Measurement of the production of neighbouring jets in lead–lead collisions at $\sqrt{^{S}NN}=2.76$ TeV with the ATLAS detector

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Physics Letters B

www.elsevier.com/locate/physletb

Measurement of the production of neighbouring jets in lead–lead collisions at √

s

= ² . 76 TeV with the ATLAS detector

.ATLASCollaboration^

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

Articlehistory:

Received29June2015

Receivedinrevisedform3October2015 Accepted22October2015

Availableonline27October2015 Editor:D.F.Geesaman

ThisLetterpresentsmeasurementsofcorrelatedproductionofnearbyjetsinPb+^{Pbcollisionsat}√_s

NN= 2.76 TeV usingtheATLASdetectorattheLargeHadronCollider.Themeasurementwasperformedusing 0.14 nb⁻¹ofdatarecordedin2011.Theproductionofcorrelatedjetpairswasquantifiedusingtherate, RR,of“neighbouring”jetsthataccompany“test”jetswithinagivenrangeofangulardistance,R,in the pseudorapidity–azimuthalangleplane.ThejetsweremeasuredintheATLAScalorimeterandwere reconstructedusingtheanti-kt algorithmwithradiusparametersd=⁰.2,0.3,and0.4.RR wasmea- suredindifferentPb+Pb collisioncentralitybins,characterizedbythetotaltransverseenergymeasured intheforwardcalorimeters.A centralitydependenceofRR isobservedforallthreejetradiiwithRR foundtobelowerincentralcollisionsthaninperipheralcollisions.TheratiosformedbytheRR values indifferentcentralitybinsandthevaluesinthe40–80%centralitybinarepresented.

©^{2015CERNforthebenefitoftheATLAS}Collaboration.PublishedbyElsevierB.V.Thisisanopen accessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP³.

1. Introduction

Experimental studies of jet production in Pb+Pb collisions at the LHC can directly reveal the properties of the quark–gluon plasma created in the collisions. One predicted consequence of quark–gluonplasmaformationis“jetquenching”thatreferstothe modification of parton showers initiated by hard-scattering pro- cesseswhich takeplacein thequark–gluonplasma [1].Measure- mentsofjetpairsattheLHCprovidedthefirstdirectevidenceof jet quenching [2,3]. Inthose measurements, theenhancement of transverse momentum imbalance of dijets in central Pb+^{Pb col-} lisions was observed. Measurements at the LHC of inclusive jet suppression [4,5] and the variation of the suppression with jet azimuthal angle withrespect to the elliptic flow plane [6] have shownthatthetransverse energyofjetsissignificantly degraded andthat theenergy lossdependson the pathlength ofthe par- tonshowerintheplasma.Thesedijetandsingle-jetmeasurements providecomplementary informationaboutthejet quenchingpro- cess.Thesinglejetmeasurementsaresensitivetotheaveragepar- tonicenergy-loss whilethe dijetmeasurements probe differences inthequenchingbetweenthetwo partonshowerstraversingthe medium.Thosedifferencescanarisefromtheunequalpathlengths oftheshowersinthemediumorfromfluctuationsinthe energy lossprocessitself.

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

To help disentangle the contributions of these factors to the observed dijetasymmetries, themeasurement of thecorrelations between jets that are at small relative angles was performed.

Neighbouringjetpairsincludejetsoriginatingfromthesamehard interaction,butalsojetsfromdifferenthardinteractions.Thelatter are not of interest in this analysis, and are subtracted statisti- cally. The remaining neighbouring jet pairs result primarily from hard radiation by the parton that occurs early in the process of the shower formation. Generally, two neighbouring jets originat- ing fromthesamehard scatteringshould havemoresimilar path lengthsin themedium comparedto thetwo jetsintheprevious dijet measurement. Therefore measuring neighbouring jetscould probe differencesin their quenchingthat do not resultprimarily from difference in pathlength. More generally, measurements of the correlatedproductionofjetsinthesame partonshowermay provide moredetailedinsightintothe modificationoftheparton showerinthequark–gluonplasmabeyondthesubsequentquench- ingoftheresultingjets.

