Measurement of photon-jet transverse momentum correlations in 5.02 TeV Pb + Pb and pp collisions with ATLAS

Physics Letters B 789 (2019) 167–190

Contents lists available atScienceDirect

Physics

Letters

B

www.elsevier.com/locate/physletb

Measurement

of

photon–jet

transverse

momentum

correlations

in 5

.

02 TeV

Pb

+

Pb

and

pp collisions

with

ATLAS

.

The

ATLAS

Collaboration



a

r

t

i

c

l

e

i

n

f

o

a

b

s

t

r

a

c

t

Articlehistory:

Received19September2018

Receivedinrevisedform19November2018

Accepted10December2018

Availableonline13December2018 Editor:W.-D.Schlatter

Jets createdin associationwith a photon can beused as acalibrated probe tostudyenergy loss in themediumcreatedinnuclearcollisions.Measurementsofthetransversemomentumbalancebetween isolatedphotonsandinclusivejetsarepresentedusingintegratedluminositiesof0.49 nb−1ofPb+Pb collisiondataat√sNN=5.02 TeV and25 pb−1ofpp collisiondataat√s=5.02 TeV recordedwiththe ATLASdetectorattheLHC.Photonswithtransverse momentum63.1<pγ_T<200 GeV and

η

γ_<2.37 are pairedwith all jets inthe event that have pjet_T >31.6 GeV and pseudorapidity 

η

jet_{<2.8. The} transverse momentum balance given by the jet-to-photon pT ratio, xJγ, is measured for pairs with azimuthal opening angle φ >7

π/

8. Distributions ofthe per-photonjet yield as a functionof xJγ, (1/Nγ)(dN/dxJγ), are corrected for detector effects via a two-dimensional unfolding procedure and reportedattheparticlelevel.Inpp collisions,thedistributionsarewelldescribedbyMonteCarloevent generators.InPb+Pbcollisions,thexJγ distributionismodifiedfromthatobservedinpp collisionswith increasingcentrality,consistentwiththepictureofpartonenergylossinthehotnuclearmedium. The dataarecomparedwithasuiteofenergy-lossmodelsandcalculations.

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

1. Introduction

The energy loss of fast partons traversing the hot, decon-finedmediumcreatedinnucleus–nucleuscollisionscanbestudied in a controlled and systematic way through the analysis of jets produced in association with a high transverse momentum (pT)

promptphoton [1–7].Atleadingorderinquantum chromodynam-ics, the photon andleading jet are produced back-to-back inthe azimuthal plane, with equal transverse momenta. Measurements ofprompt photon productionin Au

+

Au collisions at the Rela-tivisticHeavyIonCollider(RHIC) [8] andPb

+

Pbcollisionsatthe LargeHadronCollider(LHC) [9] haveconfirmedthat,sincephotons donot participateinthestronginteraction,their productionrates are not modified by the medium [10]. Thus, photons provide an estimateofthepTanddirectionofthepartonproducedinthe

ini-tialhard-scatteringbefore it haslost energythrough interactions withthemedium. Measurementsof jetproductionwithdifferent requirements on the photon kinematics can therefore shed light onhowtheabsoluteamountofpartonenergylossdependsonthe initialpartonpT.

Furthermore,photon–jet events offeraparticularly usefulway toprobethedistributionofenergylostbyjetsinindividualevents,

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

andarecomplementarytomeasurementssuchasthedijetpT

bal-ance [11–13].Whereasthosemeasurementsreporttheratioofthe transverse momenta of two final-state jets, both of which may have lost energy, photon–jet events provide an alternative sys-temin whichone high-pT objectis certain toremain unaffected

by the hot nuclear medium. Finally, jetsproduced in association withaphotonaremorelikelytooriginatefromquarksthanthose produced in dijetevents atthe same pT.Thus, when considered

together withmeasurements of dijets or ofinclusive jet [14–16] andhadron [17–19] productionratesinPb

+

Pbcollisions, analy-sis ofphoton–jet eventscan helpto furtherconstrain theflavour (i.e.quarkversusgluon)dependenceofpartonenergyloss.

Studies of photon–hadron correlations, in which high-pT

hadrons are usedas a proxyfor the jet, were first performedat RHIC [20–22], and measurements using fully reconstructed jets havesincebegunattheLHC [23,24].IntheLHCstudies,the distri-bution ofthephoton–jet azimuthal separation,

φ

,was foundto beconsistentwiththatinsimulatedphoton–jeteventsembedded intoaheavy-ionbackground,andthejet-to-photontransverse mo-mentumratio,xJγ

=

pjetT

/p

γ

T,was studiedforinclusivephoton–jet

pairs. Theper-photon jetyield

(

1

/

Nγ

)(

dN

/

dxJγ

)

distributionwas

shiftedtosignificantlysmallervaluesinPb

+

Pbdata.

In these previous measurements, the xJγ distributions in

Pb

+

Pbeventswere notcorrectedfordetectorresolutioneffects, which led to a substantial broadening of the reported

distribu-https://doi.org/10.1016/j.physletb.2018.12.023

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

168 The ATLAS Collaboration / Physics Letters B 789 (2019) 167–190

tionsindata.As aresult, qualitativecomparisonswithmodelsor evenwiththeanalogousdistributionsinproton–proton(pp)data couldonlybeaccomplishedbyapplyinganadditionalsmearingto thecomparisondistributions tointroduce detectoreffects.Recent measurements ofdijet pT correlations [12] and inclusivejet

frag-mentationfunctionsatlargelongitudinalmomentumfraction [25] inPb

+

Pbcollisionsusedunfoldingprocedurestocorrectfor bin-migrationeffectsandreturnthedistributionstotheparticlelevel, i.e.freefromdetectoreffects.

ThisLetterreportsastudyofphoton–jetcorrelationsinPb

+

Pb collisions at a nucleon–nucleon centre-of-mass energy

√

sNN

=

5

.

02 TeV and pp collisions at the same centre-of-mass energy

√

s

=

5

.

02 TeV. The data were recorded in 2015 with the AT-LAS detector at the LHC and correspond to integrated luminosi-ties of 0

.

49 nb−1 and 25 pb−1, respectively. Events containing a prompt photon with 63

.

1

<

pγ_T

<

200 GeV and pseudorapidity



η

γ



<

2

.

37 (excludingtheregion1

.

37

<



η

γ



<

1

.

52) arestudied. The pT balanceof photon–jet pairs forjetswith pjetT

>

31

.

6 GeV

and



η

jet



_<

₂

_.

_{8 which are approximately back-to-back with the}

photon in the transverse plane,

φ >

7

π

/

8,is analysed through theper-photonyield ofjetsasafunctionofxJγ ,withall jetsthat

meetthisselectionrequirementcountedseparately.InMonteCarlo simulations,thefractionofphotonspairedwithmorethanonejet rises from1%to

≈

15% overthe reportedphoton pT ranges.The

particularphotonandjet pT rangesusedinthemeasurementare

chosen tobe evenlyspacedonlogarithmic scales tofacilitatethe unfoldingproceduredescribedbelow.

