Measurement of jet suppression in central Pb-Pb collisions at root s(NN)=2.76 TeV

(1)

SISTEMA DE BIBLIOTECAS DA UNICAMP

REPOSITÓRIO DA PRODUÇÃO CIENTIFICA E INTELECTUAL DA UNICAMP

Versão do arquivo anexado / Version of attached file:

Versão do Editor / Published Version

Mais informações no site da editora / Further information on publisher's website:

https://www.sciencedirect.com/science/article/pii/S0370269315002828

DOI: 10.1016/j.physletb.2015.04.039

Direitos autorais / Publisher's copyright statement:

©2015 by Elsevier. All rights reserved.

DIRETORIA DE TRATAMENTO DA INFORMAÇÃO

Cidade Universitária Zeferino Vaz Barão Geraldo

CEP 13083-970 – Campinas SP

Fone: (19) 3521-6493

http://www.repositorio.unicamp.br

(2)

Contents lists available atScienceDirect

Physics

Letters

B

www.elsevier.com/locate/physletb

Measurement

of

jet

suppression

in

central

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

r

a

c

t

Articlehistory: Received 6 February 2015

Received in revised form 25 March 2015 Accepted 20 April 2015

Available online 22 April 2015 Editor: L. Rolandi

Thetransversemomentum (pT)spectrumandnuclearmodiﬁcationfactor(RAA)ofreconstructedjetsin 0–10%and10–30%centralPb–Pbcollisionsat√sNN=2.76 TeV weremeasured.Jetswerereconstructed usingtheanti-kTjetalgorithmwitharesolutionparameterofR=0.2 fromchargedandneutralparticles, utilizing theALICEtracking detectorsand ElectromagneticCalorimeter (EMCal).Thejet pTspectraare reported in the pseudorapidity interval of |

η

jet|<0.5 for 40<pT,jet<120 GeV/c in 0–10%and for 30<pT,jet<100 GeV/c in 10–30% collisions. Reconstructed jets wererequired to contain aleading chargedparticlewithpT>5 GeV/c tosuppressjetsconstructedfromthecombinatorialbackgroundin Pb–Pb collisions.The leading chargedparticle requirementappliedto jetspectraboth inppand Pb– Pb collisionshad anegligibleeffectonthe RAA.Thenuclear modiﬁcationfactorRAA wasfound tobe 0.28±0.04 in0–10%and0.35±0.04 in10–30%collisions,independentofpT,jetwithintheuncertainties ofthemeasurement.Theobservedsuppressionisinfairagreementwithexpectationsfromtwomodel calculationswithdifferentapproachestojetquenching.

1. Introduction

DiscreteformulationsofQuantumChromodynamics (QCD) pre-dict the existence of a cross-over transition from normal nu-clear matter to a new state of matter called the Quark–Gluon Plasma (QGP), where the partonic constituents, quarks and glu-ons,are deconﬁned.The QGPstateis expectedtoexistatenergy densitiesabove0

.

5 GeV

/

fm3andtemperaturesabove160 MeV[1], whichcanbereachedincollisionsofheavy-ionsatultra-relativistic energies. The existence of the QGPis supported by the observa-tionsreportedbyexperimentsattheRelativisticHeavyIonCollider (RHIC)[2–5]andattheLargeHadronCollider(LHC)[6–17].

One way to characterize the properties of the QGP is to use partonsfromthehardscatteringofthepartonicconstituentsinthe collidingnucleons asmediumprobes.Hardscatteringisexpected tooccurearlyinthecollisionevolution,producinghightransverse momentum(pT) partons,whichpropagatethrough theexpanding mediumandeventuallyfragmentintojetsofhadrons.

Dueto interactions ofthe high-pT partons withthe medium, theenergyof thepartons isreducedcompared to proton–proton (pp) collisions due to medium-induced gluon radiation and col-lisionalenergy loss (jetquenching) [18,19]. The productioncross section of the initial hard scattered partons is calculable using

_E-mail_address:_{alice-publications@cern.ch}_.

perturbative QCD (pQCD), and the contribution from the non-perturbativehadronizationcanbewellcalibratedviajet measure-mentsinppcollisions.

Jet quenching has been observed at RHIC [20–29] and at the LHC[8,16,17,30–41]viathemeasurementofinclusivehadronand jet productionathigh pT,di-hadron angularcorrelationsandthe dijet energy imbalance. In all cases, the measured observable is found to be stronglymodiﬁed in centralheavy-ion collisions rel-ative to pp collisions, when compared to expectations based on treating heavy-ioncollisions asan incoherentsuperpositionof in-dependentnucleon–nucleoncollisions.

Measurementsof the jet kinematics are expectedto be more closely correlated to the initial partonkinematics than measure-mentsofhigh-pThadrons.Jetsareusuallyreconstructedby group-ing measured particles within a given distance,e.g. a cone with radius R.Theinteraction withthemedium canresultina broad-ening of the jet shape, a softening of the jet fragmentation [42] leadingtoanincreaseofout-of-conegluonradiation[43]with re-spect to jets reconstructed in pp collisions [17]. Therefore, for a givenjetresolutionparameter R andaﬁxedinitialpartonenergy, theenergyofjetsreconstructedinheavy-ioncollisionsisexpected tobesmallerthanthosereconstructedinppcollisions.

Jetmeasurementsinheavy-ioncollisionsarechallengingsincea singleeventcanhavemultiple,possiblyoverlapping,jetsfrom in-dependentnucleon–nucleonscatters,aswellascombinatoric“jets” fromthe large,partially correlatedandﬂuctuatingbackground of

http://dx.doi.org/10.1016/j.physletb.2015.04.039

0370-2693/©2015 CERN for the beneﬁt of the ALICE Collaboration. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP3_.

(3)

lowtransversemomentumparticles.Consequently,jet reconstruc-tioninheavy-ion collisionsrequires arobust jet-signaldeﬁnition, and a procedure to correct for the presence of the large back-groundanditsassociatedregion-to-regionﬂuctuations[44].

The resultsreported in thisletter are fromlead–lead (Pb–Pb) collision data atan energy per nucleon pairof

√

sNN

=

2

.

76 TeV recordedby theALICEdetectorin2011. Chargedparticles are re-constructed with the Inner Tracking System (ITS) and the Time ProjectionChamber (TPC)downto pT of0.15 GeV/c.Neutral par-ticles, excluding neutrons and K0

Ls, are reconstructed with the Electromagnetic Calorimeter (EMCal) down to a transverse en-ergy ofthe EMCalclustersof 0.3 GeV.Forjet reconstruction, we followed the approach applied in Refs. [45,46], where the aver-age energy density of the event was subtracted from the signal jets on a jet-by-jet basis, and the detector and background ef-fectswerecorrectedonanensemblebasisviaanunfolding proce-dure.Thesignaljetswereobtainedusingtheanti-kTjetalgorithm

[47] with a resolutionparameter of R

=

0

.

2 inthe pseudorapid-ity range of

|

η

jet

|

<

0

.

5. Signal jets were required to contain at least one charged particle with pT

>

5 GeV

/

c. The corrected jet pT spectraandnuclearmodiﬁcationfactors(RAA)arereportedfor 40

<

pT,jet

<

120 GeV

/

c in0–10%andfor30

<

pT,jet

<

100 GeV

/

c in10–30%centralPb–Pb collisionsandthecorrectedjet pT spec-trumfor20

<

pT,jet

<

120 GeV

/

c inppcollisionsat

√

s

=

2

.

76 TeV from13.6 nb−1 recordedin 2011.The RAAiscompared to expec-tationsfrom two jet quenchingmodelcalculationswith different approaches, described later, inorderto test thesensitivityofthe observable to the energy density via the centrality dependence, andtothepartonenergyscaleviathemomentumdependence.

2. Experimentalsetup

ForacompletedescriptionoftheALICEdetectorandits perfor-manceseeRefs. [48]and[49],respectively.Theanalysispresented herereliesmainlyon theALICEtrackingsystemandEMCal,both of which are located inside a large solenoidal magnetwith ﬁeld strength0.5 T.

