Measurement of prompt photon production in √

Contents lists available atScienceDirect

Physics Letters B

www.elsevier.com/locate/physletb

Measurement of prompt photon production in √

s

_NN

= ⁸ . 16 TeV p + ^Pb collisions with ATLAS

.TheATLAS Collaboration^

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

Articlehistory:

Received7March2019

Receivedinrevisedform19June2019 Accepted15July2019

Availableonline17July2019 Editor: M.Doser

The inclusiveproduction rates of isolated, prompt photons in p+Pb collisions at √_s

NN=⁸.16 TeV are studied withthe ATLAS detectoratthe LargeHadronColliderusing adatasetwith anintegrated luminosity of165 nb⁻¹ recorded in2016. Thecross-section and nuclearmodification factorRpPb are measured asafunctionofphoton transverse energyfrom20 GeV to550 GeV and inthreenucleon–

nucleon centre-of-mass pseudorapidity regions, (−².83,−².02), (−¹.84,0.91), and (1.09,1.90). The cross-sectionandRpPbvaluesarecomparedwiththeresultsofanext-to-leading-orderperturbativeQCD calculation,withand withoutnuclearpartondistributionfunctionmodifications,andwithexpectations basedonamodeloftheenergylossofpartonspriortothehardscattering.Thedata disfavouralarge amountofenergylossandprovidenewconstraintsonthepartondensitiesinnuclei.

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

1. Introduction

Measurements of particle and jet production rates at large transverse energy are a fundamental method of characterising hard-scatteringprocessesin allcollision systems.In collisions in- volving large nuclei, production rates are modified from those measured in proton + ^{proton (pp) collisions dueto a combina-} tion of initial- and final-state effects. The former arise from the dynamicsofpartonsinthenucleipriortothehard-scatteringpro- cess, while the latter are attributed to the strong interaction of the emerging partons with the hot nuclear medium formed in nucleus–nucleus collisions. Modification due to the nuclear en- vironment is quantified by the nuclear modification factor, R_AA, defined as the ratio of the cross-section measured in A + ^{A to} thatin pp collisions,scaledby theexpectedgeometric difference betweenthesystems.

Measurementsofprompt photonproductionrates offera way to isolate the initial-state effects because the final-state photons donotinteractstrongly.Theseinitial-stateeffectsincludethede- gree to which parton densities are modified in a nuclear envi- ronment [1–3], as well as potential modification due to an en- ergy loss arising through interactions of the partons traversing thenucleusprior tothehardscattering [4,5].Constraintsonsuch initial-stateeffectsareparticularlyimportantforcharacterisingthe observedmodificationsofstronglyinteractingfinal states,such as jetandhadron production [6,7],sincetheyaresensitive toeffects

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

fromboth initial- andfinal-state.Due tothesignificantly simpler underlying-eventconditionsinproton–nucleuscollisions,measure- ments ofphotonratescan beperformedwithbettercontrol over systematicuncertaintiesthaninnucleus–nucleuscollisions,allow- ingamorepreciseconstraintontheseinitial-stateeffects.

Prompt photon production has been extensively measured in pp collisionsata varietyofcollisionenergies [8–12] attheLarge HadronCollider (LHC).It wasalsomeasured inlead–lead(Pb+Pb) collisions at a nucleon–nucleon centre-of-mass energy √

sNN = 2.76 TeV [13,14] attheLHC,andingold–goldcollisionsat√

sNN= 200 GeV attheRelativisticHeavy IonCollider(RHIC) [15],where thedatafrombothcollidersindicatethatphotonproductionrates areunaffected bythepassageofthephotonsthroughthehotnu- clear medium. At RHIC, photon production rates were measured in deuteron–goldcollisions at√

s_NN=200 GeV [16,17] and were found to be in good agreement with perturbative QCD (pQCD) calculations. Additionally, jet production [18,19] and electroweak boson production [20–22] were measured in28 nb⁻¹ ofproton–

lead (p+^{Pb) collision data at}√

sNN=⁵.02 TeV recorded at the LHC;theformerisastronglyinteractingfinalstate,whilethelatter isnot.Allmeasurementsprovidedsomeconstraintsoninitial-state effects.