The yields are corrected via data-driven techniques for back-ground arising from combinatoric pairings of each photon with unrelatedjetsin Pb

+

Pbevents andfromthecontamination by neutralmesons inthe photonsample. The resulting xJγ

distribu-tions are corrected forthe effects ofthe experimental resolution onthephotonandjet pT viaatwo-dimensional unfolding

proce-duresimilar tothatusedinRef. [12]. Duetohigher-ordereffects, photon–jet eventsdonotgenerallyhavethe back-to-backleading ordertopologymentionedabove.Thusthepp data,whichincludes theseeffects,providesthereferencedistributionsagainstwhichto interpretthe resultsinPb

+

Pbevents.ThisLetter directly com-paresphoton–jet datainPb

+

Pband pp events,andwithMonte Carloeventgeneratorsandanalyticcalculations [26–29].

2. Experimentalset-up

The ATLAS experiment [30] is a multipurpose particle detec-torwitha forward–backwardsymmetriccylindricalgeometryand nearly4

π

coverage.1_{Thisanalysisreliesontheinnerdetector,the}

calorimeterandthedataacquisitionandtriggersystem.

Theinnerdetectorcomprisesthreemajorsubsystems:thepixel detector and the silicon microstrip tracker, which extend out to

|

η

| =

2

.

5, and the transition radiation tracker which extends to

|

η

| =

2

.

0. The inner detector covers the full azimuth and is im-mersed ina 2 T axial magneticfield. The pixeldetector consists of four cylindrical layers in the barrel region and three disks in each endcapregion.Thesilicon microstriptrackercomprises four cylindricallayers(ninedisks)ofsiliconstripdetectorsinthebarrel (endcap)region.

1 _{ATLASusesaright-handed coordinatesystemwith itsoriginat thenominal}

interactionpoint(IP)inthecentreofthedetectorandthez-axisalongthebeam pipe.Thex-axispointsfromtheIPtothecentreoftheLHCring,andthe y-axis

pointsupward.Cylindricalcoordinates (r,φ)areusedinthe transverseplane,φ beingtheazimuthalanglearoundthez-axis.Thepseudorapidityisdefinedinterms ofthe polarangleθas η= −ln tan(θ/2). Transversemomentumandtransverse energyaredefinedas pT=psinθandET=E sinθ,respectively.R isdefinedas



(η)2_{+ (φ)}2_.

Thecalorimeterisalarge-acceptance,longitudinally-segmented sampling detector covering

|

η

| <

4

.

9 with electromagnetic (EM) and hadronicsections.The EM calorimeteris a lead/liquid–argon samplingcalorimeterwithanaccordion-shapedgeometry.Itis di-vided intoa barrel region,covering

|

η

| <

1

.

475, andtwo endcap regions, covering1

.

375

<

|

η

| <

3

.

2.TheEMcalorimeterhasthree primary sections,longitudinalin showerdepth,called“layers”, in the barrel region andup to

|

η

| =

2

.

5 in the end cap regions. In the barreland first partof theend cap(

|

η

| <

2

.

4), with the ex-ception of the regions 1

.

4

<

|

η

| <

1

.

5, the first layer has a fine segmentation in

η

(



η

=

0

.

003–0

.

006) to allow the discrimina-tionofphotonsfromthetwo-photondecaysof

π

0 _and

_η

_mesons.

Overmostoftheacceptance,thetotalmaterialupstreamoftheEM calorimeterrangesfrom2

.

5 to6 radiationlengths.Inthetransition regionbetweenthebarrelandendcapregions(1

.

37

<

|

η

| <

1

.

52), the amount of material rises to 11.5 radiation lengths, and thus thisregionisnotusedforthedetectionofphotons. Thehadronic calorimeterislocated outsidetheEMcalorimeter.It consistsof a steel/scintillator-tile sampling calorimeter covering

|

η

| <

1

.

7 and a liquid–argon calorimeter with copper absorber covering 1

.

5

<

|

η

| <

3

.

2.

The forward calorimeter (FCal) is a liquid–argon sampling calorimeterlocatedoneitherside oftheinteractionpoint.It cov-ers 3

.

1

<

|

η

| <

4

.

9 and each half is composed of one EM and two hadronic sections, with copper and tungsten serving as the absorber material, respectively. The FCal is used to characterise the centrality of Pb

+

Pb collisions as described below. Finally, zero-degree calorimeters(ZDC)are situatedatlarge pseudorapid-ity,

|

η

| >

8

.

3,andareprimarilysensitivetospectatorneutrons.

Atwo-leveltriggersystemisusedtoselectevents,witha first-level trigger implemented in hardware followed by a software-based (high-level) trigger. Data for this measurement were ac-quiredusinga high-level photontrigger [31] coveringthe central region (

|

η

| <

2

.

5). At the first-level trigger stage, the transverse energyofEMshowersiscomputedwithinregions of

φ

× 

η

=

0

.

1

×

0

.

1, and those showers which satisfy an ET threshold are

usedtoseedthehigh-leveltriggerstage.Atthisnextstage, recon-structionalgorithmssimilartothoseappliedintheofflineanalysis usethefulldetectorgranularitytoformthefinaltriggerdecision. Thetriggerwasconfiguredwithanonlinephoton-pT thresholdof

30 GeV (20 GeV)inthepp (Pb

+

Pb)runningperiodandrequired thecandidatephotontosatisfyasetofloosecriteriaforthe elec-tromagnetic showershape [31]. ForthePb

+

Pbdata-taking, the high-level trigger included a procedure to estimate and subtract theunderlyingevent(UE)contributiontothe ET measuredinthe

calorimeter [9],ensuring highefficiency inhigh-activity Pb

+

Pb events.

Inadditionto thephotontrigger,Pb

+

Pbdatawererecorded withminimum-biastriggers;theseeventsareusedtocharacterise thecentralityofPb

+

Pbcollisionsasdescribed inSection3.The minimum-bias triggers arebased on thepresence ofa minimum amount ofapproximately50 GeV of transverseenergy inall sec-tions of the calorimetersystem(

|

η

| <

3

.

2) or, forevents that do not meet this condition, on substantial energy deposits in both ZDC modules andan inner-detector trackidentified by the high-leveltriggersystem.

3. DataselectionandMonteCarlosamples

Photon–jeteventsin pp andPb

+

Pbcollisionsareinitially se-lected foranalysisby thehigh-leveltriggers describedabove. The typical number ofinteractions per bunch crossing in the pp and

Pb

+

Pbdata-takingwereoneandsmallerthan10−4,respectively. Events are required to satisfy detector and data-quality require-ments, andto contain a vertex reconstructed from tracks in the

The ATLAS Collaboration / Physics Letters B 789 (2019) 167–190 169

inner detector. An additional requirement in Pb

+

Pb collisions, basedon the correlation of the signalsin the ZDCandthe FCal, is used to reject a small number of recorded events consistent withtwoPb

+

Pbinteractions inthe samebunchcrossing (pile-up) [32]. The pile-up rate is largest in the most central events, where it is at most0.1% and rejected withan efficiency greater than98%.Nopile-uprejectionisappliedin pp collisions.

