Measurement of jet radial profiles in Pb–Pb collisions at √sNN = 2.76 TeV

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Physics Letters B

www.elsevier.com/locate/physletb

Measurement of jet radial profiles in Pb–Pb collisions at √

s _NN = ² . 76 TeV

.ALICE Collaboration

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

Articlehistory:

Received6May2019

Receivedinrevisedform2July2019 Accepted8July2019

Availableonline12July2019 Editor: L.Rolandi

Thejetradialstructureandparticletransversemomentum(pT)compositionwithinjetsarepresentedin centrality-selectedPb–Pb collisionsat√_s

NN=².76 TeV.Track-basedjets,whicharealsocalledcharged jets,werereconstructedwitharesolutionparameterofR=⁰.3 atmidrapidity|ηch jet|<0.6 fortransverse momenta pT,ch jet=30–120 GeV/c. Jet–hadron correlations in relative azimuth and pseudorapidity space (ϕ,η)are measuredtostudythedistributionoftheassociated particlesaround thejetaxis for different pT,assoc-ranges between 1 and 20 GeV/c.The data in Pb–Pb collisions are compared to reference distributions for pp collisions, obtained using embedded PYTHIA simulations. The number of high-pT associate particles (4<pT,assoc<20 GeV/c)in Pb–Pb collisionsis foundto besuppressed comparedtothereferenceby30to10%,dependingoncentrality.Theradialparticledistributionrelative to the jet axisshows amoderate modification in Pb–Pb collisions with respect to PYTHIA. High-pT associate particles are slightlymore collimatedin Pb–Pb collisions comparedto the reference, while low-pT associateparticles tend to be broadened. The results,which are presented for the first time downto pT,ch jet=^{30 GeV/cin}Pb–Pb collisions,arecompatiblewithbothpreviousjet–hadron-related measurementsfromtheCMSCollaborationandjetshapemeasurementsfromtheALICECollaborationat higherpT,andaddfurthersupportfortheestablishedpictureofin-mediumpartonenergyloss.

©^{2019TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense} (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP³.

1. Introduction

Atenergydensitiesaboveapproximately0.5 GeV/fm³andtem- peraturesaboveapproximately160 MeV [1],QuantumChromody- namics (QCD) calculationson the lattice predict the existence of a phasetransition fromnormalnuclear matter toa newstate of mattercalledtheQuark–Gluon Plasma (QGP),wherethe partonic constituents,quarksandgluons,arenolongerconfinedinhadrons.

Thereiscompellingevidencefromobservationsreportedbyexper- iments atthe Relativistic Heavy IonCollider (RHIC) [2–5] and at theLargeHadronCollider (LHC)[6–17] thattheQGPiscreatedin nuclearcollisionsathighcollisionenergies.

A unique wayto characterize the propertiesof the QGP isto utilize jets as a probe of the medium. Hard scatterings are ex- pected to occur early in the collision evolution, producing high transverse momentum (pT) partons that propagate through the expandingmedium andeventually fragment intojetsof hadrons.

High-pT partonsloseenergyininteractions withthemediumdue toelasticscatteringandinducedgluonradiation [18,19].Besidesa reductionofthejetenergy,thiscanresultinabroadeningofthe transversejetprofileandasofteningofthefragmentation [20].

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

Jet quenchinghas been observed at RHIC [21–34] and at the LHC [8,16,17,35–47], e.g. via inclusiveyield andcorrelation mea- surementsofhigh-p_T hadronsandreconstructedjets. Thesemea- surementsprovideinsightsintothemechanismsofpartonenergy lossinthemediumandeventuallyintothemediumitself.

More differential measurements of the jet modification in a medium, i.e.measurements ofmodificationsofjet angularprofile andparticlecomposition,canprovidecomplementaryinformation to observablesthatfocus ontheoverall yieldchangelike nuclear modification factors. Measurements of correlated associated par- ticle production relative to jets or high-pT particles allow for a detailed measurement of the redistribution of quenched energy around the jet. An excess of low-pT particles in and around the jetuptolargedistances,aswellasasuppressionofhigh-p_T parti- cles,havebeenreported [17,48–50].Two-particlecorrelationsand jet–hadroncorrelationsshowanangularbroadeningoflow-pTpar- ticles below 3 GeV/c in heavy-ion collisions with respect to pp collisions [50].Forlow-p_T two-particlecorrelations,measurements alsoindicateanasymmetryintheshapeofthenear-sidejetpeaks:

they are broader in

η

compared to

ϕ

[48,49]. The variables

η

and

ϕ

arethedistance inpseudorapidity

η

andazimuth

ϕ

relative to the near-side jet. At the same time, measurements of theradial momentofjetspointtoageneralcollimationofjetsin Pb–Pb collisions [51].

https://doi.org/10.1016/j.physletb.2019.07.020

0370-2693/©^{2019TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense}(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP³.

Usingjetsinsteadofhigh-pTparticlesasareference(trigger)to studyangular correlations—as done in this analysis—shouldhave the advantage that jet properties better reflect the initial parton energy.Thisanalysisextendsthestudyofjet–hadroncorrelations intoaregimeoflowtrack-basedjet pT,ch jet notyetexploredwith thesetechniquesattheLHC.

In this paper, we study the correlation of charged particles (associates) with the direction of reconstructed track-based jets (triggers) in the

ϕ

-

η

plane in the same event. The jets are reconstructed using charged particles above a certain transverse momentum pT,const. The analysis focuses on two aspects of the modificationofjetswithinthemediumcreatedinPb–Pb collisions comparedto a PYTHIA [52] reference. First, theoverall modifica- tionoftheassociatedparticleyieldanditsjet-energydependence is studied. Second, the modification of the radial distribution of associatedparticleswithrespecttothejetaxisisstudiedbycom- paring the Pb–Pb results to the PYTHIA reference. Both aspects areanalyzed indetailforseveraljet transversemomenta pT,ch jet and low and high p_T of associated charged particles. PYTHIA is used as vacuum baseline, because the size of the pp dataset at

√s=².76 TeVisinsufficientforthisanalysis.

The paper is structured as follows. In Sec. 2, details on the detectorandgeneraldatareconstruction willbegiven.The corre- lationanalysis,whichservesasbasisforthispaper,ispresentedin Sec.3.Subsequently,jetreconstructionwillbedescribedinSec.4, followed by a discussion on the embedded PYTHIA reference in Sec.5.Before theresults willbe presentedinSec. 8,the observ- ables are introduced in Sec. 6 and systematic uncertainties are discussedinSec.7.AsummaryconcludesthepaperinSec.9.