ThedatausedinthismeasurementwerecollectedwiththeAT- LAS detectorduring the p+Pb collisionrunning periodin2016, andcorrespondto anintegratedluminosity of165 nb⁻¹,approx- imately sixtimeslargerthanthemeasurementsmadeinthepre- vious 5.02 TeV data. The proton and lead beams hadan energy of 6.5 TeV and 2.51 TeV per nucleon respectively, resulting in a nucleon–nucleon centre-of-masscollision energyof 8.16 TeVand https://doi.org/10.1016/j.physletb.2019.07.031

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

arapidityboost ofthisframe by±⁰.465 unitsrelativeto theAT- LASlaboratoryframe,dependingonthedirectionofthePbbeam.¹ By convention, the results are reported as a function of photon pseudorapidity in the nucleon–nucleon collision frame, η^∗, with positive η^∗ correspondingtotheprotonbeamdirection,andnega- tive η^∗correspondingtothePbbeamdirection.

Atleadingorder,theprocess p+^Pb→γ+^{X has}contributions fromdirectprocesses,inwhichthephotonisproducedinthehard interaction,andfromfragmentationprocesses,inwhichitis pro- ducedin thepartonshower.Beyondleading orderthedirectand fragmentationcomponentsarenotseparableandonlytheirsumis aphysicalobservable.

To reduce contamination from the dominant background of photonsmainlyfromlight-mesondecaysinjets,themeasurements presented here require the photons to be isolated from nearby particles.Thisrequirementalsoactstoreduce therelativecontri- bution of fragmentation photons in the measurement, and thus, the same fiducial requirement must be imposed on theoretical models when comparing with the data. Specifically, as in pre- vious ATLAS measurements [9,10], the sumof energy transverse to the beamaxis within a cone of R≡

(η)²+ (φ)²=⁰.4 aroundthephoton,E^iso_T ,isrequiredtobesmallerthan4.8+⁴.2× 10⁻³Eγ

T [GeV], where Eγ

T isthe transverseenergyof thephoton.

Atparticlelevel,E^iso_T iscalculatedasthesumoftransverseenergy ofallparticleswithadecaylengthabove10mm,excludingmuons andneutrinos.Thissumiscorrectedfortheambientcontribution fromunderlying-eventparticles,consistentwiththepreviousmea- surements [9,10].

This letter reports a measurement of the cross-section for prompt,isolated photonsin p+Pb collisionsat√

sNN=⁸.16 TeV.

Photons are measured with Eγ

T >20 GeV, the isolation require- mentdetailedabove,andinthreenucleon–nucleoncentre-of-mass pseudorapidity (η^∗) regions, −².83<η^∗<−².02, −¹.84<η^∗<

0.91, and 1.09<η^∗<1.90. In addition to the cross-section, the dataare comparedtoa pp referencecross-sectionderivedfroma previous measurement of promptphoton productionin pp colli- sionsat√

s=^{8 TeVthatusedtheidenticalisolation}condition [9].

The nuclear modification factor R_pPb is derived in each pseudo- rapidity region, using an extrapolation for the different collision energy and centre-of-mass pseudorapidity selection, and is re- portedinthe region Eγ

T >25 GeV where referencedatais avail- able.Furthermore,theratioof RpPbinthe forwardregion tothat inthebackward regionispresented.Themeasurementsare com- pared with next-to-leading-order (NLO) pQCD predictions from Jetphox [23] using parton distribution functions (PDF) extracted fromglobalanalysesthatincludenuclearmodificationeffectsanal- yses [24,25].Additionally,thedataarecomparedwithpredictions fromamodelincludinginitial-stateenergyloss [4,5,26].

