Centrality and rapidity dependence of inclusive jet production in √s[subscript NN] = 5.02 TeV proton–lead collisions with the ATLAS detector

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Centrality and rapidity dependence of inclusive

jet production in √s[subscript NN] = 5.02 TeV

proton–lead collisions with the ATLAS detector

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Citation

Aad, G., et al. “Centrality and Rapidity Dependence of Inclusive Jet

Production in √s[subscript NN] = 5.02 TeV Proton–lead Collisions

with the ATLAS Detector.” Physics Letters B 748 (September 2015):

392–413. © 2015 CERN for the benefit of the ATLAS Collaboration

As Published

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

Publisher

Elsevier

Version

Final published version

Citable link

http://hdl.handle.net/1721.1/98207

Terms of Use

Creative Commons Attribution

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Contents lists available atScienceDirect

Physics

Letters

B

www.elsevier.com/locate/physletb

Centrality

and

rapidity

dependence

of

inclusive

jet

production

in

√

s

_NN

=

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√sNN=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 _with_the _ATLAS_detector _at_the _Large_Hadron_Collider

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.

1. Introduction

Proton–lead (p+Pb) collisions at the Large Hadron Collider (LHC) provide an excellent opportunity to study hard scattering processesinvolvinganucleartarget[1].Measurementsofjet pro-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)collisionsatthe Rel-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-mail_address:_{atlas.publications@cern.ch}_.

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-twist contri-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

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compared toanucleon–nucleonreferenceconstructedfroma mea-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]wasusedtodeterminetheaverage num-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)(d2Njet/dpTd y∗), were measured as a function of jet centre-of-massrapidity,2 _y∗_,_and_transverse_momentum,_p_T_,_where

Njet isthe numberof jetsmeasured in Nevt p+Pb events anal-ysed. The centrality dependence of the per-event jet yields was evaluatedusingthenuclearmodiﬁcationfactor,

RpPb≡

1

TpA

(1/Nevt)d2Njet/dpTd y∗_cent

d2_σpp

jet/dpTd y∗

, (1)

foragivencentralityselection“cent”,whered2_σpp

jet/dpTd y∗is de-terminedusingthe jetcross-section measured in pp collisionsat √

s=2.76 TeV. The factor RpPb quantiﬁes the absolute

modiﬁca-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, RCP≡

1

Rcoll

(1/Nevt)d2Njet/dpTd y∗_cent

(1/Nevt)d2Njet/dpTd y∗_peri

, (2)

whereRcoll representstheratioofNcollinagivencentrality in-tervaltothatinthemostperipheralinterval,Rcoll≡

N_collcent/Nperi_coll. TheRCPratioissensitivetorelativedeviationsinthejetratefrom thegeometric expectationbetween the p+Pb event centralities. The RpPb and RCP measurements are presented as a function of inclusivejet y∗andpT.

Forthe2013 p+Pb run,theLHCwas conﬁguredwitha4 TeV protonbeam and a 1.57 TeVper-nucleon Pb beamthat together producedcollisions with √sNN=5.02 TeV anda rapidity shiftof thecentre-of-massframeof0.465 unitsrelativetotheATLAS rest frame.The run wassplit intotwo periods,withthe directionsof

1 _ATLAS_uses_a_right-handed_coordinate_system_with_its_origin_at_the_nominal

in-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 _The_jet_rapidity_y∗_is_deﬁned_as_y∗₌₀_.₅_lnE+pz

E−pz whereE andpzarethe

en-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 alwaysreferstothedownstream pro-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,siliconmicrostrip de-tectors,andastraw-tubetransitionradiationtracker,allimmersed ina2 Taxialmagneticﬁeld[17].Thecalorimetersystemconsists ofa liquidargon(LAr)electromagnetic(EM) calorimetercovering |η|<3.2,a steel/scintillatorsamplinghadroniccalorimeter cover-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 calorimeterusessteelastheabsorberandhasthreesegments lon-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-kt 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 whichvariedwithtimeto accommo-datetheevolutionoftheluminositywithinanLHCﬁll.

3. Dataselection

In the oﬄine analysis, charged-particle tracks were recon-structed inthe IDwiththe samealgorithm usedin pp collisions

[19].Thep+Pb eventsusedforthisanalysiswererequiredtohave

3 _An_exception_is_the_third_(outermost)_sampling_layer,_which_has_a_segmentation

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Fig. 1. Distributionof EPb

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 MBTS ho-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)was53 bil-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.Nootherrequirementswereapplied. 4. Centralitydetermination

The centrality of the p+Pb events selected for analysis was characterisedbythetotaltransverseenergy EPb_T intheFCal mod-uleonthePb-goingside.The EPb_T distributionforminimum-bias

p+Pb collisionspassingtheeventselectiondescribedinSection3 ispresentedinFig. 1.Followingstandardtechniques[20], central-ityintervalsweredeﬁnedintermsofpercentilesofthe EPb

T dis-tributionafteraccountingforanestimatedineﬃciencyof(2±2)% forinelastic p+Pb collisionstopass theappliedeventselection. Thefollowingcentralityintervalswereusedinthisanalysis,in or-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

AverageRcollandTpAvaluesforthecentrality

inter-valsusedinthisanalysisalongwithtotalsystematic uncertainties. The Rcoll valuesare with respectto

60–90%events,whereNcoll =2.98+0₋₀..2129.

Centrality Rcoll TpA[mb−1] 0–90% – 0.107+0₋₀..005003 60–90% – 0.043+0.003 −0.004 40–60% 2.16+0.08 −0.07 0.092+0 .004 −0.006 30–40% 3.00+0₋₀..2114 0.126+0 .003 −0.004 20–30% 3.48+0.33 −0.18 0.148+ 0.004 −0.002 10–20% 4.05+0₋₀..4921 0.172+0 .007 −0.003 0–10% 4.89+0₋₀..8327 0.208+0 .019 −0.005

p+Pb collisions andcalculatethe probability distributionof the number of nucleon participants Npart, P(Npart). 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 EPb

T distribution for nucleon–nucleon collisions,tobeusedasinputtotheGlaubermodel.This distribu-tionwasﬁttoagammadistribution.

Then, an extension ofthe wounded-nucleon(WN) [24] model thatincludedanon-lineardependenceof EPb_T onNpart wasused todeﬁneNpart-dependentgammadistributionsfor EPb_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 shiftoftheproducedsoft par-ticleswithrespecttothenucleon–nucleonframe[25].Thegamma distributions were summed over Npart witha P(Npart) weighting toproduceahypothetical EPb_T distribution.Thatdistributionwas fittothemeasured EPb_T distributionshowninFig. 1withthe pa-rameters of the extended WN model allowed to vary freely. The best fit, which contained a significant non-linear term, success-fully described the EPb

T distribution in dataover several orders ofmagnitude. Fromtheresultsoftheﬁt,thedistributionof Npart valuesandthecorrespondingNpart

werecalculatedforeach cen-tralityinterval.TheresultingRcollandTpA valuesand

correspond-ingsystematicuncertainties,whicharedescribedinSection8,are showninTable 1.

