Measurement of the cross section for inclusive isolated-photon production in pp collisions at √s = 13 TeV using the ATLAS detector

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Reference

Measurement of the cross section for inclusive isolated-photon production in pp collisions at √s = 13 TeV using the ATLAS detector

ATLAS Collaboration

ANCU, Lucian Stefan (Collab.), et al.

Abstract

Inclusive isolated-photon production in pp collisions at a centre-of-mass energy of 13 TeV is studied with the ATLAS detector at the LHC using a data set with an integrated luminosity of 3.2 fb−1 . The cross section is measured as a function of the photon transverse energy above 125 GeV in different regions of photon pseudorapidity. Next-to-leading-order perturbative QCD and Monte Carlo event-generator predictions are compared to the cross-section measurements and provide an adequate description of the data.

ATLAS Collaboration, ANCU, Lucian Stefan (Collab.), et al . Measurement of the cross section for inclusive isolated-photon production in pp collisions at √s = 13 TeV using the ATLAS

detector. Physics Letters. B , 2017, vol. 770, p. 473-493

DOI : 10.1016/j.physletb.2017.04.072

Available at:

http://archive-ouverte.unige.ch/unige:94583

Disclaimer: layout of this document may differ from the published version.

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Contents lists available atScienceDirect

Physics Letters B

www.elsevier.com/locate/physletb

Measurement of the cross section for inclusive isolated-photon production in pp collisions at √

s = 13 TeV using the ATLAS detector

.TheATLASCollaboration

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

Articlehistory:

Received25January2017

Receivedinrevisedform31March2017 Accepted27April2017

Availableonline2May2017 Editor:M.Doser

Inclusive isolated-photonproductionin pp collisions atacentre-of-mass energyof 13 TeV isstudied withtheATLASdetectorattheLHCusingadatasetwithanintegratedluminosityof3.2 fb⁻¹.Thecross sectionismeasuredasafunctionofthephotontransverse energyabove125 GeV indifferentregions of photon pseudorapidity. Next-to-leading-order perturbative QCD and Monte Carlo event-generator predictions are comparedto the cross-section measurements and provide anadequate description of thedata.

©²⁰¹⁷^TheÂuthor(s).^Published^byÊlsevier^B.V.^Thisîsânôpenâccessârticleûnder^the^CC^BY^license (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP³.

1. Introduction

Theproduction ofprompt photonsinproton–proton (pp) collisions, pp→γ ₊ X, provides a testing ground for perturbative QCD (pQCD) with a hard colourless probe. All photonspro- ducedinppcollisionsthatarenotsecondariesfromhadrondecays areconsidered as“prompt”. Twoprocessescontribute toprompt- photonproductionin pp→γ + ^X:^the ^direct^process,ⁱⁿ^which thephoton originates directlyfromthe hard interaction, andthe fragmentation process, in which the photon is emitted in the fragmentationof a hightransverse momentum (pT) parton[1,2].

Measurementsof inclusive prompt-photon production were used recentlytoinvestigatenovelapproachestothedescriptionofpar- tonradiation[3]andtheimportanceofresummationofthreshold logarithms in QCD and of the electroweak corrections [4]. Com- parisonsof prompt-photondata andpQCD are usually limitedby the theoretical uncertainties associated with the missing higher- order terms in the perturbative expansion. The extension of the recentnext-to-next-to-leading-order(NNLO)pQCDcalculationsfor jetproduction[5]toprompt-photonproduction¹willallowamore stringent test of pQCD. To make such a test with small experi- mentalandtheoreticaluncertainties,itisoptimaltoperformmea- surementsofprompt-photonproductionathighphotontransverse energiesandatthehighestpossiblecentre-of-massenergyofthe collidingparticles.

Sincethe dominantproduction mechanismin ppcollisions at the LHC proceeds via the qg →^qγ process, measurements of

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

1 After completion of the work presented here, ﬁrst NNLO calculations for prompt-photonproductionhavebeencompleted[6].

prompt-photon production are sensitive at leading order (LO) to the gluon density in the proton [7–16]. Although prompt photon data were initiallyincluded in thedetermination ofthe pro- tonpartondistributionfunctions(PDFs),theirusewasabandoned someyearsago.Sincethen,theoreticaldevelopments[13,14]have shown ways to improve the description of the data in terms of pQCD,andarecentstudyquantiﬁedtheimpactofprompt-photon datafromhadroncollidersonthegluondensityintheproton[15].