2. Experimentalset-up

The ATLAS detector [27] is a multipurpose detector with a forward–backwardsymmetric cylindricalgeometry. Forthis mea- surement, its relevant components include an inner tracking de- tector surrounded by a thin superconducting solenoid, andelec- tromagneticandhadroniccalorimeters.Theinner-detectorsystem

1 ATLASusesaright-handedcoordinatesystemwithitsoriginatthe nominal interactionpoint(IP)inthecentreofthedetectorandthez-axisalongthebeam pipe.Thex-axispointsfromtheIPtothecentreoftheLHCring,andthe y-axis pointsupward.Cylindricalcoordinates(r,φ)areusedinthe transverseplane,φ beingtheazimuthalanglearoundthez-axis.Thepseudorapidityisdefinedinterms ofthepolarangleθasη= −ln tan(θ/2)andtherapidityofthecomponentsofthe beam,y,aredefinedintermsoftheirenergy,E,andlongitudinalmomentum,pz, asy=⁰.5ln^E_E⁺₋^p_p^z

z.

is immersedin a 2 T axial magnetic field andprovides charged- particle tracking in the pseudorapidity range η^lab<2.5 in the laboratory frame. In order of closest to furthest from the beam pipe, itconsists ofa high-granularitysiliconpixel detector,a sil- icon microstrip tracker, and a transition radiation tracker. Addi- tionally,thenewinsertableB-layer [28] hasbeenoperatingasthe innermostlayer ofthetrackingsystemsince 2015. Thecalorime- ter system covers the range η^lab<4.9. In the region η^lab<

3.2, electromagnetic calorimetry is provided by barrel and end- caphigh-granularitylead/liquid-argon(LAr)samplingcalorimeters.

An additional thin LAr presampler covers η^lab<1.8 to cor- rect for energy loss in material before the calorimeters. The LAr calorimetersaredividedintothreelayersinradialdepth.Hadronic calorimetryisprovidedby asteel/scintillator-tilecalorimeter,seg- mented into three barrelstructures within η^lab<1.7, andtwo copper/LAr hadronicendcapcalorimeters,which coverthe region 1.5<η^lab<3.2. Finally, the forward calorimeter covers 3.2<

η^lab<4.9 andisdividedintothreecompartments.Thefirstcom- partment is a copper/LAr electromagnetic calorimeter, while the remainingtwotungsten/LAr calorimetercompartmentscollectthe hadronicenergy.

Duringdata-taking,eventswereinitiallyselectedusingalevel-1 trigger, implemented in custom electronics, based on energyde- positionintheelectromagneticcalorimeter.Thehigh-level trigger [29] was then used to select events consistent with a high-Eγ T photon candidate.The highleveltriggerwas configuredwithfive online Eγ

T thresholds from 15 GeV to 35 GeV. Each trigger is used foran exclusiveregion of the Eγ

T spectrum, starting 5 GeV above the trigger threshold because there the trigger is fullyef- ficient.The highest-thresholdtrigger isused inthemeasurement over the whole Eγ

T rangeabove 40 GeV andis unprescaled. The lower-threshold,prescaled, triggers areusedto performthemea- surementforEγ

T intherangeof20–40 GeV.

Data-taking was divided into two periods with different con- figurations of the LHC beams. In the first period, the lead ions circulatedinbeam1(clockwise)andprotonscirculatedinbeam2, whileinthesecondperiodthebeamswerereversed.Theseperiods correspondedtointegratedluminositiesof57 nb⁻¹ and108 nb⁻¹ respectively.

3. Photonreconstructionandidentification

Photons are reconstructed following a procedure used exten- sively in previous ATLAS measurements [10], of which only the mainfeaturesaresummarisedhere.

Photoncandidatesarereconstructedfromclustersofenergyde- positedintheelectromagneticcalorimeterinthree regionscorre- spondingtothelaboratory-frame(η^lab)positionsofthebarreland forwardandbackwardendcapsη^lab<2.37.Thetransitionregion betweenthebarrelandendcapcalorimeters,1.37<η^lab<1.56, isexcluded dueto its higherlevelofinactive material.The mea- surementof the photon energyis based on theenergy collected incalorimetercellsinanareaofsizeη× φ =⁰.075×⁰.175 in thebarrelandη_{× φ =}0.125×⁰.125 intheendcaps.Itiscor- rectedvia adedicated energycalibration [30] whichaccountsfor lossesinthematerialbeforethecalorimeter,bothlateralandlon- gitudinal leakage, andforvariation of thesampling-fraction with energyandshowerdepth.