2. Experimentalsetup

ForacompletedescriptionoftheALICEdetectoranditsperfor- manceseeRefs. [53] and [54],respectively.

Thedatawererecordedin2011forPb–Pb collisionsat√ sNN= 2.76 TeVusingasetofcentralitytriggersbasedonthehitmulti- plicitymeasured bytheV0detector,whichconsistsofsegmented scintillatorscoveringthefullazimuthover2.8<

η

<5.1 (V0A)and

−³.7<

η

<−¹.7 (V0C).Events were selected with V0multiplic- ities corresponding to the 0–50% most central events using the centrality determination as described in Ref. [55]. The accepted events,reconstructed asdescribed in Ref. [56], were required to haveaprimaryreconstructedvertexwithin10 cmofthecenterof thedetectoralongthebeamaxis.Forthisanalysis,atotalof12M eventswereused.

TheanalysispresentedherereliesmainlyonthecentralALICE trackingsystems,whicharelocatedinsidealargesolenoidalmag- netwithafieldstrengthof0.5 T.TheyconsistoftheInnerTracking System(ITS), ahigh-precisionsix-layer cylindricalsilicondetector systemwiththe inner layerat a radiusof 3.9 cm andthe outer layerat43 cmfromthebeamaxis,andtheTimeProjectionCham- ber(TPC)witharadialextentof85–247 cm,whichprovidesupto 159independentspacepointspertrack.

To ensure a good track-momentum resolution for jet recon- struction, all reconstructed tracks were required to have atleast threehitsintheITS.FortrackswithoutanyhitintheSiliconPixel Detector(SPD),whichprovidesthetwoinnermostlayersoftheITS, thelocationoftheprimaryvertexwasusedinadditiontothehits intheTPCandITS.Thisimprovesthetrack-momentumresolution andreducestheazimuthaldependenceofthetrackreconstruction efficiencydueto the non-uniform SPDresponse. Accepted tracks were required to be measured with 0.15<pT<100 GeV/c in

|

η

|<0.9, andto have at least 70 TPC space-points and no less than80%ofthegeometricallyfindablespace-pointsintheTPC.

Thesingle-tracktrackingefficiencywasestimatedfromthede- tector response of HIJING [57] events reconstructed to detector level using GEANT3 [58] for the particletransport. In the 0–10%

centrality class, it is about 56% at 0.15 GeV/c, about 83% at 1.5 GeV/c andthen decreases to 81% at 3 GeV/c, after which it increasesandlevels offtoabout83% atabove 6.5 GeV/c.Forthe 10–30% most central collisions, the tracking efficiency follows a similar p_T-dependence pattern, with absolutevalues of the effi- ciencythatare1 to2%highercomparedtothe0–10%mostcentral collisions. The momentum resolution,which was estimatedon a track-by-trackbasis usingthecovariancematrixofthetrackfit,is about1% at1 GeV/c andabout3%at50 GeV/c.Thecontamination by secondary particles [59] produced inparticle-material interac- tions,conversions,andweak-decayproductsoflong-livedparticles isontheleveloffewpercent.

3. Correlationanalysis

The two-dimensional associated per-trigger yield Y(

ϕ

,

η

) measures the distribution of particles relative to the jet axes in bins of

ϕ

,

η

, event centrality, and trigger and associate transverse momenta pT,assoc [60]. This distribution serves as the basis of the analysis and is formed using so-called same and mixed event correlations. Correlations from the same event are the actual correlations of trigger jets and associated particles, calculated for each selected event. In the mixed event tech- nique, jets are correlated with particles from a pool containing tracks from different events with similar trigger jet p_T, vertex z, and centralities. For vertex z, there are six bins in this pool, whose boundaries are given by (−¹⁰,−⁵,−²,0,2,5,10) in cm.

The boundariesforthe centralitypercentile binning are givenby (0,1,2,3,4,5,10,20,30,40,50).

Themixed-event-correctedassociatedper-triggeryieldforgiven jet pT-range,associatepT-range,andcentralityselectionisdefined as

Y

( ϕ , η ) =

¹ N_trig

d²N_assoc d

η

d

ϕ

=

¹ N_trig

cent,z

d²N_same d

η

d

ϕ

α

^d

2N_mixed d

η

d

ϕ

,

(1)

where the ratios in the sum are formed differentially in bins of centralityandvertexz.

The factor

α

in Eq. (1) is chosen such that the mixed-event correlations are normalized to unity in the region |

η

|<0.2,

|

ϕ

|<0.2 around thenear-sidejetpeakwheretheefficiencyfor pairs ofparalleljetsandassociates islargest.Thecontribution of thestatisticaluncertaintyofthisnormalizationtothetotalstatis- ticaluncertaintyisnegligible.Thefinitetrackingefficiencyandthe contamination by secondaries (seeSec. 2) are takenintoaccount anda correction hasbeenperformedforassociated tracksdiffer- entiallyin

η

,pT,centrality,andvertexzforsameandmixedevent correlations in Eq. (1). The efficiency maps were created using MonteCarlosimulations forthe sametrackdefinitionanddetec- torconditions.However,thiscorrectionturns outtobe negligible forallobservablesexceptfortheabsolutejet-associatedyields,be- cause its effect mostly cancels in the used relative observables, whichwillbedefinedinSec.6.

Inaddition tothecorrection fordetectorinhomogeneities and acceptance effects,the correlation also needs to be corrected for background.Theunderlyingbackgroundforthechosenobservables mainly consists of the uncorrelated particle background baseline from soft processes and the flow modulation in the correlation function. The background was found to be independent of

η

Fig. 1.Illustrationofper-triggeryieldsforthetwodifferentjetdefinitions(furtherdiscussedbelow):high-pTassociatesofjetswithpT,const≥⁰.15 GeV/candpT,ch jet=^60–80 GeV/c(left)andlow-pTassociatesofjetswithpT,const≥³.0 GeV/candpT,ch jet=30–40 GeV/c(right).Nobackgroundsubtractionwasapplied.

within |

η

_|<0.9 [61] and istherefore estimatedas a function of

ϕ

forthewhole

η

-rangeasB(

ϕ

).Toavoidincludingpartsof thejetsignal, B(

ϕ

)iscalculatedin1.0<|

η

_|<1.4,wherethe contributionfromthejet isexpectedto besmall, basedon mea- surementsinppcollisions.