The centrality of Pb

+

Pb events is defined using the total transverseenergymeasured intheFCal,evaluated atthe electro-magnetic scale and denoted by



ET. The same observable was

used to characterise 2010 and 2011 Pb

+

Pb data at

√

sNN

=

2

.

76 TeV [33] and a similar procedure, based on Monte Carlo Glaubermodeling [34],isfollowedin2015data [35].Inthis anal-ysis, Pb

+

Pb eventswithin five centralityranges are considered that represent0–10%(largest



ET values anddegree ofnuclear

overlap),10–20%,20–30%,30–50%and50–80%(smallest



ET

val-uesand degree of nuclear overlap) of the population. The mean numberofparticipatingnucleonsinminimum-biasPb

+

Pb colli-sions,Npart,rangesfrom33

.

3

±

1

.

5 in50–80%eventsto358

.

8

±

2

.

3

in0–10%events.

MonteCarlosimulationsof

√

s

=

5

.

02 TeV pp photon–jetevents are used to correct the data for bin migration and inefficiency effects,andforcomparisonwithdistributionsmeasuredinpp

col-lision data. For all the samples described below, the generated eventswere passed through a full Geant 4 simulation [36,37] of theATLASdetectorunderthesameconditionspresentduring data-takingandwere digitised andreconstructed in thesame wayas thedata.

Fortheprimarysimulationsamples,the Pythia 8.186 [38] gen-eratorwasusedwiththeNNPDF23LOpartondistributionfunction (PDF)set [39],andgeneratorparameterswhichweretunedto re-producea set of minimum-biasdata (the “A14” tune) [40]. Both the direct and fragmentation photon contributions are included in the simulation. Six million pp events were generated with a generator-level photon in the pT range 50 GeV to 280 GeV.

Ad-ditionally,a sampleof18million eventswere producedwiththe samegenerator,tune andPDF, andwere overlaidatthe detector-hit level with minimum-bias Pb

+

Pb events recorded during the 2015 run. The relative contribution of events in this “data-overlay” sample were reweighted on an event-by-event basis to matchthe



ETdistributionobservedinthephoton–jet eventsin

Pb

+

Pbdataselected foranalysis. Thus thePb

+

Pbsimulation samplescontainunderlying-eventactivitylevelsandkinematic dis-tributionsofjets(usedinthecombinatoricphoton–jetbackground estimation)identicaltothoseindata.

Additional samples of 0

.

3 million pp events and 6 million events overlaid with Pb

+

Pb data were produced with the Sherpa2.1.1 [41] generatorusing theCT10PDF set [42], aswere 0

.

6 million pp Herwig 7 [43] eventswiththeMMHTUEtuneand PDF set [44]. The Sherpa samples were generated with leading-ordermatrixelementsforphoton–jetfinalstateswithuptothree additional partons, which were merged with the Sherpa parton shower. The Herwig events were generated in a way that in-cludesthedirectandfragmentationphotoncontributions.Boththe Sherpaand Herwig samples were filtered for the presence of a photon in the required kinematic region, and are used because theycontain differentphoton

+

multijettopological distributions andjet-flavourcompositions.

At generatorlevel, photons are required tobe isolated by re-quiringthesumofthetransverseenergycarriedby primary par-ticles2 _{ina coneofsize}

_R

₌

₀

_.

_{3 aroundthephoton, E}iso

T ,to be

2 _{Primaryparticlesaredefinedasthosewithapropermeanlifetime,τ,exceeding}

cτ=10 mm.ForthejetandisolationETmeasurements,muonsandneutrinosare excludedfromthedefinition.

smaller than 3 GeV. In the analysis, the background subtraction, describedbelow,removesphotonswhichpasstheisolationcut in data but fail this isolation requirement at the particle level. Jets are definedbyapplying theanti-kt algorithm [45,46] withradius

parameter R

=

0

.

4 to primary particleswithin

|

η

| <

4

.

9.In simu-lation,thejetflavour,i.e.whetheritisquark- orgluon-initiated,is definedastheflavour ofthehighest-pT partonthat pointsto the

generator-leveljet [47].

4. Eventreconstruction 4.1. Photonreconstruction

Photoncandidatesarereconstructedfromclustersofenergy de-posited in EM calorimeter cells, following a procedure used for previous measurements of isolated prompt photon production in Pb

+

Pbcollisions [9].The procedure is similar tothat used ex-tensively in pp collisions [48,49], but isapplied to the calorime-ter cells after an event-by-event estimation and subtraction of the pile-upand UE contributionto the depositedenergy ineach cell [14]. InPb

+

Pbcollisions, all photon candidatesare treated asiftheywereunconvertedphotons.Photonidentificationisbased primarilyonshowershapesinthecalorimeter [50],selectingthose candidates which are compatible with originating from a single photon impacting the calorimeter. The measurement of the pho-ton energyis based onthe energycollected in a smallregion of calorimetercellscentredonthephoton(



η

× φ =

0

.

075

×

0

.

175 inthe barreland



η

× φ =

0

.

125

×

0

.

125 inthe endcaps),and is corrected via a dedicated calibration [51], which accounts for upstream losses and both lateral and longitudinal leakage. The sum of transverse energy in calorimeter cells inside a cone size of

R

=

0

.

3 centred on thephoton candidate, excluding a small centralareaofsize



η

× φ =

0

.

125

×

0

.

175,isusedtocompute the isolation energy Eiso_T .It is correctedforthe expectedleakage ofthephotonenergyintotheisolationcone.

Reconstructed photon candidates are required to satisfy iden-tification and isolation criteria. The identification working point (called “tight”)includes requirementson each of several shower-shape variables [50]. These criteria reject two-photon decays of neutral mesons using information in the finely segmented first calorimeterlayers, and rejecthadrons whichbegan showering in the EM section using informationfrom thehadronic calorimeter. The isolation energy is required to be Eiso_T

<

3 GeV in pp

col-lisions. In Pb

+

Pbcollisions, where UE fluctuationssignificantly broaden thedistributionof Eiso_T values,thisrequirementissetto approximatelyonestandarddeviationoftheGaussian-likepartof thedistributioncentredatzero,Eiso_T

<

8 GeV.

In simulation, prompt photons in pp collisions have a total reconstruction and selection efficiency greater than 90%. At low

pT

≈

60 GeV inthemostcentralPb

+

Pbcollisions,thisefficiency

is

≈

60%,risingwithincreasing pT andinlesscentralcollisions.In

all events, the pT scale, defined as the mean ratio of measured

photon pT to the generator-level pT, for photons which satisfy

thesecriteriais within 0

.

