϕ ) ,

(3)

where B(

ϕ

) denotes the background contribution. In pp colli- sions,typically auniformbackground (B0) originatingfromcom- binatoricsisconsidered,andestimatedemploying thezero-yield- at-minimum (ZYAM) method [29], i.e. essentially by estimating B within 1<|

ϕ

_|< ^π₂. In Pb–Pb collisions, in addition to a largecombinatorial background,two-particle correlationsare sig- nificantly affected by anisotropic flow [71]. The anisotropic az- imuthalcorrelationsmodulatethebackgroundaccordingto

B

( ϕ ) =

^B0

1

+

²

n

V_ncos

(

n

ϕ )

,

(4)

where Vn≈^vn^trig·^vassocn isapproximatelygivenby theproductof anisotropicflowcoefficientsfortriggerandassociatedparticlesat theirrespectivemomenta.Inthesubtraction,wetakeintoaccount themostdominantcontributions, v₂ to v₅,ignoring smalldevia- tionsfromfactorization[72].The dataof v2 forchargedparticles andfor charged pions, which are used instead of the v2 of

π

⁰, aretakenfrom Ref.[73].For v3 to v5 thedatafromRef.[71] are usedforboththeneutralpionsandchargedparticles.Theconstant B0 isdeterminedbyanaverageofthreewaystoobtaintheZYAM value, namely by i) a fit in 1<|

ϕ

_|< ^π₂,ii) smallest 8 (outof 60)valuesinfull

ϕ

range,andiii) minimawithin1<|

ϕ

_|<^π₂ plusthetwosmallestpointswithin 0.2 aroundtheminimum.Fi- nally,thejet-likecorrelationyieldsonthenearandawaysideare estimatedfromEq.(3)by integratingaregion of|

ϕ

_|<0.7 and

|

ϕ

₋

π

_|<1.1,respectively.Modificationofthejet-likepairyields canthenbequantifiedastheratiooftheintegratedjet-likeyields inAAoverpp,as

IAA

=

X

JAA

( ϕ )

d

ϕ /

X

Jpp

( ϕ )

d

ϕ ,

(5)

where X denotes eitherthe near-side (NS)ortheaway-side (AS) region.

4. Results

The per-trigger yields for neutral pion trigger particles with 8<p^trig_T <16 GeV/c andassociatedchargedparticles with0.5<

p^assoc_T <1,1<p^assoc_T <2,2<p^assoc_T <4 and4<p^assoc_T <6 GeV/c arepresented inFig. 2 forpp andin Fig. 3 for0–10% most cen- tral Pb–Pb collisions. The estimated background from the ZYAM procedure is indicated by the dashed lines. As explained in the previous section,a uniformbackgroundis considered inthecase

Table 1

Summaryofsourcesandassignedsystematicuncertaintiesfortheper-triggeryield inpp,and0–10%Pb–Pb collisions,aswellasIAA.Foreachsourceofsystematicun- certaintyandthetotaluncertaintylisted,themaximumvaluesofallp^assoc_T intervals aregiven.UncertaintiesontrackingefficiencyandMCclosurearecorrelatedinϕ. ForIAA,ppandPb–Pb yielduncertaintiesareassumedtobeindependent.

Source Y(ϕ)pp Y(ϕ)Pb–Pb IAA(NS) IAA(AS)

Tracking efficiency 5.4% 6.5% 8.5% 8.5%

MC closure 1.0% 2.0% 1.2% 1.2%

TPC-only tracks 1.0% 3.5% 4.3% 3.8%

Track contamination 1.0% 0.9% 1.1% 1.1%

Shower shape (σ_long² ) 1.2% 0.7% 3.4% 2.6%

Invariant mass window 1.3% 1.0% 3.5% 3.3%

Neutral pion purity 0.3% 1.1% 0.6% 0.5%

PairpTresolution 1.0% 1.1% 0.3% 0.3%

Pedestal determination – – 9.4% 11.7%

Uncertainty onvn – – 7.1% 5.1%

Total 6.7% 7.4% 12.6% 15.0%

ofpp,whileforPb–Pb datainadditiontheanisotropicflowcontri- butionsaretakenintoaccount.Sincethevn coefficientsaresmall at high-p^trig_T and p^assoc_T , a nearly flat background is observed for the4<p^assoc_T <6 GeV/ccase,eveninPb–Pb collisions.