Thebackgroundisdirectlysubtractedfromthecorrelationfunc- tion.The background-correctedper-triggeryield servesasa basis forallsubsequentmeasurements.Itisdefinedas

Ycorr

( ϕ , η ) =

Y

( ϕ , η ) −

B

( ϕ ).

(2) To illustrate the impact of the backgroundon the per-trigger yields, the uncorrected per-trigger yields can be found in Fig. 1 for high- and low-p_T associates. The background is nearly neg- ligible for high-pT associates and it is sizeable for low-pT asso- ciates.Inthe illustratedexampleforlow-p_T associates,thesignal to signal+^{background ratio, i.e. the percentage of the signal in} themeasuredobservable,isroughly0.1within aradiusofr<0.3 aroundthenear-sidejet peak.Notealsothatthebackgroundcor- rection removes all

η

-independent correlations, including the away-side ridge which is not investigated in the presentedanal- ysis.

4. Jetreconstruction

The measurement of jets in heavy-ion collisions is challeng- ingsinceasingleeventcancontainmultiple,possiblyoverlapping, jetsfromindependenthard nucleon–nucleonscatterings.Inaddi- tion,lowtransversemomentumparticlesoriginatingfromsoftpro- cesseslead toafluctuating backgroundwhichstronglyinfluences thejetreconstruction.Therelativeeffectislargestforlow-pT jets andmostcentralevents.Consequently,jetreconstructioninheavy- ioncollisions requires a robustjet definition, anda procedureto correctforthepresenceofthelargebackground [62].

Jets were reconstructed using the anti-kT or the kT algo- rithms [63] intheFastJet package [64] witharesolutionparameter of R=⁰.3. Onlythose jets whoseaxis was reconstructed within

|

η

_|<0.6 werekeptintheanalysistoassurethenominaljetcone is fully contained within the track acceptance of |

η

|<0.9. This limitstheeffectoftheacceptanceboundariesonthemeasuredjet spectrum. Jetsreconstructed by the anti-k_T algorithm were used to quantify signal jets, while jets reconstructed by the kT algo- rithmwereusedtoquantifythecontributionfromtheunderlying event [65].

Twodifferentjetdefinitionsareusedinthisanalysis:formea- surements at high associate-p_T, jets are measured with a con- stituent cut pT,const≥⁰.15 GeV/c, measurements at low asso- ciate-p_T are performed for jetsmeasured witha constituent cut

pT,const≥³.0 GeV/c. Jets with pT,const≥⁰.15 GeV/c are recon- structedusingallchargedparticlesavailableforjetreconstruction and,thus,thefragmentationbiasisassmallaspossible.Thisbias is causedby only includingcertain particles ofthejet andcould leadtoasample ofharderfragmentingjetswhenleavingoutpar- ticles at low pT. On the other hand, using all charged particles availableforjetreconstructionalsoincludesparticlesinthecorre- lationanalysiswhichwerealreadyusedinthejetfindingprocess.

Thejetfindingalgorithmselectsregionsinmomentumspacewith largeenergyflow.Thisimpliesthatthedistributionofchargedpar- ticles insidethejet isbiased. Forexample,theradial distribution ofparticleswithrespecttothejetaxiswillshowasmalldepletion atdistancesjustoutsidethejetconeradiusR.Thisparticularlyaf- fectstheshapeofthejet,i.e.howtheconstituentsaredistributed relativetothejetaxis,leadingtoanautocorrelationbias.

Therefore,thejetsthemselvesandinparticulartheirshapesare intimately connectedtothe jet definition.Forhigh-pT associates, theautocorrelationbiascannotbeavoidedandhastobeaccepted asapartofthejetdefinition.

Low-pT associatesarebroadlydistributeduptolargedistances relativetothejet.Since thejet findingalgorithmclustersthejets roughly intocones witha nominal radius of R=⁰.3, it strongly affectstheshapeofthejet.Whenmeasuringpropertiesoflow-pT associates, we avoidthe autocorrelation bias by adapting the jet definition: Trigger jets and associates can be decoupled by us- ing jets with constituents above a certain threshold and asso- ciates below the threshold. Therefore, for measurements at low associate-pT,jetsarereconstructed withpT,const≥^{3 GeV/c.Using} ageometricalmatchingprocedurethatisperformedontwocollec- tionsofthedifferentlydefinedjetswhicharereconstructedineach eventit was checkedthatthe jetaxes forboth jetdefinitions do not stronglychange.Forinstance, forjetswith pT,const≥^{3 GeV/c} and pT,ch jet>30 GeV/c the meanandstandard deviationof the matchedjetdistancedistributionareapproximatelygivenby0.016 and0.014,respectively.However,itmustbeemphasizedthatthese jet definitions select two different jet samples and that the au- tocorrelation biaswas avoided hereat theexpense of a possible fragmentationbias.

Thetransversemomentumofreconstructedjetsincludingcon- stituentsaslowas0.15 GeV/c isaffectedbythecontributionfrom theunderlyingevent.Inordertosuppressthecontributionofsuch background to the measurement of the jet momentum, we fol- lowedtheapproachdescribedinRefs. [65,66],whichaddressesthe averageadditivecontributiontothejetmomentumonajet-by-jet basis. Theunderlyingbackgroundmomentumdensity

ρ

wasesti- matedevent-by-eventusingthemedianofp^raw_T,jet/A_jet,wherep^raw_T,jet

Table 1

TruejetpopulationspT,trueinGeV/ccorrespondingtogivenpT,ch jet-rangesfordifferenteventcentralityclasses.Therangesaregivensuchthattheycontainatleast68%of thejetpopulation.Themostprobablevaluesofthedistributionsaregiveninparentheses.

pT,const-cut 0.15 GeV/c 3 GeV/c

pT,ch jet(GeV/c) 40–60 60–80 80–120 30–40 40–60

0–5% 11–87 (44) 22–111 (64) 49–144 (94) 7–59 (32) 21–88 (46)

5–10% 11–86 (46) 24–112 (66) 52–146 (94) 8–61 (32) 22–89 (46)

0–10% 11–86 (46) 25–113 (68) 54–147 (94) 10–63 (32) 24–91 (48)

10–30% 13–86 (50) 33–117 (70) 63–149 (98) 15–69 (32) 30–94 (48)

30–50% 25–91 (52) 47–118 (82) 75–147 (98) 23–73 (32) 36–95 (52)

istheuncorrected jet transverse momentumand A_jet isthe area ofjetsreconstructedwiththekT algorithm.