5%(1%)of unityin thebarrel(endcap). The pT resolutiondecreasesfrom3%to2%overthemeasured pT

range.

4.2. Jetreconstruction

Jetsarereconstructedfollowing theprocedurepreviously used in 2.76 TeV and 5.02 TeV pp and Pb

+

Pbcollisions [14,15,52], whichisbrieflysummarisedhere.Theanti-kt algorithm [46] with R

=

0

.

4 is appliedto energydeposits inthe calorimetergrouped into towers ofsize



η

× φ =

0

.

1

×

0

.

1. An iterative procedure,

170 The ATLAS Collaboration / Physics Letters B 789 (2019) 167–190

based entirely on data, is used to obtain an event-by-event es-timate of the average

η

-dependent UE energy density, including thatfrompile-up,whileexcludingfromtheestimatethe contribu-tionfromjetsarisingfromahardscattering.An updatedestimate ofthe jet four-momentum isobtainedby subtracting the UE en-ergy from the constituent towers of the jet. This procedure is also applied to pp data. The pT values of the resulting jets are

corrected for the average calorimeter response using an

η

- and

pT-dependent calibration derived from simulation. An additional

correction,derivedfrominsitu studiesofeventswitha jet recoil-ingagainstaphotonorZ bosonandfromthedifferencesbetween theheavy-ionreconstructionalgorithmandthat normallyusedin the13 TeV pp data [53],isapplied.Afinalcorrectionatthe anal-ysis level is applied to correctfor a deficiency in jet calibration dueto itbeingderived fromaneventsamplewithadifferentjet flavourcomposition.

The distribution of reconstructed jet pT values was studied

in simulation asa function of generator-level jet pT. In pp and

Pb

+

Pbcollisions,thejetpTscaleiswithin1%ofunity.Inpp

col-lisions,thejet pTresolutiondecreasesfrom15%atpT

≈

30 GeV to

10%atpT

≈

200 GeV.InPb

+

Pbcollisions,theresolutionatfixed

jet pT becomes worseinmore centralcollisions inaway

consis-tent with the increasing magnitude ofUE fluctuationsin the jet cone.In themostcentral eventsandat thelowest jet-pT values,

theresolution reaches50%. Athigh pT, theresolution

asymptoti-cally becomes centrality-independent and, at200 GeV,consistent withthat in pp collisions.Moreinformation aboutthejet recon-struction and jet performance in this dataset may be found in Ref. [54].

5. Dataanalysis

5.1. Photonpurityandyield

After applying the identification and isolation selection crite-ria in pp collisions, approximately 19500, 7800, 4100 and 400 photons are selected with pγ_T

=

63

.

1–79

.

6 GeV, 79

.

6–100 GeV, 100–158 GeV and 158–200 GeV, respectively. In Pb

+

Pb colli-sions,theanalogousyieldsare15400,6300,3500 and300.These rawyields are determined asafunction of pγ_T andare then cor-rectedforbackgroundandfortheeffectsof pTbinmigration.

First, the selected photon sample is corrected for the back-groundcontribution,primarilyfrommisidentifiedneutralhadrons. For each pγ_T and centrality range,the purity of prompt photons withinthisrangeisestimatedwithadouble-sidebandapproach [9,

48,49],whichissummarisedinthefollowing.

In addition to the nominal selection, background-enhanced samples of photon candidates are defined by selecting photons failing at least one of four specific shower-shape requirements (referred to as the “non-tight” selection), or by requiring that they are not isolated such that Eiso_T

>

5 GeV in pp collisions or

Eiso_T

>

10 GeV in Pb

+

Pb collisions. Regions A and B are de-fined as those containing tight photons which are isolated and non-isolated,respectively,withregion A correspondingtothe sig-nalphoton selection.Regions C and D containnon-tight photons which are isolated andnon-isolated, respectively. The numberof photon candidatesineach regionis generallyamixture ofsignal andbackgroundphotons,i.e.thosearisingfromneutralmesons in-sidejets.The Eiso

T distributionforbackgroundphotonsisexpected

tobethesameforthetightandnon-tightselectionssuchthatthe distributionofbackgroundphotons“factorises”alongisolationand identificationaxes.Separately,theprobability thata prompt pho-tonisfound inregions B, C or D isdetermined fromsimulation. This information andthe background factorisation assumption is

thenappliedtothedatatodeterminethepurityofphotonsin re-gion A, definedas the ratioof the number of signal photons to all selected photons. The purity increasessystematically with pγ_T

over themeasured pT range.In pp collisions,itrises from

≈

85%

at pγ_T

=

80 GeV tomore than95% at100 GeV,while inPb

+

Pb collisionsitistypically

≈

75–90%overthesamekinematicrange.

The background-corrected promptphoton yields are then cor-rectedfortheresolutionofthepγ_T measurement.Thisisperformed by comparingtheyields,evaluated separatelyasafunctionof re-constructed andgenerator-level pT,insimulation.Giventhegood pT resolution,thesedifferby2%atmost,andthissmallresulting

correctionisappliedtotheyieldsindata.

5.2. Jetbackgroundsubtraction

Therawjetyields,measuredasafunctionofxJγ ,arecorrected

for two background components using data-driven methods. The correctionsareperformedseparatelyforeach pγ_T intervaland sep-aratelyinpp collisionsandPb

+

Pbcollisionsofdifferent central-ityranges.

The firstbackgroundarisesfromthecombinationofahigh-pT

photon withjetsunrelatedto thephoton-producing hard scatter-ing. Theseinclude jetsfromseparate hard parton–parton scatter-ings and UE fluctuations reconstructed as jets. This background is negligible in pp collisions. Because of the inclusive jet selec-tion inthe analysis, the combinatoricbackgroundis purely addi-tive and can be statistically subtracted after scaling to the total photon yield. The combinatoric jet yields are determined in the data-overlay simulation, by examining the yield of reconstructed jets separatedfrom a generator-level photon by

φ >

7

π

/

8. Re-constructedjetsthat arenotconsistent withagenerator-level jet, i.e.nogenerator-level jetwith pT

>

20 GeV within

R

<

0

.

4,are

deemedtoarisefromtheoriginalPb

+

Pbdataeventandarethus labelledas“combinatoric”jets.Thecombinatoricjetyieldsare sub-tractedfromthemeasuredxJγ distributionsindata.

The second background is related to the estimated purity of the selectedphotons. The xJγ yields forphoton candidates in

re-gion A containan admixture ofdijets, specificallyjets correlated with misidentified neutral mesons. Since thesehadrons pass ex-perimental isolation requirements,they maybe, forexample,the leadingfragment insideajet.Theshapeofthisbackgroundinthe

xJγ distributionisdeterminedbyrepeatingtheanalysisforphoton

candidates in region C , since this region contains mostly neutral mesons that remain isolated at the detector level. The resulting per-photon xJγ distributions are scaled to match the number of

backgroundphotons,asdeterminedaboveinSection5.1,andtheir yieldsarestatisticallysubtractedfromthejetyieldsforphotonsin region A.