5. MonteCarlosimulation

Theperformanceofthejetreconstructionprocedurewas evalu-atedusingasampleof36 millioneventsinwhichsimulated√s=

5.02 TeV pp hard-scatteringeventswereoverlaid with minimum-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])andthedetectoreffectswerefully sim-ulatedusingGEANT4[28,29].Theseeventswereproducedfor dif-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-axis asymme-tries inthe detector.For each beamdirection, the four-momenta ofthegeneratedparticleswerelongitudinallyboostedbya rapid-ity of ±0.465 to match the corresponding beam conditions. The eventsweresimulatedusingdetectorconditionsappropriatetothe two periods ofthe 2013 p+Pb runandreconstructed usingthe samealgorithmsaswereappliedtotheexperimentaldata.A

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sep-arate9-million-event sample of fullysimulated 2.76 TeV PYTHIA

pp hardscatteringevents(withthesameversion,tuneandparton distributionfunctionset)wasusedtoevaluatethejetperformance in√s=2.76 TeV pp collisionsduring2013data-taking.

6. Jetreconstructionandperformance

Thejetreconstructionandunderlyingeventsubtraction proce-dureswereadaptedfromthoseusedbyATLASinPb+Pb collisions, whicharedescribedindetailinRefs. [30,31],andaresummarised here along with any substantial differences from the referenced analyses.

An iterative procedure was used to obtain an event-by-event estimate of the underlyingevent energydensity while excluding contributionsfromjetstothatestimate.Themodulationofthe un-derlyingeventenergydensitytoaccountforpotentialellipticﬂow was not included in this analysis. Jets were reconstructed from the anti-kt algorithm with R=0.4 applied to calorimeter cells

groupedintoη× φ =0.1×0.1 towers,withtheﬁnal jet kine-matics calculated from the background-subtracted energy in the cellscontainedin thejet.The rateofjetsreconstructedfromthe underlyingeventﬂuctuationsofsoftparticleswasnegligibleinthe kinematicrangestudied andthereforenoattempt toreject them wasmade. Themean subtractedtransverseenergyin p+Pb col-lisions was 2.4 GeV (1.4 GeV) forjets with|y∗| <1 ( y∗>3). In

pp collisions,thisproceduresimplysubtractstheunderlyingevent pedestaldepositedinthecalorimeterwhichcanarise,inpart,from thepresenceofadditionalpp interactionsinthesamecrossing (in-timepileup).

Followingtheabovejetreconstruction,asmallcorrection, typ-ically a few percent, was applied to the transverse momentum ofthose jetswhich didnot overlap witha regionexcluded from thebackgrounddeterminationandthuswereerroneouslyincluded inthe initial estimate ofthe underlyingeventbackground. Then, thejet energieswerecorrectedtoaccountforthecalorimeter en-ergyresponseusingan η- and pT-dependentmultiplicativefactor thatwasderivedfromthesimulations[32].Followingthis calibra-tion,aﬁnalmultiplicativeinsitu calibrationwasappliedtoaccount fordifferencesbetweenthesimulateddetectorresponseanddata. Themeasured pTofjetsrecoilingagainstobjectswithan indepen-dentlycalibratedenergyscale–suchasZ bosons,photons,orjets inadifferentregionofthedetector–wasinvestigated.Theinsitu

calibration,whichtypically differed fromunity by afew percent, was derived by comparing this pT balance in pp data with that insimulationsina fashionsimilar tothat usedpreviously within ATLAS[33].

Thejetreconstructionperformance wasevaluatedinthe simu-latedsamplesbyapplyingthesamesubtractionandreconstruction procedureaswasappliedtodata.Theresultingreconstructedjets withtransverse momentum preco

T were compared withtheir cor-respondinggenerator jets, which were produced by applyingthe anti-kt algorithm totheﬁnal-stateparticles produced byPYTHIA,

excluding muonsandneutrinos. Each generator jet was matched toareconstructedjet,andthe pT differencebetweenthetwojets wasstudiedasa functionofthegeneratorjet transverse momen-tum,pgen_T ,andgeneratorjetrapidityy∗,andinthesixp+Pb event centralityintervals.

The reconstruction eﬃciency for jets having pgen_T >25 GeV was found to be greater than 99%. The performance was quan-tiﬁed by the means and standard deviations of the pT/pT

=preco_T /pgen_T −1distributions,referredtoasthejetenergyscale closureandjetenergyresolutionrespectively.Theclosureinp+Pb events was less than 2% for pgen_T >25 GeV jets and was better than1%forpgen_T >100 GeV jets.Atlow pgen_T ,theenergyscale clo-sureandresolutionexhibitedaweak p+Pb centralitydependence,

withdifferencesintheclosureofupto 1%anddifferencesinthe resolution ofup to 2% in the mostcentral 0–10% eventsrelative tothe60–90%peripheralevents.Athighjet pT,theresponsewas centralityindependentwithinsensitivity.In pp events,theclosure waslessthan1%intheentirekinematicrangestudied.

In order to quantify the degree of pT-bin migration intro-ducedbythedetectorresponseandreconstruction procedure, re-sponse matrices were populated by recording the pT values of eachgenerator–reconstructedjetpair.Separatematriceswere con-structed for each y∗ interval and p+Pb centrality interval used intheanalysis.The pT binsusedwerechosen toincreasewith pT such thatthe widthofeachbinwas ≈0.25 of thebinlow edge. Using this binning, the proportion of jetswith reconstructed pT in the same bin as their truth pT monotonically increased with truth pTandwas50–70%.

7. Dataanalysis

Acombinationofminimum-biasandjettriggeredp+Pb events were selected for analysis as described in Section 2. The sam-pled luminosity (defined asthe luminosity divided by the mean luminosity-weightedprescale)ofthejettriggersincreasedwith in-creasingpTthreshold.Offlinejetswereselectedfortheanalysisby requiringamatchtoanonlinejettrigger.Theefficiencyofthe var-ious triggers was determined withrespect to the minimum-bias triggerandtolowerthresholdjettriggers. Forsimplicity,each pT bin usedjets selected by only one jet trigger. In a given pT bin, jets were selected by the highest-threshold jet trigger for which the efficiencywas determinedto be greater than99% in thebin. Noadditionalcorrectionsforthetriggerefficiencywereapplied.

Thedouble-differentialper-eventjetyieldsin p+Pb collisions wereconstructedvia

1 Nevt d2_Njet dpTd y∗= 1 Nevt Njet pTy∗ , (3)

whereNevtisthetotal(unprescaled)numberofMB p+Pb events sampled, Njet istheyield ofjetscorrectedforall detectoreffects andtheinstantaneoustriggerprescaleduringdata-taking,andpT and y∗ are the widths of the pT and y∗ bins. The centrality-dependentyields wereconstructedby restrictingNevt andNjet to come from p+Pb events within a givencentrality interval. The double-differential cross-section in pp collisions was constructed via d2_σ dpTd y∗= 1 Lint Njet pTy∗ , (4)

whereLintisthetotalintegratedluminosityofthejettriggerused in thegiven pT bin.The pT binning inthe pp cross-section was chosen such that the xT=2pT/

√

s binning between the p+Pb andpp datasetsisthesame.

Both the per-event yields in p+Pb collisions and the cross-sectionin pp collisionswererestrictedtothepTrangewherethe MC studies described in Section 6 show that the eﬃciencyfora truth jet to remain in the same pT binis ≥50%. This pT range was rapidity dependent, withthe lowest pT bin edgeused rang-ing from50 GeV in the mostbackwardrapidity intervalsstudied to25 GeV inthemostforwardintervals.

