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

Contents lists available atScienceDirect

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

r

a

c

t

Articlehistory:

Received29August2016

Receivedinrevisedform10October2016 Accepted18October2016

Availableonline24October2016 Editor:L.Rolandi

We presentmeasurementsoftwo-particlecorrelationswithneutralpiontrigger particlesoftransverse momenta8<ptrig_T <16 GeV/c andassociated chargedparticlesof0.5<passoc

T <10 GeV/c versus the azimuthalangledifference

ϕ

atmidrapidityinppandcentralPb–Pbcollisionsat√sNN=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 passoc

T >3 GeV/c,whilewithdecreasing momentaanenhancementdevelopsreachingabout 5 at low passoc

T .Onthenearside,anenhancementofIAAbetween1.2 atthehighestto1.8 atthelowestpassocT is observed. Thedata are comparedto parton-energy-loss predictionsof the JEWELand AMPT event generators,aswellastoaperturbativeQCDcalculationwithmedium-modifiedfragmentationfunctions. Allcalculationsqualitativelydescribetheaway-sidesuppressionathighpassoc

T .OnlyAMPTcapturesthe enhancementatlow passoc_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 Ion Col-lider (RHIC)[1–4] andatthe LargeHadronCollider (LHC)[5–13]. Among others, jet quenching [14,15],the phenomenon that high transversemomentum (pT)partonssufferenergylossby medium-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 and di-jetenergyimbalance,andviathemodificationofjetfragmentation functions.

Inparticular,measurementsusingtwo-particleangular correla-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 (ptrig_T ) and

associ- _{E-mailaddress:alice-publications@cern.ch.}

ated (passoc_T ) particles one can probe different momentum scales to study theinterplay of softandhard processes.At RHIC, for a relatively low momentum range of ptrig_T and passoc

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 ˇCerenkovgluon ra-diation[52],largeanglegluonradiation[53,54],Machcone shock-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-state sym-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}

_<

ptrig_T

<

16 GeV

/

c astriggerandchargedhadronsof0

.

5

<

passoc

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_Tassoc. Furthermore,using

π

0 _{as a} refer-enceavoidsadmixturesfromchangingparticlecompositionofthe triggerparticle,andhenceshould simplifycomparisons with cal-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

/

c andsubstracttheharmonicflowcontributionsuptothe fifth order. Besides providingaccess to medium properties, mea-surementsof

π

0_{-hadron correlationsdetermine the most} impor-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 IAA measurements. Section 4presents thedata and compar-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,and nom-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 trackingand parti-cleidentificationbymeasuring the curvatureofthetracks inthe magneticfield andthe specificenergy lossdE

/

dx.The combined informationoftheITSandTPCallows one todetermine the mo-mentaofchargedparticlesintheregionof0

.

15 to100 GeV

/

c with

aresolutionof1 to10%,respectively.TheEMCalisaPb-scintillator sampling calorimeter used primarily to measure the energy de-posit (cluster)inducedbyelectrons,positronsandphotons.It con-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

₊

B2 E

+

C2 E2 with

A

=

1

.

68,B

=

11

.

27 andC

=

4

.

84 forthedepositedenergyE given

inGeV [64].The V0detectors, whichare primarily usedfor trig-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

√

sNN

=

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,thedatawereselectedbyanonlinetrigger de-signedtoselectcentralcollisions.Thetriggerwasselectingevents based onthe sumofamplitudesintegrated inone LHC clock cy-cle (25 ns)onlineintheforwardV0detectorsaboveafixed thresh-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 withrespecttothe nomi-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 decay pho-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,isappliedtothereconstructedcluster en-ergy.Clustersfromneutralparticlesareidentifiedbyrequiringthat thedistancebetweentheextrapolatedtrackpositions onthe EM-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,withthe pri-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 comparedbetweenreconstructedπ0candidatesin

dataandclustersoriginatingfromπ0_{inHIJINGfor0–10% Pb–Pb collisions.Thedistributionsareshownafterapplyingtheenergy-dependentselectionsonσ}2

longandMγ 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

θ

12 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 (N}

LM

=

2), whilewith increas-ingenergytheshowersfurthermerge,leading tomergedclusters from

π

0_{withmainlyonelocalmaximum (N}

LM

=

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

and

φ

,

σ

2

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

σ

2

long isgenerallyrequiredtobegreaterthan0

.

3.Fromdetector simula-tionswededucedatighterselection,requiring

λ

min

<

σ

long2

< λ

max, wherethe minimumandmaximum ranges areparameterized by exp

(

a

+

b E

)

+

c

+

dE

+

e

/

E as afunctionofcluster energy E (in GeV).For

λ

min,weusea

=

2

.