Fig.1showsthesizeofthesebackgroundsinthelowest-pγ_T in-terval,wheretheyarethelargest.Thecombinatoricjetbackground for Pb

+

Pb collisions contributesprimarily to kinematicregions populated by pjet_T

<

50 GeV. It also dependsstrongly on central-ity,beinglargestin0–10% collisionsbutnearly negligiblealready in 30–50%collisions.The dijetbackgroundcontributestoa broad rangeofpjet_T valuesincludingtheregionxJγ

>

1,sincethepTratio

ofajettooneofthehadronsinthebalancingjetcangenerallybe aboveunity.Thisbackgroundhasasimilarshapeinalleventtypes. However,sincethephotonpurityislowerinPb

+

Pbeventsthan in pp events,thiscorrectionislargerintheformer.

5.3. Unfolding

The background-subtracted xJγ yields are corrected for

bin-migration effects due to detector resolution via a Bayesian un-folding procedure [55,56]. To accomplish this, the reconstructed

The ATLAS Collaboration / Physics Letters B 789 (2019) 167–190 171

Fig. 1. Distributionsofthephoton–jetpT-balancexJγ forthephotontransversemomentumintervalpγT=63.1–79.6 GeV for(left)pp,(centre)50–80%centralityand(right) 0–10%centralityPb+Pbevents.Solidgrey,dottedred,anddashedbluehistogramsshowtherawjetyields,theestimateofthecombinatoricbackground(non-existent forpp events),andthedijetbackground,respectively.Blackpointsshowthebackground-subtracteddatabeforeunfolding,withtheverticalbarsrepresentingthecombined statisticaluncertaintyfromthedataandbackgroundsubtractionprocedure.

yields are arranged in a two-dimensional

(p

γ_T

,

xJγ

)

matrix with

binedgesthat are evenlyspacedon logarithmicscales (andwith valuesmatchingthose usedinpreviousjet measurements),anda two-dimensionalunfoldingisperformedsimilartothatfordijetpT

correlationsinRef. [12].TheunfoldingisperformedinxJγ directly

topreservethefinecorrelationbetweenpjet_T andpγ_T whichwould bewashed out ifthe unfolding wereperformed in

(p

γ_T

,

pjet_T

)

. Al-thoughthemigrationalongthepγ_T axisissmall,itisnecessaryto includeitsincethedegreeofbinmigrationinxJγ depends onthe pTofthejets.

Tofullyaccountfortheeffectsofbinmigrationacrossthe anal-ysisselection, the axes of the matrix are extended over a larger rangeofpγ_T andxJγ thanthefiducialregioninwhichtheresults

are reported.A responsematrix isdetermined by matching each pairof

(p

γ_T

,

xJγ

)

valuesatthegeneratorleveltotheircounterparts

atthereconstruction level,separately for pp events andforeach Pb

+

Pbcentrality.

TheBayesianunfoldingmethodrequiresachoiceforthe num-ber of iterations, niter, and an assumption for the prior for the

initial particle-level distribution. The Pythia simulation does not include the effects of jet energy loss, and thus the underlying particle-leveldistribution indataisexpectedtohaveashape dif-ferentfromthedefaultprior inthe simulation.An initial unfold-ing usingthe default Pythia prior is performedforeach central-ityselection, and the ratios of the unfolded distributions to the generator-level priors in Pythia are fitted with a smooth func-tioninxJγ in each pγT interval. Thisfunctionis evaluatedto give

aweight w

=

w(xJγ

,

pγT

)

that isused to reweightthe

generator-leveldistributioninsimulationandthusconstructanominalprior. Alternativereweightings, usedinevaluating thesensitivity tothe choice of prior, are determined by applying

√

w (the geometric meanof thenominal reweighting andno reweighting)and w3/2

tothesample.Thereconstruction-levelxJγ distributionsin

simula-tionaftereachofthesereweightingswereexaminedtoensurethat theyspanareasonablerangeofvaluescomparedtothatobserved atthereconstructionlevelindata.

Beforeapplyingtheunfolding proceduretodata,it wastested on simulation. After the nominal reweighting, the Monte Carlo samplesweresplit intotwo statisticallyindependent subsamples. Onesubsamplewas usedto populatethe responsematrix,which was then used to unfold the reconstruction-level distribution in theothersubsample.Theunfolded resultwascomparedwiththe original generator-level distribution in the latter sample, which

werefoundtoberecoveredwithinthelimitsofthestatistical pre-cisionofthesamples.

The values of niter used for the nominal results are chosen

following the sameprocedure as inRef. [12]. For each centrality selection,theunfoldeddistributionsareexaminedasafunctionof

niter.Foreachvalueofniter,a totaluncertaintyisformedbyadding

two components in quadrature: (1) the statistical uncertainty of theunfoldeddata,whichgrowsslowly withniter,and(2)thesum

ofsquare differencesbetweentheresultsandthoseobtainedwith an alternative prior, which decreases quickly withniter.The final

values ofniter are chosen to minimise the total uncertainty, and

arebetweentwoandfour.

TheunfoldedxJγ resultsarecorrectedforthejetreconstruction

efficiency,evaluatedinsimulationasthepγ_T-dependentprobability that ageneratedjet atthegivenxJγ is successfullyreconstructed

within the total

(p

γ_T

,

xJγ

)

range used in the unfolding. This

effi-ciency is typically

>

99% for all events in the kinematic regions populatedbyjetswithpT

>

50 GeV.Inpp collisions,thisefficiency

falls to

≈

96% in the lowest-xJγ region for each pγT interval. In

Pb

+

Pbcollisions,theefficiencyatfixed xJγ decreases

monotoni-callyinincreasinglycentralevents,reaching aminimumof

≈

75% inthelowest-xJγ regionin0–10%centralityevents.