Themeasured p+Pb and pp yieldswerecorrectedforjet en-ergy resolution and residual distortions of the jet energy scale which result in pT-bin migration. For each rapidity interval, the yieldwascorrectedbytheuseofpT-dependent(and,inthep+Pb case,centrality-dependent)bin-by-bin correctionfactorsC(pT,y∗) obtained fromthe ratioof the reconstructed to the truth jet pT distributionsforjetsoriginatinginatrue y∗bin,accordingto

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C(pT,y∗)= N

jet

truth(pT,y∗) Njetreco(pT,y∗)

, (5)

whereNjet_truth (Njetreco)isthenumberoftruthjetsinthegivenptruthT (preco

T )bininthecorrespondingMCsamples.

Since the determination of the correction factors C(pT,y∗) is sensitiveto theshape ofthejet spectrumin theMC sample,the responsematricesusedtogeneratethemwerereweightedto pro-vide a better match between the reconstructed distributions in data and simulated events. The spectrum of generator jets was weighted jet-by-jetbythe ratioofthereconstructed spectrumin data to that in simulation. This ratio was found to be approx-imately linear in the logarithm of reconstructed pT. A separate reweighting was performedforthe p+Pb jetyield ineach cen-trality interval, resulting in changes of ≤10% from the original correctionfactors beforereweighting. Theresultingcorrectionsto the p+Pb and pp yieldswere at most30%, and were typically ≤10% forjetswithpT>100 GeV.Thesecorrectionswereapplied tothedetector-levelyieldNrecojet togivetheparticle-levelyieldvia

Njet=C(pT,y∗)Njetreco. (6)

A√s=5.02 TeV pp referencejetcross-sectionwasconstructed throughtheuseofthecorrected 2.76 TeV pp cross-sectionanda previousATLASmeasurementofthexT-scalingbetweenthe inclu-sivejetcross-sectionsat√s=2.76 TeV (measuredusing0.20 pb−1 of data collected in 2011) and 7 TeV (measured using 37 pb−1 of data collected in 2010) [34]. In this previous analysis, the √

s-scaled ratio ρ ofthe 2.76 TeV cross-section to that at 7 TeV wasevaluatedatﬁxedxT,

ρ(xT;y∗)= 2.76 TeV 7 TeV 3_d2_σ2.76 TeV_/_dp Td y∗ d2_σ7 TeV_/_dp Td y∗ , (7)

where d2_σ√s_/_{dpTd y}∗ _is _the _{pp jet} _{cross-section} _at _the _given

centre-of-mass energy √s, and the numerator and denominator are each evaluated at the same xT (but different pT=xT√s/2). Equation(7)canberearrangedtodeﬁnethecross-sectionat√s=

7 TeV intermsofthatat2.76 TeV timesamultiplicativefactorand dividedby ρ.

The √s=5.02 TeV pp cross-sectionat each pT and y∗ value wasconstructedbyscalingthecorrected√s=2.76 TeV pp cross-sectionmeasuredattheequivalentxT accordingto

d2σ5.02 TeV dpTd y∗ = ρ(xT;y∗)−0.643 2.76 TeV 5.02 TeV 3_d2_σ2.76 TeV dpTd y∗ , (8)

where the power −ln(2.76/5.02)/ln(2.76/7)≈ −0.643 interpo-latesbetween2.76 TeV and 7 TeV to5.02 TeV usinga power-law collisionenergydependenceateach pT and y∗.Sincethejet en-ergy scaleand xT-interpolationuncertainties are large forthe pp dataat largerapidities (|y∗|>2.8), a √s=5.02 TeV reference is notconstructedinthatrapidityregion.

The pp jetcross-section at √s=2.76 TeV measuredwiththe 2013datawas foundto agreewiththeprevious ATLAS measure-mentof thesame quantity [34] within the systematic uncertain-ties.

8. Systematicuncertainties

The RCPand RpPb measurementsaresubjecttosystematic

un-certaintiesarisingfromanumberofsources:thejetenergyscale andresolution,differencesinthespectralshapebetweendataand simulationaffectingthe bin-by-bincorrection factors,residual in-eﬃciency in the trigger selection, and the estimates of the ge-ometric quantities Rcoll (in RCP) and TpA (in RpPb). In addition

to these sources of uncertainty, which are common to the RCP andRpPbmeasurements,RpPbisalsosubjecttouncertaintiesfrom

the xT-interpolationofthe √

s=2.76 TeV pp cross-section tothe √

s=5.02 TeV centre-of-massenergyandfromtheintegrated lu-minosityofthepp dataset.

Uncertainties in the jet energy scale and resolution inﬂuence thecorrectionofthep+Pb andpp jetspectra.Theuncertaintyin thescalewastakenfromstudiesoftheinsitu calorimeterresponse andsystematicvariationsofthejetresponseinsimulation[32],as wellasstudiesoftherelativeenergyscaledifferencebetweenthe jet reconstruction procedure inheavy-ion collisions and the pro-cedure usedbyATLAS forinclusivejet measurementsin2.76 TeV and7 TeV pp collisions[34,35].Thetotalenergyscaleuncertainty inthemeasuredpTrangewas4% forjetsin|y∗| <2.8,and7% for jetsin |y∗| >2.8. The sensitivity ofthe results to the uncer-taintyintheenergyscalewasevaluatedseparatelyfortendistinct sources of uncertainty. Eachsource was treatedas fully uncorre-latedwithanyother source,butfullycorrelatedwithitselfin pT,

η,and√s.Theuncertaintyintheresolutionwastakenfrominsitu

studiesofthedijetenergybalance[36].Theresolutionuncertainty was generally<10%, exceptforlow-pT jetswhere itwas <20%. The effects on the RCP and RpPb measurements were evaluated

throughanadditionalsmearingoftheenergyofreconstructedjets inthe simulationsuch thatthe resolutionuncertaintywas added totheoriginalresolutioninquadrature.

The resultingsystematicuncertaintieson RCP (δRCP) and RpPb

(δRpPb) were evaluated by producing new response matrices in accordance witheach sourceof theenergyscale uncertaintyand the resolutionuncertainty,generatingnewcorrection factors,and calculating the new RCP and RpPb results. Eachenergy scaleand

resolution variation was applied to all rapidity bins andto both the p+Pb and pp responsematrices simultaneously.The uncer-tainty on RCP and RpPb from the total energy scale uncertainty

was determinedbyaddingtheeffectsofthetenenergyscale un-certaintysourcesinquadrature.Sincethecorrectionfactorsforthe

p+Pb spectra indifferent centralityintervalswere affected to a similardegreebyvariationsintheenergyscaleandresolution,the effects tended tocancel in the RCP ratio,and the resulting δRCP weresmall.TheresultingδRpPbvaluesweresomewhatlargerthan the δRCP values dueto therelative centre-of-mass shiftbetween the p+Pb andpp collisionsystems.Thecentralitydependenceof theenergyscaleandresolutionuncertaintiesin p+Pb eventswas negligible.

To achieve better correspondence with the data, the simu-latedjetspectrumwasreweightedtomatchthespectralshapein databeforederivingthebin-by-bincorrectionfactorsasdescribed above.Todeterminethesensitivityoftheresultstothis reweight-ingprocedure,theslopeoftheﬁttotheratioofthedetector-level spectrum in datato that insimulation was varied by the ﬁt un-certainty, and the correction factors were recomputed with this alternativeweighting.TheresultingδRpPbandδRCPfromthe nom-inalvalueswereincludedinthetotalsystematicuncertainty.