Newmeasurementsofprompt-photonproductionathighercentre- of-massenergies areexpectedtofurtherconstrainthegluonden- sityintheprotonwhencombinedwithpreviousdata.

ThesemeasurementscanalsobeusedtotunetheMonteCarlo (MC)modelstoimprovetheunderstandingofprompt-photonproduction.Inaddition,precisemeasurements oftheseprocesses aid thosesearchesforwhichthey arean importantbackground,such asthesearchfornewphenomenainﬁnalstateswithaphotonand missingtransverse momentum.

Measurements of prompt-photon production at a hadron col- lider require isolated photons to avoid the large contribution of photons from decays of energetic π⁰ and η mesons inside jets.

The production of inclusive isolated photons in pp collisions at centre-of-massenergiesof√

s=^{7 and}^{8 TeV was}^measured^by^the ATLAS[17–20]andCMS[21,22]collaborations.

Thispaper presentsmeasurements of isolated-photon production in pp collisions at √

s=13 TeV with the ATLAS detector at theLHCusingadatasetwithanintegratedluminosityof3.2 fb⁻¹ collected during 2015. These measurements are performed in a phase-spaceregionoverlappingwiththatusedinthepreviousAT- LAS measurementat√

s=^{8 TeV}^[20].^Cross^sections^as^functions

http://dx.doi.org/10.1016/j.physletb.2017.04.072

0370-2693/©²⁰¹⁷^TheÂuthor(s).^Published^byÊlsevier^B.V.^Thisîsânôpenâccessârticleûnder^the^CC^BY^license(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP³.

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ofthephotontransverseenergy² (Eγ

T)are measuredintherange Eγ

T >125 GeV for different regions ofthe photon pseudorapidity (η^γ).Thethresholdin Eγ

T ischosensoastoavoidthelow-Eγ T re- gionwherebothsystematicandtheoreticaluncertaintiesincrease.

Next-to-leading-order(NLO)pQCDandMCevent-generatorpredic- tionsarecomparedtothemeasurements.

2. TheATLASdetector

The ATLAS detector [23] is a multi-purpose detector with a forward-backward symmetric cylindrical geometry. It consists of an inner tracking detector surrounded by a thin supercon- ductingsolenoid, electromagneticand hadroniccalorimeters, and a muon spectrometer incorporating three large superconducting toroid magnets. The inner-detector system is immersed in a 2 T axialmagnetic ﬁeldandprovides charged-particle trackinginthe range|η_|<2.5.Thehigh-granularitysiliconpixeldetectorisclos- esttotheinteractionregion andprovides fourmeasurements per track; the innermostlayer, known asthe insertable B-layer [24], was addedin2014andprovides high-resolution hitsatsmallra- dius to improve the tracking performance. The pixel detector is followed by the silicon microstrip tracker, which typically pro- videsfourthree-dimensionalmeasurementpointspertrack.These silicon detectors are complemented by the transition radiation tracker, which enables radially extended track reconstruction up to |η|=².0. The calorimeter system covers the range |η|<4.9.

Within the region |η_|<3.2, electromagnetic calorimetry is pro- vided by barrel and endcap high-granularity lead/liquid-argon (LAr) electromagnetic calorimeters, with an additional thin LAr presamplercovering|η_|<1.8 tocorrectforenergylossinmaterial upstreamofthecalorimeters;for|η_|<2.5 theLArcalorimetersare dividedintothreelayersindepth.Hadroniccalorimetryisprovided bya steel/scintillator-tilecalorimeter,segmentedinto threebarrel structureswithin |η_|<1.7,and two copper/LArhadronic endcap calorimeters,whichcovertheregion1.5<|η|<3.2.Thesolidan- glecoverageiscompletedoutto|η|=⁴.9 withforwardcopper/LAr and tungsten/LAr calorimeter modules, which are optimised for electromagnetic andhadronic measurements, respectively. Events areselectedusingaﬁrst-leveltriggerimplementedincustomelec- tronics,which reduces the maximumevent rateof 40 MHzto a design value of 100 kHz using a subset of detector information.

