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Physics Letters B
Measurement of prompt photon production in √
= 8 . 16 TeV p + Pb collisions with ATLAS
a r t i c l e i n f o a b s t ra c t
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.
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, RAA, 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
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) ,where thedatafrombothcollidersindicatethatphotonproductionrates areunaffected bythepassageofthephotonsthroughthehotnu- clear medium. At RHIC, photon production rates were measured in deuteron–goldcollisions at√
sNN=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
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 hascontributions 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,EisoT ,isrequiredtobesmallerthan4.8+4.2× 10−3Eγ
T [GeV], where Eγ
T isthe transverseenergyof thephoton.
Atparticlelevel,EisoT 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√
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 TeVthatusedtheidenticalisolationcondition .
The nuclear modiﬁcation factor RpPb 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  using parton distribution functions (PDF) extracted fromglobalanalysesthatincludenuclearmodiﬁcationeffectsanal- yses [24,25].Additionally,thedataarecomparedwithpredictions fromamodelincludinginitial-stateenergyloss [4,5,26].
The ATLAS detector  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=0.5lnEE+−ppz
is immersedin a 2 T axial magnetic ﬁeld andprovides charged- particle tracking in the pseudorapidity range ηlab<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  hasbeenoperatingasthe innermostlayer ofthetrackingsystemsince 2015. Thecalorime- ter system covers the range ηlab<4.9. In the region ηlab<
3.2, electromagnetic calorimetry is provided by barrel and end- caphigh-granularitylead/liquid-argon(LAr)samplingcalorimeters.
An additional thin LAr presampler covers ηlab<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 ηlab<1.7, andtwo copper/LAr hadronicendcapcalorimeters,which coverthe region 1.5<ηlab<3.2. Finally, the forward calorimeter covers 3.2<
ηlab<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  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.
Photons are reconstructed following a procedure used exten- sively in previous ATLAS measurements , of which only the mainfeaturesaresummarisedhere.
Photoncandidatesarereconstructedfromclustersofenergyde- positedintheelectromagneticcalorimeterinthree regionscorre- spondingtothelaboratory-frame(ηlab)positionsofthebarreland forwardandbackwardendcapsηlab<2.37.Thetransitionregion betweenthebarrelandendcapcalorimeters,1.37<ηlab<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  whichaccountsfor lossesinthematerialbeforethecalorimeter,bothlateralandlon- gitudinal leakage, andforvariation of thesampling-fraction with energyandshowerdepth.
The photonsare identiﬁed usingthe tight calorimetershower shaperequirementsdescribed inRef. .Thetight requirements selectclusterswhicharecompatiblewithoriginatingfromasingle photon impacting the calorimeter.The information usedincludes thatfromthehadroniccalorimeter,thelateralshowershapeinthe second layer ofthe electromagneticcalorimeter,andthe detailed showershapeintheﬁnelysegmentedﬁrstlayer.
Theisolationtransverseenergy,EisoT ,iscomputedfromthesum ofET valuesintopologicalclustersofcalorimetercells  inside acone ofsizeR=0.4 centred onthephoton. Thisconesizeis chosen tobecompatiblewithaprevious measurementofphoton production in pp collisions at √
s=8 TeV , which is used to constructthereferencespectrumforthe RpPb measurement. This estimateexcludesanareaofη× φ =0.125×0.175 centredon thephoton,andiscorrectedfortheexpectedleakageofthephoton energyfromthisregionintotheisolationcone.
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  generatorwasusedtogeneratethenom- inal set of MC events, with the NNPDF23LO parton distribution function (PDF) set  anda set of generator parameters tuned toreproduceminimum-biaspp eventswiththesamecollisionen- ergyasthatinthep+Pb data(“A14”tune) .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.
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  eventgeneratorproducesfragmentationphotonsinadif- ferentwayfrom Pythia andwas thuschosen forthecomparison.
The NNPDF3.0NNLOPDF set  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 dataeventandthetwowerereconstructed 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<|ηlab|<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 .
Thedifferential cross-sectionis calculatedforeach Eγ T and η∗
dEγT = 1 Lint
where Lint isthe integratedluminosity,Eγ
T is thewidth ofthe Eγ
T bin, Nsig isthe yieldof photon candidatespassing identiﬁca- tion and isolation requirements, Psig 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. CMC 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 . In thisanalysistheyaretakenas trig=1,andanyuncertaintyisne- glectedasbeingsub-dominanttootheruncertainties.
Thepurity Psigisdeterminedviaadouble-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. ,and those that passtheloose requirementsofRef.  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%atEγT =100 GeV andreaches99%atEγ
T =300 GeV.
Fig. 1showsexample EisoT 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,