As the jet triggers used forthe dataselection were evaluated to havegreaterthan 99%eﬃciencyin the pT regionswhere they are used to selectjets, an uncertainty of 1% was chosen for the centrality selected p+Pb yields andthe pp cross-section in the range20<pT<125 GeV.Thisuncertaintywastakentobe uncor-related betweenthecentrality-selected p+Pb yieldsandthe pp

cross-section,resultingina1.4%uncertaintyonthe RCPand RpPb

measurements.

The geometric quantities Rcoll and TpA andtheiruncertainties are listed inTable 1.Theseuncertainties arise fromuncertainties in thegeometric modellingof p+Pb collisionsandinmodelling the Npart dependence of the forward particle production mea-sured by EPb

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Uncertaintiesin Rcoll were largest forthe ratioof the most cen-tral to the most peripheral interval (0–10%/60–90%), where they were +17/−6%, andsmallest inthe40–60%/60–90% ratio,where theywere+4/−3%.UncertaintiesinTpA werelargestinthemost central(0–10%)andmostperipheral(60–90%)centralityintervals, where the upper or lower uncertainty was as high as 10%, and smallerforintervalsinthemiddleofthe p+Pb centralityrange, wheretheyreachedaminimumof+3/−2% forthe20–30% inter-val.

ThexT-interpolationofthe √

s=2.76 TeV pp jetcross-section to5.02 TeV issensitivetouncertaintiesin ρ(xT,y∗),the√s-scaled ratioofjetspectraat2.76and7 TeV.FollowingEq.(8),the uncer-taintyintheinterpolated pp cross-section(δσ5.02 TeV₎ _at_ﬁxed_x

T isrelatedtotheuncertaintyin ρ(δρ)via(δσ5.02 TeV_/_σ5.02 TeV₎₌ 0.643(δρ/ρ),whereδρwastakenfromRef.[34].Thevaluesofδρ

rangedfrom5%to23%intheregionofthemeasurementandwere generallylargeratlower xT andatlargerrapidities.

Theintegrated luminosity forthe 2013 pp dataset was deter-minedby measuringtheinteraction ratewithseveralATLAS sub-detectors.Theabsolutecalibrationwasderivedfromthreevander Meerscans[37] performedduring the pp data-taking in2013in afashionsimilartothatusedpreviously withinATLAS[38]forpp

data-takingathigherenergies. Thesystematicuncertainty onthe integratedluminositywasestimatedtobe3.1%.

The uncertainties from the jet energy scale, jet energy res-olution, reweighting and xT-interpolation are pT and y∗ depen-dent,whiletheuncertaintiesfromthetrigger,luminosity,and ge-ometric factors are not. The total systematic uncertainty on the

RpPb measurement ranges from 7% at mid-rapidity and high pT to 18% at forward rapidities andlow pT. In most pT and rapid-itybins,thedominantsystematicuncertaintyon RpPb isfromthe

xT-interpolation.The pT- and y∗-dependentsystematic uncertain-tieson RCParesmall.Nearmid-rapidityorathighpT,theyare2%, risingtoapproximately12% atlow pT inforwardrapidities.Thus, inmostofthekinematicregionstudied,thedominantuncertainty onRCPisfromthegeometricfactorsRcoll.

9. Results

Fig. 2presentsthefullycorrectedper-eventjetyieldasa func-tionof pT in 0–90% p+Pb collisions, foreach ofthe jet centre-of-massrapidity rangesused inthisanalysis. Atmid-rapidity, the yieldsspanovereightordersofmagnitude.

ThejetnuclearmodiﬁcationfactorRpPbfor0–90%p+Pb events

ispresentedinFig. 3 inthe eightrapidity binsforwhich the pp

reference was constructed. At most rapidities studied, the RpPb

values show a slight (≈10%) enhancement above one, although manybins areconsistentwithunitywithin thesystematic uncer-tainties.At mid-rapidity, the RpPb valuesreach a maximumnear

100 GeV.No large modiﬁcation of the total yield of jets relative tothegeometricexpectation(underwhich RpPb=1)isobserved.

The data in Fig. 3 are compared to a next-to-leading order per-turbativeQCDcalculationofRpPbwiththeEPS09parameterisation

of nuclear parton distribution functions [9], using CT10 [39] for the free proton parton distribution functions and following the procedureforcalculatingjet productionratesin p+Pb collisions described in Refs. [1,40]. The data are slightly higher than the calculation,butgenerallycompatiblewithitwithinsystematic un-certainties.

Thecentral-to-peripheralratio RCPforjetsinp+Pb collisions issummarised in Fig. 4,where the RCP values forthree central-ityintervalsareshowninallrapidityrangesstudied.The RCPratio showsa strongvariation withcentrality relativetothe geometric expectation,under which RCP=1.The jet RCP for0–10%/60–90% eventsis smaller than one atall rapidities forjet pT>100 GeV

Fig. 2. Inclusivedouble-differentialper-eventjetyieldin0–90%p+Pb collisions asafunctionofjet pTindifferenty∗bins.Theyieldsarecorrectedforall

detec-toreffects.Verticalerrorbarsrepresentthestatisticaluncertaintywhiletheboxes representthesystematicuncertainties.

andatall pT atsuﬃcientlyforward(proton-going, y∗>0) rapidi-ties.Nearmid-rapidity,the40–60%/60–90% RCPvaluesare consis-tentwithunityupto100–200 GeV,butindicateasmall suppres-sion athigher pT.In all rapidity intervals studied, RCP decreases withincreasing pTandinincreasinglymorecentralcollisions. Fur-thermore, at ﬁxed pT, RCP decreases systematically at more for-ward rapidities. At the highest pT in the most forward rapidity bin,the0–10%/60–90% RCP valueis≈0.2.Inthebackward rapid-itydirection(lead-going, y∗<0), RCP isfoundtobeenhancedby 10–20%forlow-pTjets.

Fig. 5summarises thejet RpPb incentral,mid-centraland

pe-ripheral eventsin all rapidity intervalsstudied. The patterns ob-servedinthecentrality-dependent RpPb valuesareaconsequence

of the near-geometric scaling of the minimum-bias RpPb values

along with the strong modiﬁcations of the central-to-peripheral ratio RCP.At suﬃcientlyhigh pT, RpPb incentral eventsisfound

to be suppressed (RpPb<1) andin peripheral events to be

en-hanced(RpPb>1).Generally,theserespectivedeviationsfromthe

geometricexpectation(underwhichRpPb=1 forallcentrality

in-tervals)increasewith pT and,atﬁxed pT,increaseastherapidity becomes moreforward.Thus, the largeeffects in RCP are consis-tent withacombinationof modiﬁcationsthat haveopposite sign in the centrality-dependent RpPb values but have little effecton

thecentrality-inclusive(0–90%)RpPbvalues.Atbackward-going

ra-pidities ( y∗<0) the RpPb value for low-pT jets in all centrality

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Fig. 3. MeasuredRpPbvaluesforR=0.4 jetsin0–90%p+Pb collisions.Eachpanel

showsthejetRpPbinadifferentrapidityrange.Verticalerrorbarsrepresentthe

statisticaluncertaintywhiletheboxesrepresentthesystematicuncertaintiesonthe jetyields.TheshadedboxattheleftedgeoftheRpPb=1 horizontallineindicates

thesystematicuncertaintyonTpAandthepp luminosityinquadrature.Theshaded

bandrepresentsacalculationusingtheEPS09nuclearpartondistributionfunction set.