6. Systematicuncertainties

Theprimary sourcesofsystematicuncertaintycanbe grouped intothreemajorcategories:themeasurementofpjet_T ;theselection ofthephotonandmeasurementofpγ_T;themodellingand subtrac-tionofthecombinatoricbackground;andtheunfoldingprocedure. Foreachvariationdescribedbelow,theentireanalysisisrepeated includingthebackgroundcorrectionsteps andunfolding.The dif-ferences betweenthe resulting xJγ values andthe nominal ones

aretakenasanestimateoftheuncertaintyfromeachsource. Astandard setofuncertainties in thejet pT scale and

resolu-tion, followingthe strategy described in Ref. [57] andcommonly used for measurements in 2015 Pb

+

Pb and pp data [54,58], are usedinthis analysis.The impact oftheuncertainties is eval-uated by modifying the response matrix according to the given variations in the reconstructed jet pT. These include

uncertain-tiesinthe pT scalederived frominsitu studiesofthecalorimeter

response [47,59], an uncertainty in the resolution derived using data-driventechniques [60],anduncertaintiesinbothwhichresult froma small relativeenergy-scale difference betweenthe heavy-ion jet reconstruction procedure and that used in

√

s

=

13 TeV

172 The ATLAS Collaboration / Physics Letters B 789 (2019) 167–190

Fig. 2. Unfoldeddistributionsandsummaryofsystematicuncertaintiesintheper-photonjet-yieldmeasurementforpγT=63.1–79.6 GeV in(left)pp eventsand(right)0–10% centralityPb+Pbevents.Toppanelsshowthephoton–jetpT-balancexJγ distributionsandtotaluncertainties,whilethebottompanelsshowtheabsoluteuncertainties

fromjet-related,photon-related,andmodellingorunfoldingsources,aswellasthetotaluncertainty. pp collisions [53].All oftheabove uncertainties apply equallyto

jetsin pp and Pb

+

Pb events. A separate, centrality-dependent uncertainty isincluded in 0–60% Pb

+

Pb collisions. This uncer-taintyaccountsforapossiblemodificationofthejetresponseafter energy loss and is evaluated through insitu comparisons of the charged-particletrack-jetandcalorimeter-jetpTvaluesindataand

simulation. More details are provided in Refs. [54,57]. No addi-tionaluncertaintyisincludedfor60–80%centralityevents.

Uncertaintiesinthe photonpurityestimateare determinedby varying the non-tight identificationand isolation criteriaused to selecthadronbackgroundcandidatesandbyconsideringapossible non-factorisationofthehadronbackgroundalongtheaxesusedin thedouble-sidebandprocedure.Thesensitivitytothemodellingof photonshowershapesinsimulationisevaluatedbyremovingthe data-driven correctionsto these quantities [50]. Finally,the pho-ton pTscaleandresolutionuncertaintiesaredescribedindetailin

Ref. [51],andtheirimpactisevaluatedbyapplyingthemas varia-tionstotheresponsematricesusedinunfolding.

Modelling- or unfolding-related systematic uncertainties arise fromseveralsources.Theestimateofthecombinatoricphoton–jet rateinthedata-overlaysimulationissensitivetotherequirement on the minimum pT of a generator-level jet in the classification

ofa given reconstructedjet asa combinatoric jet, asopposedto aphoton-correlatedjet.Toprovideoneestimate ofthesensitivity tothisthreshold,itisvaried intherange20

±

10 GeV.Toassess thesensitivitytothechoiceofprior,theunfoldingisrepeated us-ing the alternative priors which are systematically closer to and farther from the original Pythia prior. The sensitivity to statisti-cal limitations of the simulation samples is determined through pseudo-experiments, resampling entriesin the response matrices accordingtotheir uncertainty.Finally,theanalysisisrepeated us-ingthe Sherpa simulationtoperformthecorrectionsand unfold-ing,sincethisgeneratorprovidesadifferentdescriptionofphoton– jetproductiontopologies.

Fig. 2 summarises the systematic uncertainties in each cate-gory, aswell asthe total uncertainty, forthe lowest-pγ_T interval inpp and0–10%Pb

+

Pbevents.Thejet-relateduncertaintiesare generally the dominant ones, except in more central events and

lower-pγ_T intervals, wheretheunfoldingandmodelling uncertain-tiesbecomeco-dominant.

AsanadditionalcheckonthefeaturesintheunfoldedxJγ

dis-tributions observed in data, the analysis was repeated with two modificationswhichchangethesignalphoton–jetdefinition.First, the photon–jet

φ

requirement was changed from

>

7

π

/

8 to

>

3

π

/

4.Withthisalteration,thecorrelatedjetyieldchangesonly by a smallamount,while thecombinatoricbackground,which is constant in

φ

, doubles. Second, the analysis was repeated, but selecting only the leading (highest-pT) jet in the event if it fell

withinthe

φ

window.Inthiscase,thecombinatoricbackground contributionisnolongerpurelyadditiveandtheinefficiencywhen a higher-pT uncorrelated jet is selected instead of the

photon-correlated jetmust be accountedfor,similarto Ref. [12].In both cases, thedistributions in Pb

+

Pb exhibit a qualitatively similar modification pattern compared to themain results asa function of xJγ .

7. Results

The unfolded

(

1

/

Nγ

)(

dN

/

dxJγ

)

distributions in pp collisions

are shownforeach pγ_T intervalinFig.3.Thedistributions are re-portedforall xJγ binswherethejet minimum pT requirementis

fully efficient. Also shown are the corresponding generator-level distributions from the Pythia, Sherpa and Herwig samples.Each generator describes the data fairly well, with Herwig generally overpredictingtheyieldatlarge-xJγ and Sherpa showingthebest

agreementoverthefullxJγ range.

The unfolded

(

1

/

Nγ

)(

dN

/

dxJγ

)

distributions in Pb

+

Pb

col-lisions arepresentedinFigs.4through 7,witheach figure repre-sentingadifferentpγ_T interval.Sincetheresultsarefullycorrected, they maybe directly compared withthe analogous xJγ

distribu-tions in pp collisions, which are reproduced in each panel for convenience.

Forall pγ_T intervals,thexJγ distributionsinPb

+

Pbcollisions

evolvesmoothlywithcentrality.Forperipheralcollisionswith cen-trality50–80%,theyaresimilartothosemeasuredinpp collisions.

However, inincreasingly more centralcollisions, thedistributions become progressively more modified. For the pγ_T

<

100 GeV

in-The ATLAS Collaboration / Physics Letters B 789 (2019) 167–190 173

Fig. 3. Photon–jetpT-balancedistributions(1/Nγ)(dN/dxJγ)inpp collisions,eachpanelshowingadifferentphoton-pTinterval.Theunfoldedresultsarecomparedwith theparticle-leveldistributionsfromthreeMonteCarloeventgenerators.Bottompanelsshowtheratiosofthegeneratorstothepp data.Totalsystematicuncertaintiesare shownasboxes,whilestatisticaluncertaintiesareshownasverticalbars.

tervalsshown inFigs. 4 and5,the xJγ distributions in the most

central0–10% eventsare sostrongly modifiedthat they decrease monotonically over the measured xJγ range and no peak is

ob-served. For the pγ_T

>

100 GeV region shown in Fig. 6, the xJγ

distributionsretain a peak atornearxJγ

≈

0

.

9 eveninthe most

centralcollisions.However,themagnitudeofthepeakislowerand significantlywiderthanthesharppeakinpp events.Inbothcases, thejetyield atsmallxJγ issystematically higherthanthat in pp

collisions,byup toafactoroftwo.Inlesscentral events,a peak-likestructure develops atthe same positionas the maximumin

pp events, near xJγ

≈

0

.