The tracking system consists of the ITS, a high-precision six-layer silicondetector system withthe inner layer at 3.9 cm and theouter at43 cm fromthecenter ofthe detector,andthe TPC witha radial extent of85–247 cm, provides up to 159 indepen-dent spacepoints per track.The two innermostlayers ofthe ITS consistoftheSiliconPixelDetector(SPD),whichprovidestwo lay-ersof silicon pixel sensors at radii 3.9 cm and7.6 cm fromthe beamaxisandcoversthefullazimuthover

|

η

|

<

2 and

|

η

|

<

1

.

4, respectively. The combined information of the ITS and TPC can determine the momenta of charged particles from low momen-tum (pT

≈

0

.

15 GeV

/

c) to highmomentum (pT

≈

100 GeV

/

c) in

|

η

|

<

0

.

9 andfullazimuth.

TheEMCalisaPb-scintillatorsamplingcalorimeter,which cov-ers107 degreesinazimuthand

|

η

|

<

0

.

7.Itconsistsof10 super-moduleswithatotalof11520 individualtowerseachcoveringan angularregion

η

×

ϕ

=

0

.

014

×

0

.

014 which are read out by avalanchephotodiodes.

Thedatawererecordedin2011forPb–Pbcollisionsat

√

sNN

=

2

.

76 TeV using a set of triggers based on the hit multiplicity recordedbytheVZEROdetector,whichconsistsofsegmented scin-tillators covering the full azimuth over 2

.

8

<

η

<

5

.

1 (VZERO-A) and

−

3

.

7

<

η

<

−

1

.

7 (VZERO-C).

3. Dataanalysis

A total of 11.5M (15 μb−1₎ _and_5.7M _{(3.7 μb}−1₎ _events _with VZERO multiplicities corresponding to 0–10% and 10–30% most centraleventswereselectedusingthecentralitydeterminationas

described inRef. [50]. The accepted events,reconstructed as de-scribedinRef.[51],wererequiredtohaveaprimaryreconstructed vertexwithin10 cmofthecenterofthedetector.

Reconstructedtrackswererequiredtohaveatleast3hitsinthe ITSusedintheﬁttoensureadequatetrackmomentumresolution for jet reconstruction.For trackswithout anyhit in the SPD, the primary vertex location was usedin addition tothe TPCandITS hits forthe momentum determinationof thetrack. Thisreduced the azimuthal dependence of the track reconstruction eﬃciency duetothenon-uniformSPDresponse,withoutcreatingtrack col-lectionswithdrasticallydifferingmomentumresolutions.Accepted trackswere requiredtobemeasured with0

.

15

<

pT

<

100 GeV

/

c in

|

η

|

<

0

.

9,andtohaveatleast70TPCspace-pointsandnoless than80%ofthegeometricallyﬁndablespace-pointsintheTPC.The trackingeﬃciencywasestimatedfromsimulationsofthedetector response using GEANT3 [52] with the HIJING [53] event genera-tor asinput. In 0–10% collisions, it is about56% at 0

.

15 GeV

/

c,

about83%at1

.

5 GeV

/

c andthendecreasesto81%at3 GeV/c,after whichitincreasesandlevelsofftoabout83%atabove 6.5 GeV/c. In 10–30% collisions, the tracking eﬃciency follows a similar pT dependence pattern, with absolute values of the eﬃciency that are 1 to 2% higher compared to 0–10% collisions. The momen-tum resolution

δ

pT

/

pT,estimated ona track-by-track basis using the covariance matrix of the track ﬁt, is about 1% at 1

.

0 GeV

/

c

andabout3% at50 GeV

/

c.Trackswith pT

>

50 GeV

/

c wereonly a smallcontributionto theinclusivejet populationconsidered in this analysis, for example only 20% of the jetswith pT,jet larger than100 GeV

/

c werefoundtocontainatrackabove50 GeV

/

c.

EMCal cells witha calibrated response of more than 50MeV were clusteredprior toinclusion inthe jet ﬁnderby aclustering algorithm whichrequiredeach cluster toonlyhavea single local maximum [49]. Interactions of slow neutrons or highly ionizing particles in the avalanche photodiodes createclusters with large apparent energy, but anomalously small number of contributing cells, and are removed from theanalysis. Anon-linearity correc-tion,derivedfromelectrontestbeamdata,ofabout7%at0.5 GeV andnegligibleabove3 GeV,was appliedtotheclusters’ energies. The energy resolution obtained from electron test beam data is about15%at0.5 GeVandbetterthan5%above3 GeV.

Unlike electrons and photons, which deposittheir full energy intheEMCalvia electromagneticshowering, chargedhadrons de-posit energy in the EMCal, mostly via minimum ionization, but also via nuclear interactions which generate hadronic showers. To avoiddouble counting, the energy depositedin the EMCalby charged particles that were already reconstructed as tracks, the clusters’ energieswerecorrectedbythefollowingprocedure[54]: All tracks with pT

>

0

.

15 GeV

/

c were propagated to the aver-age cluster depthwithin the EMCal,andthen associatedto clus-ters with ET

>

0

.

15 GeV within the window

|

η

|

<

0

.

015 and

|

ϕ

|

<

0

.

025. Trackswere always matched to their nearest clus-ter, while clusters were allowed to havemultiple track matches. Clusters withmatched tracks were correctedforcharged particle contamination by removing the fraction f

=

100% of the sumof the momenta of all matched tracks from the cluster energy, as done in [54]. Clusters with ET

>

0

.

30 GeV after this correction wereusedinthisanalysis.

ThecollectionoftracksandcorrectedEMCalclusterswas then assembled intojetsusing theanti-kT or thekT algorithms inthe FastJetpackage [55] witha resolutionparameterof R

=

0

.

2.Only those jetsthat were at least R away fromthe EMCalboundaries of

|

η

|

<

0

.

7 and1

.

4

< φ <

π

,andthusfullycontainedwithin the EMCal acceptance,were kept intheanalysis whichlimitsthe ef-fect ofthe acceptanceboundarieson themeasured jet spectrum. Jetsreconstructedby theanti-kT algorithmwere usedtoquantify

(4)

signaljets,whilejetsreconstructedbythekT algorithmwereused toquantifythecontributionfromtheunderlyingevent.

Thesignal spectrum formedfromthe reconstructed jetsis af-fected by the contribution from the underlying event. In order to suppress the contribution of the background to the measure-ment of the jet energy, we followed the approach described in Refs. [45,46], which addresses the average additive contribution tothe jet momentum ona jet-by-jetbasis. The underlying back-ground momentum density was estimated event-by-event using themedianof praw

T,jet

/

Ajet,where prawT,jet is theuncorrected energy and Ajet is the area of jets reconstructed withthe kT algorithm. Due to the limited acceptanceof the EMCal,

ρch

, the medianof theevent-by-eventmomentumdensitydistributionobtainedfrom chargedjets(i.e.jetsreconstructedfromtracksonly)in

η

jet

<

0

.

5 and full azimuthal acceptance was used. Then,

ρscaled

was de-terminedby scaling

ρch

usinga centrality-dependent factor.This factorisobtained fromaparametrization ofthemeasurement of the charged-to-neutral energy ratio, using tracks and corrected clusters in the EMCal acceptance. In 0–10% central Pb–Pb colli-sions, the average charged background momentum density was

ρ

ch

≈

110 GeV

/

c.Afterscalingtoincludetheneutralcomponent weobtained

ρ

scaled

≈

190 GeV

/

c,whichcorrespondstoan aver-agecontribution ofthe underlyingevent ofabout24 GeV

/

c ina cone of R

=

0

.

2. In 10–30%central Pb–Pb collisions

ρ

scaled

de-creases to

≈

130 GeV

/

c. For every signal jet reconstructed with theanti-kT algorithm, the backgrounddensityscaled by the area of the reconstructed signal jet was subtracted from the recon-structed transverse momentum of the signal jet according to

preco_T_,_jet

=

praw_T_,_jet

−

ρ

scaled

·

Ajet.

