Study of jet quenching with isolated-photon plus jet correlations in PbPb and pp collisions at root s(NN)=5.02 TeV

Contents lists available atScienceDirect

Physics

Letters

B

www.elsevier.com/locate/physletb

Study

of

jet

quenching

with

isolated-photon

+

jet

correlations

in

PbPb

and

pp

collisions

at

√

s

_NN

=

5

.

02 TeV

.

The

CMS

Collaboration



CERN,Switzerland

a

r

t

i

c

l

e

i

n

f

o

a

b

s

t

r

a

c

t

Articlehistory:

Received27November2017 Receivedinrevisedform25July2018 Accepted26July2018

Availableonline15August2018 Editor:M.Doser Keywords: CMS Heavyion Physics Photon Jet Jetquenching

Measurements ofazimuthalangleandtransversemomentum(pT)correlationsofisolatedphotonsand

associatedjetsarereportedforpp andPbPb collisionsat√sNN=5.02 TeV.Thedatawererecordedwith

theCMSdetectorattheCERNLHC.Foreventscontainingaleadingisolatedphotonwithpγ_T>40 GeV/c

and anassociatedjetwithpjet_T >30 GeV/c,thephoton+jetazimuthalcorrelation and pTimbalancein

PbPb collisionsare studiedasfunctions ofcollisioncentralityand pγ_T.Theresultsare comparedtopp referencedatacollectedatthesamecollisionenergyandtopredictionsfromseveraltheoreticalmodels forpartonenergyloss.Noevidenceofbroadeningofthephoton+jetazimuthalcorrelationsisobserved, while the ratio pjet_T/pγ_T decreases significantlyfor PbPb datarelative to the pp reference. Allmodels considered agree within uncertainties withthe data. The number ofassociated jets perphoton with

pγ_T>80 GeV/c isobservedtobeshiftedtowardslower pjet_T valuesincentralPbPb collisionscompared topp collisions.

©2018TheAuthor.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3_.

1. Introduction

Quantum chromodynamics predicts that in relativistic heavy ion collisions a state of deconfined quarks and gluons known as the quark–gluon plasma (QGP) can be formed [1,2]. Parton scatterings with large momentum transfer, which occur very early (

≈

0

.

1 fm

/

c) compared to the timescale of QGP formation (

≈

1 fm

/

c), provide tomographic probes of the plasma [3]. The outgoing partons interact strongly with the QGP and lose en-ergy [4–9].Thisphenomenon,knownas“jetquenching”,hasbeen observedthrough measurements ofhadronswithhightransverse momentum(pT) [10–15] andofjets [16–22],bothcreatedbythe

fragmentationofthehigh-momentumpartons.

Since electroweak bosons do not interact strongly with the QGP [23–26], measurements of jets produced in the same hard scattering in conjunction with thesebosons have, in contrast to dijetmeasurements, a controlled configurationof theinitial hard scattering [27–29].The electroweakboson pT reflects,onaverage,

theinitialenergyoftheassociatedpartonthatfragmentsintothe jet,beforeanymedium-inducedenergylosshasoccurred [30,31]. At LHC energies,the production ofjetswith pT

>

30 GeV

/

c that

are associated with electroweak bosons is dominated by quark

 _{E-mailaddress:cms-publication-committee-chair@cern.ch.}

fragmentation [32]. Hence, the study ofcorrelations in boson-jet events, such as the azimuthal angle (

φ

) difference and pT ratio

between theboson and the associatedjets, opens the possibility forin-depthstudies ofthepartonenergylossmechanisms utiliz-ingtheoreticallywell-controlledinitialproductionprocesses.These studies also facilitate the extraction of QGP properties via com-parisonswiththeoreticalmodels [31,33–37].Measurementsofthis kindwerefirstperformedinPbPb collisionsatanucleon–nucleon center-of-massenergy

√

sNN

=

2

.

76 TeV withisolated-photon

+

jet

events [38] and at 5.02 TeV with Z-jet events [39] by the CMS Collaboration at the CERN LHC. The precision of these previous measurements was limited by the available number ofboson-jet pairs.

In theresultsreportedinthispaper,the electroweakbosonis an isolated photon, whichis selectedexperimentally by usingan isolationrequirement,namelythattheadditionalenergyinacone offixedradiusaroundthedirectionofthereconstructedphotonis lessthana specifiedvalue [23,24]. Thisrestriction suppressesthe backgroundcontributionsfromphotonsoriginatingfromdecaysof neutral mesons (“decay photons”), and gives a sample contain-ingmostlypromptphotons.Promptphotonsarephotonsproduced directlyinthehardscatteringprocess,oremittedinthe fragmen-tation ofa high-pT parton(“fragmentation photons”). This Letter

reports the measurement ofcorrelations of isolated photonsand associatedjetsinPbPb andpp collisionsat

√

sNN

=

5

.

02 TeV. The https://doi.org/10.1016/j.physletb.2018.07.061

0370-2693/©2018TheAuthor.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP3_.

energyloss through the

φ

and pT correlations between isolated

photons and the associated jets. The azimuthal angle difference

φ

jγ

= |φ

jet

−φ

γ

|

,thepTratioxjγ

=

pjetT

/

p

γ

T anditsaverage



xjγ



,

theaveragenumberofassociatedjetsperphoton, Rjγ ,andthe

ra-tiooftheyieldofassociatedjetsinPbPb datatopp data,Ijet_AA,are presented.TheresultsfromPbPb collisionsarecomparedtothose frompp collisions,withthepp data servingasareferenceto ex-tractinformationaboutthe modifications dueto the presenceof theQGP.

2. TheCMSdetector

The central feature of the CMS apparatus is a superconduct-ingsolenoidof 6 m internaldiameter,providing amagnetic field of3.8 T.Withinthesolenoidvolumeare asilicon pixelandstrip tracker which measures charged particles within the pseudora-pidity range

|

η

|

<

2

.

5, a lead tungstate crystal electromagnetic calorimeter(ECAL), anda brass andscintillator hadron calorime-ter(HCAL),each composed of a barreland two endcapsections. Thebarrelandendcapcalorimetersprovide

|

η

|

coverageoutto3. Photoncandidatesusedinthisanalysisarereconstructedusingthe energydepositedinthebarrelregionoftheECAL,whichcoversa rangeof

|

η

|

<

1

.

48.Hadronforward(HF)calorimetersextendthe

|

η

|

coverage of theHCAL to

|

η

|

=

5

.

2. In PbPb collisions,the HF calorimetersareusedtodeterminethecentralityofthecollisions, whichisrelatedto theimpact parameterofthe twocollidingPb nuclei [16],andtheazimuthal angleofmaximumparticledensity (theeventplane) [40].Muonsaredetectedingas-ionization cham-bersembedded inthesteelflux-returnyokeoutsidethesolenoid. Amoredetaileddescriptionof theCMSdetector,together witha definition of the coordinate system used and the relevant kine-maticvariables,canbefoundinRef. [41].

3. Analysisprocedure 3.1.Eventselection

Events containing high-pT photon candidates are selected by

theCMStriggersystem,whichconsistsofalevel-1(L1)anda high-level trigger (HLT) [42]. Events are first selected by requiring an ECALtransverseenergydepositlargerthan21 (20) GeV duringthe PbPb (pp)data-taking period.Photon candidates are then recon-structedattheHLTusingthe“island”clusteringalgorithm [24,43], whichisappliedtoenergydepositsintheECAL.TheHLTselection efficiency was determined in data and was found to be greater than98% foreventscontaining aphotonwithpγ_T

>

40 GeV

/

c and

|

η

γ

_|

_<

₁

_.