Severalsourcesofsystematicuncertaintyhavebeenconsidered.

Since thereis a pT dependenceon theuncertainties, their maxi- mumcontributiontotheper-triggeryieldsinppandPb–Pb colli- sions,aswell asonthe IAA furtherdiscussedbelow,are givenin Table 1.Thelargesteffecttotheper-triggeryieldsarisesfromthe uncertainty on the charged-particle tracking efficiency estimated from variations of the trackselection andresidual differences of MC closure tests. These uncertainties are correlated in

ϕ

, and theirvalues (addedinquadrature)areexplicitlyreportedinFig. 2 andFig. 3.Uncertaintiesrelatedto charged-particletrackingwere further explored by repeating the full analysis with tracks re- constructed only by the TPC. Systematic uncertainties related to the

π

⁰ identification were obtained by varying the criteria for

σ

_long² selection and the invariant mass window. Uncertainties re- lated to

π

⁰ purity and p_T resolution were assessed by varying the parameterizations, which were obtainedfrom detectorsimu- lationsandusedfortherespectivecorrections.Totaluncertainties were computedby addingtheindividualcontributions inquadra- ture.

The modification ofthe per-trigger yield can be quantified as theratio,IAA,oftheintegratedjet-likecorrelationyieldsinPb–Pb over pp,asexplained inthe previous section (see Eq.(5)).Fig. 4 presents the IAA on the nearside for|

ϕ

_|<0.7 and away side for|

ϕ

₋

π

_|<1.1.The uncertaintyon IAA (reportedin Table 1) isdominatedbytheuncertaintyonthedeterminationofB0 (esti- mated fromthedifference ofthe 3methods to extractthe base- line)andthemeasureduncertaintiesonv_n,andhenceitislargely uncorrelated across p^assoc_T . On the nearside, the IAA is found to besignificantlylargerthanunity.Theenhancementincreasesfrom IAA≈¹.2 athighp^assoc_T to1.8 atlowp^assoc_T .Thedataareconsistent with our previous results extracted from di-hadron correlations above 3 GeV/c [40]. On theaway side, IAA isstrongly enhanced below 3 GeV/c, reaching values up to IAA≈^{5 at lowest passoc}T , while above 4 GeV/c it is suppressed to about 0.6. As before, the data are compared to previous results using di-hadron cor- relations [40], which were obtained within a smaller integration region (|

ϕ

_|<0.7) andonly takingintoaccount v2 intheZYAM subtraction. For p^assoc_T >4 GeV/c, there is good agreement be- tweenthetwosetsofdata,whileforsmallerp^assoc_T theaway-side peaksbecome wideranddetails oftheZYAMsubtractionaswell asthesizeoftheintegrationregionmatter.Ontheawayside,the suppression athighp^assoc_T is understoodtooriginate fromparton energyloss[14–19],whiletheenhancementatlow p^assoc_T mayin-

Fig. 2.Charged-particleassociatedyieldsrelativetoπ⁰ triggerparticlesversusϕinppcollisionsat√

sNN=2.76 TeV.Theπ⁰triggermomentumrangeis8<p^trig_T <

16 GeV/c,andassociatedchargedparticlerangesare0.5<p^assoc_T <1,1<p^assoc_T <2,2<p^assoc_T <4 and4<p^assoc_T <6 GeV/c.Thebarsrepresentstatisticaluncertainties,the boxesuncorrelatedsystematicuncertainties.DashedlinescorrespondtotheestimatedbackgroundusingtheZYAMproceduredescribedinthetext.Therangeofthevertical axisisadjustedforeachpanel,and“zero”isnotshowninallcases.