Theaveragerawbackgroundmomentumdensity

ρ

decreases towards more peripheral collisions. It is

ρ

≈110, 65, and 25 GeV/c in the 0–10%, 10–30%, and 30–50% most central Pb–Pb collisions, respectively. The background momentum density is a detector-levelquantitythatdependsonthetrackingefficiencyand trackdefinition.Forsignaljetsreconstructedwiththeanti-kTalgo- rithmandconstituentsabove0.15 GeV/c,thebackgrounddensity scaledby the area ofthe reconstructed signal jet was subtracted fromtheraw reconstructedtransverse momentum (p^raw_T,jet) ofthe signaljetaccordingto p_{T,ch jet}=^praw_T,jet−

ρ

_·Ajet.

Due to region-to-region variations of the background, the background-corrected jet transverse momenta are affected by residual fluctuations. To give an estimate for these fluctuations for the jet definition used, cones with radius R=⁰.3 are ran- domlyplacedineachevent.Inthesecones,thetrackmomentaare summedandthebackgroundissubtractedtocalculateδp_T:

δ

p_T

=

cone

p_T,track

− ρ ·

^A

,

(3)

whereA istheareaofthecone.

For the 0–10%, 10–30%, and 30–50% most central collisions, the standard deviation of the δpT-distribution as a measure for themagnitudeofthefluctuationshasbeenevaluated to 6.7, 5.1, and 3.3 GeV/c, respectively. Since the δp_T-distribution also con- tainsthejetsignal,thestandarddeviationofthefulldistributionis impactedbyit.Alowerlimitofthesefluctuationsisgivenbyper- formingaGaussianfitoftheleft-handsideoftheδpT-distribution.

The Gaussian widths were evaluated to 5.5, 4.0, and 2.3 GeV/c forthe 0–10%, 10–30%, and 30–50% most central collisions. The sample of jets that only uses constituents above 3 GeV/c is not correctedforthe underlyingevent astheconstituentcut already strongly suppresses the contribution from the background such thatitisnegligible.

In addition to background fluctuations, also the finite detec- tor resolution and single particle efficiency influence the mea- surement. To quantifyboth effects,the ratio of reconstructed jet momentum p_T,recandtruejet momentum p_T,true was calculated taking into account the detector resolution by using a response matrixandbackgroundfluctuationsgivenbytheδpTdistributions.

TheresponsematrixwascreatedfromMonteCarlosimulationsfor whichthetruejet momentumisknown bygeometrically match- ingparticle-levelPYTHIAjetswiththecorrespondingdetector-level jetsreconstructedusingafulldetectormodelinGEANT3.Morede- tailedstudieshavebeenperformedforjetsonthesamedatasetin Ref. [66].

There are two effects contributing to the jet momentum res- olution: detector effects and underlying event fluctuations. The detectoreffectslead toa jet momentum responsethat ispeaked atpT,rec=^pT,true,buthasa tailtolowervaluesofdetectorlevel momentum due to tracking inefficiency. The tracking efficiency

changes by onlya few percentfromperipheral tocentral events.

Background fluctuations produce an approximately Gaussian re- sponse,withawidththatdependsstronglyoncentrality.Thecom- bined effect leads to a standard deviation in the jet momentum resolutionof30%(20%)forjetswithp_{T,ch jet}=^{30 GeV/c and27%}

(27%)forjetswithp_{T,ch jet}=^{120 GeV/cforthe0–10%(10–30 and} 30–50%)mostcentralevents.

It should be emphasized that p_{T,ch jet} refers to the jet trans- versemomentumatdetectorlevel,correctedforbackgroundonly.

Since within-event fluctuations of the background are not cor- rectedfor,the meanofthegiven pT,ch jet-rangeis slightlyhigher than that of the underlying true p_T distribution for more cen- tralcollisions wherefluctuations aredominant. Hence,duetothe steeply-fallingjetspectrum,fluctuationsleadtoashiftofthespec- trum to larger values. For more peripheral collisions where de- tector effects are dominant, there is the opposite effect, i.e. the spectrumisshiftedtosmallervalues.Thefractionofpurelycom- binatorial jetsinthemomentum rangesused intheanalysiswas foundtobenegligible.

To give a rough estimate of the true jet populations for a given reconstructed jet momentum range, projections of the re- sponse matrices, introduced above, are used [67]. For measured p_{T,ch jet}-distributions,approximaterangesaregiveninTable1asa measure forthe truejet momentum distributions. The truepop- ulations are defined as the smallest possible ranges around the pT,ch jet-rangeinwhich atleast68% ofthe jet populationcan be found.

5. ConstructionofPYTHIAbaseline

In thisanalysis, reconstructeddetector-level PYTHIA-jetsserve as vacuumbaseline, because the size of the pp dataset at √

s= 2.76 TeVisinsufficientforthispurpose.

ToaccountforthefluctuationsoftheunderlyingeventinPb–Pb collisions,PYTHIAjetsembedded inrealPb–Pb collisionsareused as a reference. Jets reconstructed in this reference dataset still show thesamebaseline jet propertiesbutalsoincludethe effect of background fluctuations from the Pb–Pb event. To create this reference dataset, the following procedure is applied. Events are simulated withPYTHIA6 (Perugia-0 [68], version 6.421) followed bytransportinthedetectorusingGEANT3andfullresponsesimu- lationandreconstruction simulatingthesamedetectorconditions as inthe Pb–Pb dataset. The reconstructed tracks are embedded intoPb–Pb events,i.e.they arecombinedwithtracksfromPb–Pb events.InordertosimulatethesameconditionsasinPb–Pb, the tracking efficiency in pp is decreased to the level expected in Pb–Pb.SincethetrackingefficiencyinppishigherthaninPb–Pb, 2%of the PYTHIAtracks are randomly discarded before they are embedded [54]. Jet finding algorithms are applied tothe PYTHIA eventandalsotothecombinedPYTHIA+Pb–Pb event.Jetsfound inthecombinedeventareonlyacceptedforthereferencedataset if they can be matched geometrically with those in the PYTHIA event. A matched embedded jet needs to be less than R=⁰.3 awayfromaPYTHIAjet.

DuetotheveryhighparticleoccupancyofthePb–Pb collision system,theprobabilitytoreconstructaPYTHIAjetintheembed- dedeventismuchlower thantheprobability toreconstructajet of samemomentum by overlapping a jet that alreadyexisted in thePb–Pb event,evenafterapplyingageometricalmatchingpro- cedure.Therefore,withoutanyfurtherintervention,theembedded jetsamplewouldconsistmostlyofPb–Pb-jetsoverlappinglow-p_T PYTHIAjets.