_{44 reconstructedoffline. The}

_η

_{γ interval ofthe photons}

usedinthisanalysisisrestrictedtothebarrelregionoftheECAL, whichhasthebestperformanceintermsofphotonreconstruction andtriggeringandhasthelowestrateofmisreconstructedtracks.

Apure sample ofinelastic hadronicpp and PbPb collisionsis obtainedwithfurtherofflineselectioncriteriaappliedtothe trig-geredevents [16,44].Notableamongthese, areconstructedevent vertexand at least three (one) calorimeter towers in the HF on each side of the interaction point with energy

>

3 GeV are (is) required in the PbPb (pp) analysis. Events with spurious energy depositionsin the HCAL(i.e., sporadicuncharacteristic noise and signalsfrommalfunctioning calorimeterchannels) arerejectedby established algorithms that flag such events, to remove possible contamination ofthe jet sample [45]. Events withmultiple colli-sionshavea negligibleeffectonthemeasurementsincethe

aver-centiles of the distribution ofthe total energymeasured inboth HFcalorimeters.Theeventcentralityobservablecorrespondstothe fractionofthetotalinelastichadroniccrosssection,starting at0% forthemostcentralcollisions,i.e.,thosewiththesmallestimpact parameterandthelargestnuclearoverlap [16].

3.2. Jetreconstruction

Offlinejetreconstruction isperformedusingtheCMS particle-flow (PF)algorithm [46]. Bycombininginformationfromall sub-detectorsystems,thePFalgorithmidentifiesfinal-stateparticlesin an event, classifyingthem aselectrons, muons, photons, charged hadrons,orneutralhadrons.Toformjets,thesePFobjectsare clus-tered using the anti-kT sequential recombination algorithm

pro-videdinthe FastJet framework [47,48].Asmalljetradius param-eter of R

=

0

.

3 is chosen to minimize the effects of heavy ion backgroundfluctuations(

∼

10 GeV incentralPbPb collisions)and forconsistencywiththepreviousmeasurementat2.76 TeV [38].

For the PbPb data, the underlying background from soft col-lisions (i.e., the underlying event, UE) is subtracted during jet reconstruction by employing the iterative algorithm described in Ref. [49], using the same implementation as in the PbPb analy-sis of Ref. [16]. In pp collisions, jets are reconstructed without UE subtraction. For pp and PbPb samples, the reconstructed jet energies are corrected to the energies of final-state particle jets usingafactorizedmultistepapproach [50].Thecorrectionsare de-rivedusingsimulateddijetandphoton

+

jeteventsgeneratedwith the pythia 8.212 [51] (CUETP8M1 tune [52]) Monte Carlo (MC) eventgenerator which,for thecase ofPbPb corrections,are em-beddedintoasimulatedunderlyingbackgroundeventfrom hydjet 1.9 [53].Thebackgroundsimulationistunedtoreproducethe ob-servedcharged-particlemultiplicityandpTspectruminPbPb data.

Reconstructed jetsare requiredtohave

|

η

jet

_|

_<

₁

_.

_{6 and corrected} pjet_T

>

30 GeV

/

c, to ensure that the jet reconstruction efficiency andenergyresolution(JER)are wellunderstood,i.e., resultsfrom dataareinagreementwithexpectationsfromMC.

3.3. Photonreconstruction

Photoncandidatesarereconstructedfromclustersofenergy de-positedintheECAL.The “hybrid”algorithm usedfortheanalysis inpp collisionsisdetailedinRef. [43],whilethedescriptionofthe island clustering algorithm optimized for high-multiplicity PbPb collisionscanbefoundinRef. [24].

In order to reduce electron contamination, photon candidates are discarded if the differences in pseudorapidity and azimuthal angle between the photon candidate and any electron candidate trackwithpT

>

10 GeV

/

c are lessthan0.02 and0.15radians,

re-spectively [24].Thesematchingwindows areconservativechoices based onthe detectorangular resolution.The relatively large az-imuthal angle window allows for the curvature of the electron trajectories.Anomaloussignalscausedbytheinteractionofhighly ionizing particles directly with thesilicon avalanche photodiodes usedfortheECALbarrelreadoutareremoved usingthe prescrip-tion given in Ref. [24]. The energy of the reconstructed photons is corrected to account for the effects of the material in front of the ECAL and for the incomplete containment of the shower energy. For PbPb data, an additional correction is applied to ac-countforenergycontaminationfromtheUE.Themagnitudeofthe combinedenergycorrection forisolated photonsvariesfrom0to 10%,dependingonthecentralityofthecollisionand pγ_T.The

cor-Fig. 1. ThecentralitydependenceoftheshowershapevariableσηηforphotonswithpTγ>60 GeV/c.TheblackpointsshowthePbPb experimentalresults,theredhistograms arethesignaltemplatesfrom pythia+hydjetsimulations,andthegreenhistogramsarethebackgroundtemplatesobtainedfromanonisolatedsidebandregionindata.(For interpretationofthecolorsinthefigure(s),thereaderisreferredtothewebversionofthisarticle.)

rections are obtainedfrom simulated pythia and pythia

+

hydjet photonevents.

SimilartoRef. [54],a generator-levelphoton candidateis con-sidered isolated ifthe pT sum of final-state generated particles,

excludingneutrinos,inaconeofradius



R

=



(

η

)

2

_{+ (φ)}

2

₌

0

.

4 around the direction of the candidate, SumIso, is less than 5 GeV

/

c.Fora reconstructedphotoncandidate,thecorresponding isolation variable,SumIsoUE−sub_{,is calculatedwithrespect tothe}

centroidofthecluster,notincludingthepToftheclusterandafter

correctingfortheUE (only inPbPb collisions), andisrequiredto belessthan1 GeV

/

c.Theisolationcriterionforreconstructed pho-tonsistighterthanforgeneratedphotonstominimize theimpact of UE fluctuations in PbPb collisions, where a downward fluctu-ation in the UE could inadvertently allow a nonisolated photon candidate to pass the isolation criteria. A systematic uncertainty isassignedtoaccountfortheeffectofthisdifferenceonthefinal observables,asdetailedinSection3.5.

Imposingtheisolation requirementsuppressesthe background contributions from fragmentation and decay photons, resulting in a sample enriched in isolated prompt photons. The dominant remaining backgrounds for isolated photon candidates are ECAL showers initiated by isolated hadrons, and real photons that are decayproducts ofisolatedneutralmesons,e.g.,

π

0_,

_η

_,and

_ω

_.The

hadron-inducedshowersarerejectedusingtheratioofHCALover ECALenergy insidea cone ofradius



R

=

0

.

15 around the pho-ton candidate, H

/

E. Only photon candidates with H

/

E

<

0

.

1 are selected forthis analysis. The decayphotons can be significantly reducedusingacutontheshowershape,ameasureofhowenergy depositedintheECALisdistributedin

φ

and

η

[54],asdiscussed inSection3.4.Theefficienciesofthesecriteriainselectingphotons areextractedfromsimulations asafunction of pγ_T andcorrected forincollisiondata.