9.Forthe lowest-pγT interval,this occurs

onlyfor50–80%centralityevents,whileinthehighesttwopγ_T in-tervalsthedistributionin0–10%eventsis consistentwithalocal peak.

Asanother wayof characterising howthe modified xJγ

distri-butions depend oncentrality and pγ_T,Fig. 8 presentstheir mean value,



xJγ



, and integral, Rγ , with both values calculated in the

region xJγ

>

0

.

5.Thesequantities areshownasa functionof the

meannumberofparticipatingnucleonsNpart inthecorresponding

centralityselection,andareplottedforthefirstthree pγ_T intervals wheretheyhavesmallstatisticaluncertainties.Whenmeasuredin theregionxJγ

>

0

.

5,thevalueof



xJγ



inpp collisionsisobserved to be

≈

0

.

89 for all pγ_T intervals. Simulation studies show that, at generator level,the jet yield atxJγ

>

0

.

5 corresponds to only

theleading(highest-pT)photon-correlatedjetineachevent.Thus,



xJγ



can be interpretedas aconditional per-jet fractional energy loss,and Rγ canbeinterpreted asthefractionofphotonswitha leading jetabove xJγ

=

0

.

5.In pp collisions, Rγ rangesfrom0

.

65

to0

.

75 inthethree pγ_T intervalsshown,whichisbelowunitydue tothejetselectioncriteria(

φ >

7

π

/

8,

|

η

| <

2

.

8).

InPb

+

Pbevents,



xJγ



decreasesmonotonicallyfromthevalue inpp collisionsasthecollisionsbecomemorecentral.Inthemost central collisions,it isbelowthe pp value by0.04–0.06, depend-ing onthe pγ_T interval, whilein peripheralcollisions itreachesa

174 The ATLAS Collaboration / Physics Letters B 789 (2019) 167–190

Fig. 4. Photon–jetpT-balancedistributions(1/Nγ)(dN/dxJγ)inPb+Pbevents (redcircles)witheachpanelshowingadifferentcentralityselectioncomparedtothatin

pp events(bluesquares).ThesepanelsshowresultsforpγT=63.1–79.6 GeV.Totalsystematicuncertaintiesareshownasboxes,whilestatisticaluncertaintiesareshownas verticalbars.

Fig. 5. Photon–jetpT-balancedistributions(1/Nγ)(dN/dxJγ)inPb+Pbevents (redcircles)witheachpanelshowingadifferentcentralityselectioncomparedtothatin

pp events(bluesquares).ThesepanelsshowresultsforpγT=79.6–100 GeV.Totalsystematicuncertaintiesareshownasboxes,whilestatisticaluncertaintiesareshownas verticalbars.

valuewhichisstatisticallycompatiblewiththatin pp events.The

Rγ value also decreases monotonically as the collisions become morecentral, reflectingthe overall shiftofthe xJγ value of

lead-ingjetsbelowxJγ

=

0

.

5.Atlow pγT incentral Pb

+

Pbcollisions, Rγ reachesthevalueof0

.

5,whichisonly

≈

75%ofitsvalueinpp

collisions.

Theresultsarecomparedwiththefollowingtheoretical predic-tions whichinclude MonteCarlogenerators andanalytical calcu-lations ofjet energy loss: (1)a pQCD calculationwhich includes Sudakov resummation to describe the vacuum distributions and energy loss in Pb

+

Pb collisions as described in the BDMPS-Z formalism [26], (2) a perturbative calculation within the frame-work of soft-collinear effective field theory with Glauber gluons (SCETG)inthesoftgluonemission(energy-loss)limit [27],(3)the

JEWELMonteCarloeventgeneratorwhichsimulatesQCDjet evo-lutioninheavy-ioncollisionsandincludesenergy-losseffectsfrom

radiative andelasticscatteringprocesses [28], and(4)theHybrid Strong/WeakCoupling model [29] which combinesinitial produc-tion using Pythia witha parameterisation ofenergy lossderived fromholographicmethods,andincludesback-reactioneffects.

Figs. 9 and 10 compare a selection of the measured xJγ

dis-tributions withtheresultsofthesetheoretical predictions,where possible. Before testing the description of energy-loss effects in Pb

+

Pbevents,thepredictedxJγ distributionsarecomparedwith pp datainFig.9.TheHybridmodeland JEWEL,whichuse Pythia forthephoton–jet productioninvacuum, givea gooddescription of pp events over the measured xJγ range in both pγT intervals

shown. TheBDMPS-ZandSCETG perturbative calculationscapture

the generalfeatures but predict distributions that are more and lesspeaked,respectively,thanthoseindata.

In Pb

+

Pb events with low pγ_T, shown in the left panel of Fig.10,theJEWEL,Hybrid,andSCETG modelssuccessfullycapture

The ATLAS Collaboration / Physics Letters B 789 (2019) 167–190 175

Fig. 6. Photon–jetpT-balancedistributions(1/Nγ)(dN/dxJγ)inPb+Pbevents (redcircles)witheachpanelshowingadifferentcentralityselectioncomparedtothatin

pp events(bluesquares).Thesepanelsshowresultsfor pγT=100–158 GeV.Totalsystematicuncertaintiesareshownasboxes,whilestatisticaluncertaintiesareshownas verticalbars.

Fig. 7. Photon–jetpT-balancedistributions(1/Nγ)(dN/dxJγ)inPb+Pbevents (redcircles)witheachpanelshowingadifferentcentralityselectioncomparedtothatin

pp events(bluesquares).Thesepanelsshowresultsfor pγT=158–200 GeV.Totalsystematicuncertaintiesareshownasboxes,whilestatisticaluncertaintiesareshownas verticalbars.

severalkey featuresofthe xJγ distribution,includingtheabsence

ofavisiblepeak,andthemonotonicallyincreasingbehaviourwith decreasingxJγ .TheBDMPS-Zmodelpredictsasuppressionofthe

yieldnearxJγ

≈

0

.

9 relativetowhatispredictedinpp events,

con-sistentwiththetrendindata.However,itunderestimatestheyield atlow xJγ in both pp and Pb

+

Pbcollisions. In the higher-pγT

interval, the Hybrid model and JEWEL successfully describe the reappearanceofalocalisedpeaknearxJγ

≈

0

.

9.However,noneof

themodelsconsidered heredescribe theincrease ofthejet yield at xJγ

<

0

.

5 above that observed in pp events. Additional

com-parisons between these data and theoretical calculations which are differential in both pγ_T and centrality will further constrain the description of the strongly coupled medium in these mod-els.

8. Conclusion

This Letter presents a study of photon–jet transverse mo-mentum correlations for photons with 63

.

1

<

pγ_T

<

200 GeV in Pb

+

Pbcollisions at

√

sNN

=

5

.

02 TeV and pp collisionsat

√

s

=

5

.