Fig. 3.Charged-particleassociatedyieldsrelativetoπ⁰triggerparticlesversusϕin0–10% mostcentralPb–Pb collisionsat√_s

NN=².76 TeV.SeecaptionofFig. 2formore information.

volveaninterplayofvariouscontributions,suchaskT broadening, medium-excitation,aswellasfragmentsfromradiatedgluons[53, 61,74–76]. Theenhancement onthenearside, firstobserved and discussed in Ref. [40], may also be related to the hot medium, inducinga changeofthe fragmentationfunctionorthe quark-to- gluonjetratio.

The observation of IAA>1 at low pT is consistent with the measured enhancement of low-p_T particles from jet fragmenta- tion inPb–Pb relative to pp [48,49].At RHIC in Au–Au collisions at 200 GeV for a similar range of p^trig_T as used in the present measurement, IAA onthe away side was found toreach at most 2–3[35],neglecting v3 andhigherordersharmonicsintheback- groundsubtraction,whileonthenearsidenosignificantenhance- mentwasreported.

In Fig. 5 the data are compared to calculations using the JEWEL[60]andAMPT [61]eventgenerators,aswellaspQCDcal- culation[62].JEWEL[60]addressestheparton–mediuminteraction by giving a microscopic description of the transport coefficient, ˆ

q,which essentiallydefines theaverageenergy lossper unit dis- tance. Hard scattersare generated accordingto Glauber collision geometry,andpartonssufferfromelasticandradiativeenergyloss in the medium, including a MonteCarlo implementationof LPM interference effects. TheJEWEL calculation includes theso called

“recoilhadrons”,whichareproducedbyfragmentingmediumpar- tonsthatinteractedwiththepropagatinghardparton. AMPT[77]

uses initial conditionsof HIJING, followed by parton andhadron cascades withelastic scatterings for final-stateinteraction. String melting with a parton interaction cross section of 1.5 mb and

Fig. 4.Per-triggeryieldmodification, IAA,onthenearside (left)and awayside (right)with triggerπ⁰ particle at8<p^trig_T <16 GeV/c for0–10% Pb–Pb collisionsat

√sNN=².76 TeV.Thedatafromourpreviousmeasurementusingdi-hadroncorrelations[40]areslightlydisplacedforbettervisibility.Thebarsrepresentstatisticalandthe boxessystematicuncertainties.

Fig. 5.Per-triggeryieldmodification, IAA,onthenearside (left)and awayside (right)with triggerπ⁰ particle at8<p^trig_T <16 GeV/c for0–10% Pb–Pb collisionsat

√_s

NN=².76 TeV.Thedataarecomparedtomodelcalculations[60–62]asexplainedinthetext.Thebarsrepresentandtheboxessystematicuncertainties.

partonrecombination for hadronization is used with parameters fromRef.[78].ThepQCDcalculation[62]isperformedatnext-to- leading order (NLO). Ituses nuclear partondistribution functions for initial-state cold nuclear matter effects, and a phenomeno- logicalmodel for medium-modified fragmentationfunctions. The evolutionofbulk mediumisdonewitha3+1 dimensionalideal hydrodynamic model, and the value qˆ is consistent with that of the JET collaboration, which was extracted using experimental data[79].ThepredictionforIAAisonlyavailablefortheawayside, anddonefollowingRef.[80].