Software algorithms with access to the full detector information are then used inthe high-level trigger to yielda recorded event rateofabout1 kHz[25].

3. Dataselection

The data usedin this analysiswere collected withthe ATLAS detectorduringtheppcollisionrunningperiodof2015,whenthe LHCoperatedwithabunchspacingof25 nsandacentre-of-mass energy of √

s=^{13 TeV.} Ônly êvents ^taken ⁱⁿ ^stable ^beam ^con- ditions andsatisfying detectoranddata-quality requirements are considered. The total integratedluminosity of the collected sample amounts to 3.16±⁰.07 fb⁻¹ [26,27]. Events were recorded using a single-photon trigger, with a transverse energy threshold of 120 GeV. The trigger efficiency for isolated photons with

2 ATLASusesaright-handedcoordinatesystemwithitsoriginatthenominalin- teractionpoint(IP)inthecentreofthedetectorandthez-axisalongthebeampipe.

Thex-axispointsfromtheIPtothecentreoftheLHCring,andthey-axispoints upwards.Cylindricalcoordinates(r,φ)areusedinthe transverseplane,φ being theazimuthalanglearoundthez-axis.Thepseudorapidityisdeﬁnedintermsof thepolarangleθas η= −ln tan(θ/2).Angulardistanceismeasuredinunitsof

R≡

(η)²+(φ)².ThetransverseenergyisdeﬁnedasET=^E^sinθ,whereE istheenergy.

Eγ

T >125 GeV and |η^γ|<2.37, excluding 1.37<|η^γ|<1.56, is higherthan99%.

Eventsarerequiredtohaveareconstructedprimaryvertex.Pri- maryverticesare formedfromsetsoftwoormorereconstructed tracks, each with pT>400 MeV and |η_|<2.5,that are mutually consistent with having originatedat the same three-dimensional point within the luminous region of the collidingproton beams.

If multiple primary vertices are reconstructed, the one with the highestsum ofthe p²_T oftheassociated tracks isselected asthe primaryvertex.

Photon and electron candidates are reconstructed from clusters ofenergydeposited inthe electromagneticcalorimeter.Can- didates without a matching track or reconstructed conversion vertex³ in the inner detector are classified as unconverted photons[28].Thosewithamatchingreconstructedconversionvertex oramatchingtrackconsistentwithoriginatingfromaphotoncon- version are classified asconverted photons. Those matched to a trackconsistentwithoriginatingfromanelectronproducedinthe beaminteractionregionareclassifiedaselectrons.

The photonidentificationisbasedprimarily onshowershapes inthecalorimeter[28].Aninitialselectionisderivedusingthein- formation from the hadronic calorimeter and the lateral shower shape in the second layer of the electromagnetic calorimeter, wheremostofthephoton energyiscontained.The finaltightse- lection applies stringent criteria[28] to thesevariables, different for converted andunconverted photon candidates. It also places requirements on the shower shape in the finely segmented first calorimeter layer to ensure the compatibility of the measured shower profilewiththatoriginatingfromasingle photonimpact- ingthecalorimeter.Whenapplyingthephotonidentificationcrite- riatosimulatedevents,correctionsaremadeforsmalldifferences intheaveragevaluesoftheshower-shapevariablesbetweendata andsimulation.Theefficiencyofthephotonidentificationvariesin the range92–98%for Eγ

T =125 GeV and 86–98%for Eγ

T =^{1 TeV,} depending on η^γ andwhetherthe photoncandidate isclassiﬁed as unconverted or converted [28,29].For Eγ

T >125 GeV, the un- certaintyinthephotonidentiﬁcationeﬃciencyvariesbetween1%

and5%,dependingon η^γ andEγ T.

Thephotonenergymeasurementismadeusingcalorimeterand tracking information. A dedicated energycalibration [30] is then appliedtothecandidatestoaccountforupstreamenergylossand bothlateralandlongitudinalleakage;amultivariateregressional- gorithm to calibrate electron and photon energy measurements wasdevelopedandoptimisedonsimulatedevents.Thecalibration ofthelayer energiesinthecalorimeterisbasedon themeasure- ment performed with2012 dataat √

s=^{8 TeV} ^[30]. ^The ^overall energyscale indataandthedifferenceinthe energyresolution’s constant term⁴ between data andsimulation are estimated with a sample of Z-boson decays to electrons recorded in 2012 and reprocessed using the same electron reconstruction and calibra- tion scheme as used for the 2015 data taking and event pro- cessing. The energyscale and resolution corrections are checked using Z-bosondecaystoelectrons recordedin the2015dataset.