Two approaches have been tested which ensure that the jet sample showsPb–Pb-event-like fluctuationsof a PYTHIAjet, and notjetsfromthePb–Pb event.Theanalysisbaselinetechniqueuses acutonthefractionofthejet pTthatoriginatesfromthematched jetinPYTHIA.Theappliedcutvaluesaremotivatedbytheunder- lying truejet distribution that showstwo separated populations:

jetsmostlyconsistingofparticlesfromPYTHIAorfromPb–Pb.The cutvaluewaschosentoachievethebestseparationofthetwodis- tributions.Inthe0–10%mostcentral collisions,itisrequiredthat atleast20% ofthejet constituents’pT originatefromthePYTHIA jet.Formoreperipheralcollisions,thisfractionisincreasedto25%.

For jetswith p_T,const≥^{3 GeV/c,which were measured down to} 30 GeV/c,acut of50%is applied.However,thisprocedure might imposea biasontheimplicitlyacceptedbackgroundfluctuations.

Therefore,variationsaroundthesenominalvalueswereconsidered forthe evaluation ofsystematic uncertainties. Alternatively, a jet vetotechnique has been used: an embedded jet is not accepted if it overlaps with an already existing jet of sizeable transverse momentum p_{T,ch jet} in the Pb–Pb event. Several veto cut values between15and40GeV/cweretested.Eventually,itturnsoutthat bothapproaches yieldvery similarresults. Thereconstructed jets whichsurvivetheMCpercentagecutserveasaninputtothenext analysisstepswhicharethesameasinthedataanalysis.

6. Observables

In this analysis, two features of particle jets are probed in Pb–Pb collisions: changes in the particle p_T composition of jets andtheirradialdistributionrelativetothejetaxis.

Toproberelative changes inthechargedparticle pT composi- tionofjetsinasurroundingconewithR=⁰.3,thejet-associated yield ratio is measured. The ratiois formed fromthe integrated jet-associated per-trigger yields YPbPb and Yemb which represent the integrals of the per-trigger yield in the jet cone for a given pT,assoc-range as introduced in Eq. (2). Technically, the integral is the sum over the entries of all (

η

,

ϕ

)-bins whose center is within distances of up to R=⁰.3 around the jet axis in the background-correctedper-triggeryieldhistogram.

The jet-associated yield ratio is defined by RY =^YPbPb/Y_emb. Itdirectly comparesintegratedjet-associated per-triggeryields in Pb–Pb tothesameyieldsforembeddedPYTHIAjets.Anenhance- mentorsuppressioninassociatedyieldsisdirectlyseenasadevi- ationfromunityintheratio.

Therelativeradialparticledistributionaroundthejetisdirectly derivedfromthejet-associatedyields.Itshowstherelativedistri- butionofparticleyields insidethejetcone.Thus, itisameasure forthebroadeningorcollimationofconstituentswithcertainmo- menta in or around the jet cone. As for the jet-associated yield ratio, this measurement is performed for high- and low-pT jet- associated yields. The radial shape is normalized to represent a probability distribution.Itis definedinbinsof r=

η

²+

ϕ

², thedistancetothejetaxis,toexploit theradialsymmetryofthe jet peak. InRefs. [48,49], an asymmetricbroadening ofthe near- sidejetpeakisobservedintwo-particlecorrelations.Itisstrongest forlowassociateandtriggermomentaandvanishesforhighermo- menta.Therefore,in theanalysispresentedhere, theinfluence of thisasymmetryonjet–hadroncorrelationswastestedtocheckthe

radial symmetry of the jet peak. Even for the lowest accessible jetandassociatedtrackmomenta,nojetpeakasymmetrywasob- served.Measurementsin

η

and

ϕ

leadtothesameconclusions within statisticalprecision, whichjustifiesthepresentationofthe jetradialshapeinbinsofr.Thecorrelationfunctionwhichisused to obtain theshape isoriginally binned in

η

and

ϕ

.The binning waschosenfineenoughtoavoidsignificantbinningeffects.

For a given centrality-bin, and trigger and associate pT, it is definedbythefollowingformula:

S

(

r_min

,

r_max

) =

¹ A

rmax

r_min

Y_corr

(

r

)

dr

,

(4)

where Y_corr(r) represents the background-corrected per-trigger yield, rmin and rmax the bin edges, and A=rrange

0 Ycorr(r)dr the integral for the self-normalization of the radial shape. The up- per limitinthe integralusedfortheself-normalizationischosen to reflect the different ranges of the shown radial shape and is rrange=⁰.3 forthejetswithpT,const≥⁰.15 GeV/c andrrange=⁰.9 forjetswith pT,const≥^{3 GeV/c.The} statisticaluncertaintyiscal- culatedtakingintoaccounttheself-normalization.

7. Systematicuncertainties

Severalsourcesofsystematicuncertaintiescontributetothefull uncertaintyofthemeasurement andtheevaluated individualun- certaintiesarecombinedusingaquadraticsum,assumingtheyare uncorrelated. Uncertaintiesforthefollowinganalysisaspectshave beentakenintoaccount:thenon-jet-relatedbackgroundcorrection technique, themixed-eventcorrection,theselectionofembedded jets, the tracking efficiency, and the impact of using a PYTHIA referenceinstead ofa measured referenceinpp atthe sameen- ergy. The uncertainties are partly correlated point-to-point. The discusseduncertaintiesaresummarizedinTables2–4.

To correctforthe non-jet-correlated backgroundin thecorre- lationfunction,thebackgroundisevaluatedonthesidebandsand subtractedin

ϕ

,asdescribedinSec.3.Differentunderlyingback- groundmethodsforthecorrelationfunctionshavebeentested:for systematicuncertainties,thedefinitionofthesidebandrangewas variedto1.1<|

η

|<1.3 insteadof1.0<|

η

|<1.4.Inaddition, asimplermethodthatapproximatesthebackgroundbyaconstant baseline(B(

ϕ

=const))hasbeenused.

The mixed-event acceptance/inhomogeneity correction is a small correction. Two variations are considered for systematic uncertainties. First, the mixed-event correction is calculated in- clusivelyforall

ϕ

.Second,thenormalizationofthemixed-event correlations isperformedfor|

η

|<0.3 and full |

ϕ

| ^insteadof usingtheplateauin|

η

_|<0.2 and|

ϕ

_|<0.2.

