Centrality and rapidity dependence of inclusive jet production in √sNN=5.02 TeV proton-lead collisions with the ATLAS detector

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Physics Letters B

www.elsevier.com/locate/physletb

Centrality and rapidity dependence of inclusive jet production in √

s

= ⁵ . 02 TeV proton–lead collisions with the ATLAS detector

.ATLASCollaboration^

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

Articlehistory:

Received12December2014 Receivedinrevisedform16April2015 Accepted14July2015

Availableonline17July2015 Editor:D.F.Geesaman

Measurementsofthecentralityandrapiditydependenceofinclusivejetproductionin√_s

NN=⁵.02 TeV proton–lead (p+^{Pb) collisions and the jet} cross-section in √

s=².76 TeV proton–proton collisions are presented.Thesequantitiesare measuredindatasetscorresponding toanintegratedluminosityof 27.8 nb⁻¹ and 4.0 pb⁻¹,respectively,recorded withthe ATLASdetector atthe LargeHadronCollider in 2013.The p+Pb collisioncentralitywascharacterisedusingthetotaltransverseenergymeasuredin the pseudorapidityinterval−⁴.9<η<−³.2 inthe directionofthe leadbeam.Results are presented for the double-differentialper-collisionyields as afunctionofjetrapidity and transversemomentum (pT)forminimum-biasandcentrality-selected p+Pb collisions,andare comparedtothejetratefrom the geometricexpectation.The totaljetyield inminimum-biasevents isslightlyenhanced abovethe expectationinapT-dependentmannerbutisconsistentwiththeexpectationwithinuncertainties.The ratios of jetspectra fromdifferent centralityselections show astrong modificationof jetproduction at all pT atforward rapiditiesand for large pT atmid-rapidity, whichmanifests as asuppression of the jetyield incentralevents andan enhancementinperipheralevents.Theseeffects implythatthe factorisation between hard and soft processes is violated at an unexpected level in proton–nucleus collisions. Furthermore,themodificationsatforward rapiditiesarefoundtobe afunctionofthetotal jetenergyonly,implyingthattheviolationsmayhaveasimpledependenceonthehardparton–parton kinematics.

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

1. Introduction

Proton–lead (p+^{Pb) collisions at the Large Hadron Collider} (LHC) provide an excellent opportunity to study hard scattering processesinvolvinganucleartarget[1].Measurementsofjetpro- duction in p+Pb collisions provide a valuable benchmark for studies of jet quenching in lead–lead collisions by, for example, constraining the impact of nuclearparton distributions on inclu- sivejet yields. However, p+Pb collisionsalsoallowthe studyof possibleviolationsoftheQCDfactorisationbetweenhardandsoft processeswhichmaybeenhancedincollisionsinvolvingnuclei.

Previousstudiesindeuteron–gold(d+^{Au)collisionsattheRel-} ativisticHeavyIonCollider(RHIC)observedsuchviolations,mani- festedinthesuppressedproductionofveryforwardhadronswith transverse momenta up to 4 GeV [2–4]. Studies of forward di- hadron angularcorrelationsatRHIC alsoshowed amuch weaker dijetsignal ind+Au collisions thanin pp collisions [4,5]. These

 E-mailaddress:[email protected].

effects have been attributed to the saturation of the parton dis- tributions in the gold nucleus [6–8], to the modification of the nuclearpartondistributionfunction[9],tothehigher-twistcontri- butions to the cross-section enhanced by theforward kinematics of the measurement [10], or to the presence of a large nucleus [11]. The extended kinematic reach of p+Pb measurements at theLHCallowsthestudyofhardscatteringprocessesthatproduce forwardhadronsorjetsoveramuchwiderrapidityandtransverse momentumrange.Suchmeasurementscandeterminewhetherthe factorisation violationsobserved atRHIC persist athigherenergy and, ifso, how the resulting modifications vary as a function of particle orjet momentum andrapidity.The resultsofsuch mea- surements could test the competingdescriptions ofthe RHIC re- sultsand, moregenerally,providenewinsightinto thephysicsof hardscatteringprocessesinvolvinganucleartarget.