Giventheobservedsuppressionpatternasafunctionofjet ra-pidity,inwhichthesuppressionin RCP atﬁxed pT systematically increasesat more forward-going rapidities,it is natural to ask if it is possible to ﬁnd a single relationship between the RCP val-ues in the different rapidity intervals which is a function of jet kinematicsalone.Totestthis, the RCP valuesineachrapidity bin wereplottedagainstthequantity pT×cosh(y∗)≈E,wherey∗ isthe centreofthe rapidity binand E is thetotal energyofthe jet.Inrelativistickinematics,thetotalenergyofaparticleisgiven by E=mTcosh(y∗), where the transverse mass mT=

m2₊_p2 T. Inthekinematicrangestudied,themassofthetypicaljet is suf-ﬁciently smallrelative to its transverse momentum that approx-imatingthe transverse mass, mT, with the pT is reasonable. The 0–10%/60–90% RCPversus pT×cosh(y∗)isshownforallten ra-pidityrangesinFig. 6.Whenplottedagainstthisvariable,the RCP valuesineachoftheﬁveforward-goingrapidities( y∗>+0.8)fall along thesame curve,which isapproximatelylinear inthe loga-rithmofE.Thistrendisalsoobservedinthetwomostforwardof theremaining rapidity intervals(−0.3<y∗<+0.8), butthe RCP valuesatbackwardrapidities( y∗<−0.3)donotfollowthistrend. This pattern is also observed in other centrality intervals, albeit withadifferentslopeinln(E)foreachcentralityinterval.

Thesepatternssuggestthattheobservedmodiﬁcationsmay de-pend on the initial parton kinematics, such as the longitudinal momentum fraction of the parton originating in the proton, xp.

Inparticular,adependenceonxp wouldexplainwhythedata

fol-Fig. 4. MeasuredRCP valuesforR=0.4 jetsinp+Pb collisionsincentral(stars),

mid-central(diamonds)andmid-peripheral(crosses)events.Eachpanelshowsthe jetRCPinadifferentrapidityrange.Verticalerrorbarsrepresentthestatistical

un-certaintywhiletheboxesrepresentthesystematicuncertaintiesonthejetyields. TheshadedboxesattheleftedgeoftheRCP=1 horizontallineindicatethe

system-aticuncertaintyonRcollfor(fromlefttoright)peripheral,mid-centralandcentral

events.

low a consistent trend vs. pT×cosh(y∗) at forward rapidities (where jet production at a given jet energy E is dominated by

xp∼E/(

√

s/2)partons in theproton) but donot do so at back-wardrapidities(wherethelongitudinalmomentumfractionofthe partonoriginatingintheleadnucleus, xPb,aswell asxp areboth

neededtorelatethejetandpartonkinematics).

ByanalogywithFig. 6wheretheRCPvaluesareplottedversus

pT×cosh(y∗), the RpPb valuesin the fourmost forward-going

bins studied are plottedagainst this variable in Fig. 7. The RpPb

values incentral andperipheral eventsare shownseparately. Al-though the systematic uncertainties are larger on RpPb than on

RCP, the observed behaviour for jetswith pT>150 GeV is con-sistent withthenuclearmodiﬁcations depending onlyonthe ap-proximate totaljet energypT×cosh(y∗).Incentral(peripheral) events, the RpPb values atforward rapidities are consistent with

a rapidity-independent decreasing (increasing) function of pT× cosh(y∗).Thus,thesingletrendinRCPversus pT×cosh(y∗)at forwardrapiditiesappearstoarisefromoppositetrendsinthe cen-tralandperipheralRpPb,bothasinglefunctionofpT×cosh(y∗).

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Fig. 5. MeasuredRpPbvaluesforR=0.4 jetsinp+Pb collisionsincentral(stars),

mid-central(diamonds)andperipheral(crosses)events.Eachpanelshowsthejet

RpPbinadifferentrapidityrange.Verticalerrorbarsrepresentthestatistical

uncer-taintywhiletheboxesrepresentthesystematicuncertaintiesonthejetyields.The shadedboxesattherightedgeoftheRpPb=1 horizontallineindicatethe

system-aticuncertaintiesonTpAandthepp luminosityaddedinquadraturefor(fromleft

toright)peripheral,mid-centralandcentralevents.

The results presented here use the standard Glauber model withfixed σNN toestimatethegeometricquantities.Theimpactof geometricmodelswhichincorporateevent-by-eventchangesinthe configurationoftheprotonwavefunction[41]hasalsobeen stud-ied.Usingtheso calledGlauber–Gribov ColourFluctuation model todeterminethe geometric parametersamplifiesthe effectsseen with the Glauber model. In this model, the suppression in cen-tral events and the enhancement in peripheral events would be increased.

10. Conclusions

Thispaperpresents theresults ofa measurement ofthe cen-tralitydependenceofjetproductioninp+Pb collisionsat√sNN= 5.02 TeV over a wide kinematic range. The data were collected withthe ATLASdetectorattheLHC andcorrespond to27.8 nb−1 of integrated luminosity. The centrality of p+Pb collisions was characterised using the total transverse energy measured in the forward calorimeter on the Pb-going side covering the interval −4.9<η<−3.2. The average numberof nucleon–nucleon colli-sions and the mean nuclear thickness factor were evaluated for eachcentralityintervalusingaGlauberMonteCarloanalysis.

Resultsare presented forthenuclear modiﬁcation factor RpPb

withrespect to a measurement of the inclusive jet cross-section in √s=2.76 TeV pp collisions corresponding to 4.0 pb−1 of in-tegrated luminosity. The pp cross-section was xT-interpolated to 5.02 TeV usingpreviousATLASmeasurementsofinclusivejet

pro-ductionat2.76 and7 TeV.Resultsarealsoshownforthe central-to-peripheral ratio RCP. The centrality-inclusive RpPb results for

0–90%collisionsindicateonlyamodestenhancementoverthe ge-ometricexpectation.ThisenhancementhasaweakpTandrapidity dependenceandisgenerallyconsistent withpredictions fromthe modiﬁcation of the parton distribution functions in the nucleus, which is small in the kinematicregion probed by this measure-ment.

TheresultsoftheRCPmeasurementindicateastrong centrality-dependentreduction intheyieldofjetsincentralcollisions rela-tivetothatinperipheralcollisions,afteraccountingfortheeffects of the collision geometries. In addition, the reduction becomes more pronounced with increasing jet pT and at more forward (downstream proton) rapidities. These two results are reconciled by the centrality-dependent RpPb results,which show a

suppres-sionincentralcollisionsandenhancementinperipheralcollisions, apatternwhichissystematicinpT andy∗.

The RCPand RpPb measurementsatforwardrapiditiesarealso

reportedasafunctionofpT×cosh(y∗),theapproximatetotaljet energy.Whenplottedthisway,theresultsfromdifferentrapidity intervalsfollowasimilartrend.Thissuggeststhatthemechanism responsible forthe observed effectsmay depend onlyon the to-taljetenergyor,moregenerally,ontheunderlyingparton–parton kinematics such asthe fractional longitudinalmomentum ofthe partonoriginatingintheproton.