3.4. Photon

+

jetpairselection

Toformphoton

+

jet pairs,thehighestpT isolatedphoton

can-didatethat passes the selection criteriais paired withall jetsin thesameevent. ThecombinatorialbackgroundinPbPb collisions, whichincludes misidentified jetsthat arise fromUE fluctuations, aswellasjetsfrommultiplehardparton–partonscatteringsinthe same collision,needs to be subtracted in orderto study the en-ergylosseffectsonthejetsproducedinthesamehardscattering asthe photon. Thisbackground subtractionis performedby cor-relatingeachleadingisolatedphotoncandidatewithreconstructed jetsfoundin40differentevents,randomlyselectedfromminimum biasPbPb datasuchthattheeventcentrality,theinteractionvertex positionalongthebeamaxis,andtheeventplane, arewithin5%, 5 cm,and

π

/

10,respectively,ofthosefromthesignal event.The valueswereoptimizedsuchthatthestatisticaluncertaintydueto

thesubtractionisnegligiblecomparedtothestatisticaluncertainty ofthephotonsample.

The backgroundcontributionfrompairs ofdecayphotonsand jetsissubtractedwithaprocedurebasedoncollisiondata,usinga two-componenttemplate fitoftheelectromagneticshower shape variable

σ

ηη ,whichisdefinedasamodifiedsecondmomentofthe

ECAL energycluster distribution around its mean

η

position [54,

55]:

σ

_ηη2

=



5×5 i wi

(

η

i

−

η

5×5

)

2



5×5 i wi

,

wi

=

max



0

,

4

.

7

+

ln Ei E5×5



,

(1)

where Ei and

η

i are the energy deposit and

η

of the ith ECAL

crystal within a 5

×

5 crystal array centered around the electro-magnetic cluster, and E5×5 and

η

5×5 are the total energy and

mean

η

ofthe 5

×

5 crystalmatrix,respectively. Theshape ofthe signal distributionisobtainedfrom pythia

+

hydjet _{simulationsof} isolatedpromptphoton

+

jetprocesses,whilethebackground tem-plates are obtained from a nonisolated sideband region in data, 10

<

SumIsoUE−sub

<

20 GeV

/

c. The purity ofthe photon sample (fraction of prompt photons within the remaining collection of candidates) is determinedfrom thefit.Examples ofthe template fits are shownin Fig. 1for the lowest pγ_T photons andthe four centrality intervals used in thisanalysis. The purity decreases in morecentralcollisions,reflectinganincreaseinthebackgrounds.

Theyieldsandkinematiccharacteristicsofthebackground aris-ing from pairs of decay photons and jetsare estimated by ana-lyzing eventswitha larger photon shower width(0

.

011

<

σ

ηη

<

0

.

017), which are dominated by decay photons. The background contributionfractionisthensubtractedfromtheyieldforthe sig-nal events, which have a smaller photon shower width (σηη

<

0

.

01),accordingtothepurityobtainedfromthetemplatefits. The detector response for low-pT jets can exhibit significant

nonlinearity and biases because of the background subtraction procedure ofthe currentjet algorithm, aswell asthe high mag-netic field of theCMS detector.Thisis neither well-modelednor well-understood.Hence,thedistributionsarenotunfoldedforthe detectorresolution,buttheapproachinsteadistosmear,i.e., con-volve withaGaussian resolutionadjustmentterm, thejet energy inpp eventstomatchtheJERineachofthePbPb centralityclasses inwhichthecomparisonismade.Thisisdoneineveryfigure ex-cept Fig.10.The JER

σ

(

pgen_T

)

isdefined as theGaussian standard deviationofthepreco

T

/

p gen

T ratio,whereprecoT istheUE-subtracted,

detector-level jet pT,and pgen_T isthe generator-level jet pT

with-out any contributions from a PbPb UE. For PbPb (pp) collisions, theJERiscalculatedfrom pythia

+

hydjet (pythia)eventsthat are

Centrality [%] C S [(GeV/c)1/2_{] N [GeV/c]} pp – 0.06 0.95 0 PbPb 0–30 0.06 1.24 6.83 30–100 0 0–10 0.06 1.24 8.42 10–30 5.54 30–50 2.37 50–100 0

propagatedthroughthe Geant4 [56] package.TheUEproducedby hydjet_{with Geant4simulationhasbeencomparedtodataby} ob-servingtheenergycollected insiderandomlyorientedconeswith thesameradiusasthedistanceparameterofthejetalgorithm.The MCsimulationisfoundtobeingoodagreementwiththe experi-mentalresults.TheJERisparametrizedusingtheexpression

σ



pgen_T



=



C2

₊

S2 pgen_T

+

N2



pgen_T



2

.

(2)

Thestochastic term S describes the pT dependenceofthe jet

energy resolution, the constant term C represents the high-pT

limit of the resolution, andthe noise term N reflects the effect of UE fluctuations on the energy resolution. All parameters for

σ

(

pgen_T

)

aredetermined using pythia and pythia

+

hydjetsamples with their numerical values provided in Table 1. Following the smearingto0–30%PbPb data,theenergyresolutions ofjetswith

pjet_T

=

30

(

60

)

GeV

/

c measuredinpp datachanges from18%(14%) to35%(22%)respectively.ComparedtotheJER,thejet

φ

resolution hasanegligibleeffect.

3.5.Systematicuncertainties

Systematic uncertainties are estimated separately for the pp andPbPb analyses.Theuncertaintiesaredeterminedforeach cen-trality and pγ_T interval using similar procedures as described in Ref. [38].Sevensourcesofuncertaintyareconsidered:photon pu-rity,isolation definition,photonenergyscale,electron contamina-tion, photon efficiency, JER, and jet energy scale (JES). The total systematicuncertainties arecalculatedbysumminginquadrature theuncertaintiesfromallsources.

Theuncertaintyonthephoton purityestimateisevaluated by varyingthenonisolatedsidebandregionsusedtoobtainthe back-groundtemplate.Themaximumdeviationfromthenominalvalues is

±

10% (

±

6%)forcentral(peripheral)PbPb collisions,and

±

5%in pp collisions.The varied purity valuesare then used to perform thebackgroundsubtraction,andthemaximumdifferencefromthe nominalresultsis quotedastheuncertainty.Theuncertaintydue totheisolated photondefinitionis determinedby comparingthe photon

+

jetobservables whenusinggenerator-level and detector-leveldefinitionsoftheisolationvariables.Thephotonenergyscale uncertaintyisbasedontheresidualdata-to-simulationphoton en-ergyscaledifferenceafterapplyingthephotonenergycorrections, amounting to about1%, independent of pγ_T and eventcentrality. Theuncertaintyduetoelectroncontaminationisevaluatedby re-peatingtheanalysiswithoutapplyingelectronrejection,and scal-ingthedifferenceinthefinalobservablestotheresidualelectron contamination after applying electron rejection. The electron re-jectionefficiency is determined to be 66% fromMC studies. The uncertaintyon the photon efficiencycorrection is determined by varying theselection criteria formatching reconstructedphotons withgenerator-level photons.The uncertaintyon theJERhastwo

evaluatedbypropagatingtheeffectsofhavingaJERthatdiffersby 15%relativetothenominalvalue.Thesecondsource(7%)accounts fortheuncertaintyintheresolutionandthemodelingofthe JER distributions,andwas obtainedbyconsideringthedifferences be-tweentheextractedJERineach pgen_T binandtheparametrization usingEq. (2),anddeterminingthevalueatonestandarddeviation ofthatdistribution,assumingthatthedifferencesarenormally dis-tributed.