135,b

= −

0

.

245,c

=

d

=

e

=

0,while for

λ

max 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 NLM

=

2.Within 8

<

pT

<

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 areobtainedfrom detectorsimulations forNLM

=

1 and 2,respectively, andarethe same for pp and Pb–Pb data. In the pT range relevant for the

analysis, the parametersfor



M



are a

=

0

.

044 and b

=

0

.

005 for

NLM

=

1,anda

=

0

.

115,b

=

0

.

001 for NLM

=

2,whilefor

σ

they are a

=

0

.

012 andb

=

0 for NLM

=

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 NLM

=

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 Ntrig d2Nassoc d



ϕ

d



η

=

S

(

η

, 

ϕ

)

M

(

η

, 

ϕ

)

(2)

isdefinedasthenumberofassociatedparticlesinintervalsof az-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 S andmixedeventsM.Thesignaldistribution S

(

η

,



ϕ

)

=

1

/

Ntrigd2Nsame

/

d



η

d



ϕ

istheassociatedyield pertrigger parti-cleforparticlepairsfromthesameevent.Thebackground distri-butionM

(

η

,



ϕ

)

=

α

d2_N

mixed

/

d



η

d



ϕ

correctsforpair accep-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

)

=

1

Ntrig

dNassoc

dϕ ,for

8

<

ptrig_T

<

16 GeV

/

c andvarious passoc_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 pT 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 pT resolution ofreconstructed

π

0 _{estimatedfromdetectorsimulationsisabout} 5% and10% forpp andPb–Pb collisions, respectively, slightly in-creasingwith pT.Thecharged-particletrackingefficiencyisabout 75–85% dependingon pT andcollisionsystem. Thecontamination bysecondaryparticlesfromparticle–materialinteractions, conver-sions,andweak-decay productsoflong-livedparticlesisbetween 4–8%. Both the tracking inefficiency and contamination, are cor-rectedforinthemeasured correlationsinintervalsof passoc

T .The trigger- and associated-particlepair pT resolutions lead to a cor-rectionoflessthan2

.

5%.

Toobtain thejet-relatedcontribution fromthe measured per-triggeryields,oneusuallysubtractsnon-jetrelatedsourcesof par-ticleproduction,

J

(

ϕ

)

=

C

(

ϕ

)

−

B

(

ϕ

) ,

(3)

where B

(

ϕ

)

denotes the background contribution. In pp colli-sions,typically auniformbackground (B0) originatingfrom com-binatoricsisconsidered,andestimatedemploying the zero-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 Vncos

(

n



ϕ

)



,

(4)

where Vn

≈

vntrig

·

vassocn isapproximatelygivenby theproductof anisotropicflowcoefficientsfortriggerandassociatedparticlesat theirrespectivemomenta.Inthesubtraction,wetakeintoaccount themostdominantcontributions, v2 to v5,ignoring small devia-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

|

<

π 2 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

<

ptrig_T

<

16 GeV

/

c and associatedchargedparticles with0

.

5

<

passoc

T

<

1,1

<

passocT

<

2,2

<

passocT

<

4 and4

<

passocT

<

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.Foreachsourceofsystematic

un-certaintyandthetotaluncertaintylisted,themaximumvaluesofallpassocT 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 (σ2 long) 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% Pair pTresolution 1.0% 1.1% 0.3% 0.3%

Pedestal determination – – 9.4% 11.7%

Uncertainty on vn – – 7.1% 5.1%

Total 6.7% 7.4% 12.6% 15.0%

ofpp,whileforPb–Pb datainadditiontheanisotropicflow contri-butionsaretakenintoaccount.Sincethevn coefficientsaresmall at high-ptrig_T and passoc_T , a nearly flat background is observed for the4

<

passoc_T

<

6 GeV

/

c case,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

2

long 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 detector simu-lationsandusedfortherespectivecorrections.Totaluncertainties were computedby addingtheindividualcontributions in quadra-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)andthemeasureduncertaintiesonvn,andhenceitislargely uncorrelated across passoc_T . On the nearside, the IAA is found to besignificantlylargerthanunity.Theenhancementincreasesfrom

IAA

≈

1

.

2 athighpassoc_T to1

.