Iftherelationshipbetweenthecentralityintervalsandproton– lead collision impact parameter determined by the geometric models is correct, these results imply large, impact parameter-dependent changes in the number of partons available for hard scattering. However, they mayalso be theresult ofa correlation betweenthekinematicsofthescatteringandthesoftinteractions resulting inparticle production atbackward (Pb-going) rapidities [42,43].

Recently, theeffects observedherehavebeen hypothesised as arising froma suppressionofthe softparticle multiplicityin col-lisions producinghighenergyjets[44].Independently,it hasalso beenarguedthatprotonconﬁgurationscontainingalarge-x parton interactwithnucleonsinthenucleuswithareducedcross-section, resultingintheobservedmodiﬁcations[45].Inanycasethe pres-enceofsuchcorrelationswouldchallengetheusual factorisation-basedframeworkfordescribinghardscatteringprocessesin colli-sionsinvolvingnuclei.

Acknowledgements

We thank CERN forthe very successfuloperation of the LHC, aswell as thesupport staff fromour institutionswithout whom ATLAScouldnotbeoperatedeﬃciently.

We acknowledge the support of ANPCyT, Argentina; YerPhI, Armenia; ARC, Australia;BMWFW andFWF,Austria; ANAS, Azer-baijan; SSTC, Belarus; CNPq and FAPESP, Brazil; NSERC, NRC and CFI, Canada; CERN; CONICYT, Chile; CAS, MOST and NSFC, China; COLCIENCIAS, Colombia; MSMT CR, MPO CR and VSC CR, Czech Republic; DNRF, DNSRC and Lundbeck Foundation, Den-mark; EPLANET, ERC and NSRF, European Union; IN2P3-CNRS, CEA-DSM/IRFU,France; GNSF,Georgia;BMBF,DFG,HGF, MPGand AvHFoundation,Germany;GSRTandNSRF,Greece;ISF,MINERVA, GIF, I-CORE and Benoziyo Center, Israel; INFN, Italy; MEXT and JSPS, Japan; CNRST, Morocco; FOM and NWO, Netherlands; BRF and RCN, Norway; MNiSW and NCN, Poland; GRICES and FCT, Portugal; MNE/IFA, Romania; MES of Russia andROSATOM, Rus-sian Federation; JINR; MSTD, Serbia; MSSR, Slovakia; ARRS and MIZŠ, Slovenia; DST/NRF, South Africa; MINECO, Spain; SRC and Wallenberg Foundation, Sweden;SER, SNSF and Cantons of Bern and Geneva, Switzerland; NSC, Taiwan; TAEK, Turkey; STFC, the

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Fig. 6. MeasuredRCPvaluesforR=0.4 jetsin0–10%p+Pb collisions.Thepanelontheleftshowstheﬁverapidityrangesthatarethemostforward-going,whilethepanel

ontherightshowstheremainingﬁve.TheRCPvaluesateachrapidityareplottedasafunctionofpT×cosh(y∗),wherey∗isthemidpointoftherapiditybin.Vertical

errorbarsrepresentthestatisticaluncertaintywhiletheboxesrepresentthesystematicuncertaintiesonthejetyields.Theshadedboxattheleftedge(intheleftpanel) andrightedge(intherightpanel)oftheRCP=1 horizontallineindicatesthesystematicuncertaintyonRcoll.

Fig. 7. MeasuredRpPbvaluesfor R=0.4 jetsinp+Pb collisionsdisplayedformultiplerapidityranges,showing0–10%eventsintheleftpaneland60–90%eventsinthe

rightpanel.TheRpPbateachrapidityisplottedasafunctionofpT×cosh(y∗),wherey∗isthemidpointoftherapiditybin.Verticalerrorbarsrepresentthestatistical

uncertaintywhiletheboxesrepresentthesystematicuncertaintiesonthejetyields.TheshadedboxattheleftedgeoftheRpPb=1 horizontallineindicatesthesystematic

uncertaintiesonTpAandthepp luminosityaddedinquadrature.

Royal Society and Leverhulme Trust, United Kingdom; DOE and NSF,UnitedStatesofAmerica.

The crucial computingsupport fromall WLCG partners is ac-knowledged gratefully, in particular from CERN and the ATLAS Tier-1 facilities at TRIUMF (Canada), NDGF (Denmark, Norway, Sweden), CC-IN2P3 (France), KIT/GridKA (Germany), INFN-CNAF (Italy),NL-T1(Netherlands),PIC(Spain),ASGC (Taiwan),RAL(UK) andBNL(USA)andintheTier-2facilitiesworldwide.

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ATLASCollaboration

G. Aad84,B. Abbott112, J. Abdallah152, S. Abdel Khalek116, O. Abdinov11,R. Aben106,B. Abi113, M. Abolins89, O.S. AbouZeid159,H. Abramowicz154, H. Abreu153, R. Abreu30, Y. Abulaiti147a,147b, B.S. Acharya165a,165b,a, L. Adamczyk38a, D.L. Adams25, J. Adelman177, S. Adomeit99, T. Adye130, T. Agatonovic-Jovin13a,J.A. Aguilar-Saavedra125a,125f,M. Agustoni17,S.P. Ahlen22,F. Ahmadov64,b, G. Aielli134a,134b, H. Akerstedt147a,147b, T.P.A. Åkesson80,G. Akimoto156, A.V. Akimov95,

G.L. Alberghi20a,20b, J. Albert170,S. Albrand55,M.J. Alconada Verzini70, M. Aleksa30, I.N. Aleksandrov64, C. Alexa26a,G. Alexander154, G. Alexandre49,T. Alexopoulos10, M. Alhroob165a,165c, G. Alimonti90a, L. Alio84,J. Alison31,B.M.M. Allbrooke18, L.J. Allison71,P.P. Allport73,J. Almond83,A. Aloisio103a,103b, A. Alonso36, F. Alonso70,C. Alpigiani75, A. Altheimer35,B. Alvarez Gonzalez89, M.G. Alviggi103a,103b, K. Amako65,Y. Amaral Coutinho24a,C. Amelung23,D. Amidei88,S.P. Amor Dos Santos125a,125c, A. Amorim125a,125b,S. Amoroso48, N. Amram154,G. Amundsen23, C. Anastopoulos140,L.S. Ancu49, N. Andari30, T. Andeen35, C.F. Anders58b,G. Anders30,K.J. Anderson31, A. Andreazza90a,90b,

V. Andrei58a, X.S. Anduaga70, S. Angelidakis9,I. Angelozzi106, P. Anger44, A. Angerami35, F. Anghinolﬁ30,A.V. Anisenkov108,c, N. Anjos125a,A. Annovi47,A. Antonaki9,M. Antonelli47, A. Antonov97, J. Antos145b, F. Anulli133a,M. Aoki65,L. Aperio Bella18, R. Apolle119,d, G. Arabidze89, I. Aracena144, Y. Arai65, J.P. Araque125a,A.T.H. Arce45,J-F. Arguin94, S. Argyropoulos42, M. Arik19a, A.J. Armbruster30, O. Arnaez30, V. Arnal81,H. Arnold48,M. Arratia28,O. Arslan21, A. Artamonov96, G. Artoni23,S. Asai156, N. Asbah42,A. Ashkenazi154,B. Åsman147a,147b, L. Asquith6, K. Assamagan25, R. Astalos145a,M. Atkinson166, N.B. Atlay142,B. Auerbach6,K. Augsten127,M. Aurousseau146b,