The photonsare identified usingthe tight calorimetershower shaperequirementsdescribed inRef. [31].Thetight requirements selectclusterswhicharecompatiblewithoriginatingfromasingle photon impacting the calorimeter.The information usedincludes thatfromthehadroniccalorimeter,thelateralshowershapeinthe second layer ofthe electromagneticcalorimeter,andthe detailed showershapeinthefinelysegmentedfirstlayer.

Theisolationtransverseenergy,E^iso_T ,iscomputedfromthesum ofET valuesintopologicalclustersofcalorimetercells [32] inside acone ofsizeR=⁰.4 centred onthephoton. Thisconesizeis chosen tobecompatiblewithaprevious measurementofphoton production in pp collisions at √

s=^{8 TeV [9], which is used to} constructthereferencespectrumforthe RpPb measurement. This estimateexcludesanareaofη× φ =⁰.125×⁰.175 centredon thephoton,andiscorrectedfortheexpectedleakageofthephoton energyfromthisregionintotheisolationcone.

4. Simulatedeventsamples

Samples of Monte Carlo (MC) simulated events were gener- atedtostudythedetectorperformanceforsignalphotons.Proton–

proton generators were used as the source of events containing photons.Toincludetheeffectsofthe p+Pb underlying-eventen- vironment,thesesimulatedpp eventswerecombinedwithp+^Pb eventsfromdatabeforereconstruction.Inthisway,thesimulated eventscontaintheeffectsofthe p+Pb underlying-eventidentical tothoseobservedindata.

The Pythia 8.186 [33] generatorwasusedtogeneratethenom- inal set of MC events, with the NNPDF23LO parton distribution function (PDF) set [34] anda set of generator parameters tuned toreproduceminimum-biaspp eventswiththesamecollisionen- ergyasthatinthep+^{Pb data(“A14”}tune) [35].Acentre-of-mass boostwasapplied tothegeneratedeventstobringthemintothe same laboratory frame as the data. The generator simulates the directphoton contribution and, through final-stateQEDradiation in 2→^{2 QCD processes,also includes the} fragmentationphoton contributions;thesecomponentsaredefinedtobesignalphotons.

Eventsweregeneratedinsixexclusive Eγ

T rangesfrom17 GeVto 500 GeV.

An additional MC sample was used to assess the sensitiv- ity of the measurement to this choice of generator. The Sherpa 2.2.4 [36] eventgeneratorproducesfragmentationphotonsinadif- ferentwayfrom Pythia andwas thuschosen forthecomparison.

The NNPDF3.0NNLOPDF set [37] was used, andthe eventswere generated in the same kinematic regions as the Pythia events.

Theseeventswere generated withleading-ordermatrix elements forphoton-plus-jetfinalstateswithuptothreeadditionalpartons, whichwere mergedwiththe Sherpa parton shower.The Sherpa sampleproducedresultsconsistentwith Pythia,and,thus,nocor- rectionoruncertaintyisapplied.

The Pythia and Sherpa pp eventswere passed through a full Geant4simulationoftheATLASdetector [38,39].Tomodeltheun- derlyingeventeffects,eachsimulatedeventwascombinedwitha minimum-biasp+^{Pb dataeventandthetwowere}reconstructed together asa singleevent, usingthesamealgorithms asused for the data. These events were split between the two beam con- figurations in a proportion matched to that in data-taking. The underlying event activity levels, as characterized by the sum of the transverse energy in the outgoing-Pb-beam side of the for- wardcalorimeter(3.1<|η^lab_|<4.9), are differentinthephoton- containing data eventsfrom the minimum-biasdata eventsused inthesimulation.Thus,thesimulatedeventswereweightedona per-eventbasistomatchtheunderlyingeventactivitydistribution indata.Furthermore,thephotonshowershapesandidentification efficiency in simulation were adjusted for small differences pre- viouslyobserved betweenthesequantities indataandin Geant4 simulation [31].