Allcalculations are ableto qualitatively describe the suppres- sionof IAA athigh p^assoc_T ontheaway side,further corroborating theidea that the suppression iscaused by partonenergyloss in hot matter. JEWEL and the pQCD calculation do not exhibit an increase at low pT, while AMPT quantitatively describes the en- hancementatthenear(exceptatlowest p^assoc_T ) andawayside.In AMPTthelow-p^assoc_T enhancement isattributedtotheincreaseof softparticles asaresultofthejet-mediuminteractions. However, inparticularon thenearside forp^assoc_T >5 GeV/c AMPT predicts a strong suppression of I_AA down to about 0.6, which clearly is notseeninthedata.AlsoontheawaysideAMPT tendstounder- predictthe IAA for p^assoc_T >5 GeV/c.Both defects,which maybe relatedtothefactthatAMPTwasfoundtooverpredictthesingle- particlesuppressionincentral Pb–Pb collisions[81], indicatethat thedescriptionimplementedinAMPTisnotcomplete.

5. Summary

Two-particlecorrelationswithneutralpionsoftransversemo- menta 8<p^trig_T <16 GeV/c as trigger and charged hadrons of 0.5<p^assoc_T <10 GeV/c asassociated particles versus azimuthal

angle difference

ϕ

at midrapidity in pp (Fig. 2) and central Pb–Pb (Fig. 3) collisionsat√

s_NN=².76 TeV have beenmeasured.

The per-triggeryields havebeen extractedfor|

ϕ

_|<0.7 on the nearandfor|

ϕ

₋

π

_|<1.1 ontheawayside,aftersubtractingthe contributionsoftheflowharmonics, v₂ uptov₅ (Fig. 3).Theper- triggeryieldmodificationfactor,IAA,quantifiedastheratioofper- triggeryieldsinPb–Pb tothatinppcollisions,hasbeenmeasured for the near and away side in 0–10% most central Pb–Pb colli- sions (Fig. 4).Ontheawayside,theper-triggeryieldsinPb–Pb are stronglysuppressedtothelevelofIAA≈⁰.6 for p^assoc_T >3 GeV/c, whilewithdecreasingmomentaanenhancement developsreach- ing about5.2 atlowest p^assoc_T .Onthenearside, anenhancement of IAA between1.2 to1.8 at lowest p^assoc_T is observed. The data are compared to predictions ofthe JEWEL andAMPT event gen- erators, as well as a pQCD calculation at next-to-leading order withmedium-modifiedfragmentationfunctions (Fig. 5).Allcalcu- lationsareabletoqualitativelydescribetheaway-sidesuppression athigh p^assoc_T .OnlyAMPT is ableto capturetheenhancement at low p^assoc_T , both on nearand away side. However, it also under- predicts IAA above 5 GeV/c, in particular on the near-side. The coincidenceoftheaway-sidesuppressionathighpT andthelarge enhancement atlow p_T on thenear andaway side issuggestive ofa commonunderlyingmechanism, likely relatedtothe energy lost by highmomentum partons.The data henceprovide agood testinggroundto constrainmodelcalculationswhich aimtofully describejet–mediuminteractions.

Acknowledgements

WethankHanzhongZhangandGuo-LiangMaforprovidingthe AMPTandpQCDpredictions,respectively.

The ALICE Collaboration would like to thank all its engineers andtechnicians fortheir invaluablecontributionstotheconstruc- tion of the experiment and the CERN accelerator teams for the outstanding performance of the LHC complex. The ALICECollab- oration gratefully acknowledges the resources and support pro- videdbyall GridcentresandtheWorldwide LHCComputingGrid (WLCG) collaboration. The ALICE Collaboration acknowledges the followingfundingagencies fortheirsupport inbuildingandrun- ningtheALICEdetector:A.I.AlikhanyanNationalScience Labora- tory(YerevanPhysicsInstitute)Foundation(ANSL),StateCommit- teeofScienceandWorldFederationofScientists(WFS),Armenia;