This Letter presents measurements of the production rate of neighbouring jetsin Pb+Pb collisions at √

s_NN=².76 TeV char- acterized by the quantity R_R introduced in Ref. [7]. The R_R variable quantifies the rate of neighbouring jets that accom- pany “test”jetswithin a givenrange ofangular distance,R, in the pseudorapidity–azimuthal angle (η–φ) plane,¹ where R =

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

http://dx.doi.org/10.1016/j.physletb.2015.10.059

0370-2693/©^{2015CERNforthebenefitoftheATLAS}Collaboration.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP³.

(η)²+ (φ)^2.Jetswere reconstructedwiththe anti-kt [8]al- gorithm using radius parameter values d=⁰.2, 0.3, and 0.4. In eventswithtestjetswithtransverseenergy ET>70 GeV,further jetsare searched for within a certain angular distance from the testjet.

The rate of the neighbouring jets that accompany a test jet, RR,isdefinedas

R_R(E^test_T ,E^nbr_T )=

^Ntestjet

i=1 N^nbr_jet,i(E^test_T ,E^nbr_T , R)

N_jet^test(E^test_T ) , (1)

where E^test_T and E^nbr_T are the transverse energies of thetest and neighbouringjet,respectively;N^test_jet isthenumberoftestjetsina given E^test_T binandN_jet^nbr is thenumberofneighbouringjets. Fur- ther,the R_R quantitywas usedtodefineper-test-jetnormalized spectraofneighbouringjetsas

dR_R dE^nbr_T = ¹

N^test_jet

N^test_jet



i=¹ dN_jet^nbr_,i

dE^nbr_T (E^test_T ,E^nbr_T , R). (2) Previousmeasurements ofthecorrelated productionofneigh- bouring jetswere performedby the DØ experiment in p^{p colli-}¯ sionsattheTevatron[7].ThemeasurementsbyDØwereintended tomeasurethestrongcouplingconstant, αs,andtotestitsrunning overalargerangeofmomentumtransfers.Themeasurementspre- sentedin thisLetter use similar techniquesand follownotations introducedinthatmeasurement.

2. Experimentalsetup

ThemeasurementspresentedinthisLetterwereperformedus- ingtheATLASinnerdetector,calorimeter,triggeranddataacquisi- tionsystems[9].Theinner detector[10]measures chargedparti- cleswithintheinterval |η_|<2.5.The innerdetectoriscomposed ofsiliconpixeldetectorsintheinnermostlayers,followedbysili- conmicrostripdetectorsandastraw-tubetracker,allimmersedin a2 Taxialmagneticfieldprovidedbyasolenoid. Thecalorimeter system consists of a high-granularity liquid argon (LAr) electro- magnetic (EM) calorimeter covering |η_|<3.2, a steel/scintillator samplinghadroniccalorimetercovering |η|<1.7,a LAr hadronic calorimetercovering1.5<|η|<3.2.Thehadroniccalorimeterhas three sampling layers longitudinal in shower depth and has a

η_{× φ} granularityof 0.1×⁰.1 for |η_|<2.5 and 0.2×⁰.2 for 2.5<|η|<4.9.² The EM calorimeters are segmented into three shower-depthcompartmentswithanadditionalpre-samplerlayer.

Theforwardregions areinstrumented withcopper/LArandtung- sten/LArforwardcalorimeters (FCal)covering 3.2<|η|<4.9,op- timized for electromagnetic and hadronic energy measurements, respectively.Twominimum-biastriggerscintillators(MBTS)coun- tersare locatedon eachside at3.56m alongthe beamline from the centre ofthe ATLAS detector. The MBTS detect charged par- ticlesin therange 2.1<|η|<3.9. EachMBTS counter is divided into16sections,eachofwhichprovidesmeasurementsofboththe pulseheightsandarrivaltimesofenergydeposits.Thezero-degree calorimeters(ZDCs)arelocatedsymmetricallyatz= ±^{140 mand} cover |η|>8.3. In Pb+Pb collisions the ZDCs measure primarily

Thex-axispointsfromtheIPtothecentreoftheLHCring,andthe y-axis points upward.Cylindricalcoordinates(r,φ)areusedinthetransverseplane,φbeingthe azimuthalanglearoundthebeampipe.Thepseudorapidityisdefinedintermsof thepolarangleθasη= −ln tan(θ/2).