5. Dataanalysis

Thedifferential cross-sectionis calculatedforeach Eγ T and η^∗

binas

dσ

dE^γ_T = ¹ Lint

1

E^γ_T

NsigPsig

^{sel trigCMC,}

where Lint isthe integratedluminosity,Eγ

T is thewidth ofthe Eγ

T bin, Nsig isthe yieldof photon candidatespassing identifica- tion and isolation requirements, P_sig is the purity of the signal selection, ^sel is the combined reconstruction, identification and isolationefficiencyforsignalphotons, ^trigisthetriggerefficiency, and CMC is a MC derived bin-by-bin correction for the change in the Eγ

T spectrum fromphotonsmigratingbetweenbins inthe spectrum duetothewidthin theenergyresponse. C_MC isdeter- minedafterallselectioncriteriaatbothreconstructionandparticle levelsareimposed.

Triggerefficiencies ^trig arestudiedusingeventsselectedwith minimum-biastriggers,level-1triggerswithoutadditionalrequire- ments, and photon high-level triggers without identification re- quirements. They are greater than 99.5% for all triggers [29]. In thisanalysistheyaretakenas ^trig=^1,andanyuncertaintyisne- glectedasbeingsub-dominanttootheruncertainties.

Thepurity P_sigisdeterminedviaadouble-sideband procedure used extensively in previous measurements of cross-sections for processes witha photon inthe final state [9,10,40,41] andsum- marised here. In the procedure, four regions are defined which categorisephoton candidatesalong two axes:(1) isolation,corre- sponding toan isolated andan inverted “non-isolated”selection;

(2) identification, corresponding to photons that pass the tight identification requirementsdescribed inRef. [31],and those that passtheloose requirementsofRef. [31] butfailcertaincomponents of the tight requirements, designedto mostly selectbackground.

Themajorityofsignalphotonsareinthetight,isolatedregion,de- fined to be the signal region, while the other three regions are dominated by the background. Photon candidates that comprise thebackgroundareassumedtobedistributedinawaythatisun- correlated along thetwo axes.The yields inthe threenon-signal sidebandsareusedtoestimatetheyieldofbackgroundinthesig- nal regionandiscombinedwiththeyield inthesignal regionto extractthepurity.Theprocedurealsoaccountsforthesmallfrac- tion of signal photonswhich are reconstructed inthe non-signal sidebands;thesequantities,knownasleakagefractions,aredeter- minedfromthesimulationsamplesdescribedinSection4.Thepu- rityistypically45%atEγ

T =^{20 GeV,risesto80%atEγ}T =^{100 GeV} andreaches99%atEγ

T =300 GeV.

Fig. 1showsexample E^iso_T distributions foridentified andiso- latedphotons,thecorrespondingdistributionsforbackgroundpho- tons with the normalisation determined by the double-sideband method, andthe resulting signal-photondistributions afterback- groundsubtraction, comparedwiththose forgenerator-level pho- tons in MC simulation. The figure showsthe shape of the back- ground distribution within the signal region, and the correspon- dencebetweenthebackground-subtracteddataandthesignal-only Pythia 8distributions givesconfidencethat thesimulationsaccu- ratelyrepresentthedata.

The photon selection efficiency is determined from MC sim- ulations. Generated prompt photons are required to be isolated at the generator level, after an estimate of the underlyingevent hasbeensubtractedfromtheisolationenergy,asdescribedabove.

Reconstruction efficiency isdetermined by requiring a photon to have been reconstructed within R=⁰.2 of the generated pho- ton. Reconstructed photons matching to a generated photon are further required to satisfy tight identificationand isolation crite- riadefinedinSection1.Thecombinedefficiencyofsignalphotons to passall reconstructionlevel selections, ^sel, istypically90% at all Eγ

T and η^∗,exceptatEγ

T ≈^{20 GeVwhereitdecreasestoabout} 80%.Fig.2summarisesthedifferentcomponentsofthetotalselec- tionefficiency.Thereconstructionefficiencyis96–99%everywhere,