Intheembedding, acutmotivatedby studyingtheunderlying true jet distributions is applied on the fraction of jet p_T origi- nating from the PYTHIAevent, as described in Sec. 5.Instead of cuttingat20%for0–10%centrality,and25%forothercentralities, thecut isvariedto15%and25%for0–10%centrality,andto20%

and 30% for other centralities. As described above, for jets with pT,const≥^{3 GeV/c a baseline cut value of 50% is used. For sys-} tematicvariation,thecut isperformedat15%and60%for0–10%

centrality,20%and60%forothercentralities.

The detectorhasa finitesingle trackreconstruction efficiency, whichisonlyknownwithfiniteprecision.Sinceallobservablesare corrected forthe trackingefficiency, they areall directly affected byitsuncertainty.Detailedstudiesofthetrackingefficiencyuncer- tainty havebeen performedto evaluatethe sizeof itssystematic uncertainty [54,66]. The studies indicate that the (absolute) un- certainty is 4% for Pb–Pb collisions, mainly dueto an imperfect

Table 2

Tableofsystematicuncertaintiesforjet-associatedyieldsinPb–Pb,embeddedPYTHIA,andtheirratioforhigh-pTassociates(4–20 GeV/c)andlow-pTassociates(1–2 GeV/c) andforthe0–10%mostcentralcollisions.Uncertaintiesaregivenasrelativeuncertaintiesinpercentages.

pT,assoc(GeV/c) Observable 4–20 1–2

pT,ch jet(GeV/c) 40–60 60–80 80–120 30–40 40–60

Background (%) Pb–Pb 0.3–0.6 0.7–1.5 1.5–2.0 6.9 8.0

Embedded 0.3–0.7 0.7–1.0 1.0–1.1 6.8 6.7

Ratio 0.4–0.7 0.1–0.7 0.4–1.6 6.9 9.6

Mixed event correction (%) Pb–Pb 0.2 0.3 0.5 0.2 0.2

Embedded 0.7 0.4 0.4 0.1 <0.1

Ratio 0.7 0.5 0.3 0.2 0.2

Embedding (%) Pb–Pb – – – – –

Embedded 0.1–2.3 0.1–0.4 0.1–0.3 5.0 2.7

Ratio 0.1–2.3 0.1–0.4 0.1–0.3 4.6 2.7

Tracking efficiency (%) Pb–Pb 4.0 4.0 4.0 4.0 4.0

Embedded 4.0 4.0 4.0 4.0 4.0

Ratio – – – – –

Tracking PYTHIA (%) Pb–Pb – – – – –

Embedded 2.0 2.0 2.0 2.0 2.0

Ratio 2.0 2.0 2.0 2.0 2.0

PYTHIA vs. pp (%) Pb–Pb – – – – –

Embedded 5.0 5.0 5.0 2.0 2.0

Ratio 5.0 5.0 5.0 2.0 2.0

Total (%) Pb–Pb 4.0–4.1 4.1–4.3 4.3–4.5 8.0 9.0

Embedded 6.8–7.2 6.8 6.8 9.8 8.7

Ratio 5.5–5.9 5.4–5.5 5.4–5.6 8.8 10.3

Table 3

Tableofsystematicuncertaintiesforjetradialshapesforhigh-pTassociates(4–20 GeV/c)inPb–Pb andembeddedPYTHIAforthe0–10%mostcentralcollisions.Uncertainties aregivenasrelativeuncertaintiesinpercentages.Notethatrelativeuncertaintiesgrowforhigherrvalues.

Data sample Pb–Pb Embedded PYTHIA

pT,ch jet(GeV/c) 40–60 60–80 80–120 40–60 60–80 80–120

Background (%) 0.1–6.5 0.1–13.0 0.1–19.2 0.0–6.9 0.0–10.8 0.0–14.5

Mixed event corr. (%) <0.1 <0.1 <0.1 <0.1 <0.1 <0.1

Embedding (%) – – – 1.0–13.9 0.4–3.1 0.1–0.8

PYTHIA vs. pp (%) – – – 2.0 2.0 2.0

Total (%) 0.1–6.5 0.1–13.0 0.1–19.2 2.2–15.7 2.0–11.5 2.0–14.6

Table 4

Table of systematic uncertainties for jet radial shapes for low-pT associates (1–2 GeV/c,2–3 GeV/c)inPb–Pb andembeddedPYTHIAforjetswith pT,ch jet= 40–60GeV/candforthe0–10%mostcentralcollisions.Uncertaintiesaregivenas relativeuncertaintiesinpercentages.Notethatrelativeuncertaintiesgrowforhigher rvalues.

Data sample Pb–Pb Embedded PYTHIA

pT,assoc(GeV/c) 1–2 2–3 1–2 2–3

Background (%) 1.6–7.5 0.4–8.8 2.2–11.9 1.0–4.2

Mixed event corr. (%) <0.1 <0.1 <0.1 <0.1

Embedding (%) – – 1.2–7.4 0.8–11.3

PYTHIA vs. pp (%) – – 2.0–10.0 2.0–10.0

Total (%) 1.6–7.5 0.4–8.8 6.2–13.0 4.5–15.7

descriptionoftheITS-TPCmatchingefficiency.Anotheruncertainty fromthe trackingefficiencycorrection entersthisanalysisdueto the usage of PYTHIA simulations. The tracking efficiency of the PYTHIAdataisartificiallyloweredby2% beforeembeddingtoac- count for the lower tracking efficiency in Pb–Pb collisions. As a conservative estimate, a relative uncertainty of 100% isassigned to this value. Both components of the tracking efficiency uncer- taintyare takenintoaccount asindependentcontributions tothe uncertainty,i.e.addedinquadraturetothefulluncertainty.These uncertaintiesare directlyused asuncertainties forthe yields,see Table2. Forthe jet-associatedyield ratio,the uncertaintyon the

tracking efficiency in Pb–Pb cancels, because it is correlated in Pb–Pb and theembedded PYTHIAreference. Fortheradial shape distribution,achangeinthe trackingefficiencyhasnoimpact ei- ther, since these observables are relative quantities that do not depend onthe globalmagnitudeofthe trackingefficiency.As an alternative approach to estimate the impact of these two uncer- tainties of the tracking efficiencies on the observables, the full analysiswasredoneusingcorrectionsthatassumetheabovegiven lowertrackingefficiencies.Therewasnosignificantimpactonthe presentedresults.