G. Avolio30, G. Azuelos94,e, Y. Azuma156, M.A. Baak30,A.E. Baas58a, C. Bacci135a,135b, H. Bachacou137, K. Bachas155,M. Backes30,M. Backhaus30,J. Backus Mayes144, E. Badescu26a,P. Bagiacchi133a,133b, P. Bagnaia133a,133b,Y. Bai33a,T. Bain35, J.T. Baines130,O.K. Baker177,P. Balek128,F. Balli137, E. Banas39, Sw. Banerjee174,A.A.E. Bannoura176,V. Bansal170, H.S. Bansil18,L. Barak173, S.P. Baranov95,

E.L. Barberio87, D. Barberis50a,50b, M. Barbero84,T. Barillari100,M. Barisonzi176,T. Barklow144,

N. Barlow28, B.M. Barnett130,R.M. Barnett15,Z. Barnovska5, A. Baroncelli135a,G. Barone49, A.J. Barr119, F. Barreiro81,J. Barreiro Guimarães da Costa57, R. Bartoldus144, A.E. Barton71,P. Bartos145a,

V. Bartsch150, A. Bassalat116,A. Basye166, R.L. Bates53, J.R. Batley28,M. Battaglia138,M. Battistin30, F. Bauer137,H.S. Bawa144,f,M.D. Beattie71,T. Beau79,P.H. Beauchemin162, R. Beccherle123a,123b, P. Bechtle21,H.P. Beck17,g,K. Becker176, S. Becker99,M. Beckingham171,C. Becot116,A.J. Beddall19c, A. Beddall19c,S. Bedikian177, V.A. Bednyakov64,C.P. Bee149, L.J. Beemster106,T.A. Beermann176,

M. Begel25,K. Behr119,C. Belanger-Champagne86,P.J. Bell49, W.H. Bell49, G. Bella154, L. Bellagamba20a, A. Bellerive29,M. Bellomo85, K. Belotskiy97,O. Beltramello30,O. Benary154,D. Benchekroun136a, K. Bendtz147a,147b, N. Benekos166,Y. Benhammou154, E. Benhar Noccioli49, J.A. Benitez Garcia160b, D.P. Benjamin45,J.R. Bensinger23,K. Benslama131, S. Bentvelsen106,D. Berge106,

E. Bergeaas Kuutmann167,N. Berger5, F. Berghaus170, J. Beringer15,C. Bernard22,P. Bernat77, C. Bernius78,F.U. Bernlochner170,T. Berry76,P. Berta128, C. Bertella84,G. Bertoli147a,147b, F. Bertolucci123a,123b,C. Bertsche112, D. Bertsche112,M.I. Besana90a, G.J. Besjes105,

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R.M. Bianchi124, L. Bianchini23,M. Bianco30,O. Biebel99,S.P. Bieniek77, K. Bierwagen54,J. Biesiada15, M. Biglietti135a, J. Bilbao De Mendizabal49, H. Bilokon47, M. Bindi54,S. Binet116, A. Bingul19c,

C. Bini133a,133b,C.W. Black151,J.E. Black144, K.M. Black22,D. Blackburn139,R.E. Blair6, J.-B. Blanchard137,T. Blazek145a,I. Bloch42,C. Blocker23,W. Blum82,∗,U. Blumenschein54, G.J. Bobbink106,V.S. Bobrovnikov108,c_, _{S.S. Bocchetta}80_,_{A. Bocci}45_, _{C. Bock}99_, _{C.R. Boddy}119_,

M. Boehler48,T.T. Boek176, J.A. Bogaerts30, A.G. Bogdanchikov108,A. Bogouch91,∗,C. Bohm147a, J. Bohm126,V. Boisvert76, T. Bold38a,V. Boldea26a, A.S. Boldyrev98, M. Bomben79,M. Bona75, M. Boonekamp137, A. Borisov129,G. Borissov71,M. Borri83, S. Borroni42,J. Bortfeldt99,

V. Bortolotto135a,135b,K. Bos106, D. Boscherini20a,M. Bosman12, H. Boterenbrood106, J. Boudreau124, J. Bouffard2,E.V. Bouhova-Thacker71, D. Boumediene34,C. Bourdarios116, N. Bousson113,

S. Boutouil136d, A. Boveia31, J. Boyd30, I.R. Boyko64,J. Bracinik18,A. Brandt8, G. Brandt15,O. Brandt58a, U. Bratzler157,B. Brau85,J.E. Brau115, H.M. Braun176,∗, S.F. Brazzale165a,165c, B. Brelier159,

K. Brendlinger121,A.J. Brennan87,R. Brenner167, S. Bressler173, K. Bristow146c, T.M. Bristow46, D. Britton53, F.M. Brochu28,I. Brock21, R. Brock89,C. Bromberg89,J. Bronner100, G. Brooijmans35, T. Brooks76,W.K. Brooks32b,J. Brosamer15,E. Brost115,J. Brown55, P.A. Bruckman de Renstrom39, D. Bruncko145b, R. Bruneliere48,S. Brunet60,A. Bruni20a, G. Bruni20a, M. Bruschi20a, L. Bryngemark80, T. Buanes14, Q. Buat143, F. Bucci49,P. Buchholz142, R.M. Buckingham119,A.G. Buckley53,S.I. Buda26a, I.A. Budagov64,F. Buehrer48,L. Bugge118, M.K. Bugge118,O. Bulekov97,A.C. Bundock73,H. Burckhart30, S. Burdin73, B. Burghgrave107,S. Burke130,I. Burmeister43, E. Busato34,D. Büscher48,V. Büscher82, P. Bussey53, C.P. Buszello167,B. Butler57, J.M. Butler22,A.I. Butt3,C.M. Buttar53,J.M. Butterworth77, P. Butti106,W. Buttinger28,A. Buzatu53,M. Byszewski10,S. Cabrera Urbán168, D. Caforio20a,20b, O. Cakir4a, P. Calaﬁura15,A. Calandri137, G. Calderini79,P. Calfayan99, R. Calkins107, L.P. Caloba24a, D. Calvet34, S. Calvet34,R. Camacho Toro49,S. Camarda42,D. Cameron118,L.M. Caminada15, R. Caminal Armadans12, S. Campana30,M. Campanelli77,A. Campoverde149, V. Canale103a,103b, A. Canepa160a, M. Cano Bret75,J. Cantero81,R. Cantrill125a, T. Cao40, M.D.M. Capeans Garrido30, I. Caprini26a,M. Caprini26a, M. Capua37a,37b, R. Caputo82,R. Cardarelli134a,T. Carli30,G. Carlino103a, L. Carminati90a,90b,S. Caron105,E. Carquin32a,G.D. Carrillo-Montoya146c,J.R. Carter28,

J. Carvalho125a,125c,D. Casadei77,M.P. Casado12,M. Casolino12,E. Castaneda-Miranda146b,A. Castelli106, V. Castillo Gimenez168,N.F. Castro125a,P. Catastini57,A. Catinaccio30,J.R. Catmore118,A. Cattai30, G. Cattani134a,134b,J. Caudron82,S. Caughron89,V. Cavaliere166,D. Cavalli90a,M. Cavalli-Sforza12, V. Cavasinni123a,123b,F. Ceradini135a,135b,B.C. Cerio45,K. Cerny128,A.S. Cerqueira24b,A. Cerri150, L. Cerrito75, F. Cerutti15, M. Cerv30,A. Cervelli17, S.A. Cetin19b,A. Chafaq136a, D. Chakraborty107, I. Chalupkova128,P. Chang166, B. Chapleau86,J.D. Chapman28,D. Charfeddine116,D.G. Charlton18, C.C. Chau159, C.A. Chavez Barajas150, S. Cheatham86, A. Chegwidden89,S. Chekanov6,