Region-to-regionbackgroundfluctuationsleadtoasmearingof the reconstructed jet energy. Their magnitude was estimated as described in Refs. [45,46] in two different ways: (1) by taking the scalar sum of the pT of all particles found in a cone ran-domly placed in the event, referred to as random-cone method, and(2) embedding a single particle in the eventand inspecting theanti-kTjetthatcontainsthatembeddedparticle,referredtoas embedded track method. The first method does not rely on any assumptionsaboutthestructureofthebackgrounditselfandgives approximately the same background fluctuation as embedding a trackwithinfinitemomentumforanti-kT jets.Thesecondmethod shouldbeabletoreproducethebackgroundasseenbytheanti-kT algorithmmore directly.The background fluctuationswere quan-tifiedby

δ

pT

=

pconeT

−

ρ

scaled

· π

R2 fortherandom-conemethod, and

δ

pT

=

preco_T_,_jet

−

pprobeT forthe embedded-trackmethod witha minimumof pprobe_T

=

10 GeV

/

c forthe pT oftheembedded track. Above10 GeV

/

c theresulting

δ

pTdistributiondoesnotdependon thepToftheembeddedparticle.The

δ

pTdistributionsforthetwo methodsinthe10%mostcentralcollisionsareshowninFig. 1 for

pprobe_T

=

60 GeV

/

c.The twomethods appeartoprovide thesame quantitative response to the background ﬂuctuations, with only marginal differencesmainly due to smalljet area ﬂuctuationsin theembeddingtrackmethod.Thewidthsofthe

δ

pT distributions areabout6 GeV/c.The left-handside (LHS)ofthe distributionis Gaussian-likeandisdominatedbysoftparticleproduction.To de-termine its width, the distributions were ﬁtted recursively with a Gaussian function in the range

[

μ

LHS

₋

₃

_σ

LHS

_,

_μ

LHS

₊

1

2

σ

LHS

]

usingthemeanandwidthofthe

δ

pT distributionasstarting val-uesfor

σ

and

μ

.The LHSwidthis about5 GeV

/

c in 0–10% and about3

.

5 GeV

/

c in 10–30%events.Theright-hand sidehas addi-tionalcontributionsfromhardscatteringprocesses,andthe result-ing non-Gaussian tailathigh

δ

pT isdueto overlapping jets. The random-conemethodwasusedasthebaselineinthisanalysisfor creatingthe response matrix used in unfolding, while the single

Fig. 1. TheδpTdistribution for R=0.2 with the random-cone and the

embedded-track methods in the 10% most central events, with pprobeT =60 GeV/c for

the

embedded-track method.

particle embedding method was used to study the sensitivity of theresultstothemethod.

Additionally, signal jets were required to contain a charged trackwithatransversemomentumofatleast5 GeV

/

c anda min-imumbackgroundsubtracted preco

T,jet of30 GeV

/

c for0–10%andof 20 GeV

/

c for 10–30% most central events, which roughly corre-spondsto5

σ

ofthe

δ

pTdistribution,inordertosuppressthe con-tributionofcombinatorialjets, i.e.fromjetsreconstructedmainly fromupwardﬂuctuationsofthesoft-particlebackground.

Both the averagebackground andthe backgroundﬂuctuations are averaged over all possible orientations of the event plane, namely it is assumed that the signal jet sample being analyzed is isotropicallydistributedwithrespect to theeventplane. How-ever, the jet sample may show some degree of correlation with theeventplane,bothforphysicalreasons(e.g. pathlength depen-denceofjetenergyloss) orasaresultofthecutsappliedinthe analysis(mostnotablytherequirementontheleadinghadron pT). Sincethe backgroundisalsocorrelatedwiththeeventplane due to ﬂow (v2) [10],a question mayarise aboutthe validity ofthis approach. Upper limits on the magnitude of these effects have beenestimatedby usingrandom cones biasedtowardsthe event plane, either by requiringthe presence ofa 5 GeV

/

c track orby weighting the distribution using an upper limit on the jet v2 of 0.1. Inboth cases,the upperlimitson theshiftof thejet energy scale (JES)werefoundtobesmallerthan0

.

1 GeV

/

c.

4. Unfolding

The measuredjet spectraare distortedby theresponseofthe detectors used in the measurement and the background ﬂuctu-ations in the underlying event. To correct for these effects we usedan “unfolding”procedure,asdescribed inRef.[46].The cor-recteddistribution ptrue_T_,_jetandthemeasured distribution preco_T_,_jetare related by a convolution through the response matrix RMtot

=

RMbkg

×

RMdet, where RMdet parametrizes the detectorresponse and RMbkg the backgroundﬂuctuations. The unfoldingprocedure operates under the assumption that preco_T_,_jet

=

RMtot

×

ptrue_T_,_jet. Both backgroundﬂuctuationsandthedetectorresponsetojetsare uni-formwithinthe

η

and

ϕ

acceptances,whichisa preconditionfor thefactorizedapproachusedinbuildingRMtot.

The detectorresponse for jet reconstruction was obtained us-ing pp eventssimulated withthePYTHIA 6[56] event generator

(5)

(tune A [57]). Jets were reconstructed both at “generator level” andat“detectorlevel”usingtheanti-kT algorithm.Generator-level simulationsutilizedonlypromptparticlesoriginatingfromthe col-lision(withc

τ

<

1 cm),directlyfromtheeventgeneratoroutput, withoutaccountingfordetectoreffects;detector-levelsimulations also includeda detailedparticle transport and detectorresponse simulationbasedonGEANT3[52]withthedetectorresponsesetto the Pb–Pb configuration. During detector-level jet reconstruction, an additional pT-dependent tracking inefficiency was introduced inordertoaccountforthelargerinefficiencyduetothelarger oc-cupancy effects in central Pb–Pb events compared to pp events. Occupancy effects have been estimated comparing the tracking performance in PYTHIA and HIJING simulations, which represent ppandPb–Pbevents[53].TheoccupancyeffectsincentralHIJING eventsarelargerfor pT

<

0

.

5 GeV

/

c wheretheeﬃciencyisabout 4% lower compared to PYTHIA, and then levels off to about 2% lowerfor pT

>

2 GeV

/

c.Insemi-centralHIJINGevents,occupancy effects onthe tracking eﬃciencyamount to no morethan 2% at low pT andabout1% for pT

>

2 GeV

/

c. Other than thistracking eﬃciencycorrection,thedetectorresponsetojetswasassumedto bethesameinPb–PbeventsasinthePYTHIAsimulatedpp colli-sions.

Thegenerator-levelanddetector-leveljetswerematchedbased onthe Euclideandistancebetweentheirjet axes in pseudorapid-ityandazimuthal angle.It wasensured thatthematching opera-tionis bijective:each generator-leveljet was matchedto atmost onedetector-leveljet [46]. Everymatchedjet paircorrespondsto an entryin the detector response matrix, RMdet. An unmatched generator-leveljetrepresentsajetthatwasnotreconstructed,and thisdistributionwasusedtodeterminethejetreconstruction eﬃ-ciency.In0–10%Pb–Pbevents,thedetectorjetreconstruction eﬃ-ciencywasfoundtobe90%at40 GeV

/

c and95%above70 GeV

/

c,

limitedmainlybythetrackreconstructioneﬃciencyoftheleading charged particle. As described above, at detector level the con-stituentcut was 150 MeV

/

c for tracks,and300 MeV forclusters after the cluster energy is corrected for charged particle energy contamination.However,atgeneratorlevelnosuchcutisapplied, and hence the reconstructed jets are corrected to a constituent chargedparticlemomentumof0 MeV

/

c andtoaconstituent clus-terenergyof0 MeVintheunfoldingprocess.Anetnegative shift oftheJESatdetectorlevelwasobtained, whichoriginatesmainly from tracking ineﬃciency and unreconstructed particles, such as neutronsandK0

L,thoughthesubtractionprocedureforenergy de-posits by charged particles in the EMCal and missing secondary particles from weak decays contribute to the shift [54]. The JES correction applied through the response matrix is about 23% at

ptrue_T_,_jet of40 GeV

/

c and29%at120 GeV

/

c independentof central-ity.

The RMbkg matrix was constructed row-by-row by takingthe

δ

pTdistributionandshiftingitalongthepreco_T_,_jetaxisbytheamount ptrue_T_,_jet corresponding to each row (Toeplitz matrix). This matrix construction method assumes that the response of the jet spec-trumtobackgroundﬂuctuationsisindependentofthejet momen-tum.