Finally, an uncertainty is assigned since PYTHIA is used as a baselineinsteadofameasuredppreference.Includingthisuncer- tainty, the conclusions are also validfor a pp referenceand not only for an embedded PYTHIA reference. In order to do so, the presented observableswere calculated andcompared for PYTHIA eventsandpp collisions at7TeV. Withinthestatisticalprecision ofthiscomparison, itisonly possibletogive an estimate forthe inclusive pT,ch jet-range. Therelativedeviationsofeachobservable between both datasets enter directly asa systematic uncertainty andareonthelevelofafewpercent,cf.Tables2–4.

8. Results

Figs. 2 and 3 depict the jet-associated yields (left) and yield ratios (right)for high-p_T andlow-p_T associated particles,respec-

Fig. 2.Centrality dependenceofjet-associatedyields(left)and yieldratios(right)forhigh-pT associates.Boxesrepresentsystematicuncertainties,errorbarsrepresent statisticaluncertainties.Observablesarecorrectedforacceptanceandbackgroundeffects.

Fig. 3.Centralitydependenceofjet-associatedyields(left)andyieldratios(right)forlow-pT associates.Boxesrepresentsystematicuncertainties,errorbarsrepresent statisticaluncertainties.Observablesarecorrectedforacceptanceandbackgroundeffects.

tively.Bothquantitiesare shownasa functionofeventcentrality andforseveralselectedjettransversemomenta.

Thejet-associatedyieldratioshowsasuppressionwithasignif- icanceofseveralstandarddeviationsinthecentralityrange0–50%

for the considered high-pT associated particles. In the probed jet momentum range, no significant pT,ch jet-dependence is ob- served. The centrality-dependent linear slope of the distribution for p_{T,ch jet}= ^{40–60 GeV/c is more than one standard deviation} awayfromzero,takingintoaccount statisticalandsystematicun- certaintiesaddedin quadrature,indicating that thereisa slightly strongersuppression formore centralcollisions inthiscase. Asa crosscheck,thesameobservablewasalsomeasured forjetswith severalhigherminimumpT,const-cuts,i.e.1,2,and3GeV/c,which arelessaffectedbytheunderlyingevent.Theyleadtosimilarcon- clusions.

The jet-associated yield ratio forlow-p_T associates has much larger statistical and systematic uncertainties than the ratio of high-pTconstituents,thusitisnotpossibletodrawadefinitecon- clusion.

ThemeasuredjetrelativeradialshapesarepresentedinFigs.4 and 5. The top panels show the self-normalized distributions, the difference and the ratio of the shapes in Pb–Pb and em- bedded PYTHIA can be found in the two lower panels. The jet radial shapes of high-p_T associates are measured for p_{T,ch jet}= 40–60 GeV/c,60–80 GeV/c, and80–120 GeV/c.Shapes oflow-pT associatesarepresentedforjetswith pT,ch jet=30–40 GeV/c and pT,const>3 GeV/c for associates with pT,assoc= ^{1–2 GeV/c and} p_T,assoc=^{2–3 GeV/c.}

Ingeneral,theradialshapemeasurementsindicatethatall jet- associated yields are similarly distributed relative to the jet axis inPb–Pb andembedded PYTHIA.The yieldsofhigh-p_T associates appear to be slightly more collimated near the core for jets in Pb–Pb,thoughtheabsoluteeffectissmall.Whiletheshapeisnot significantly changedforjettransversemomentabetween40 and 60 GeV/c in Pb–Pb compared to the reference, there is a visible collimation forhigher jet momentaabove 60 GeV/c. This can be seenbestinthedifferencedistributions_PbPb−emb ofFig.4which show that a larger fraction of the associated yield can be found nearthecoreinPb–Pb collisions.

The ratio distributions show that the collimation effect per- sists up to r=⁰.2, which is best visiblefor jets with p_{T,ch jet}= 60–80 GeV/c.IntheCMSmeasurement [50],nosignificantchange of thenear-side jetpeak widthis observedinPb–Pb forhigh-p_T associates and jets above 120 GeV/c. However, the magnitude of the effect observed here is compatible with the observations withinuncertainties.AlsonotethattheCMSdatahintsaswelltoa smallcollimationofthepeakforhigher-pT associates(4–8 GeV/c).

Possibleeffectswhichmightleadtoacollimationincludearelative changeinthequark/gluoncontentinPb–Pb comparedtotherefer- ence [69],aswellasasuppressionoflarge-anglesoftradiationin thecoherentjetenergylosspicture [70,71].Low-pT jet-associated yieldspresentedinFig.5aremeasureduptoadistanceofr=⁰.9 relative to thejet since inthiscasethe associates aredecoupled fromthetriggerjets.

For pT,assoc= ^{1–2 GeV/c,a hintofa broadeningofthe radial} shapeisobservedforjetswithmomentabetween30 and40 GeV/c

Fig. 4.Jetrelativeradialshapedistributions,differences,andratiosforthe0–10%mostcentralcollisionsforhigh-pTconstituents,shownfordifferentjettransversemomenta.

Boxesrepresentsystematicuncertainties,shadedboxesincludeuncertaintiesfromPYTHIA/ppcomparison,anderrorbarsrepresentstatisticaluncertainties.Observablesare correctedforacceptanceandbackgroundeffects.

for the given definition. The broadening is visible in the differ- ence distribution of the left plot in Fig. 5: in Pb–Pb collisions, a smaller fraction of particles can be found directly next to the jet axis. For higher associate transverse momenta, i.e. pT,assoc= 2–3 GeV/c,thereisnosignificantmodificationofthelow-pTradial shapeof jetsinPb–Pb collisions within the largecurrent experi- mentaluncertainties.Arobustmeasurementofthisobservablefor p_{T,ch jet}= 40–60 GeV/c orhighermomentaisnot possibledueto theinsufficientsizeofthedataset.Forhigherjet momentaabove 120 GeV/c,CMSmeasuresasignificantbroadeningofthenear-side jetpeak.

9. Summary

Thepresentedresultsconstitute thefirstattempttostudyjet–

hadroncorrelationswithtrack-basedjetsdowntotransversemo- menta of 30 GeV/c in Pb–Pb collisions — a challenging regime dueto thelargeunderlyingeventandits fluctuations.The jetra-

dial shapesand the change in the particle pT composition were measured in Pb–Pb collisions at √

sNN=².76 TeV forhigh- and low-pT associates and compared to embedded PYTHIA simula- tions. The number of high-pT associates in Pb–Pb collisions is suppressed comparedto the referenceby roughly 30to 10%,de- pending on centrality. The radial particle distribution relative to the jet axis shows a moderate modification in Pb–Pb collisions with respect to PYTHIA. High-pT associate particles are slightly more collimated in Pb–Pb collisions compared to the reference.