Finally,theJESuncertaintyarisesfromthreecontributionsthat are added inquadrature forthe final value. Twoare commonto both thepp andPbPb samples:the residualdeviationfromunity in simulation (i.e., the closure) of the JES after applying all jet energycorrections(2%)andthedifferencebetweendataand sim-ulation (2%).These two effectsare independent ofcentrality and together amount to 2.8%. The closure of theJES depends on the flavor of the fragmentingparton: simulations show that the en-ergy scale ofquark jetsis consistentlyhigher than that ofgluon jets. For pp collisions, the fragmentation dependence of the JES hasbeenstudiedandisaccountedforintheuncertaintyfromthe difference betweendata and simulation. However, in PbPb colli-sions,theratioofquarksandgluonscanbedifferentfrompp data because of expected differences in centrality-dependent quench-ing of jets initiated by quarks or gluons. The subtractionof the UE in PbPb collisions results in the JES having a larger depen-denceon the fragmentationpatternthan found forpp collisions, since one can only distinguish between soft particles from the jet fragmentation and the underlying event on average. Hence, an additionaluncertainty,evaluated usingcollisiondata and sim-ulation, is applied in PbPb collisions to account for these frag-mentation effects on the JES arising from the subtraction algo-rithm, underlying event, and quenching. The photon-tagged jet fragmentation function in PbPb data is constructed and fit by a two-component model of the jet fragmentation functions for quarkandgluonjetsthatwereobtainedfromMCsimulations.For

pγ_T

>

60 GeV

/

c, the results show that the fraction of jets origi-nating from gluon fragmentation in data can be constrained to between0%andapproximately26%,whichcorrespondstothe frac-tionfoundin pythia

+

hydjet_{MCsamples.Hence,inthiskinematic} region, thedifference between theJES fora pure quark jet sam-pleandtheinclusivesampleisusedintheuncertaintyestimation. For40

<

pγ_T

<

60 GeV

/

c,wheretheresultsofthetemplatefitare inconclusive becauseof thelarge statisticaluncertainties, thefull difference intheJESbetweenhaving0% and100%gluonjet frac-tion is used. This difference is approximately 2–5% (1.5–2.5%) in central(peripheral)collisions.Thefinal systematicuncertainty as-sociated withtheunknown quark–gluonratioindata istakenas themaximumdeviationfromvaryingtheJESupanddown accord-ingtothequark–gluonratioconstraintsmentionedaboveforeach

pγ_T interval.

Asummary of thesystematicuncertainties for Rjγ ,



xjγ



,and

φ

jγ in PbPb collisions is shown in Tables 2 and 3, averaged

over multiple pγ_T and/or event centralityintervals. The dominant sources of uncertainties in both pp andPbPb collisions are from JESandphotonpurityestimation.Thesystematicuncertaintiesfor PbPb andpp collisionsareconsidereduncorrelated.

4. Resultsanddiscussion

4.1. Photon

+

jetazimuthalcorrelation

Possible modification of the back-to-back photon and recoil-ing jet alignment by the medium can be studied by comparing

Table 2

Summaryoftherelativesystematicuncertainties(in%)forpγT>40 GeV/c. Sourceofsystematic

uncertainty[%]

pp PbPb

0–30% centrality 30–100% centrality

xjγ Rjγ xjγ Rjγ xjγ Rjγ

Photon energy scale <0.5 <0.5 0.7 <0.5 <0.5 0.5

Photon isolation 0.8 0.9 0.8 1.0 0.8 0.7

Photon purity <0.5 0.5 3.1 3.5 2.0 2.2

Photon efficiency <0.5 <0.5 <0.5 <0.5 <0.5 <0.5

Electron contamination <0.5 <0.5 0.5 0.9 <0.5 0.9

Jet energy scale 1.9 1.8 2.8 7.3 2.8 5.1

Jet energy resolution 0.9 1.1 2.3 3.6 1.0 1.5

Table 3

Summaryoftheabsolutesystematicuncertaintieson(1/Njγ)(dN/dφjγ)forpγT>40 GeV/c,averagedovertheφjγ distributions.

Sourceofsystematic uncertainty

pp PbPb

0–30% centrality 30–100% centrality Photon energy scale <0.01×10−2 ₂

.12×10−2 ₀ .08×10−2 Photon isolation 0.27×10−2 _0.26×10−2 _0.16×10−2 Photon purity 0.13×10−2 _0.78×10−2 _0.61×10−2 Photon efficiency <0.01×10−2 _0.09×10−2 _0.03×10−2 Electron contamination 0.05×10−2 _0.19×10−2 _0.14×10−2 Jet energy scale 0.23×10−2 _1.63×10−2 _0.86×10−2

Jet energy resolution 0.31×10−2 ₀

.46×10−2 ₀

.48×10−2

Fig. 2. TheazimuthalcorrelationofphotonsandjetsinfivepγT intervalsfor0–30%centrality(top,fullcircles)and30–100%centrality(bottom,fullsquares)PbPb collisions. Thesmearedpp data(opensymbols)areincludedforcomparison.Theverticallines(bands)throughthepointsrepresentstatistical(systematic)uncertainties.

therelative azimuthal angle(

φ

jγ )distributions in pp andPbPb

collisions [16,17]. The distributions are normalized by the num-ber of photon

+

jet pairs. The shape of the

φ

jγ distribution in

pp and PbPb collisions is studied in intervals of leading photon

pT andtwo eventcentrality classes,as shownin Fig. 2. The

ex-ponentially falling region (

φ

jγ

>

2π

/

3) is fit to a normalized

exponential function,asinRef. [38], andthe valuesofthe expo-nentsinPbPb andpp collisionsfromthefitsarecompared.Within thequotedstatisticalandsystematicuncertainties,thePbPb results withdifferentphoton pT andeventcentralityselectionsare

con-sistentwiththecorrespondingsmearedpp referencedata,i.e.,no broadeningofthedistributionsisobserved.

4.2. Photon

+

jettransversemomentumimbalance

The asymmetry ratio xjγ

=

pjetT

/

p γ

T is used to quantify the

photon

+

jet pT imbalance dueto in-medium parton energy loss.

In addition to the photon and jet selections used in the

φ

jγ

study,a

φ

jγ

> (

7π

)/

8 selectionisappliedtoselectback-to-back

photon

+

jet topologies, suppressing the contributions from back-ground jets as well as photon-multijet events. Fig. 3 shows the

xjγ distributionsfordifferentcentralityandpγT regionsinpp and

PbPb collisions, normalizedby thenumber of photons. In 0–30% centrality PbPb collisions, significant modifications (lower mean andsmallerintegral values)ofthexjγ spectrawithrespecttothe

smeared pp reference dataare observed,while the modifications aresmallerinthe30–100%centralityPbPb collisions.

Themeanvalues,



xjγ



(in effect,atruncatedmeanbecauseof

the pjet_T threshold),ofthe xjγ distributions are shownasa

func-tion of pγ_T in Fig.4 (top).The



xjγ



valuesin PbPb andsmeared

pp collisions are consistent with each other within the quoted uncertainties overthewhole pγ_T intervalprobed in30–100% cen-tralityPbPb collisionsandintheregion pγ_T

<

60 GeV

/

c for0–30%

Fig. 3. Distributionofxjγ=pjetT/p

γ

T infivep

γ

T intervalsfor0–30%centrality(top,fullcircles)and30–100%centrality(bottom,fullsquares)PbPb collisions.Thesmearedpp data(opensymbols)areincludedforcomparison.Theverticallines(bands)throughthepointsrepresentstatistical(systematic)uncertainties.

Fig. 4. Thexjγvalues (top)and Rjγ, the numberofassociated jetsper

pho-ton(bottom),in0–30%centrality(left,fullcircles)and30–100%centrality(right, fullsquares)PbPb collisions.Thesmearedpp data(opensymbols)areaddedfor comparison.Theverticallines(bands)throughthepointsrepresentstatistical (sys-tematic)uncertainties.

centralityPbPb collisions. Athigher pγ_T inthe morecentral PbPb events,the



xjγ



valueislowerthaninpp data.