This paper reports the centrality dependence of inclusive jet production in p+Pb collisions at a nucleon–nucleon centre- of-mass energy √

sNN =⁵.02 TeV. The measurement was per- formed using a dataset corresponding to an integrated luminos- ity of 27.8 nb⁻¹ recorded in 2013. The p+^{Pb jet yields were}

http://dx.doi.org/10.1016/j.physletb.2015.07.023

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

compared toanucleon–nucleonreferenceconstructedfromamea- surement of jet production in pp collisions at a centre-of-mass energy √

s=².76 TeV using a dataset corresponding to an inte- grated luminosity of 4.0 pb⁻¹ also recorded in 2013. Jets were reconstructed from energy deposits measured in the calorimeter usingtheanti-kt algorithmwithradiusparameter R=⁰.4[12].

Thecentralityof p+Pb collisionswas characterisedusingthe total transverse energymeasured in the pseudorapidity¹ interval

−⁴.9<η<−³.2 in the direction of the lead beam. Whereasin nucleus–nucleuscollisionscentralityreflectsthedegreeofnuclear overlapbetweenthecollidingnuclei,centralityin p+Pb collisions is sensitive to the multiple interactions betweenthe protonand nucleonsintheleadnucleus.Centralityhasbeensuccessfullyused atlower energiesind+Au collisionsatRHICasan experimental handleonthecollisiongeometry[2,13,14].

AGlaubermodel[15]wasusedtodeterminetheaveragenum- ber of nucleon–nucleon collisions, ^Ncoll^{, and the mean value} of the overlap function, TpA(b)=_+∞

−∞ ρ(b,z)dz, where ρ(b,z) is the nucleon density at impact parameter b and longitudi- nal position z, in each centrality interval. Per-event jet yields, (1/Nevt)(d²N_jet/dpTd y^∗), were measured as a function of jet centre-of-massrapidity,² y^∗,andtransversemomentum,p_T,where N_jet isthe numberof jetsmeasured in Nevt p+^{Pb events anal-} ysed. The centrality dependence of the per-event jet yields was evaluatedusingthenuclearmodificationfactor,

R_pPb≡ ¹ T_pA

(1/Nevt)d²N_jet/dpTd y^∗

cent

d²σ_jet^pp/dp_Td y^∗ , (1)

foragivencentralityselection“cent”,whered²σ_jet^pp/dp_Td y^∗isde- terminedusingthe jetcross-section measured in pp collisionsat

√s=².76 TeV. The factor RpPb quantifies the absolutemodifica- tionofthe jet raterelative tothe geometric expectation.In each centrality interval, the geometric expectationis the jet rate that wouldbeproducedbyanincoherentsuperpositionofthenumber ofnucleon–nucleoncollisions corresponding to the meannuclear thicknessinthegivenclassofp+Pb collisions.

Resultsarealsopresentedforthecentral-to-peripheralratio,

R_CP≡ ¹ R_coll

(1/N_evt)d²N_jet/dp_Td y^∗

cent

(1/N_evt)d²N_jet/dp_Td y^∗

peri

, (2)

whereR_coll representstheratioof^Ncoll^{inagivencentralityin-} tervaltothatinthemostperipheralinterval,Rcoll≡

N_coll^cent /N^peri_coll. TheR_CPratioissensitivetorelativedeviationsinthejetratefrom thegeometric expectationbetween the p+^{Pb event} centralities.

The RpPb and RCP measurements are presented as a function of inclusivejet y^∗andp_T.