02 TeV.Thedatawere recordedwiththe ATLASdetectoratthe LHC andcorrespond to integratedluminosities of 0

.

49 nb−1 _and

25 pb−1_{,respectively. The data are correctedfor thepresence of}

combinatoricphoton–jetpairsandofdijetpairswhereoneofthe jetsismisidentifiedasaphoton.Themeasured quantitiesindata are fullycorrected for detectoreffects andreportedat the parti-cle level. Per-photon distributions of the jet-to-photon pT ratio, xJγ

=

pjetT

/p

γ

T,aremeasuredforpairswithanazimuthallybalanced

configuration,

φ >

7

π

/

8. In pp events, thedata arewell repro-ducedbyeventgeneratorsormodelsthatdependonthem,butare

176 The ATLAS Collaboration / Physics Letters B 789 (2019) 167–190

Fig. 8. Summaryof(left)themeanjet-to-photonpTratio



xJγand(right)thetotalper-photonjetyieldRγ,calculatedintheregionxJγ>0.5.Thevaluesarepresentedasa functionofthemeannumberofparticipatingnucleonsNpartintoppanels.EachcolourandsymbolrepresentsadifferentpγT interval,wherethelowestandhighestintervals aredisplacedhorizontallyforclarity.Thepointsplottedat Npart=2 correspondtopp collisions.ThebottompanelsshowthedifferencebetweenthePb+Pbcentrality selectionandpp collisions.Boxesshowthetotalsystematicuncertaintywhiletheverticalbarsrepresentstatisticaluncertainties.

Fig. 9. Photon–jetpT-balancedistributions(1/Nγ)(dN/dxJγ)inpp collisionsfor(left)pγT=63.1–79.6 GeV and(right)p

γ

T=100–158 GeV.Theunfoldedresultsarecompared withthetheoreticalcalculationsshownasdashedcolouredlines(seetext).Totalsystematicuncertaintiesareshownasboxes,whilestatisticaluncertaintiesareshownas verticalbars.

Fig. 10. Photon–jetpT-balancedistributions(1/Nγ)(dN/dxJγ)in0–10%Pb+Pbcollisionsfor(left)pTγ =63.1–79.6 GeV and(right) p

γ

T =100–158 GeV.Theunfolded

resultsarecomparedwiththetheoreticalcalculationsshownasdashedcolouredlinesdenotingcentralvaluesorcolouredbandswhichcorrespondtoarangeoftheoretical parameters(seetext).Totalsystematicuncertaintiesareshownasboxes,whilestatisticaluncertaintiesareshownasverticalbars.

The ATLAS Collaboration / Physics Letters B 789 (2019) 167–190 177

notfullydescribed indetailby approachesbasedon perturbative calculations.

InPb

+

Pbcollisions, xJγ distributionsareobserved tohavea

significantlymodified total yieldandshape compared withthose in pp collisions. These modifications have a smooth onset as a function of Pb

+

Pb event centrality and pγ_T. In peripheral col-lisions at high pγ_T, the distributions in Pb

+

Pb are statistically compatiblewiththosein pp.In themostcentral Pb

+

Pbevents atlow pγ_T,the yield decreasesmonotonically withincreasing xJγ

overthemeasuredrange,instrongcontrasttothesharplypeaked distributions in pp events. However, in less central events or in higher-pγ_T intervals,the xJγ distributionsretaina peak-likeexcess

atan xJγ valuesimilar tothatin pp collisionsbutwithasmaller

per-photonyield.Thislastobservationsuggeststhattheamountof energylostbyjetsinsingleeventshasabroaddistribution,witha smallbutsignificantpopulationofjetsretaininga pp-like pT

cor-relationwiththephoton becausetheydonot loseanappreciable amountofenergy.

Theseresultsaresensitivetohowpartonsinitiallyproduced op-posite toa high-pT photon loseenergyin their interactions with

thehotnuclearmedium.Takentogetherwithothermeasurements ofsingle-jetanddijetproduction, thedata providenew, comple-mentaryinformation abouthow energyloss inthe strongly cou-pledmediumvarieswiththeinitialpartonflavourandpT.

Acknowledgements

We thankCERN for thevery successful operation ofthe LHC, aswell asthe support stafffromour institutions without whom ATLAScouldnotbeoperatedefficiently.

WeacknowledgethesupportofANPCyT,Argentina;YerPhI, Ar-menia; ARC, Australia; BMWFW and FWF, Austria; ANAS, Azer-baijan;SSTC, Belarus; CNPq and FAPESP, Brazil; NSERC, NRC and CFI,Canada; CERN; CONICYT,Chile; CAS, MOSTandNSFC, China; COLCIENCIAS, Colombia; MSMT CR, MPO CR and VSC CR, Czech Republic;DNRFandDNSRC,Denmark;IN2P3-CNRS,CEA-DRF/IRFU, France; SRNSFG, Georgia; BMBF, HGF, andMPG, Germany; GSRT, Greece;RGC,HongKong SAR,China;ISFandBenoziyo Center, Is-rael; INFN, Italy; MEXT and JSPS, Japan; CNRST, Morocco; NWO, Netherlands;RCN, Norway;MNiSW andNCN, Poland;FCT, Portu-gal; MNE/IFA, Romania; MES of Russiaand NRC KI, Russian Fed-eration; JINR; MESTD, Serbia; MSSR, Slovakia; ARRS and MIZŠ, Slovenia;DST/NRF,SouthAfrica;MINECO,Spain;SRCand Wallen-berg Foundation, Sweden; SERI, SNSF and Cantons of Bern and Geneva, Switzerland; MOST, Taiwan; TAEK, Turkey; STFC, United Kingdom;DOEandNSF, UnitedStatesofAmerica. Inaddition, in-dividualgroupsandmembershavereceivedsupportfromBCKDF, Canarie,CRCandComputeCanada,Canada;COST,ERC,ERDF, Hori-zon2020, andMarie Skłodowska-Curie Actions, European Union; Investissementsd’ Avenir Labex andIdex, ANR,France; DFG and AvH Foundation, Germany; Herakleitos, Thales and Aristeia pro-grammesco-financedbyEU-ESFandtheGreekNSRF,Greece; BSF-NSF andGIF, Israel; CERCA Programme Generalitat de Catalunya, Spain;TheRoyalSocietyandLeverhulmeTrust,UnitedKingdom.

The crucialcomputing support fromall WLCG partners is ac-knowledged gratefully,in particularfromCERN, the ATLAS Tier-1 facilities at TRIUMF (Canada), NDGF (Denmark, Norway, Swe-den),CC-IN2P3(France),KIT/GridKA(Germany),INFN-CNAF(Italy), NL-T1(Netherlands),PIC(Spain),ASGC(Taiwan),RAL(UK)andBNL (USA),theTier-2facilitiesworldwideandlargenon-WLCGresource providers.Majorcontributorsofcomputingresources arelistedin Ref. [61].

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