Withajet pT thresholdof30 GeV

/

c,the



xjγ



valuesobserved

fortheselectedphoton

+

jetpairs likelyunderestimates theactual imbalance.Photon

+

jet pairsforwhichthe momentumofthe as-sociated jets falls below the jet pT threshold do not contribute

to the



xjγ



value. To assess how the “missing” jets might

af-fect the



xjγ



results,the average number of associated jets per

photonpassingtheanalysisselections,Rjγ ,isshowninFig.4

(bot-tom).Inthe 0–30%mostcentralPbPb collisions,the valueof Rjγ

isfound to be lower than in the smeared pp data in all leading photon pTintervals.Theabsolutedifferenceisapproximately

con-stant asa function of pγ_T,but the relative difference islarger at lower pγ_T,since the Rjγ in pp collisionsisitselflower inthat

re-gion.

4.3. Jetyieldratio

Fig.5shows, asa functionof pjet_T forseveral pγ_T intervalsand two PbPb eventcentralityintervals, theratiooftheassociatedjet yieldsinPbPb andsmearedpp events,Ijet_AA:

Ijet_AA

=



1 Nγ_PbPb dNjet_PbPb dpjet_T

 

1 Nγ_pp dN_ppjet dpjet_T



.

(3)

Thisvariable reflectsthe modificationof theassociated jet pT

spectra by the medium. In 30–100% PbPb events, the Ijet_AA val-ues are slightly suppressed for photon candidates with pγ_T

<

80 GeV

/

c,andconsistent withunityfor photoncandidates with

pγ_T

>

80 GeV

/

c.For0–30% centralityPbPb events, a suppression ofapproximately a factorof 2is observedat low pγ_T. As the pγ_T

increases, the larger phase spaceallows quenchedjets toremain abovethekinematicselections,whichtranslatestoaslightexcess ofquenchedjetsappearingatlow p_Tjet.Thisisseeninthetoprow, whereIjet_AAforlow pjet_T increaseswithpγ_T whiletheIjet_AAatlargepjet_T

staysroughlyconstant.

4.4. Centralitydependence

ThecentralitydependenceinPbPb collisionsofxjγ spectrafor pγ_T

>

60 GeV

/

c is shown in Fig. 6. In the most peripheral colli-sions (50–100% centrality), the xjγ distribution agrees with the

smearedpp referencedata.Ascollisionsbecomemorecentral,the PbPb distributionsshifttowardslower xjγ andtheintegralsofthe xjγ spectra become smaller. This is consistent with the

expecta-tion that a larger amountof parton pT is transportedout of the

jet cone asa consequenceofthe larger averagepath length that thepartonneedstotravelthroughinmorecentralPbPb collisions [57,58].

Fig.7shows



xjγ



andRjγ inpp andPbPb collisionsasa

func-tion ofeventcentrality, quantified by



Npart



,which isthe mean

number ofparticipating nucleons within a givencentrality inter-val.The



Npart



valuesareestimatedfromaMCGlaubermodel [15, 59]. In central collisions, a suppression ofboth



xjγ



and Rjγ is

con-Fig. 5. TheIjet_AAvs. pjetT for0–30%centrality(top)and30–100%centrality(bottom)PbPb collisions.Theverticallines(bands)throughthepointsrepresentstatistical(systematic) uncertainties.

Fig. 6. Thecentralitydependenceofxjγ ofphoton+jetpairsnormalizedbythenumberofphotonsforPbPb (fullmarkers)andsmearedpp (openmarkers)data.Thevertical

lines(bands)throughthepointsrepresentstatistical(systematic)uncertainties.

Fig. 7. Thexjγ(top)andRjγ (bottom)asafunctionofNpartforpγT>60 GeV/c (left)andpγT>80 GeV/c (right).ThePbPb results(fullmarkers)arecomparedtopp results(openmarkers)smearedbytherelativejetenergyresolutioncorresponding toeachcentralityinterval.Theverticallines(bands)throughthepointsrepresent statistical(systematic)uncertainties.

sistent withsignificant in-medium energy loss of the associated jets.

4.5. Comparisontotheoreticalmodels

TheresultsforPbPb collisionspresentedinFig.2for

φ

jγ and

Fig. 3 for xjγ are compared with several theoretical calculations

withdifferentapproachestomodelingthejetenergylossinFigs.8

and9,respectively.Thexjγ distributionsassumedbythedifferent

modelcalculationsinpp collisionsarecomparedtotheunsmeared pp data inFig. 10.The jewel modelis a dynamical,perturbative framework for jet quenching, which has been extended to sim-ulate boson-jet events [37,60]. The LBT 2017 model [34] uses a linearizedBoltzmanntransportmodelforjetpropagationthrough themedium,includingtherecoiledmediumpartonsinthe recon-structionofthepartonicjets.Thehybridmodel [35,36] combinesa perturbative descriptionoftheweaklycoupledphysicsofjet pro-duction andevolution withagauge/gravityduality descriptionof thestronglycoupleddynamicsofthemedium,andofthesoft ex-changes between the jet andthe medium. The calculationsfrom the jewel and hybrid models have been smeared to the corre-spondingJERinpp orPbPb collisions.

Predictionsfromthe jewel andhybridmodels havepreviously shown reasonable agreement withmeasurements of inclusivejet nuclear modification factors [36,61]. For the results reported in thisLetter,allmodelsdescribewell thepp results.Theyalso cap-ture the general features of the 0–30% PbPb data, although the hybridmodelappearstobetterdescribethexjγ results.Asshown

in Fig.9,the jewel and LBTmodels appeartounderestimate the

col-Fig. 8. TheazimuthalcorrelationofphotonsandjetsinfivepγT intervalsfor0–30%centrality(top,fullcircles)and30–100%centrality(bottom,fullsquares)PbPb collisions. ThedatapointsshownareidenticaltothoseinFig.2.Theoreticalcalculationsfrom jewel [37,60],LBT [34],andhybridmodel [35,36] areincludedforcomparison.

Fig. 9. Thexjγ distributionsinfivepγT intervalsfor0–30%centrality(top,fullcircles)and30–100%centrality(bottom,fullsquares)PbPb collisions.Thedatapointsshown areidenticaltothoseinFig.3.Theoreticalcalculationsfrom jewel [37,60],LBT [34],andhybridmodel [35,36] areincludedforcomparison.

Fig. 10. Thexjγ distributionsinfivepγT intervalsforunsmearedpp data(fullsquares).Thexjγ distributionsinpp collisionsassumedbythe jewel [37,60],LBT [34],and

hybridmodels [35,36] discussedinthisLetterarealsoshownforcomparison.

lisions,whichsuggeststhattheamountofenergytransportedout ofthe jet cone is larger in these models than indata. A similar effectis also hintedat inthe 30–100% PbPb data, which can be attributed to the fact that those distributions are dominated by eventsinthe30–50%centralityinterval,whereenergylosseffects

are still significant. The models are also consistent withdata in that noneofthemshow a broadeningoftheobserved

φ

jγ

dis-tributionsinPbPb comparedtopp collisionsinthephotonandjet kinematicranges presented,despite their implementing contribu-tionsfrompartoniccollisions.

5. Summary

Correlations of isolated photons with transverse momentum

pγ_T

>

40 GeV

/

c andpseudorapidity

|

η

γ

_|

_<

₁

_.

_{44 andassociatedjets}

with pjet_T

>

30 GeV

/

c and

|

η

jet

_|

_<

₁

_.