The pT-dependenceofthejetmomentumresolution

σ

(

preco_T_,_jet

)/

ptrue_T_,_jet is different for the background and detector contributions [46]. The contributionfrom background ﬂuctuations is dominant atlow ptrue

T,jetandisproportionalto1

/

p true

T,jet,whereasthe contribu-tionfromdetectoreffectsisfairlyconstant withptrue_T_,_jet.The cross-overbetweenthetwocontributionshappensatptrue_T_,_jet

≈

30 GeV

/

c.

Thecombinedjet momentumresolution isabout23% at ptrue_T_,_jet of 40 GeV

/

c and 20% at 120 GeV

/

c for 0–10% collisions, while itis 24%atptrue

T,jetof30 GeV

/

c and20%at100 GeV

/

c for10–30%.

Two unfolding algorithms withdifferent regularization proce-dureswereusedforcorrectingthemeasuredjetspectrum:the

χ

2 minimization method [58] with a log–log-regularization and the generalized Singular Value Decomposition (SVD) method [59],as implemented in RooUnfold [60], which was used for the default value ofthe data points. The measured spectrum usedas an in-put to the unfolding was in the range 30

<

pT,jet

<

120 GeV

/

c for 0–10% and 20

<

pT,jet

<

100 GeV

/

c for 10–30% collisions. A smoothed version of the measured spectrum was used as the prior, sothat thestatisticalﬂuctuationswithin thedatawere not magniﬁed inthe unfolding process. The regularization parameter used for SVD unfolding is k

=

5. The value of k is chosen such that it corresponds to the d vector magnitude of 1, and Pearson coeﬃcientswhichdonotshowalargevariationinthecorrelation betweenneighboring pTbins.

The corrected jet spectra are reported for 40

<

pT,jet

<

120 GeV

/

c in 0–10%,and for30

<

pT,jet

<

100 GeV

/

c in10–30% wherethe eﬃciencyduetothesekinematiccutsishigh, approx-imately 90%. Itwas veriﬁedthat thecut on thereconstructedjet

pT hasanegligibleeffectinthereportedpT regionoftheﬁnal re-sult,aslongastherequirementontheleadingchargedtrackpTis atleast5 GeV

/

c.Ifthisthresholdisreduced,thecutonthe mini-mumreconstructedjetpTbecomescrucialforunfoldingstability.

Theanalysisproceduresinthe10%mostcentralcollisionswere testedwithtwodifferentMonte Carlo(MC)models,whereevents were constructedby embedding jetsinto a softbackground.The ﬁrst testveriﬁedthe robustnessoftheunfolding frameworkwith the inclusionof fake“jets”that are clusteredfromthesoft back-ground, whichdid notoriginate froma hardprocess. Thesecond modeltestedtheassumptionthatthebackgroundanddetector re-sponsescanbefactorized.

In the ﬁrst model, the soft background of both charged and neutral particles was modeled with3100

<

Ntracks

<

5150 where theparticletransversemomentaweretakenfromaBoltzmann dis-tribution with a temperature of 550 MeV. This model created a ﬂuctuating background similar to that ofthe 0–10% Pb–Pb data; e.g. thebackground ﬂuctuations, asestimated via the

δ

pT distri-butions, coincide within few percent. Jets were reconstructed at generator levelin PYTHIA-onlyeventsandatdetectorlevel,with the added background.The first model validatedthe background subtractiontechnique,andinparticularthestabilityofthe unfold-ingmethodagainstthecontributionfromtheresidual combinato-rial background.In the second model,the backgroundwas taken fromreal0–10%Pb–Pbevents.Thechargedparticlecorrectionfor the EMCal clusters was applied after embedding. Only jets with at least 1 GeV/c oftransverse momentum coming from the em-bedded PYTHIAevent were selected for the test. This is needed to reject the signal from hard scatterings in the data, but also removes mostofthe combinatorialjets fromthePb–Pb underly-ing event. The secondmodel was usedto testthe validityof the charged particlecorrection applied to theEMCal clusters, in par-ticularintheinterplaybetweentheunderlyingeventandthejets. It alsovalidatescertain aspects ofthecorrectionsapplied forthe backgroundfluctuations, e.g. theunsmearing ofthejet pT dueto backgroundfluctuationsortheoverlapwithlow momentumjets. Backgroundtracksandclusterscouldbematchedtojettracksand clusters orvice versa, so that the correction forcharged particle contamination could potentially causean over-subtraction that is not corrected forin the unfolding procedure. These Monte Carlo testsshowedthat theanalysisprocedures outlinedabove, includ-ing unfolding,recovered theinput spectrumwithin thestatistical andsystematicuncertaintiesofthemodels.

(6)

Fig. 2. The

spectra of

R=0.2 jets with a leading track requirement of 5 GeV/c in 0–10% and 10–30% most central Pb–Pb collisions scaled by 1/Ncolland in inelastic

pp collisions at √sNN=2.76 TeV. The uncertainties on the normalization are about

11% for the Pb–Pb data from the uncertainty on Ncolland about 8% for the pp data

from the total inelastic cross section.

Table 1

Summary of systematic uncertainties for 0–10% most central collisions. The ﬁrst col-umn is the uncertainty at the minimum pmin

T,jetof 40 GeV/c, the second column is

the uncertainty at the maximum pmax

T,jetof 120 GeV/c. The minimum and maximum

columns give the extreme, and the last column gives the average systematic uncer-tainty over the entire pTrange. The total correlated uncertainty was calculated by

adding the components in quadrature, while the shape uncertainty was calculated as the σ of the different variations (see text for details).

Category Relative uncertainty (%) pmin

T,jet pmaxT,jet Min. Max. Avg.

Tracking eﬃciency 7.7 11.3 7.3 11.3 8.8 Track momentum resolution 1.0 1.0 1.0 1.0 1.0 Charged particle correction 0.7 2.7 0.7 6.4 3.7 EMCal clusterizer 3.2 1.8 0.1 3.2 1.4 EMCal response 4.4 4.4 4.4 4.4 4.4 Background ﬂuctuations 3.9 2.7 2.3 3.9 2.8 Jet raw pTcuts 2.6 6.7 1.5 6.7 3.6

Combinatorial jets 0.3 0.5 0.0 0.5 0.2 Total correlated uncertainty 10.6 14.5 10.6 14.5 12.0 Unfolding method 0.1 10.0 0.1 15.5 6.6 SVD reg. param. k=4 3.6 11.7 2.4 11.7 6.0 SVD reg. param. k=6 7.2 2.7 1.5 8.8 5.3 Prior choice 1 1.9 4.0 0.2 4.0 1.6 Prior choice 2 2.1 1.4 0.1 2.1 0.9 Total shape uncertainty 3.8 7.2 2.7 7.4 5.3

5. Results

Theunfoldedjetspectrain0–10%and10–30%centralcollisions aredisplayedinFig. 2.Tocomparethespectrawiththespectrum measuredinppcollisions,theyieldisdividedbythenumberof bi-narycollisions,whichisNcoll

=

1501

±

167 for0–10%and743

±

79 for 10–30% collisions, as estimated from a Glauber MC calcula-tion[50].

Thesystematic uncertainties on thejet spectrum are summa-rized in Table 1 for the 0–10% centrality class. For the 10–30% centralityclassthecorresponding uncertainties differ,onaverage, by2%orless. Thesystematicuncertaintieswere dividedintotwo categories: correlated uncertainties and shape uncertainties. The correlated uncertainties result dominantly from uncertainties on theJES,suchastheuncertaintyofthetrackingeﬃciency,thatwill shifttheentirejet spectrum inone direction, whereasthe shape

uncertaintiesarerelatedtotheunfoldingandcandistorttheslope ofthespectrum.