2 Anexceptionisthethirdsamplinglayerthathasasegmentationof0.2×⁰.1 upto|η|=¹.4.

“spectator”neutrons,whichoriginatefromoneoftheincidentnu- clei and do not interact hadronically with nucleons of the other nucleus.

Minimum-bias Pb+Pb collisions were required either to have thetransverseenergyinthewholecalorimeterexceeding50 GeV attheLevel-1triggerortohaveatrackreconstructedintheinner detectorincoincidencewithZDCsignalsonbothsides.

Eventswithhigh-pTjetswereselectedusingacombinationofa minimum-biasLevel-1triggerandHighLevelTrigger(HLT)jettrig- gers.TheLevel-1minimum-biastriggerrequiredatotaltransverse energymeasuredinthecalorimetertobelargerthan10 GeV.The HLTjettriggerusedtheofflinePb+^Pbjetreconstructiondescribed inSection4,exceptfortheapplicationofthefinalhadronicenergy scale correction. The HLT jet trigger selected eventscontaining a d=⁰.2 jetwithE_T>20 GeV.

3. Eventselectionanddatasets

This analysis used data from Pb+Pb collisions at a nucleon–

nucleon centre-of-mass energy of √

s_NN=².76 TeV recorded by ATLASin2011.Itutilizesdatasamplescorrespondingtoatotalin- tegratedluminosity of0.14 nb⁻¹.Theminimum-biassample was recordedwithdifferentprescalesdepending ontheinstantaneous luminosityintheLHCfill.Theprescaleindicateswhichfractionof eventsthatpassedthetriggerselectionwereselectedforrecording bytheDAQ.Theminimum-biastriggerrecordedaneffectivelumi- nosityof7 μb⁻¹.Eventsselectedbytheminimum-biastriggerand thejettriggerswererequiredtohaveareconstructedprimaryver- texwithatleastthree associatedtrackseachwith p_T>500 MeV and a difference between time of pulses from the two sides of the MBTS detectorofless than7 ns. A totalof 51(14.2) million minimum-biastriggered (jet-triggered) events passed theapplied eventselectionsandwereusedintheanalysis.

Inheavy-ion collisions,“centrality”reflectsthe overlapvolume ofthetwocollidingnuclei, controlledbytheimpactparameterof the collisions. The centrality of Pb+Pb collisions was character- ized by E^FCal_T , thetotal transverseenergy measured inthe FCal [11].The centralityintervalsweredefinedaccordingtosuccessive percentilesofthe E^FCal_T distributionorderedfromthemostcen- tral(highest E^FCal_T ) tothemostperipheral collisions.Production ofneighbouringjetswas measuredinfourcentralitybins: 0–10%, 10–20%,20–40%,and40–80%,withthe40–80%binservingasthe reference.ThepercentilesweredefinedaftercorrectingtheE^FCal_T distributionfora2%minimum-biastriggerinefficiencythataffects themostperipheralevents,whicharenotincludedinthisanalysis.

The performance oftheATLAS detectorandoffline analysisin measuringjetsintheenvironmentofPb+Pb collisionswasevalu- atedusingalargesampleofMonteCarlo(MC)eventsobtainedby overlayingsimulatedPYTHIA[12]hard-scatteringeventsontoran- domly selected minimum-bias Pb+^{Pb events, recorded by ATLAS} during thesamedata-taking periodasthedatausedinthisanal- ysis. PYTHIAversion 6.423 withthe AUET2Btune [13] was used.

ThreemillionPYTHIAeventswereproducedforeachoffiveinter- valsofthetransversemomentumofoutgoingpartonsinthe2→² hard-scatteringprocess,withboundaries17,35,70,140,280,and 560 GeV.The detectorresponse tothe PYTHIAevents was simu- latedusingGEANT4[14,15],andthesimulatedhitswerecombined withthe data fromthe minimum-biasPb+^{Pb events before per-} formingthereconstruction.