Uncertainties in the measurements performed with this sample are estimatedfollowing a procedure similar to that discussed in Ref. [30]. The difference between the values measured with the 2015dataandthosepredictedfromthereprocessed2012datais alsotakenintoaccountintheuncertainties.Theuncertaintyinthe photon energyscale at high Eγ

T is typically 0.5–2.0%, depending on η^γ.Eventswithatleastonephotoncandidatewithcalibrated

3 Conversionvertexcandidatesarereconstructedfrompairsofoppositelycharged tracksintheinnerdetectorthatarelikelytobeelectrons[28].

4 Therelativeenergyresolutionisparameterisedasσ(E)/E=^a/√

E⊕^c,^where^a isthesamplingtermandcistheconstantterm.

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Table 1

Kinematicrequirementsandnumberofselectedeventsindataforeachphase-spaceregion.

Phase-space region

Requirement onE^γ_T E_T^γ>125 GeV

Isolation requirement E^iso_T < 4.8+⁴.2·¹⁰⁻³·^E^γT [^GeV]

Requirement on|η^γ| |η^γ|<0.6 0.6<|η^γ|<1.37 1.56<|η^γ|<1.81 1.81<|η^γ|<2.37

Number of events 356 604 480 466 140 955 275 483

Eγ

T >125 GeV and|η^γ|<2.37 areselected.Candidatesinthere- gion1.37<|η^γ_|<1.56,whichincludes thetransitionregion be- tweenthebarrelandendcapcalorimeters,arenotconsidered.

The photon candidateis requiredto be isolated based on the amount of transverse energy inside a cone of size R=⁰.4 in the η–φplane aroundthephotoncandidate,excludingan areaof size η×φ=⁰.125×⁰.175 centred on the photon. The iso- lationtransverse energyiscomputed fromtopologicalclustersof calorimetercells [31]andisdenotedby E^iso_T .Themeasured value ofE^iso_T iscorrectedforleakageofthephoton’senergyintotheiso- lationcone and theestimated contributions fromthe underlying event(UE) andadditional inelastic pp interactions (pile-up). The lattertwo correctionsarecomputed simultaneouslyon an event- by-eventbasis[18]andthecombinedcorrectionistypically2 GeV.

Thecombinedcorrectioniscomputedusingamethodsuggestedin Refs.[32,33]:thekt jet algorithm[34,35] withjetradius R=⁰.5 isused toreconstruct all jetstakingasinput topological clusters ofcalorimetercells;noexplicittransversemomentumthresholdis applied.Theambient-transverseenergydensityforthe event(ρ), frompile-upandtheunderlyingevent,iscomputedusingtheme- dian of the distribution of the ratio between the jet transverse energy and its area. Finally, ρ is multiplied by the area of the isolation cone to compute the correction to E_Tîso. In addition, for simulatedevents,data-drivencorrectionsto Eîso_T areapplied such thatthepeakpositionintheEîso_T distributioncoincidesindataand simulation.Afterallthesecorrections, Eîso_T isrequiredtobelower thanEîso_T_,_cut(Eγ

T)[^GeV]≡ ⁴.8+⁴.2·¹⁰⁻³·Ê^γ_T [^GeV]^[20].^Theîsola- tionrequirementsignificantlyreducesthemainbackground,which consistsofmulti-jeteventswhereone jettypically containsa π⁰

orηmeson thatcarries mostofthe jetenergyandis misidenti- ﬁedasaphotonbecauseitdecaysintoanalmostcollinearphoton pair.

A small fraction of the events contain more than one photoncandidate satisfyingtheselection criteria. Insuch events,the highest-Eγ

T (leading) photon is considered for further study. The total number of data events selected by using the requirements discussed above amounts to 1253508. A summary of the kine- matic requirements aswell asthe numberof selected events in dataineach|η^γ_|regionareincludedinTable 1.Theselectedsam- pleofeventsisusedtounfoldthedistributioninEγ

T separatelyfor eachofthefourregionsin|η^γ_|indicatedinTable 1;theunfolding is performed using the samples of MC events described in Sec- tion4.1andtheresultsarecomparedtothepredictionsfromthe Pythiaand Sherpagenerators aswell asto the predictions from NLOpQCD(seeSection8).