The dominant correlated uncertainty on the jet spectrum of about 9% arises from the uncertainty on the tracking eﬃciency. It is estimatedby varying the trackingeﬃciency by 5% in deter-mining RMdet and unfolding the spectrum. The uncertainty due tothecorrectionprocedureforthechargedparticledouble count-ing intheEMCalofabout4% wasdetermined byvarying f from

100% to30% inboth the measured spectrumandthe RMdet.The determinationoftheuncertaintiesfromother EMCalresponse re-lateduncertaintiesasEMCalenergyscale,EMCalenergyresolution, andEMCalnon-linearityisoutlinedin[54]andcombinedleadsto anuncertaintyof4.4%.Theuncertaintyarisingfromthechoice of theEMCalclusteringalgorithm isdetermined byusingadifferent clusterizingmethod,thatformsﬁxed-sizeclustersfrom3

×

3 tow-ers. For the background ﬂuctuations, the response matrix RMbkg was constructedwiththesingle-track embeddingmethodfor de-termining

δ

pT, asdiscussed above. To estimate the sensitivity of the unfolding to the raw jet pT selection, the pT range ofinput spectraisvariedby extendingtherangeatboththelowandhigh endsby

±

5 GeV

/

c.Theinﬂuenceofcombinatorialjets, estimated by varying the low edge ofthe unfolded spectrum from0to up to10 GeV

/

c wasfoundtobenegligible.Sinceallsourcesof uncer-taintyareindependent,eachcontributionisaddedinquadratureto obtaintheﬁnalcorrelateduncertaintyof10.6%to14.5%aslistedin Table 1.TheuncertaintyontheJESis2.4%to3.2%andcanbe ob-tainedbydividingtheuncertaintieslistedinTable 1by4.5,where theexponentn

=

4

.

5 wasobtainedbyﬁttinga powerlawto the measuredspectrum.

Theshapeuncertaintyisdominatedbytheregularizationused in the unfolding and can be divided into two components: the method by which the solution is regularized, e.g.

χ

2 _instead _of theSVDunfolding,andthevariationoftheregularizationprocess within a given method. The regularization is done by adding a penalty terminthe

χ

2 _method _and_by_ignoring _the _components oftheSVDdecompositionthataredominatedbystatistical ﬂuctua-tions.FortheSVDmethod,theregularizationk factorisaninteger valueandthuscanonlybevariedinintegersteps.Theuncertainty relatedtothechoiceofthepriorisestimatedbyvaryingthe expo-nent ofthe powerlawfunction extractedfromthereconstructed spectrum by

±

0

.

5,which isused toconstruct theprior.The un-certaintyrelatedtothechoiceofthepriorisestimatedbyvarying the exponentn

=

4

.

5 by

±

0

.

5 toscale the prior.The differences intheunfoldedspectrumwiththesevariationsaresummarizedin Table 1. These variations in the regularization strategy are com-bined assuming that they constitute independent measurements. Theﬁnalshape uncertaintyisthusobtainedbysummingthemin quadratureanddividingbythesquarerootofthenumberof vari-ations.

The jet spectrum in pp collisions was measured in the same wayasreportedinRef.[54],butwiththe5 GeV

/

c leadingcharged particlerequirementnecessaryforthePb–Pbanalysis. The result-ing spectrum normalized per inelastic pp collision is shown in Fig. 2. In order to determine the effect of the leading track re-quirement in pp collisions, the ratio of the jet spectra with a 5 GeV

/

c leading track requirement (the biased jet sample), over thespectrumofjetswithoutaleading trackrequirement(the in-clusivejetsample)withresolutionparameter R

=

0

.

2 isshownin Fig. 3.Systematicuncertaintiesintheratiowere evaluatedby re-moving theuncertainties that are correlatedbetweenthe spectra obtainedwithandwithoutthecutontheleading particle.Ascan be seeninFig. 3,for pT,jet above 50 GeV

/

c morethan 95%ofall reconstructed jetshaveatleastone trackwitha pT greater than 5 GeV/c. PYTHIA tune A (but also other common tunes like the Perugiatunes[57])accuratelydescribes themeasuredratio.

(7)

Fig. 3. Ratio

of the jet spectrum with a leading track

pT>5 GeV/c over

the inclusive

jet spectrum for R=0.2 in pp collisions at √s=2.76 TeV.

The inﬂuence of the leading track requirement in the Pb–Pb measurement,nominally setto 5 GeV

/

c was tested byvarying it by 40%, i.e. reducing it to 3 and increasing it to 7 GeV

/

c, and withthemoreextremevaluesof0 and10 GeV

/

c.Theratiosofjet spectrawiththedifferentleading trackpT biases,afterall correc-tions,areshowninFig. 4for R

=

0

.

2 jetsin0–10%centralPb–Pb collisions at

√

sNN

=

2

.

76 TeV. The corrections to these different jetspectrawere done usingthesameunfoldingprocedure asthe nominalspectrum with leading track pT bias of 5 GeV/c,witha slightly modiﬁed response matrix which accounts for the differ-entbiases.Sincetheunfoldingprocedureweakensthecorrelation betweenthe statisticalﬂuctuationsofthe jet spectrawith differ-ent leading track requirements, the statistical uncertainties have beenadded inquadrature inthe ratio.The systematicshape un-certaintyisduetotheunfoldingprocedure,andhasbeentreated ascompletelyuncorrelatedintheratio.Thecorrelateduncertainty isprimarilyduetotheuncertaintyontheJES,whichishighly cor-relatedbetweenthe various spectra.The systematicvariations in the unfolding procedure have been applied consistently for both thedenominator(withaleadingtrack pT

>

5 GeV

/

c) andthe nu-merators (with a 0, 3, 7 and 10 GeV

/

c leading track bias), and theresultingdifferenceinthe ratioshasbeentakenasa system-atic uncertainty. The jet spectra withleading trackrequirements of 3 and0 GeV

/

c are consistent withthe baseline measurement

witha5 GeV

/

c requirement.Theunfoldingisnotasstableaswith a 5 GeV

/

c requirement,whichleads toalarger systematic uncer-taintyduetotheunfoldingcorrectionprocedure,especiallyforthe inclusive spectrum. All measurements of the ratio of jet spectra withdifferentleadingtrackbiases,particularlythosewithahigher leadingtrackpT requirementthanthenominal,arewelldescribed by PYTHIA 6 (tune A), within one sigmaof the uncertainties or less.

The nuclearmodificationfactor, RAA,isdefinedastheratioof the jet spectrum in Pb–Pb divided by the spectrum in pp colli-sions scaledby Ncoll.Itisconstructed suchthat RAA equals unity if thereis nonet nuclear modificationof thespectrum inPb–Pb collisions ascomparedtoan incoherentsuperpositionof indepen-dentpp collisions.TheresultingRAAofjetswitha5 GeV

/

c leading track requirement for R

=

0

.

2 in the 0–10% and10–30% central Pb–PbcollisionsisreportedinFig. 5.Thesystematicandstatistical uncertainties from the Pb–Pb and pp measurements (see Fig. 2) are added in quadrature. The resulting uncertainty on the nor-malization is fromscaling the pp cross section with the nuclear overlap TAA

=

23

.

5

±

0

.

87 mb−1 for0–10%and11

.

6

±

0

.

60 mb−1 for10–30%collisions. As canbe seen,jetsinthemeasured pT,jet range are strongly suppressed. The average RAA in both 0–10% and 10–30% central eventswas found to have a negligible pT,jet dependence. In the 10% mostcentral events, combining the sta-tisticalandsystematicuncertaintyinquadrature,theaverage RAA is found to be 0

.

28

±

0

.

04. The suppression is smaller in mag-nitude in the 10–30% central events, leading to an average RAA of0

.

35

±

0

.

04.Theseresultsqualitativelyagreewiththe suppres-sionobtainedfrommeasurementsusingcharged-particlejets[46], thoughthejetenergyscaleisnotthesameinbothcases,andsoa directcomparisonisnotpossible.Furthermore,theresultsare con-sistent withthe RAA reportedbyATLAS forR

=

0

.

4 jetsscaled by theratiooftheyields withthedifferentresolution parametersin different pT,jetbins[36,41].