_{6,have been studied forthe}

first time inpp and PbPb collisions at

√

sNN

=

5

.

02 TeV, usinga

largedatasamplecollectedbytheCMSexperiment.Nosignificant azimuthalangularbroadeningbetweenphotonsandtheassociated jetsisobservedinPbPb dataascomparedtopp data,forallevent centralities and multiple photon pT intervals. The xjγ

=

pjetT

/

p

γ T

and the average number of associated jets per photon, Rjγ , are

studiedindifferentleadingphoton pT andPbPb collision

central-ity intervals. For all pγ_T

>

60 GeV

/

c intervals, the



xjγ



and Rjγ

valuesinthe 0–30%mostcentral PbPb collisionsare found tobe lowerthan thoseinthecorresponding pp referencedata, indicat-ing that a larger fraction ofjets lose energyandthus fall below 30 GeV

/

c in PbPb collisions. Thedifferencesbetweenthe pp and PbPb results increase as collisions become more central. A shift ofthejet spectratowardslower pjet_T isobservedwhencomparing theyieldsofassociatedjetsinthe0–30%mostcentralPbPb colli-sionstothoseinpp collisions.Thesenewresultsarequalitatively similar tothose reportedat

√

sNN

=

2

.

76 TeV andto calculations

fromvarious theoreticalmodels.Thebetterstatisticalprecisionof thenew higherenergydata provides an opportunity to test the-oretical models against data over a wide kinematic range in pγ_T

and xjγ , andfor different event centralities, using a selection of

partonswithdefinedflavor(quark/gluon)andinitialkinematics.

Acknowledgements

WecongratulateourcolleaguesintheCERNaccelerator depart-ments for the excellent performance of the LHC and thank the technicalandadministrativestaffs atCERN andatother CMS in-stitutes for their contributions to the success of the CMS effort. Inaddition,wegratefullyacknowledgethecomputingcentersand personneloftheWorldwideLHCComputingGridfordeliveringso effectivelythe computinginfrastructureessential to ouranalyses. Finally, we acknowledge the enduring support for the construc-tionandoperation oftheLHCandthe CMSdetectorprovidedby thefollowingfundingagencies:BMWFWandFWF(Austria);FNRS and FWO (Belgium); CNPq, CAPES, FAPERJ, and FAPESP (Brazil); MES (Bulgaria); CERN; CAS, MOST, and NSFC (China); COLCIEN-CIAS(Colombia);MSESandCSF(Croatia);RPF(Cyprus);SENESCYT (Ecuador); MoER, ERC IUT, and ERDF (Estonia); Academy of Fin-land,MEC,andHIP(Finland);CEAandCNRS/IN2P3(France);BMBF, DFG, and HGF (Germany); GSRT (Greece); OTKA and NIH (Hun-gary);DAEandDST(India);IPM(Iran);SFI(Ireland);INFN(Italy); MSIPandNRF(RepublicofKorea);LAS (Lithuania);MOE andUM (Malaysia); BUAP, CINVESTAV,CONACYT, LNS, SEP, and UASLP-FAI (Mexico); MBIE (New Zealand); PAEC (Pakistan); MSHE and NSC (Poland);FCT(Portugal);JINR(Dubna);MON,ROSATOM,RAS,RFBR andRAEP(Russia);MESTD (Serbia);SEIDI,CPAN,PCTI andFEDER (Spain);SwissFundingAgencies(Switzerland);MST(Taipei); ThEP-Center, IPST, STAR, and NSTDA (Thailand); TUBITAK and TAEK (Turkey);NASUandSFFR(Ukraine); STFC(United Kingdom);DOE andNSF(USA).

Individuals have received support from the Marie-Curie pro-gramandtheEuropeanResearchCouncilandHorizon2020Grant, contract No. 675440 (European Union); the Leventis Foundation; the A. P. Sloan Foundation; the Alexander von Humboldt Foun-dation;the BelgianFederal Science Policy Office;the Fonds pour laFormation à laRecherche dans l’Industrieetdans l’Agriculture (FRIA-Belgium); the Agentschapvoor Innovatie door Wetenschap en Technologie (IWT-Belgium); the Ministry of Education, Youth

and Sports (MEYS) of the Czech Republic; the Council of Sci-ence and Industrial Research, India; the HOMING PLUS program of the Foundation for Polish Science, cofinanced from European Union, Regional DevelopmentFund, theMobilityPlusprogram of theMinistryofScienceandHigherEducation,theNationalScience Center (Poland), contracts Harmonia 2014/14/M/ST2/00428, Opus 2014/13/B/ST2/02543, 2014/15/B/ST2/03998, and 2015/19/B/ST2/ 02861,Sonata-bis2012/07/E/ST2/01406;theNationalPriorities Re-search Program by Qatar National Research Fund; the Programa Severo Ochoa del Principado de Asturias; the Thalisand Aristeia programscofinancedbyEU-ESFandtheGreekNSRF;the Rachada-pisekSompotFundforPostdoctoralFellowship,Chulalongkorn Uni-versity and the Chulalongkorn Academic into Its 2nd Century Project Advancement Project (Thailand); the Welch Foundation, contractC-1845;andtheWestonHavensFoundation(USA).