4. MonteCarlosimulationsandtheoreticalpredictions 4.1.MonteCarlosimulations

SamplesofMC events were generatedto studythe character- istics of signal events. The MC programs Pythia 8.186 [36] and Sherpa 2.1.1 [37] were used to generate the simulated events.

In both generators, the partonic processes were simulated using tree-levelmatrixelements,withtheinclusionofinitial- andﬁnal- statepartonshowers. Fragmentation into hadronswasperformed

using the Lund string model [38] in the case of Pythia, and in Sherpa events by a modiﬁed version of the cluster model [39].

TheLONNPDF2.3[40]PDFswereusedforPythia(NLOCT10[41]

for Sherpa) to parameterise the proton structure. Both samples include a simulation of the UE. The event-generator parameters were set according to the “A14” tune for Pythia [42] and the

“CT10”tuneforSherpa.Allthesamplesofgeneratedeventswere passedthroughtheGeant4-based[43]ATLASdetector- andtrigger- simulation programs [44].They were reconstructed andanalysed by the same program chain as the data.Pile-up from additional pp collisions in thesame andneighbouring bunch crossingswas simulatedbyoverlayingeachMC eventwithavariablenumberof simulatedinelastic ppcollisionsgeneratedusingPythia8withthe A2tune[45].TheMCeventswereweighted toreproducethedis- tributionoftheaveragenumberofinteractionsperbunchcrossing (μ) observedinthedata,referred to as“pile-upreweighting”;in thisprocedure, the μ value in thedata isdivided by a factor of 1.16±⁰.07, a rescaling which improves the agreement between thedataandsimulationfortheobserved numberofprimary vertices andrecoversthe fractionofvisiblecross-sectionof inelastic ppcollisionsasmeasuredinthedata[46].

ThePythiasimulationofthesignalincludesLOphoton-plus-jet events from both direct processes (the hard subprocesses qg→ qγ and qq¯ → ^gγ, called the “hard” component) and photon bremsstrahlung in QCD dijet events (called the “bremsstrahlung”

component).The Sherpasampleswere generatedwithLOmatrix elements for photon-plus-jet final states with up to three additional partons(2→ⁿ^processes ^withⁿ ^from^{2 to} ^5);^the ^matrix elementswere mergedwiththeSherpapartonshower[47] using the ME+PS@LO prescription. While the bremsstrahlung component was modelledin Pythiaby final-stateQED radiationarising fromcalculationsofall2→^{2 QCD}^processes,ît^wasâccounted^for in Sherpathrough the matrixelements of2→ⁿ ^processes ^with n≥^3;ⁱⁿ^the^generationôf^the^Sherpa^samples,ârequirementon thephotonisolationatthematrix-elementlevelwasimposedus- ingthecriteriondefinedinRef.[48].⁵

The predictions ofthe MC generators atparticle level arede- ﬁned using those particles with a lifetime τ longer than 10 ps;

these particles are referred to as “stable”. The particles associated withtheoverlaid pp collisions (pile-up) are notconsidered.

The particle-level isolation requirement on the photon was built summing the transverse energy ofall stableparticles, except for muonsandneutrinos,inaconeofsizeR=⁰.4 aroundthepho- ton direction afterthe contribution fromthe UE was subtracted;

the samesubtractionprocedure usedon datawas applied atthe particlelevel.Therefore,thecrosssectionsquotedfromMCsimula- tionsrefertophotonsthatareisolatedbyrequiringE^iso_T (particle)<

E^iso_T_,_cut(Eγ T).

5 Thiscriterion,commonlycalledFrixione’scriterion,requiresthetotaltransverse energyinsideaconeofsizeVaroundthegeneratedﬁnal-statephoton,excluding thephotonitself,tobebelowacertainthreshold,E^max_T (V)=E^γ_T((1−cosV)/(1− cosR))ⁿ^,^for^allV<R^.^The^parameters^for^the^threshold^were^chosen^to^beR= 0.3,n=^{2 and}=⁰.025.