Forthe2013 p+^{Pb run,theLHCwas configuredwitha4 TeV} protonbeam and a 1.57 TeVper-nucleon Pb beamthat together producedcollisions with √

s_NN=⁵.02 TeV anda rapidity shiftof thecentre-of-massframeof0.465 unitsrelativetotheATLAS rest frame.The run wassplit intotwo periods,withthe directionsof

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

Thex-axispointsfromtheIPtothecentreoftheLHCring,andthe y-axispoints upward.Cylindricalcoordinates(r,φ)areusedinthetransverseplane,φbeingthe azimuthalanglearoundthebeampipe.Thepseudorapidityisdefinedinlaboratory coordinatesintermsofthepolarangleθasη_{= −}ln tan(θ/2).During2013p+^Pb data-taking,thebeamdirectionswerereversedapproximatelyhalf-waythroughthe runningperiod,butinpresentingresultsthedirectionoftheprotonbeamisalways chosentopointtopositiveη.

2 Thejetrapidityy^∗isdefinedasy^∗=0.5ln^E_E⁺₋^p_p^z

z whereE andpzaretheen- ergyandthecomponentofthemomentumalongtheprotonbeamdirectioninthe nucleon–nucleoncentre-of-massframe.

theproton andlead beamsbeingreversed attheendofthefirst period. The first period provided approximately 55% of the inte- gratedluminosity withthePbbeamtravellingtopositiverapidity andthe protonbeamtonegative rapidity,andthe second period provided theremainderwiththe beamsreversed.The analysisin this paperuses the events fromboth periods of data-taking and y^∗ isdefinedsothat y^∗>0 alwaysreferstothedownstreampro- tondirection.

2. Experimentalsetup

The measurements presented in this paper were performed using the ATLAS inner detector(ID), calorimeters, minimum-bias trigger scintillator (MBTS), and trigger and data acquisition sys- tems[16].The IDmeasureschargedparticleswithin |η_|<2.5 us- inga combinationofsiliconpixeldetectors,siliconmicrostripde- tectors,andastraw-tubetransitionradiationtracker,allimmersed ina2 Taxialmagneticfield[17].Thecalorimetersystemconsists ofa liquidargon(LAr)electromagnetic(EM) calorimetercovering

|η|<3.2,a steel/scintillatorsamplinghadroniccalorimetercover- ing |η|<1.7,aLArhadroniccalorimetercovering1.5<|η|<3.2, and two LAr electromagnetic and hadronic forward calorimeters (FCal)covering3.2<|η_|<4.9.TheEMcalorimetersuseleadplates astheabsorbersandaresegmentedlongitudinallyinshowerdepth into three compartments with an additional presamplerlayer in front for|η_|<1.8. The granularity of the EM calorimeter varies withlayer andpseudorapidity. The middlesampling layer, which typicallyhasthelargestenergydepositinEMshowers,hasaη×

φ granularity of 0.025×⁰.025 within |η_|<2.5. The hadronic calorimeterusessteelastheabsorberandhasthreesegmentslon- gitudinal in shower depth with cell sizes η× φ =0.1×0.1 for |η|<2.5³ and 0.2×⁰.2 for 2.5<|η|<4.9. The two FCal modules are composed of tungsten and copper absorbers with LAr as the active medium, which together provide ten interac- tion lengthsof material.TheMBTS detects chargedparticles over 2.1<|η|<3.9 using two hodoscopes of16 counters each, posi- tionedatz= ±³.6m.

The p+^{Pb and pp events usedinthisanalysiswere recorded} usinga combinationofminimum-bias(MB) andjet triggers[18].

In p+Pb data-taking,theMB triggerrequiredhitsinatleastone counter in each side of the MBTS detector. In pp collisions the MBconditionwas thepresenceofhitsinthepixelandmicrostrip detectors reconstructed as a track by the high-level trigger sys- tem. Jetswere selected usinghigh-level jet triggers implemented witha reconstruction algorithm similar to the procedureapplied inthe offline analysis. Inparticular, it usedthe anti-k_t algorithm with R=⁰.4, abackground subtractionprocedure,anda calibra- tion of the jet energy to the full hadronic scale. The high-level jettriggers wereseededfromacombinationoflow-levelMB and jethardware-basedtriggers.Sixjettriggerswithtransverseenergy thresholds ranging from 20 GeV to 75 GeV were used to select jets within |η_|<3.2 and a separate trigger with a threshold of 15 GeV wasused toselect jetswith3.2<|η_|<4.9. Thetriggers were prescaledinafashion whichvariedwithtimetoaccommo- datetheevolutionoftheluminositywithinanLHCfill.