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TheCMSCollaboration

A.M. Sirunyan,

A. Tumasyan

YerevanPhysicsInstitute,Yerevan,Armenia

W. Adam,

F. Ambrogi,

E. Asilar,

T. Bergauer,

J. Brandstetter,

E. Brondolin,

M. Dragicevic,

J. Erö,

M. Flechl,

M. Friedl,

R. Frühwirth

1

,

V.M. Ghete,

J. Grossmann,

J. Hrubec,

M. Jeitler

1

,

A. König,

N. Krammer,

I. Krätschmer,

D. Liko,

T. Madlener,

I. Mikulec,

E. Pree,

N. Rad,

H. Rohringer,

J. Schieck

1

,

R. Schöfbeck,

M. Spanring,

D. Spitzbart,

W. Waltenberger,

J. Wittmann,

C.-E. Wulz

1

,

M. Zarucki

InstitutfürHochenergiephysik,Wien,Austria

V. Chekhovsky,

V. Mossolov,

J. Suarez Gonzalez

InstituteforNuclearProblems,Minsk,Belarus

E.A. De Wolf,

D. Di Croce,

X. Janssen,

J. Lauwers,

H. Van Haevermaet,

P. Van Mechelen,

N. Van Remortel

UniversiteitAntwerpen,Antwerpen,Belgium

S. Abu Zeid,

F. Blekman,

J. D’Hondt,

I. De Bruyn,

J. De Clercq,

K. Deroover,

G. Flouris,

D. Lontkovskyi,

S. Lowette,

I. Marchesini,

S. Moortgat,

L. Moreels,

Q. Python,

K. Skovpen,

S. Tavernier,

W. Van Doninck,

P. Van Mulders,

I. Van Parijs

VrijeUniversiteitBrussel,Brussel,Belgium

D. Beghin,

H. Brun,

B. Clerbaux,

G. De Lentdecker,

H. Delannoy,

B. Dorney,

G. Fasanella,

L. Favart,

R. Goldouzian,

A. Grebenyuk,

T. Lenzi,

J. Luetic,

T. Maerschalk,

A. Marinov,

T. Seva,

E. Starling,

C. Vander Velde,

P. Vanlaer,

D. Vannerom,

R. Yonamine,

F. Zenoni,

F. Zhang

2

UniversitéLibredeBruxelles,Bruxelles,Belgium

A. Cimmino,

T. Cornelis,

D. Dobur,

A. Fagot,

M. Gul,

I. Khvastunov

3

,

D. Poyraz,

C. Roskas,

S. Salva,

M. Tytgat,

W. Verbeke,

N. Zaganidis

GhentUniversity,Ghent,Belgium

H. Bakhshiansohi,

O. Bondu,

S. Brochet,

G. Bruno,

C. Caputo,

A. Caudron,

P. David,

S. De Visscher,

C. Delaere,

M. Delcourt,

B. Francois,

A. Giammanco,

M. Komm,

G. Krintiras,

V. Lemaitre,

A. Magitteri,

A. Mertens,

M. Musich,

K. Piotrzkowski,

L. Quertenmont,

A. Saggio,

M. Vidal Marono,

S. Wertz,

J. Zobec

UniversitéCatholiquedeLouvain,Louvain-la-Neuve,Belgium

W.L. Aldá Júnior,

F.L. Alves,

G.A. Alves,

L. Brito,

M. Correa Martins Junior,

C. Hensel,

A. Moraes,

M.E. Pol,

P. Rebello Teles

CentroBrasileirodePesquisasFisicas,RiodeJaneiro,Brazil

E. Belchior Batista Das Chagas,

W. Carvalho,

J. Chinellato

4

,

E. Coelho,

E.M. Da Costa,

G.G. Da Silveira

5

,

D. De Jesus Damiao,

S. Fonseca De Souza,

L.M. Huertas Guativa,

H. Malbouisson,

M. Melo De Almeida,

C. Mora Herrera,

L. Mundim,

H. Nogima,

L.J. Sanchez Rosas,

A. Santoro,

A. Sznajder,

M. Thiel,

E.J. Tonelli Manganote

4

,

F. Torres Da Silva De Araujo,

A. Vilela Pereira

UniversidadedoEstadodoRiodeJaneiro,RiodeJaneiro,Brazil

S. Ahuja

a

,

C.A. Bernardes

a

,

T.R. Fernandez Perez Tomei

a

,

E.M. Gregores

b

,

P.G. Mercadante

b

,

S.F. Novaes

a

,

Sandra S. Padula

a

,

D. Romero Abad

b

,

J.C. Ruiz Vargas

a

a_{UniversidadeEstadualPaulista,SãoPaulo,Brazil} b_{UniversidadeFederaldoABC,SãoPaulo,Brazil}

A. Aleksandrov,

R. Hadjiiska,

P. Iaydjiev,

M. Misheva,

M. Rodozov,

M. Shopova,

G. Sultanov

InstituteforNuclearResearchandNuclearEnergy,BulgarianAcademyofSciences,Sofia,Bulgaria

A. Dimitrov,

L. Litov,

B. Pavlov,

P. Petkov

UniversityofSofia,Sofia,Bulgaria

W. Fang

6

,

X. Gao

6

,

L. Yuan

BeihangUniversity,Beijing,China

InstituteofHighEnergyPhysics,Beijing,China

Y. Ban,

G. Chen,

J. Li,

Q. Li,

S. Liu,

Y. Mao,

S.J. Qian,

D. Wang,

Z. Xu

StateKeyLaboratoryofNuclearPhysicsandTechnology,PekingUniversity,Beijing,China

C. Avila,

A. Cabrera,

L.F. Chaparro Sierra,

C. Florez,

C.F. González Hernández,

J.D. Ruiz Alvarez,

M.A. Segura Delgado

UniversidaddeLosAndes,Bogota,Colombia

B. Courbon,

N. Godinovic,

D. Lelas,

I. Puljak,

P.M. Ribeiro Cipriano,

T. Sculac

UniversityofSplit,FacultyofElectricalEngineering,MechanicalEngineeringandNavalArchitecture,Split,Croatia

Z. Antunovic,

M. Kovac

UniversityofSplit,FacultyofScience,Split,Croatia

V. Brigljevic,

D. Ferencek,

K. Kadija,

B. Mesic,

A. Starodumov

7

,

T. Susa

InstituteRudjerBoskovic,Zagreb,Croatia

M.W. Ather,

A. Attikis,

G. Mavromanolakis,

J. Mousa,

C. Nicolaou,

F. Ptochos,

P.A. Razis,

H. Rykaczewski

UniversityofCyprus,Nicosia,Cyprus

M. Finger

8

,

M. Finger Jr.

8 CharlesUniversity,Prague,CzechRepublic

E. Carrera Jarrin

UniversidadSanFranciscodeQuito,Quito,Ecuador

E. El-khateeb

9

,

S. Elgammal

10

,

A. Ellithi Kamel

11

AcademyofScientificResearchandTechnologyoftheArabRepublicofEgypt,EgyptianNetworkofHighEnergyPhysics,Cairo,Egypt

R.K. Dewanjee,

M. Kadastik,

L. Perrini,

M. Raidal,

A. Tiko,

C. Veelken

NationalInstituteofChemicalPhysicsandBiophysics,Tallinn,Estonia

P. Eerola,

H. Kirschenmann,

J. Pekkanen,

M. Voutilainen

DepartmentofPhysics,UniversityofHelsinki,Helsinki,Finland

J. Havukainen,

J.K. Heikkilä,

T. Järvinen,

V. Karimäki,

R. Kinnunen,

T. Lampén,

K. Lassila-Perini,

S. Laurila,

S. Lehti,

T. Lindén,

P. Luukka,

H. Siikonen,

E. Tuominen,

J. Tuominiemi

HelsinkiInstituteofPhysics,Helsinki,Finland

T. Tuuva

LappeenrantaUniversityofTechnology,Lappeenranta,Finland

M. Besancon,

F. Couderc,

M. Dejardin,

D. Denegri,

J.L. Faure,

F. Ferri,

S. Ganjour,

S. Ghosh,

P. Gras,

G. Hamel de Monchenault,

P. Jarry,

I. Kucher,

C. Leloup,

E. Locci,

M. Machet,

J. Malcles,

G. Negro,

J. Rander,

A. Rosowsky,

M.Ö. Sahin,

M. Titov

A. Abdulsalam,

C. Amendola,

I. Antropov,

S. Baffioni,

F. Beaudette,

P. Busson,

L. Cadamuro,

C. Charlot,

R. Granier de Cassagnac,

M. Jo,

S. Lisniak,

A. Lobanov,

J. Martin Blanco,

M. Nguyen,

C. Ochando,

G. Ortona,

P. Paganini,

P. Pigard,

R. Salerno,

J.B. Sauvan,

Y. Sirois,

A.G. Stahl Leiton,

T. Strebler,

Y. Yilmaz,

A. Zabi,

A. Zghiche

LaboratoireLeprince-Ringuet,Ecolepolytechnique,CNRS/IN2P3,UniversitéParis-Saclay,Palaiseau,France

J.-L. Agram

12

,

J. Andrea,

D. Bloch,

J.-M. Brom,

M. Buttignol,

E.C. Chabert,

N. Chanon,

C. Collard,

E. Conte

12

,

X. Coubez,

J.-C. Fontaine

12

,

D. Gelé,

U. Goerlach,

M. Jansová,

A.-C. Le Bihan,

N. Tonon,

P. Van Hove

UniversitédeStrasbourg,CNRS,IPHCUMR7178,F-67000Strasbourg,France

S. Gadrat

CentredeCalculdel’InstitutNationaldePhysiqueNucleaireetdePhysiquedesParticules,CNRS/IN2P3,Villeurbanne,France