8 atlowpassoc_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 passocT , 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 passoc_T

>

4 GeV

/

c, there is good agreement be-tweenthetwosetsofdata,whileforsmallerpassoc_T theaway-side peaksbecome wideranddetails oftheZYAMsubtractionaswell asthesizeoftheintegrationregionmatter.Ontheawayside,the suppression athighpassoc_T is understoodtooriginate fromparton energyloss[14–19],whiletheenhancementatlow passoc

in-Fig. 2. Charged-particleassociatedyieldsrelativetoπ0 _{triggerparticlesversusϕinppcollisionsat}√_s

NN=2.76 TeV.Theπ0triggermomentumrangeis8<ptrigT <

16 GeV/c,andassociatedchargedparticlerangesare0.5<passocT <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-particleassociatedyieldsrelativetoπ0_{triggerparticlesversus}

ϕin0–10% mostcentralPb–Pb collisionsat√sNN=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-pT 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 ptrig_T as used in the present measurement, IAA onthe away side was found toreach at most 2–3[35],neglecting v3 andhigherordersharmonicsinthe back-groundsubtraction,whileonthenearsidenosignificant enhance-mentwasreported.

In Fig. 5 the data are compared to calculations using the JEWEL[60]andAMPT [61]eventgenerators,aswellaspQCD cal-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”,whichareproducedbyfragmentingmedium par-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<ptrigT <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<ptrigT <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]isperformedat next-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 q is

ˆ

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 passoc_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 passoc_T ) andawayside.In AMPTthelow-passoc

T enhancement isattributedtotheincreaseof softparticles asaresultofthejet-mediuminteractions. However, inparticularon thenearside forpassoc_T

>

5 GeV

/

c AMPTpredicts a strong suppression of IAA down to about 0

.

6, which clearly is notseeninthedata.AlsoontheawaysideAMPT tendsto under-predictthe IAA for passocT

>

5 GeV

/

c.Both defects,which maybe relatedtothefactthatAMPTwasfoundtooverpredictthe single-particlesuppressionincentral Pb–Pb collisions[81], indicatethat thedescriptionimplementedinAMPTisnotcomplete.

5. Summary

Two-particlecorrelationswithneutralpionsoftransverse mo-menta 8

<

ptrig_T

<

16 GeV

/

c as trigger and charged hadrons of 0

.

5

<

passoc

T

<

10 GeV

/

c as associated particles versus azimuthal

angle difference



ϕ

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

√

sNN

=

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).The per-triggeryieldmodificationfactor,IAA,quantifiedastheratioof per-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 passoc_T

>

3 GeV

/

c, whilewithdecreasingmomentaanenhancement develops reach-ing about5

.

2 atlowest passoc

T .Onthenearside, anenhancement of IAA between1

.

2 to1

.

8 at lowest passoc_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).All calcu-lationsareabletoqualitativelydescribetheaway-sidesuppression athigh passoc_T .OnlyAMPT is ableto capturetheenhancement at low passoc

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 pT 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 invaluablecontributionstothe construc-tion of the experiment and the CERN accelerator teams for the outstanding performance of the LHC complex. The ALICE Collab-oration gratefully acknowledges the resources and support pro-videdbyall GridcentresandtheWorldwide LHCComputingGrid (WLCG) collaboration. The ALICE Collaboration acknowledges the followingfundingagencies fortheirsupport inbuildingand run-ningtheALICEdetector:A.I.AlikhanyanNationalScience Labora-tory(YerevanPhysicsInstitute)Foundation(ANSL),State Commit-teeofScienceandWorldFederationofScientists(WFS),Armenia; AustrianAcademyofSciencesandÖsterreichischeNationalstiftung fürForschung,TechnologieundEntwicklung,Austria;Conselho Na-cionaldeDesenvolvimentoCientíficoeTecnológico(CNPq), Finan-ciadora deEstudose Projetos(Finep)andFundação de Amparoà PesquisadoEstadodeSãoPaulo(FAPESP),Brazil;Ministryof Edu-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,YouthandSportsoftheCzech Repub-lic,Czech Republic;Danish NationalResearchFoundation (DNRF), TheCarlsbergFoundationandTheDanishCouncilforIndependent Research|NaturalSciences,Denmark;HelsinkiInstituteofPhysics (HIP),Finland;Commissariatàl’EnergieAtomique(CEA)and Insti-tut Nationalde Physique Nucléaire etde Physique desParticules (IN2P3)and Centre Nationalde laRecherche Scientifique(CNRS), France; Bundesministerium für Bildung, Wissenschaft, Forschung undTechnologie (BMBF)andGSI Helmholtzzentrum für Schweri-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 and Tech-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 Asuntosdel Per-sonal Academico (DGAPA), Mexico; Nationaal instituut voor sub-atomaire fysica (Nikhef), Netherlands; The Research Council of Norway,Norway;CommissiononScienceandTechnologyfor Sus-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 EducationandScienceoftheRussianFederationandNational Re-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 InformationandNational Re-search Foundation of Korea (NRF),South Korea;Centro de Inves-tigacionesEnergéticas,MedioambientalesyTecnológicas(CIEMAT) andMinisteriodeCiencia eInnovación, Spain;Knut& Alice Wal-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. References