4. Jetreconstructionandneighbouringjetselection

Jetswerereconstructedwithinthepseudorapidityinterval|η_|<

2.8. The jet reconstruction techniques described inRef. [4]were used,andarebrieflysummarizedhere. Theanti-k_t algorithmwas

firstruninfour-vectorrecombinationmode,onη× φ =⁰.1× 0.1 logical towers. The energies in the towers were obtained by summing energies of calorimeter cells, calibrated at a scale set for electron showers, within the tower boundaries. Then, an it- erative procedure was used to estimate a calorimeterlayer- and

η-dependentunderlying event(UE) energydensity,whileexclud- ing actualjetsfromthat estimate. TheUE energywas subtracted fromeach calorimeter cellwithin thetowers included in there- constructedjet.Thesubtractionaccountedforacos 2φmodulation intheUEenergydensityduetocollectiveflow[11]ofthemedium usingameasurementoftheamplitudeandphaseofthatmodula- tioninthecalorimeter.Thejetenergiesandmomentawerecalcu- latedviaasumofallcellscontainedwithinthejets,treatingeach cellasamasslessfour-vector,usingE_TvaluesaftertheUEsubtrac- tion.A correctionwas appliedto thereconstructed jettransverse energiestoaccountforjetsnotexcludedoronlypartiallyexcluded fromthe UE estimate. The magnitudeofthe correction was typ- ically a few percent but can be as large as 10% for jets whose energies are fully included in the UE estimate. Then, a final η- andjet E_T-dependenthadronicenergyscalecalibrationfactorwas applied[4].

Separate from the calorimeter jets, “track jets” were recon- structedbyapplyingtheanti-kt algorithmwithd=⁰.4 tocharged particleshaving pT>4 GeV. Thetrackjetswereusedinconjunc- tion withelectromagnetic clusters to remove the contribution of

“UE jets” generated by fluctuations in the underlying event. The techniqueisdescribedindetailinRef.[4].

Inthe MC simulation,thekinematics ofthe referencePYTHIA generator-level jets (hereafter called “truth jets”) were recon- structed from PYTHIA final-state particles with the anti-kt algo- rithmwithradiusd=⁰.2,0.3,and0.4usingthesametechniques asappliedinpp analyses[16].PYTHIAtruthjetswerematchedto theclosestreconstructedjetofthesamed valuewithinR=⁰.2.

The resulting matched jetswere used to evaluate the jet energy resolution(JER), thejet energyscale(JES), thejet angularresolu- tion,andthejetreconstructionefficiency.

The R_R measurement was performed with the sample trig- gered by the jet triggers. The measurement was done differen- tiallyin transverseenergy ofthe test andneighbouring jets, and in collision centrality. Five E^test_T intervals 70–80, 80–90, 90–110, 110–140, 140–300 GeV and four E^nbr_T intervals, 30–45, 45–60, 60–80,80–130 GeVwere used.Furthermoreconfigurations where all the ET bins of the test jets or of the neighbouring jets have the same upper bound of 300 GeV were also used in thisanal- ysis. The number of bins and their boundaries were chosen to minimizethe impactofthelimitednumberofeventsinthedata while preserving the ability to infer the trends in the measured distributions.Foreachjetradius,neighbouringjetsareconsidered if they lie within a specific annulus in R around the test jet:

0.5< R<1.6,0.6< R<1.6,and0.8< R<1.6 ford=⁰.2, d=⁰.3,andd=⁰.4 jets,respectively.Theinneredgeofeachannu- luswaschosentoavoidpossibleoverlapoftestandneighbouring jets, and the outer edge value (π/2) rejects neighbouring jets in the hemisphere opposite to the test jet and maximizes the number of events.Choosing a maximum R of 1.6 restrictsthe pseudorapidity range of test jets to |η|<1.2, yielding approxi- mately 87×^{103 d}=⁰.4 test jetswith p_T>80 GeV analysed in 0–10%centraleventsand37×^{103 testjetsin40–80%peripheral} events.