S. Beauceron,

C. Bernet,

G. Boudoul,

R. Chierici,

D. Contardo,

P. Depasse,

H. El Mamouni,

J. Fay,

L. Finco,

S. Gascon,

M. Gouzevitch,

G. Grenier,

B. Ille,

F. Lagarde,

I.B. Laktineh,

M. Lethuillier,

L. Mirabito,

A.L. Pequegnot,

S. Perries,

A. Popov

13

,

V. Sordini,

M. Vander Donckt,

S. Viret

UniversitédeLyon,UniversitéClaudeBernardLyon1,CNRS-IN2P3,InstitutdePhysiqueNucléairedeLyon,Villeurbanne,France

A. Khvedelidze

8

GeorgianTechnicalUniversity,Tbilisi,Georgia

I. Bagaturia

14

TbilisiStateUniversity,Tbilisi,Georgia

C. Autermann,

L. Feld,

M.K. Kiesel,

K. Klein,

M. Lipinski,

M. Preuten,

C. Schomakers,

J. Schulz,

V. Zhukov

13

RWTHAachenUniversity,I.PhysikalischesInstitut,Aachen,Germany

A. Albert,

E. Dietz-Laursonn,

D. Duchardt,

M. Endres,

M. Erdmann,

S. Erdweg,

T. Esch,

R. Fischer,

A. Güth,

M. Hamer,

T. Hebbeker,

C. Heidemann,

K. Hoepfner,

S. Knutzen,

M. Merschmeyer,

A. Meyer,

P. Millet,

S. Mukherjee,

T. Pook,

M. Radziej,

H. Reithler,

M. Rieger,

F. Scheuch,

D. Teyssier,

S. Thüer

RWTHAachenUniversity,III.PhysikalischesInstitutA,Aachen,Germany

G. Flügge,

B. Kargoll,

T. Kress,

A. Künsken,

T. Müller,

A. Nehrkorn,

A. Nowack,

C. Pistone,

O. Pooth,

A. Stahl

15

RWTHAachenUniversity,III.PhysikalischesInstitutB,Aachen,Germany

M. Aldaya Martin,

T. Arndt,

C. Asawatangtrakuldee,

K. Beernaert,

O. Behnke,

U. Behrens,

A. Bermúdez Martínez,

A.A. Bin Anuar,

K. Borras

16

,

V. Botta,

A. Campbell,

P. Connor,

C. Contreras-Campana,

F. Costanza,

C. Diez Pardos,

G. Eckerlin,

D. Eckstein,

T. Eichhorn,

E. Eren,

E. Gallo

17

,

J. Garay Garcia,

A. Geiser,

J.M. Grados Luyando,

A. Grohsjean,

P. Gunnellini,

M. Guthoff,

A. Harb,

J. Hauk,

M. Hempel

18

,

H. Jung,

M. Kasemann,

J. Keaveney,

C. Kleinwort,

I. Korol,

D. Krücker,

W. Lange,

A. Lelek,

T. Lenz,

J. Leonard,

K. Lipka,

W. Lohmann

18

,

R. Mankel,

I.-A. Melzer-Pellmann,

A.B. Meyer,

G. Mittag,

J. Mnich,

A. Mussgiller,

E. Ntomari,

D. Pitzl,

A. Raspereza,

M. Savitskyi,

P. Saxena,

R. Shevchenko,

S. Spannagel,

N. Stefaniuk,

G.P. Van Onsem,

R. Walsh,

Y. Wen,

K. Wichmann,

C. Wissing,

O. Zenaiev

DeutschesElektronen-Synchrotron,Hamburg,Germany

R. Aggleton,

S. Bein,

V. Blobel,

M. Centis Vignali,

T. Dreyer,

E. Garutti,

D. Gonzalez,

J. Haller,

A. Hinzmann,

M. Hoffmann,

A. Karavdina,

R. Klanner,

R. Kogler,

N. Kovalchuk,

S. Kurz,

T. Lapsien,

D. Marconi,

M. Meyer,

M. Niedziela,

D. Nowatschin,

F. Pantaleo

15

,

T. Peiffer,

A. Perieanu,

C. Scharf,

UniversityofHamburg,Hamburg,Germany

M. Akbiyik,

C. Barth,

M. Baselga,

S. Baur,

E. Butz,

R. Caspart,

T. Chwalek,

F. Colombo,

W. De Boer,

A. Dierlamm,

N. Faltermann,

B. Freund,

R. Friese,

M. Giffels,

M.A. Harrendorf,

F. Hartmann

15

,

S.M. Heindl,

U. Husemann,

F. Kassel

15

,

S. Kudella,

H. Mildner,

M.U. Mozer,

Th. Müller,

M. Plagge,

G. Quast,

K. Rabbertz,

M. Schröder,

I. Shvetsov,

G. Sieber,

H.J. Simonis,

R. Ulrich,

S. Wayand,

M. Weber,

T. Weiler,

S. Williamson,

C. Wöhrmann,

R. Wolf

InstitutfürExperimentelleKernphysik,Karlsruhe,Germany

G. Anagnostou,

G. Daskalakis,

T. Geralis,

A. Kyriakis,

D. Loukas,

I. Topsis-Giotis

InstituteofNuclearandParticlePhysics(INPP),NCSRDemokritos,AghiaParaskevi,Greece

G. Karathanasis,

S. Kesisoglou,

A. Panagiotou,

N. Saoulidou

NationalandKapodistrianUniversityofAthens,Athens,Greece

K. Kousouris

NationalTechnicalUniversityofAthens,Athens,Greece

I. Evangelou,

C. Foudas,

P. Gianneios,

P. Katsoulis,

P. Kokkas,

S. Mallios,

N. Manthos,

I. Papadopoulos,

E. Paradas,

J. Strologas,

F.A. Triantis,

D. Tsitsonis

UniversityofIoánnina,Ioánnina,Greece

M. Csanad,

N. Filipovic,

G. Pasztor,

O. Surányi,

G.I. Veres

19 MTA-ELTELendületCMSParticleandNuclearPhysicsGroup,EötvösLorándUniversity,Budapest,Hungary

G. Bencze,

C. Hajdu,

D. Horvath

20

,

Á. Hunyadi,

F. Sikler,

V. Veszpremi

WignerResearchCentreforPhysics,Budapest,Hungary

N. Beni,

S. Czellar,

J. Karancsi

21

,

A. Makovec,

J. Molnar,

Z. Szillasi

InstituteofNuclearResearchATOMKI,Debrecen,Hungary

M. Bartók

19

,

P. Raics,

Z.L. Trocsanyi,

B. Ujvari

InstituteofPhysics,UniversityofDebrecen,Debrecen,Hungary

S. Choudhury,

J.R. Komaragiri

IndianInstituteofScience(IISc),Bangalore,India

S. Bahinipati

22

,

S. Bhowmik,

P. Mal,

K. Mandal,

A. Nayak

23

,

D.K. Sahoo

22

,

N. Sahoo,

S.K. Swain

NationalInstituteofScienceEducationandResearch,Bhubaneswar,India

S. Bansal,

S.B. Beri,

V. Bhatnagar,

R. Chawla,

N. Dhingra,

A.K. Kalsi,

A. Kaur,

M. Kaur,

S. Kaur,

R. Kumar,

P. Kumari,

A. Mehta,

J.B. Singh,

G. Walia

PanjabUniversity,Chandigarh,India

A. Bhardwaj,

S. Chauhan,

B.C. Choudhary,

R.B. Garg,

S. Keshri,

A. Kumar,

Ashok Kumar,

S. Malhotra,

M. Naimuddin,

K. Ranjan,

Aashaq Shah,

R. Sharma