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J. Adam

39

,

88

,

D. Adamová

85

,

M.M. Aggarwal

89

,

G. Aglieri Rinella

35

,

M. Agnello

31

,

112

,

N. Agrawal

48

,

Z. Ahammed

136

,

S. Ahmad

18

,

S.U. Ahn

69

,

S. Aiola

140

,

A. Akindinov

55

,

S.N. Alam

136

,

D.S.D. Albuquerque

123

,

D. Aleksandrov

81

,

B. Alessandro

112

,

D. Alexandre

103

,

R. Alfaro Molina

64

,

A. Alici

106

,

12

,

A. Alkin

3

,

J. Alme

22

,

37

,

T. Alt

42

,

S. Altinpinar

22

,

I. Altsybeev

135

,

C. Alves Garcia Prado

122

,

M. An

7

,

C. Andrei

79

,

H.A. Andrews

103

,

A. Andronic

99

,

V. Anguelov

95

,

C. Anson

88

,

T. Antiˇci ´c

100

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F. Antinori

109

,

P. Antonioli

106

,

L. Aphecetche

115

,

H. Appelshäuser

61

,

S. Arcelli

27

,

R. Arnaldi

112

,

O.W. Arnold

96

,

36

,

I.C. Arsene

21

,

M. Arslandok

61

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B. Audurier

115

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A. Augustinus

35

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R. Averbeck

99

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M.D. Azmi

18

,

A. Badalà

108

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Y.W. Baek

68

,

S. Bagnasco

112

,

R. Bailhache

61

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R. Bala

92

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S. Balasubramanian

140

,

A. Baldisseri

15

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R.C. Baral

58

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A.M. Barbano

26

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R. Barbera

28

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F. Barile

33

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G.G. Barnaföldi

139

,

L.S. Barnby

35

,

103

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V. Barret

71

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P. Bartalini

7

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K. Barth

35

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J. Bartke

119

,

i

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E. Bartsch

61

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M. Basile

27

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N. Bastid

71

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S. Basu

136

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B. Bathen

62

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G. Batigne

115

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A. Batista Camejo

71

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B. Batyunya

67

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P.C. Batzing

21

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I.G. Bearden

82

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H. Beck

95

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C. Bedda

31

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N.K. Behera

51

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I. Belikov

65

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F. Bellini

27

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H. Bello Martinez

2

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R. Bellwied

125

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E. Belmont-Moreno

64

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L.G.E. Beltran

121

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V. Belyaev

76

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G. Bencedi

139

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S. Beole

26

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I. Berceanu

79

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A. Bercuci

79

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Y. Berdnikov

87

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D. Berenyi

139

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R.A. Bertens

54

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D. Berzano

35

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L. Betev

35

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A. Bhasin

92

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I.R. Bhat

92

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A.K. Bhati

89

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B. Bhattacharjee

44

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J. Bhom

119

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L. Bianchi

125

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N. Bianchi

73

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C. Bianchin

138

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J. Bielˇcík

39

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J. Bielˇcíková

85

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A. Bilandzic

82

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36

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96

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G. Biro

139

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R. Biswas

4

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S. Biswas

80

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4

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S. Bjelogrlic

54

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J.T. Blair

120

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D. Blau

81

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C. Blume

61

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F. Bock

75

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95

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A. Bogdanov

76

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H. Bøggild

82

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L. Boldizsár

139

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M. Bombara

40

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M. Bonora

35

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J. Book

61

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H. Borel

15

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A. Borissov

98

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M. Borri

127

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84

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F. Bossú

66

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E. Botta

26

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C. Bourjau

82

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P. Braun-Munzinger

99

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M. Bregant

122

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T.A. Broker

61

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T.A. Browning

97

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M. Broz

39

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E.J. Brucken

46

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E. Bruna

112

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G.E. Bruno

33

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D. Budnikov

101

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H. Buesching

61

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S. Bufalino

31

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26

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P. Buhler

114

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S.A.I. Buitron

63

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P. Buncic

35

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O. Busch

131

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Z. Buthelezi

66

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J.B. Butt

16

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J.T. Buxton

19

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J. Cabala

117

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D. Caffarri

35

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X. Cai

7

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H. Caines

140

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A. Caliva

54

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E. Calvo Villar

104

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P. Camerini

25

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F. Carena

35

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W. Carena

35

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F. Carnesecchi

12

,

27

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J. Castillo Castellanos

15

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A.J. Castro

128

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24

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C. Ceballos Sanchez

9

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J. Cepila

39