Toquantify thecentrality dependenceof theneighbouring jet yields,theratio ρR_R≡^RR|cent/R_R|40−⁸⁰iscalculatedasthera- tioofR_R measuredineachcentralitybintoR_R measuredinthe reference(40–80%)bin.

5. Correctionstoneighbouringjetrates

Therawratesofneighbouringjetsincludea contributionfrom neighbouring jetsthat originate fromdifferent hard partonic in- teractions in the same Pb+Pb collision. Thiscombinatorial back- ground is present both in the MC simulation and in the data and must be subtracted. It is largest in the low E^nbr_T bins and it increases with increasing centrality of the collision, since the probability forthe presence oftwo independent hard scatterings in one Pb+^{Pbcollision is expectedto increase withthe number} of binary collisions. The combinatorial background is estimated usingthedifferentialyieldofinclusivejets(d³N_jet/dηdφdE_T)eval- uated in minimum-bias Pb+^{Pb events.To each eventconsidered} a weight is assigned such that the event sample obtained from the minimum-biastrigger hasthe samecentrality distributionas thesamplecollectedbythejettrigger.Thisestimatedbackground needs to be corrected fora geometrical bias present inthe case wherethecombinatorialjet overlapswitha realneighbouringjet or when two combinatorial jets overlap. These biases were re- movedbyapplyingamultiplicativecorrectionfactortobackground distributionspriortothesubtraction.Thismultiplicativefactorwas derivedfromthereconstructionefficiencyoftwoneighbouringjets evaluatedasafunctionoftheirangularseparationintheannulus.

Inthatevaluation,onejetwasrequiredtooriginatefromPYTHIA’s hard scatteringandthe other jet was required to originate from the minimum-biasdataintheoverlay. Theimpact ofthiscorrec- tiononthefinalsubtracteddistributionissmallerthan0.5%.

The combinatoric jet kinematics may also be affected by the presenceofatestjet.Tocontrolthisinfluence,astudycomparing the combinatoricjetsfromthe overlayMC eventswith thesame jetsintheoriginal minimum-biasdatawasperformed. Thisstudy resultedinanadditionalcorrection,independentofcentralityand jet ET,that decreases thecombinatorial backgroundby 1.5%. The

±^1.2% uncertainty on the correction originates from the limited number ofevents andwas included inthe systematicuncertain- ties.

Inordertoaccountfortheeffectoftheazimuthaldependence ofjet yields[6],thecombinatorialbackgroundwasreweighted to takeintoaccountthemeasuredazimuthaldistributionsoftestjets aswellascombinatorialjets.Thechangeoftherawsubtracteddis- tributionincentralcollisionsandlow E^nbr_T binsafterthereweight- ingisatthelevelof1%anddecreaseswithincreasingcentralityof thecollisionandE^nbr_T .

Thebackgroundissubtractedfromraw R_R distributions both in the data andin the MC simulation, allowing an evaluation of the effectivenessofthesubtractionusingthe MCsimulation. The signal-to-background ratio strongly depends on the centrality of the collision and E^nbr_T . In 0–10% central collisions, the signal-to- backgroundratiocanbeaslowas0.15forthemostextremecase of 30< E^nbr_T <45 GeV, and increases to approximately 0.8 for 60<E^nbr_T <80 GeV.

TherawsubtractedRR distributionsareaffectedbythejeten- ergy resolution.Thecombinationofthejetenergyresolutionand the steeply falling ET spectrum produces a net migration ofjets from lower ET to higher ET valuessuch that a jet reconstructed withagiven E^rec_T corresponds,onaverage,toalowertruthjet E_T,

^Etruth_T ^.Therelationshipbetween^Etruth_T ^{and Erec}_T ^wasevaluated in simulatedeventsforthedifferentcentrality binsandthree jet radiiusedintheanalysis.Theextractedrelationshipswereusedto correct forthe averageshift inthe test jet energy. No correction duetothejetreconstructionefficiencyforthetestjetsisneeded, since theanalysisoperates inthetransverseenergyregionwhere the jet reconstruction is fully efficient. No correction due to jet triggerefficiencyisneededeithersincetheplateauofthejettrig- ger efficiency is reached for all test jets, except for d=⁰.4 jets