3. Dataselection

In the offline analysis, charged-particle tracks were recon- structed inthe IDwiththe samealgorithm usedin pp collisions [19].Thep+^{Pb eventsusedforthisanalysiswererequiredtohave}

3 Anexceptionisthethird(outermost)samplinglayer,whichhasasegmentation of0.2×⁰.1 upto|η|=¹.7.

Fig. 1. Distributionof E^Pb_T for minimum-bias p+Pb collisionsrecordedduring the2013run,measuredintheFCalat−⁴.9<η<−³.2 inthePb-goingdirection.

Theverticaldivisionscorrespondtothesixcentralityintervalsusedinthisanalysis.

Fromrighttoleft,theregionscorrespondtocentralityintervalsof0–10%,10–20%, 20–30%,30–40%,40–60%and60–90%.

a reconstructed vertex containing at least two associated tracks with pT>0.1 GeV,atleast onehit ineach ofthetwo MBTSho- doscopes, and a difference betweentimes measured on the two MBTSsidesoflessthan10 ns.Eventscontaining multiple p+^Pb collisions(pileup)weresuppressedbyrejectingeventshavingtwo ormorereconstructedvertices,eachassociatedwithreconstructed tracks with a total transverse momentum scalar sum of atleast 5 GeV.Thefractionofeventswithone p+Pb interactionrejected by thisrequirementwas lessthan 0.1%. Events witha pseudora- pidity gap (definedby the absence ofclusters inthe calorimeter withmorethan0.2 GeV oftransverseenergy)ofgreaterthantwo units on the Pb-going side of the detector were also removed from the analysis. Such events arise primarily from electromag- netic or diffractive excitation ofthe proton.After accounting for event selection, the number of p+^{Pb events sampled by the} highest-luminosityjettrigger(whichwasunprescaled)was53bil- lion.Theeventselection criteriadescribed herewere designedto selectasampleof p+^{Pb eventstowhichacentralityanalysiscan} beappliedandforwhichmeaningfulgeometricparameterscanbe determined.

The pp events used in this analysis were required to have a reconstructed vertex, with the same definitionas the vertices in p+^{Pb eventsabove.Noother}requirementswereapplied.

4. Centralitydetermination

The centrality of the p+^{Pb events selected for analysis was} characterisedbythetotaltransverseenergy E^Pb_T intheFCalmod- uleonthePb-goingside.The E^Pb_T distributionforminimum-bias p+Pb collisionspassingtheeventselectiondescribedinSection3 ispresentedinFig. 1.Followingstandardtechniques[20],central- ityintervalsweredefinedintermsofpercentilesofthe E_T^Pbdis- tributionafteraccountingforanestimatedinefficiencyof(2±²)% forinelastic p+Pb collisionstopass theappliedeventselection.

Thefollowingcentralityintervalswereusedinthisanalysis,inor- derfromthemostcentraltothemostperipheral:0–10%,10–20%, 20–30%, 30–40%, 40–60%, and60–90%, with the 60–90% interval servingasthereferenceinthe RCPratio.Events witha centrality beyond90% werenotusedintheanalysis,sincetheuncertainties onthecompositionoftheeventsampleandinthedetermination ofthegeometricquantitiesarelargefortheseevents.

A Glauber Monte Carlo (MC) [15] analysis was used to cal- culate Rcoll and TpA for each centrality interval. First, a Glauber MC program[21] was used tosimulatethe geometryofinelastic

Table 1

AverageRcollandTpAvaluesforthecentralityinter- valsusedinthisanalysisalongwithtotalsystematic uncertainties. The Rcoll valuesare with respectto 60–90%events,where^Ncoll =².98⁺₋⁰₀^._.²¹₂₉.