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P. Cerello

112

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J. Cerkala

117

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B. Chang

126

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S. Chapeland

35

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M. Chartier

127

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J.L. Charvet

15

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S. Chattopadhyay

136

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S. Chattopadhyay

102

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A. Chauvin

96

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36

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V. Chelnokov

3

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M. Cherney

88

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C. Cheshkov

133

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B. Cheynis

133

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V. Chibante Barroso

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D.D. Chinellato

123

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51

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P. Chochula

35

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K. Choi

98

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M. Chojnacki

82

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S. Choudhury

136

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P. Christakoglou

83

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C.H. Christensen

82

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P. Christiansen

34

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T. Chujo

131

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S.U. Chung

98

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C. Cicalo

107

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L. Cifarelli

12

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27

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F. Cindolo

106

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J. Cleymans

91

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F. Colamaria

33

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D. Colella

56

,

35

,

A. Collu

75

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M. Colocci

27

,

G. Conesa Balbastre

72

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Z. Conesa del Valle

52

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M.E. Connors

140

,

ii

,

J.G. Contreras

39

,

T.M. Cormier

86

,

Y. Corrales Morales

112

,

I. Cortés Maldonado

2

,

P. Cortese

32

,

M.R. Cosentino

122

,

124

,

F. Costa

35

,

J. Crkovská

52

,

P. Crochet

71

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R. Cruz Albino

11

,

E. Cuautle

63

,

L. Cunqueiro

35

,

62

,

T. Dahms

36

,

96

,

A. Dainese

109

,

M.C. Danisch

95

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A. Danu

59

,

D. Das

102

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I. Das

102

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S. Das

4

,

A. Dash

80

,

S. Dash

48

,

S. De

122

,

A. De Caro

30

,

G. de Cataldo

105

,

C. de Conti

122

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J. de Cuveland

42

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A. De Falco

24

,

D. De Gruttola

30

,

12

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N. De Marco

112

,

S. De Pasquale

30

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R.D. De Souza

123

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A. Deisting

95

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99

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A. Deloff

78

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C. Deplano

83

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P. Dhankher

48

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D. Di Bari

33

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A. Di Mauro

35

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P. Di Nezza

73

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B. Di Ruzza

109

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M.A. Diaz Corchero

10

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T. Dietel

91

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P. Dillenseger

61

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R. Divià

35

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Ø. Djuvsland

22

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A. Dobrin

83

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35

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D. Domenicis Gimenez

122

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B. Dönigus

61

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O. Dordic

21

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T. Drozhzhova

61

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A.K. Dubey

136

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A. Dubla

99

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L. Ducroux

133

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A.K. Duggal

89

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P. Dupieux

71

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R.J. Ehlers

140

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D. Elia

105

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E. Endress

104

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H. Engel

60

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E. Epple

140

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B. Erazmus

115

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F. Erhardt

132

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B. Espagnon

52

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M. Estienne

115

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S. Esumi

131

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G. Eulisse

35

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J. Eum

98

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D. Evans

103

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S. Evdokimov

113

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G. Eyyubova

39

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L. Fabbietti

36

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96

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D. Fabris

109

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J. Faivre

72

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A. Fantoni

73

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M. Fasel

75

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L. Feldkamp

62

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A. Feliciello

112

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G. Feofilov

135

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J. Ferencei

85

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A. Fernández Téllez

2

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E.G. Ferreiro

17

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A. Ferretti

26

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A. Festanti

29

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V.J.G. Feuillard

71

,

15

,

J. Figiel

119

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M.A.S. Figueredo

122

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S. Filchagin

101

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D. Finogeev

53

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F.M. Fionda

24

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E.M. Fiore

33

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M. Floris

35

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S. Foertsch

66

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P. Foka

99

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S. Fokin

81

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E. Fragiacomo

111

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A. Francescon

35

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A. Francisco

115

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U. Frankenfeld

99

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G.G. Fronze

26

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U. Fuchs

35

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C. Furget

72

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A. Furs

53

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M. Fusco Girard

30

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J.J. Gaardhøje

82

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M. Gagliardi

26

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A.M. Gago

104

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K. Gajdosova

82

,