Inordertointerprettheresultsandmovetoamore quantita-tive understandingofjetquenchingmechanisms,acomparisonof themeasured RAA in0–10%centralcollisionstocalculationsfrom two differentmodelsisalsoshownin Fig. 5.Theﬁrstmodel, Ya-JEM[61],usesa2

+

1DhydrodynamicalcalculationandaGlauber MC for theinitial geometry,as well asa LOpQCD calculation to determine the outgoingpartons. Parton showersare modified by a medium-induced increase of the virtuality during their evolu-tion throughthemedium. TheLund modelinPYTHIAisusedfor hadronization intofinal state particles.The kinematicsofthe vir-tual partons intheevolving partonic shower weremodified with aparameterrelatedtothetwotransportcoefficients,q and

ˆ

e,

ˆ

that describeshowstronglyapartonofagivenmomentumcouplesto

Fig. 4. Ratios

of jet spectra with different leading track

pTrequirements (“0 over 5”, “3 over 5”, “7 over 5” and “10 over 5”) for R=0.2 jets in 0–10% Pb–Pb collisions at √_s

(8)

Fig. 5. RAAfor R=0.2 jets with the leading track requirement of 5 GeV/c in

0–10% (left) and 10–30% (right) most central Pb–Pb collisions compared to calculations from

YaJEM [61]and JEWEL [62]. The boxes at RAA=1 represent the systematic uncertainty on TAA.

themedium.Theparameterwasﬁxedsothatthemodelaccurately describesthe RAA for chargedhadrons at10 GeV/c [17], butno additionalchanges were made forthe prediction of the jet RAA. Thesecond model,JEWEL[62],takes a differentapproach inthe descriptionof theparton–medium interaction by givinga micro-scopicaldescriptionofthetransportcoeﬃcient,q.

ˆ

Essentiallyeach scatteringoftheinitialpartonwithmediumpartonsiscomputed andtheaverageoverallscattersdeterminesq.

ˆ

JEWELusesa com-binationofGlauberandPYTHIAtodeterminetheinitialgeometry, a 1D Bjorken expansion for the medium evolution, and PYTHIA forhadronizationintofinalstateparticles.Thetransversemedium densityprofileinJEWELisproportionaltothedensityofwounded nucleonscombinedwitha1DBjorkenexpansionforthetime evo-lution.Hardscatters aregenerated accordingto Glauber collision geometry,andsufferfromelasticandradiativeenergylossinthe medium,includingaMonte CarloimplementationofLPM interfer-enceeffects. PYTHIA is used for the hadronization of final state particles.Despitetheir differentapproaches, bothcalculationsare foundtoreproducethejetsuppression.YaJEM,however,exhibitsa slightlysteeper increasewithjet pT thanthedata.Thecalculated

χ

2 _are ₁

_.

_{690 for} _YaJEM _and ₀

_.

_{368 for} _JEWEL, _obtained_by com-paringthemodelswiththedata.Additionalmeasurementswillbe neededinordertofurtherconstrainthemodels,suchasmeasuring thejetsuppressionrelativetotheeventplaneangle,whichwould requireamoreaccurate modelingofthe path-lengthdependence ofjetquenching.

6. Summary

The transverse momentum (pT) spectrum and nuclear modi-ﬁcation factor (RAA) of jetsreconstructed from charged particles measuredby the ALICEtrackingsystemandneutralenergy mea-suredbytheALICEElectromagneticCalorimeteraremeasuredwith

R

=

0

.

2 in the range of 40

<

pT,jet

<

120 GeV

/

c for 0–10% and in30

<

pT,jet

<

100 GeV

/

c for 10–30% most central Pb–Pb colli-sionsat

√

sNN

=

2

.

76 TeV were measured.The jetswererequired to contain at least one charged particle with pT

>

5 GeV

/

c. The effectofthis requirementonthe reported RAA was evaluated by theratios ofthe jet spectrawiththe 5 GeV/c to no requirement compared to expectations on PYTHIA, andfound not to have an observableeffectwithintheuncertaintiesofthemeasurement.Jets with40

<

pT,jet

<

120 GeV

/

c arestronglysuppressed inthe 10% mostcentralevents,with RAA about0

.

28

±

0

.

04,independentof pT,jet within theuncertainties ofthe measurement.The suppres-sionin10–30%eventsis0

.

35

±

0

.

04,slightlylessthaninthemost

centralevents.Theobservedsuppressionisinfairagreementwith expectationsfromtwojetquenchingmodelcalculations.

Acknowledgements

The ALICE Collaboration would like to thank all its engineers andtechniciansfortheir invaluablecontributions tothe construc-tionoftheexperimentandtheCERNacceleratorteamsforthe out-standingperformanceoftheLHCcomplex.TheALICECollaboration gratefully acknowledges the resources and support provided by all Gridcenters and theWorldwide LHC ComputingGrid (WLCG) collaboration. The ALICE Collaboration acknowledges the follow-ing funding agencies for their support in building and running theALICEdetector:StateCommittee ofScience,WorldFederation of Scientists (WFS)and SwissFonds Kidagan, Armenia; Conselho Nacional de DesenvolvimentoCientíﬁco e Tecnológico (CNPq), Fi-nanciadorade Estudos eProjetos(FINEP),Fundação de Amparoà Pesquisa do Estado de São Paulo (FAPESP); National Natural Sci-enceFoundation ofChina (NSFC), theChinese Ministryof Educa-tion(CMOE)andtheMinistryofScienceandTechnologyofChina (MSTC); Ministry of Education andYouth ofthe Czech Republic; Danish Natural Science Research Council, the Carlsberg Founda-tion andthe DanishNationalResearch Foundation;TheEuropean ResearchCouncilundertheEuropeanCommunity’sSeventh Frame-work Programme; Helsinki Institute of Physics andthe Academy ofFinland;FrenchCNRS–IN2P3,the‘RegionPaysdeLoire’,‘Region Alsace’, ‘Region Auvergne’ andCEA, France; German Bundesmin-isterium fur Bildung, Wissenschaft, Forschung und Technologie (BMBF)andtheHelmholtzAssociation;GeneralSecretariatfor Re-searchandTechnology,MinistryofDevelopment,Greece; Hungar-ianOrszagosTudomanyosKutatasiAlappgrammok(OTKA)and Na-tionalOﬃceforResearch andTechnology (NKTH); Departmentof Atomic EnergyandDepartmentofScience andTechnologyofthe Governmentof India;Istituto Nazionale diFisica Nucleare(INFN) and Centro Fermi – Museo Storico della Fisica e Centro Studi e Ricerche“EnricoFermi”,Italy;MEXTGrant-in-AidforSpecially Pro-motedResearch,Japan;JointInstituteforNuclearResearch,Dubna; National Research Foundation of Korea (NRF); Consejo Nacional de Cienca y Tecnologia (CONACYT), Direccion General de Asun-tos del Personal Academico (DGAPA), México, Amerique Latine Formation academique–European Commission (ALFA–EC) and the EPLANETProgram(EuropeanParticle PhysicsLatin American Net-work); StichtingvoorFundamenteelOnderzoekderMaterie(FOM) andtheNederlandseOrganisatievoorWetenschappelijkOnderzoek

(9)

(NWO),Netherlands; ResearchCouncil ofNorway (NFR); National ScienceCentre ofPoland;MinistryofNationalEducation/Institute forAtomic PhysicsandConsiliul Na ¸tionalal Cercet˘arii ¸Stiin ¸tiﬁce– Executive AgencyforHigherEducationResearchDevelopmentand Innovation Funding (CNCS–UEFISCDI), Romania; Ministry of Ed-ucation and Science of Russian Federation, Russian Academy of Sciences, Russian Federal Agency ofAtomic Energy, Russian Fed-eralAgencyforScience andInnovationsandThe Russian Founda-tionforBasicResearch;MinistryofEducationofSlovakia; Depart-mentofScienceandTechnology,RepublicofSouthAfrica; Centro de Investigaciones Energeticas, Medioambientales y Tecnologicas (CIEMAT),E-InfrastructuresharedbetweenEuropeandLatin Amer-ica(EELA), MinisteriodeEconomíayCompetitividad(MINECO)of Spain,XuntadeGalicia(ConselleríadeEducación),Centrode Apli-cacionesTecnológicasyDesarrolloNuclear(CEADEN),Cubaenergía, Cuba, and IAEA (International Atomic Energy Agency); Swedish Research Council (VR) and Knut & Alice Wallenberg Foundation (KAW); Ukraine Ministry of Education andScience; United King-domScienceandTechnology FacilitiesCouncil (STFC);TheUnited States Department of Energy, the United States National Science Foundation,the StateofTexas,andtheState ofOhio;Ministry of Science,EducationandSportsofCroatiaandUnitythrough Knowl-edge Fund, Croatia. Council of Scientiﬁc and Industrial Research (CSIR),NewDelhi,India.