Centrality Rcoll TpA[mb⁻¹] 0–90% – 0.107⁺₋⁰₀^._.⁰⁰⁵₀₀₃ 60–90% – 0.043⁺₋⁰₀^._.⁰⁰³₀₀₄ 40–60% 2.16⁺₋⁰₀^._.⁰⁸₀₇ 0.092⁺₋⁰₀^._.⁰⁰⁴₀₀₆ 30–40% 3.00⁺₋⁰₀^._.²¹₁₄ 0.126⁺₋⁰₀^._.⁰⁰³₀₀₄ 20–30% 3.48⁺₋⁰₀^._.³³₁₈ 0.148⁺₋⁰₀^._.⁰⁰⁴₀₀₂ 10–20% 4.05⁺₋⁰₀^._.⁴⁹₂₁ 0.172⁺₋⁰₀^._.⁰⁰⁷₀₀₃ 0–10% 4.89⁺₋⁰₀^._.⁸³₂₇ 0.208⁺₋⁰₀^._.⁰¹⁹₀₀₅

p+Pb collisions andcalculatethe probability distributionof the number of nucleon participants N_part, P(N_part). The simulations used a Woods–Saxonnuclear densitydistribution and an inelas- tic nucleon–nucleoncross-section, σNN, of 70±^{5 mb.} Separately, PYTHIA 8 [22,23] simulations of4 TeV on 1.57 TeV pp collisions provided a detector-level E^Pb_T distribution for nucleon–nucleon collisions,tobeusedasinputtotheGlaubermodel.Thisdistribu- tionwasfittoagammadistribution.

Then, an extension ofthe wounded-nucleon(WN) [24] model thatincludedanon-lineardependenceof E^Pb_T onNpart wasused todefineNpart-dependentgammadistributionsfor E^Pb_T ,withthe constraint that the distributions reduce to the PYTHIA distribu- tion for Npart=^{2. The non-linear term accounted for the pos-} sible variation of the effectiveFCal acceptance resulting froman Npart-dependentbackwardrapidity shiftoftheproducedsoftpar- ticleswithrespecttothenucleon–nucleonframe[25].Thegamma distributions were summed over Npart witha P(Npart) weighting toproduceahypothetical E^Pb_T distribution.Thatdistributionwas fittothemeasured E^Pb_T distributionshowninFig. 1withthepa- rameters of the extended WN model allowed to vary freely. The best fit, which contained a significant non-linear term, success- fully described the E^Pb_T distribution in dataover several orders ofmagnitude. Fromtheresultsofthefit,thedistributionof Npart valuesandthecorresponding

Npart

werecalculatedforeachcen- tralityinterval.TheresultingRcollandTpA valuesandcorrespond- ingsystematicuncertainties,whicharedescribedinSection8,are showninTable 1.

5. MonteCarlosimulation

Theperformanceofthejetreconstructionprocedurewasevalu- atedusingasampleof36 millioneventsinwhichsimulated√

s= 5.02 TeV pp hard-scatteringeventswereoverlaid withminimum- bias p+^{Pb eventsrecordedduringthe2013run.Thusthesample} contains an underlying event contribution that is identical inall respects to the data. The simulatedevents were generated using PYTHIA [22] (version 6.425, AUET2B tune [26], CTEQ6L1 parton distributionfunctions[27])andthedetectoreffectswerefullysim- ulatedusingGEANT4[28,29].Theseeventswereproducedfordif- ferent pT intervalsofthegenerator-level (“truth”) R=⁰.4 jets.In total,thegenerator-levelspectrumspans10<pT<10³ GeV.Sep- arate sets of 18 million events each were generated for the two differentbeamdirectionstotakeintoaccountanyz-axisasymme- tries inthe detector.For each beamdirection, the four-momenta ofthegeneratedparticleswerelongitudinallyboostedbyarapid- ity of ±⁰.465 to match the corresponding beam conditions. The eventsweresimulatedusingdetectorconditionsappropriatetothe two periods ofthe 2013 p+^{Pb runand}reconstructed usingthe samealgorithmsaswereappliedtotheexperimentaldata.Asep-