AustrianAcademyofSciencesandÖsterreichischeNationalstiftung fürForschung,TechnologieundEntwicklung,Austria;ConselhoNa- cionaldeDesenvolvimentoCientíficoeTecnológico(CNPq),Finan- ciadora deEstudose Projetos(Finep)andFundação de Amparoà PesquisadoEstadodeSãoPaulo(FAPESP),Brazil;MinistryofEdu- cationofChina(MOEofChina),MinistryofScience &Technology of China (MOST of China) and NationalNatural Science Founda- tion of China (NSFC), China; Ministry of Science, Education and Sportand Croatian Science Foundation, Croatia; Centro de Inves- tigacionesEnergéticas,MedioambientalesyTecnológicas(CIEMAT), Cuba;MinistryofEducation,YouthandSportsoftheCzechRepub- lic,Czech Republic;Danish NationalResearchFoundation (DNRF), TheCarlsbergFoundationandTheDanishCouncilforIndependent Research|NaturalSciences,Denmark;HelsinkiInstituteofPhysics (HIP),Finland;Commissariatàl’EnergieAtomique(CEA)andInsti- tut Nationalde Physique Nucléaire etde Physique desParticules (IN2P3)and Centre Nationalde laRecherche Scientifique(CNRS), France; Bundesministerium für Bildung, Wissenschaft, Forschung undTechnologie (BMBF)andGSI Helmholtzzentrum fürSchweri- onenforschung GmbH, Germany; Ministry of Education, Research andReligiousAffairs,Greece;NationalResearch,Developmentand Innovation Office, Hungary; Department of Atomic Energy Gov- ernment of India (DAE), India; Indonesian Institute of Science, Indonesia; Centro Fermi – Museo Storico della Fisica e Centro Studi e Ricerche Enrico Fermi and Istituto Nazionale di Fisica Nucleare (INFN), Italy; Institute for InnovativeScience andTech- nology, Nagasaki Institute of Applied Science (IIST), Japan Soci- ety for the Promotion of Science (JSPS) KAKENHI and Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan; Consejo Nacionalde Cienciay Tecnología(CONA- CYT), through Fondo de Cooperación Internacional en Ciencia y Tecnología(FONCICYT)andDirecciónGeneralde AsuntosdelPer- sonal Academico (DGAPA), Mexico; Nationaal instituut voor sub- atomaire fysica (Nikhef), Netherlands; The Research Council of Norway,Norway;CommissiononScienceandTechnologyforSus- tainableDevelopmentintheSouth(COMSATS),Pakistan;Pontificia UniversidadCatólicadelPerú,Peru;MinistryofScienceandHigher Education and National Science Centre, Poland; Ministry of Ed- ucation and Scientific Research, Institute of Atomic Physics and RomanianNationalAgencyforScience,TechnologyandInnovation, Romania; Joint Institute for Nuclear Research (JINR), Ministry of EducationandScienceoftheRussianFederationandNationalRe- search Centre Kurchatov Institute, Russia; Ministry of Education, Science,Research andSportofthe Slovak Republic, Slovakia; Na- tional Research Foundation of South Africa, South Africa; Korea Institute ofScience andTechnology InformationandNationalRe- search Foundation of Korea (NRF),South Korea;Centro de Inves- tigacionesEnergéticas,MedioambientalesyTecnológicas(CIEMAT) andMinisteriodeCiencia eInnovación, Spain;Knut& AliceWal- lenberg Foundation (KAW) and Swedish Research Council (VR), Sweden;EuropeanOrganizationforNuclearResearch,Switzerland;

National Science and Technology Development Agency (NSDTA), Officeofthe Higher EducationCommissionunderNRU project of ThailandandSuranaree University ofTechnology (SUT),Thailand;

Turkish Atomic Energy Agency(TAEK), Turkey;National Academy ofSciences ofUkraine, Ukraine; ScienceandTechnology Facilities Council (STFC), United Kingdom; National Science Foundation of theUnitedStatesofAmerica(NSF)andUnitedStatesDepartment ofEnergy,OfficeofNuclearPhysics(DOENP),UnitedStates.

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