ForjetswithpT,const≥^{3 GeV/c,theradial} distributionsoflow-pT associates were measured. A hint ofa broadening ofthe low-pT radial shapesisobservedfor pT,assoc= 1–2 GeV/c.The shapefor p_T,assoc = ^{2–3 GeV/c does not show a significant} modification within its large uncertainties. The results are in line with both previous jet–hadron-related measurements fromthe CMSCollab- orationandjetshapemeasurementsfromtheALICECollaboration athigherp_T andaddfurthersupportfortheestablishedpictureof in-mediumpartonenergyloss.

Fig. 5.Jetrelativeradialshapedistributions, differences,and ratiosforthe 0–10%mostcentralcollisionsfor twodifferentlow-pT constituentranges.Boxesrepresent systematicuncertainties,shadedboxesincludeuncertaintiesfromPYTHIA/ppcomparison,anderrorbarsrepresentstatisticaluncertainties.Observablesarecorrectedfor acceptanceandbackgroundeffects.They-axisscaleoftheratioischosentofocusonr<0.3,wherethedeviationoftheratiofromunityissignificant.

Acknowledgements

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 Gridcenters and theWorldwide LHCComputingGrid (WLCG) collaboration. The ALICE Collaboration acknowledges the followingfundingagencies fortheirsupport inbuildingandrun- ningtheALICEdetector: A.I. AlikhanyanNationalScience Labora- tory(YerevanPhysicsInstitute)Foundation(ANSL),StateCommit- teeofScienceandWorldFederationofScientists(WFS),Armenia;

Austrian Academy of Sciences, Austrian Science Fund (FWF): [M 2467-N36] and Nationalstiftung für Forschung, Technologie und Entwicklung,Austria; MinistryofCommunicationsandHighTech- nologies, National Nuclear Research Center, Azerbaijan; Conselho NacionaldeDesenvolvimentoCientíficoeTecnológico (CNPq),Uni- versidadeFederal doRioGrande doSul(UFRGS), Financiadorade EstudoseProjetos(Finep)andFundaçãodeAmparoàPesquisado Estado deSão Paulo(FAPESP),Brazil; MinistryofScience &Tech- nology of China (MSTC), National Natural Science Foundation of China (NSFC) andMinistry ofEducation ofChina (MOEC), China;

Croatian Science Foundation and Ministry of Science andEduca- tion,Croatia;CentrodeAplicacionesTecnológicasyDesarrolloNu- clear(CEADEN), Cubaenergía, Cuba; Ministry ofEducation, Youth and Sports of the Czech Republic, Czech Republic; The Danish Council for Independent Research | Natural Sciences, the Carls- bergFoundationandDanishNationalResearchFoundation(DNRF), Denmark; Helsinki Institute of Physics (HIP), Finland; Commis- sariat à l’Energie Atomique (CEA), Institut National de Physique Nucléaire et de Physique des Particules (IN2P3) and Centre Na- tional de la Recherche Scientifique (CNRS) and Région des Pays de laLoire,France; Bundesministeriumfür Bildung,Wissenschaft, Forschung und Technologie (BMBF) and GSI Helmholtzzentrum fürSchwerionenforschungGmbH,Germany;GeneralSecretariatfor ResearchandTechnology,MinistryofEducation,ResearchandRe- ligions, Greece; National Research, Development and Innovation Office,Hungary;DepartmentofAtomicEnergy,GovernmentofIn- dia (DAE), Department of Science and Technology, Government

of India(DST), University Grants Commission, Government ofIn- dia(UGC)andCouncilofScientificandIndustrialResearch(CSIR), India; IndonesianInstitute of Science, Indonesia; Centro Fermi – MuseoStorico dellaFisica eCentroStudi e RicercheEnricoFermi andIstitutoNazionalediFisicaNucleare(INFN),Italy;Institutefor Innovative Science and Technology, Nagasaki Institute of Applied Science (IIST), Japan Society for the Promotion of Science (JSPS) KAKENHIandJapaneseMinistryofEducation,Culture, Sports,Sci- enceand Technology (MEXT),Japan; Consejo Nacionalde Ciencia y Tecnología (CONACYT), through Fondo de Cooperación Interna- cional enCiencia yTecnología(FONCICYT)andDirección General deAsuntosdelPersonalAcademico(DGAPA),Mexico;Nederlandse OrganisatievoorWetenschappelijkOnderzoek(NWO),Netherlands;

The ResearchCouncil ofNorway, Norway;CommissiononScience andTechnology forSustainable Developmentin theSouth (COM- SATS),Pakistan;PontificiaUniversidadCatólicadelPerú,Peru;Min- istryofScienceandHigherEducationandNationalScienceCentre, Poland;KoreaInstituteofScienceandTechnologyInformationand National Research Foundation of Korea (NRF), Republicof Korea;

Ministry ofEducation andScientific Research,Institute of Atomic Physics and Ministry of Research and Innovation and Institute of Atomic Physics, Romania; Joint Institute for Nuclear Research (JINR), Ministry ofEducation and Science of the Russian Federa- tion,NationalResearchCentreKurchatovInstitute,RussianScience Foundation and Russian Foundation for Basic Research, Russia;

Ministry of Education, Science, Researchand Sport ofthe Slovak Republic, Slovakia; NationalResearch Foundation of South Africa, South Africa; Swedish Research Council (VR) and Knut & Alice WallenbergFoundation(KAW),Sweden;EuropeanOrganizationfor Nuclear Research, Switzerland; National Science and Technology DevelopmentAgency(NSDTA),SuranareeUniversityofTechnology (SUT) andOfficeoftheHigher EducationCommissionunderNRU project of Thailand, Thailand; Turkish Atomic Energy Authority (TAEK),Turkey;NationalAcademyofSciencesofUkraine,Ukraine;

ScienceandTechnologyFacilitiesCouncil(STFC),UnitedKingdom;

NationalScienceFoundationoftheUnitedStatesofAmerica(NSF) andU.S.DepartmentofEnergy,OfficeofNuclearPhysics(DOENP), UnitedStatesofAmerica.