References

[1] HotQCD Collaboration, T. Bhattacharya, M.I. Buchoff, N.H. Christ, et al., QCD phase transition with chiral quarks and physical quark masses, Phys. Rev. Lett. 113 (Aug 2014) 082001, arXiv:1402.5175 [hep-lat], http://link.aps.org/doi/ 10.1103/PhysRevLett.113.082001.

[2]BRAHMSCollaboration,I.Arsene,etal.,Quark–gluonplasmaandcolorglass condensateatRHIC?TheperspectivefromtheBRAHMSexperiment,Nucl.Phys. A757 (1–2)(2005)1–27.

[3]PHOBOSCollaboration,B.Back,etal.,ThePHOBOSperspectiveondiscoveries atRHIC,Nucl.Phys.A757 (1–2)(2005)28–101.

[4]PHENIXCollaboration,K.Adcox,etal.,Formationofdensepartonicmatterin relativisticnucleus–nucleuscollisionsatRHIC:experimentalevaluationbythe PHENIXCollaboration,Nucl.Phys.A757 (1–2)(2005)184–283.

[5]STARCollaboration,J. Adams,etal.,Experimentalandtheoretical challenges inthesearchforthequark–gluonplasma:theSTARCollaboration’scritical as-sessmentoftheevidencefromRHICcollisions,Nucl.Phys.A757 (1–2)(2005) 102–183.

[6]ALICECollaboration,K.Aamodt,etal.,Charged-particlemultiplicitydensityat mid-rapidityincentral Pb–Pbcollisionsat√sNN=2.76 TeV,Phys.Rev.Lett.

105(2010)252301,arXiv:1011.3916[nucl-ex].

[7]ALICECollaboration,K.Aamodt,etal.,Centralitydependenceofthe charged-particle multiplicity density at mid-rapidityinPb–Pb collisions at √sNN=

2.76 TeV,Phys.Rev.Lett.106(2011)032301,arXiv:1012.1657[nucl-ex]. [8]CMSCollaboration,S. Chatrchyan,et al.,Dependence onpseudorapidityand

centrality of chargedhadron production inPb–Pb collisions at a nucleon– nucleoncentre-of-massenergyof2.76 TeV,J.HighEnergyPhys.1108(2011) 141,arXiv:1107.4800[nucl-ex].

[9]ALICECollaboration,K.Aamodt,etal.,Two-pionBose–Einsteincorrelationsin centralPb–Pbcollisionsat√sNN=2.76 TeV,Phys.Lett.B696(2011)328–337,

arXiv:1012.4035[nucl-ex].

[10]ALICECollaboration,K.Aamodt,etal.,EllipticﬂowofchargedparticlesinPb– Pbcollisionsat2.76 TeV,Phys.Rev.Lett.105(2010)252302,arXiv:1011.3914 [nucl-ex].

[11]ATLAS Collaboration,G.Aad,et al.,Measurementofthepseudorapidityand transversemomentumdependenceoftheellipticﬂowofchargedparticlesin lead–leadcollisionsat√sNN=2.76 TeV withtheATLASdetector,Phys.Lett.B

707(2012)330–348,arXiv:1108.6018[hep-ex].

[12]CMSCollaboration,S.Chatrchyan,etal.,Centralitydependenceofdihadron cor-relationsandazimuthalanisotropyharmonicsinPb–Pbcollisionsat√sNN=

2.76 TeV,Eur.Phys.J.C72(2012)2012,arXiv:1201.3158[nucl-ex].

[13]ALICECollaboration,K.Aamodt,etal.,Higherharmonicanisotropicﬂow mea-surementsofchargedparticlesinPb–Pbcollisionsat√sNN=2.76 TeV,Phys.

Rev.Lett.107(2011)032301,arXiv:1105.3865[nucl-ex].

[14]ATLASCollaboration,G.Aad,etal.,Measurementofthedistributionsof event-by-eventﬂowharmonicsinlead–leadcollisionsat 2.76 TeVwiththeATLAS detector attheLHC,J.HighEnergyPhys.1311(2013)183,arXiv:1305.2942 [hep-ex].

[15]CMSCollaboration, S. Chatrchyan,et al., Measurement ofhigher-order har-monicazimuthalanisotropyinPb–Pbcollisionsat√sNN=2.76 TeV,Phys.Rev.

C89(2014)044906,arXiv:1310.8651[nucl-ex].

[16]ALICECollaboration, K.Aamodt, etal., Suppression ofchargedparticle pro-ductionatlargetransversemomentumincentralPb–Pbcollisionsat√sNN=

2.76 TeV,Phys.Lett.B696(2011)30–39,arXiv:1012.1004[nucl-ex]. [17]CMSCollaboration,S.Chatrchyan,etal.,Observationandstudiesofjet

quench-inginPb–Pbcollisionsatnucleon–nucleoncenter-of-massenergyof2.76 TeV, Phys.Rev.C84(2011)024906,arXiv:1102.1957[nucl-ex].

[18]M.Gyulassy,M.Plumer,Jetquenchingindensematter,Phys.Lett.B243(1990) 432–438.

[19]R.Baier,Y.L.Dokshitzer,S.Peigne,D.Schiff,InducedgluonradiationinaQCD medium,Phys.Lett.B345(1995)277–286,arXiv:hep-ph/9411409.

[20]PHENIXCollaboration,K.Adcox,etal.,Suppressionofhadronswithlarge trans-versemomentumincentralAu+Aucollisionsat√sNN=130 GeV,Phys.Rev.

Lett.88(2002)022301,arXiv:nucl-ex/0109003.

[21]STARCollaboration, C. Adler, et al., Disappearance ofback-to-back high pT

hadroncorrelationsincentralAu+Aucollisionsat√sNN=200 GeV,Phys.Rev.

Lett.90(2003)082302,arXiv:nucl-ex/0210033.

[22]PHENIXCollaboration,K. Adcox,etal.,Centralitydependenceofthehigh-pT

chargedhadronsuppressioninAu+Aucollisionsat √sNN=130 GeV,Phys.

Lett.B561(2003)82–92,arXiv:nucl-ex/0207009.

[23]PHENIX Collaboration,S.S. Adler,et al., Suppressedπ0 _production _at _large

transversemomentumincentralAu+Aucollisionsat√sNN=200 GeV,Phys.

Rev.Lett.91(2003)072301,arXiv:nucl-ex/0304022.

[24]STAR Collaboration, J. Adams, et al., Transverse-momentum and collision-energydependence ofhigh-pT hadron suppressioninAu+Aucollisionsat

ultrarelativisticenergies,Phys.Rev.Lett.91(Oct2003)172302,arXiv:nucl-ex/ 0305015.

[25]STARCollaboration,J.Adams,etal.,Evidencefromd+Aumeasurementsfor ﬁnalstatesuppressionofhigh-pThadronsinAu+AucollisionsatRHIC,Phys.

Rev.Lett.91(2003)072304,arXiv:nucl-ex/0306024.

[26]PHOBOSCollaboration,B.Back,etal.,Chargedhadrontransversemomentum distributionsinAu+Aucollisionsat√sNN=200 GeV,Phys.Lett.B578(2004)

297–303,arXiv:nucl-ex/0302015.

[27]BRAHMS Collaboration, I. Arsene, et al., Transverse momentum spectra in Au+Auand d+Au collisionsat √sNN=200 GeV and the pseudorapidity

dependenceofhigh-pTsuppression,Phys.Rev.Lett.91(2003)072305,arXiv:

nucl-ex/0307003.

[28]PHENIXCollaboration,A.Adare,etal.,Systemsizeandenergydependenceof jet-inducedhadronpaircorrelationshapesinCu+CuandAu+Aucollisions at√sNN=200 and62.4 GeV,Phys.Rev.Lett.98(2007)232302,arXiv:nucl-ex/

0611019.

[29]PHENIXCollaboration,A.Adare,etal.,Quantitativeconstraintsontheopacity ofhotpartonicmatterfromsemi-inclusivesinglehightransversemomentum pionsuppressioninAu+Aucollisionsat √sNN=200 GeV,Phys. Rev.C77

(2008)064907,arXiv:0801.1665[nucl-ex].

[30]ATLASCollaboration,G.Aad,etal.,Observationofacentrality-dependentdijet asymmetryinPb–Pbcollisionsat√sNN=2.76 TeV withtheATLASdetectorat

theLHC,Phys.Rev.Lett.105(2010)252303,arXiv:1011.6182[hep-ex]. [31]ALICECollaboration,K.Aamodt,etal.,Particle-yieldmodiﬁcationinjet-like

az-imuthaldi-hadroncorrelationsinPb–Pbcollisionsat√sNN=2.76 TeV,Phys.

Rev.Lett.108(2012)092301,arXiv:1110.0121[nucl-ex].

[32]CMSCollaboration,S.Chatrchyan,etal.,Studyofhigh-pTchargedparticle

sup-pressioninPb–Pbcomparedtopp collisionsat√sNN=2.76 TeV,Eur.Phys.J.

C72(2012)1945,arXiv:1202.2554[nucl-ex].

[33]CMSCollaboration, S. Chatrchyan,et al., Jet momentumdependence of jet quenchinginPb–Pbcollisionsat √sNN=2.76 TeV,Phys. Lett.B712(2012)

176–197,arXiv:1202.5022[nucl-ex].

[34]CMSCollaboration,S.Chatrchyan,etal.,Measurementofjetfragmentationinto chargedparticlesinpp andPb–Pbcollisionsat√sNN=2.76 TeV,J.High

En-ergyPhys.1210(2012)087,arXiv:1205.5872[nucl-ex].

[35]CMS Collaboration, S. Chatrchyan, et al., Studies of jet quenching using isolated-photon+jet correlations in Pb–Pb and pp collisions at √sNN=

2.76 TeV,Phys.Lett.B718(2013)773–794,arXiv:1205.0206[nucl-ex]. [36]ATLASCollaboration,G.Aad,etal.,Measurementofthejetradiusand

trans-versemomentumdependenceofinclusivejetsuppressioninlead–lead colli-sionsat√sNN=2.76 TeV withtheATLASdetector,Phys.Lett.B719(2013)

220–241,arXiv:1208.1967[hep-ex].

[37]CMSCollaboration,S.Chatrchyan,etal.,Evidenceofb-jetquenchinginPb–Pb collisionsat√sNN=2.76 TeV,Phys.Rev.Lett.113 (13)(2014)132301,arXiv:

1312.4198[nucl-ex].

[38]CMSCollaboration, S. Chatrchyan, et al., Modiﬁcationof jet shapesin Pb– Pbcollisionsat √sNN=2.76 TeV,Phys. Lett.B730(2014)243–263, arXiv:

1310.0878[nucl-ex].

[39]CMSCollaboration,S.Chatrchyan,etal.,Measurementofjetfragmentationin Pb–Pbandppcollisionsat√sNN=2.76 TeV,arXiv:1406.0932[nucl-ex].

[40]ATLAS Collaboration, G. Aad, et al., Measurement of inclusivejet charged-particlefragmentationfunctionsinPb+Pbcollisionsat√sNN=2.76 TeV with

(10)

[41]ATLASCollaboration,G.Aad,etal.,Measurementsofthenuclearmodiﬁcation factorforjetsinPb–Pbcollisionsat√sNN=2.76 TeV withtheATLASdetector,

arXiv:1411.2357[hep-ex].

[42]C.A.Salgado,U.A.Wiedemann,Mediummodiﬁcationofjetshapesandjet mul-tiplicities,Phys.Rev.Lett.93(2004)042301,arXiv:hep-ph/0310079. [43]I.Vitev,Largeanglehadroncorrelationsfrommedium-inducedgluonradiation,

Phys.Lett.B630(2005)78–84,arXiv:hep-ph/0501255.

[44]M.Cacciari,J.Rojo,G.P.Salam,G.Soyez,Jetreconstructioninheavyion colli-sions,Eur.Phys.J.C71(2011)1539,arXiv:1010.1759[hep-ph].

[45]ALICE Collaboration, B. Abelev, et al., Measurement of event background ﬂuctuations for charged particle jet reconstruction in Pb–Pb collisions at

√_s

NN=2.76 TeV, J. High Energy Phys. 1203 (2012) 053, arXiv:1201.2423

[hep-ex].

[46]ALICECollaboration,B.Abelev,etal.,Measurementofchargedjetsuppression inPb–Pbcollisionsat√sNN=2.76 TeV,J.HighEnergyPhys.1403(2014)013,

[47]M.Cacciari,G.P.Salam,G.Soyez,Theanti-k(t)jetclusteringalgorithm,J.High EnergyPhys.0804(2008)063,arXiv:0802.1189[hep-ph].

[48]ALICECollaboration,K.Aamodt,etal.,TheALICEexperimentattheCERNLHC, J.Instrum.3(2008),S08002.

[49]ALICECollaboration,B.B.Abelev,etal.,PerformanceoftheALICEexperiment at theCERN LHC,Int.J.Mod. Phys.A 29(2014)1430044,arXiv:1402.4476 [nucl-ex].

[50]ALICECollaboration,B.Abelev,et al.,CentralitydeterminationofPb–Pb col-lisionsat √sNN=2.76 TeV with ALICE,Phys. Rev.C88 (4)(2013)044909,

[51]ALICECollaboration,B.Abelev,etal.,Centralitydependenceofcharged parti-cleproductionatlargetransversemomentuminPb–Pbcollisionsat√sNN=

2.76 TeV,Phys.Lett.B720(2013)52–62,arXiv:1208.2711[hep-ex].

[52] R. Brun, F. Carminati, S. Giani, GEANT detector description and simulation tool, CERN Program Library Long Write-up, W5013, 1994.

[53]X.-N.Wang,M.Gyulassy,HIJING:aMonte Carlomodelformultiplejet produc-tioninpp,pAandAAcollisions,Phys.Rev.D44(1991)3501.

[54]ALICECollaboration,B.Abelev,etal.,Measurementoftheinclusivedifferential jet crosssectioninpp collisionsat√s=2.76 TeV,Phys.Lett.B722(2013) 262–272,arXiv:1301.3475[nucl-ex].

[55]M.Cacciari,G.P.Salam,G.Soyez,FastJetusermanual,Eur.Phys.J.C72(2012) 1896,arXiv:1111.6097[hep-ph].

[56]T.Sjostrand,S.Mrenna,P.Z.Skands,PYTHIA6.4physicsandmanual,J.High EnergyPhys.0605(2006)026,arXiv:hep-ph/0603175.

[57]P.Z.Skands,TuningMonteCarlogenerators:thePerugiatunes,Phys.Rev.D82 (2010)074018,arXiv:1005.3457[hep-ph].

[58] C. Verkerk, CERN School of Computing, Aiguablava, Spain, 9–22 Sep 1984, Pro-ceedings.

[59]A. Hocker,V.Kartvelishvili, SVD approachtodataunfolding,Nucl. Instrum. MethodsA372(1996)469–481,arXiv:hep-ph/9509307.

[60]CERN, PHYSTAT 2011 Workshop on Statistical Issues Related to Discovery ClaimsinSearchExperimentsandUnfolding,CERN,Geneva,2011.

[61]T.Renk,PhysicsprobedbythePTdependenceofthenuclearsuppression fac-tor,Phys.Rev.C88 (1)(2013)014905,arXiv:1302.3710[hep-ph].

[62]K.C.Zapp,F.Krauss,U.A.Wiedemann,Aperturbativeframeworkforjet quench-ing,J.HighEnergyPhys.1303(2013)080,arXiv:1212.1599[hep-ph].