S.V. Chekulaev160a, G.A. Chelkov64,h,M.A. Chelstowska88,C. Chen63,H. Chen25, K. Chen149,

L. Chen33d,i,S. Chen33c,X. Chen146c,Y. Chen66,Y. Chen35, H.C. Cheng88, Y. Cheng31, A. Cheplakov64, R. Cherkaoui El Moursli136e,V. Chernyatin25,∗,E. Cheu7, L. Chevalier137,V. Chiarella47,

G. Chiefari103a,103b, J.T. Childers6, A. Chilingarov71,G. Chiodini72a, A.S. Chisholm18,R.T. Chislett77, A. Chitan26a, M.V. Chizhov64,S. Chouridou9,B.K.B. Chow99, D. Chromek-Burckhart30,M.L. Chu152, J. Chudoba126, J.J. Chwastowski39, L. Chytka114,G. Ciapetti133a,133b,A.K. Ciftci4a,R. Ciftci4a, D. Cinca53, V. Cindro74, A. Ciocio15,P. Cirkovic13b,Z.H. Citron173,M. Citterio90a,M. Ciubancan26a,A. Clark49, P.J. Clark46,R.N. Clarke15, W. Cleland124,J.C. Clemens84,C. Clement147a,147b,Y. Coadou84,

M. Cobal165a,165c,A. Coccaro139, J. Cochran63, L. Coffey23,J.G. Cogan144,J. Coggeshall166, B. Cole35, S. Cole107,A.P. Colijn106, J. Collot55, T. Colombo58c,G. Colon85, G. Compostella100,

P. Conde Muiño125a,125b, E. Coniavitis48,M.C. Conidi12, S.H. Connell146b, I.A. Connelly76,

S.M. Consonni90a,90b, V. Consorti48,S. Constantinescu26a,C. Conta120a,120b, G. Conti57,F. Conventi103a,j, M. Cooke15, B.D. Cooper77, A.M. Cooper-Sarkar119,N.J. Cooper-Smith76, K. Copic15, T. Cornelissen176, M. Corradi20a,F. Corriveau86,k, A. Corso-Radu164, A. Cortes-Gonzalez12, G. Cortiana100, G. Costa90a, M.J. Costa168,D. Costanzo140,D. Côté8, G. Cottin28,G. Cowan76,B.E. Cox83, K. Cranmer109, G. Cree29, S. Crépé-Renaudin55,F. Crescioli79, W.A. Cribbs147a,147b,M. Crispin Ortuzar119, M. Cristinziani21, V. Croft105,G. Crosetti37a,37b, C.-M. Cuciuc26a, T. Cuhadar Donszelmann140,J. Cummings177,

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M.J. Da Cunha Sargedas De Sousa125a,125b,C. Da Via83,W. Dabrowski38a,A. Daﬁnca119,T. Dai88, O. Dale14,F. Dallaire94,C. Dallapiccola85,M. Dam36, A.C. Daniells18,M. Dano Hoffmann137, V. Dao48, G. Darbo50a, S. Darmora8, J. Dassoulas42, A. Dattagupta60, W. Davey21,C. David170,T. Davidek128, E. Davies119,d,M. Davies154,O. Davignon79,A.R. Davison77,P. Davison77, Y. Davygora58a,E. Dawe143, I. Dawson140,R.K. Daya-Ishmukhametova85, K. De8,R. de Asmundis103a,S. De Castro20a,20b,

S. De Cecco79, N. De Groot105, P. de Jong106,H. De la Torre81, F. De Lorenzi63, L. De Nooij106, D. De Pedis133a, A. De Salvo133a,U. De Sanctis165a,165b, A. De Santo150,J.B. De Vivie De Regie116, W.J. Dearnaley71,R. Debbe25,C. Debenedetti138, B. Dechenaux55, D.V. Dedovich64,I. Deigaard106, J. Del Peso81,T. Del Prete123a,123b,F. Deliot137,C.M. Delitzsch49,M. Deliyergiyev74,A. Dell’Acqua30, L. Dell’Asta22,M. Dell’Orso123a,123b,M. Della Pietra103a,j, D. della Volpe49, M. Delmastro5,

P.A. Delsart55,C. Deluca106,S. Demers177,M. Demichev64,A. Demilly79, S.P. Denisov129,

D. Derendarz39,J.E. Derkaoui136d,F. Derue79, P. Dervan73,K. Desch21, C. Deterre42,P.O. Deviveiros106, A. Dewhurst130,S. Dhaliwal106,A. Di Ciaccio134a,134b,L. Di Ciaccio5, A. Di Domenico133a,133b,

C. Di Donato103a,103b, A. Di Girolamo30, B. Di Girolamo30, A. Di Mattia153,B. Di Micco135a,135b, R. Di Nardo47,A. Di Simone48,R. Di Sipio20a,20b,D. Di Valentino29,F.A. Dias46,M.A. Diaz32a, E.B. Diehl88,J. Dietrich42,T.A. Dietzsch58a,S. Diglio84, A. Dimitrievska13a, J. Dingfelder21, C. Dionisi133a,133b,P. Dita26a, S. Dita26a,F. Dittus30,F. Djama84,T. Djobava51b,J.I. Djuvsland58a,

M.A.B. do Vale24c,A. Do Valle Wemans125a,125g_, _{T.K.O. Doan}5_,_{D. Dobos}30_,_{C. Doglioni}49_, _{T. Doherty}53_,

T. Dohmae156,J. Dolejsi128, Z. Dolezal128,B.A. Dolgoshein97,∗,M. Donadelli24d, S. Donati123a,123b, P. Dondero120a,120b,J. Donini34, J. Dopke130, A. Doria103a,M.T. Dova70, A.T. Doyle53,M. Dris10, J. Dubbert88,S. Dube15, E. Dubreuil34, E. Duchovni173, G. Duckeck99,O.A. Ducu26a, D. Duda176,

A. Dudarev30,F. Dudziak63,L. Duﬂot116, L. Duguid76, M. Dührssen30,M. Dunford58a,H. Duran Yildiz4a, M. Düren52,A. Durglishvili51b,M. Dwuznik38a, M. Dyndal38a,J. Ebke99, W. Edson2, N.C. Edwards46, W. Ehrenfeld21,T. Eifert144,G. Eigen14, K. Einsweiler15,T. Ekelof167,M. El Kacimi136c,M. Ellert167, S. Elles5,F. Ellinghaus82, N. Ellis30, J. Elmsheuser99,M. Elsing30,D. Emeliyanov130, Y. Enari156,

O.C. Endner82,M. Endo117, R. Engelmann149,J. Erdmann177, A. Ereditato17,D. Eriksson147a,G. Ernis176, J. Ernst2,M. Ernst25,J. Ernwein137,D. Errede166,S. Errede166,E. Ertel82, M. Escalier116,H. Esch43, C. Escobar124,B. Esposito47,A.I. Etienvre137, E. Etzion154,H. Evans60,A. Ezhilov122,L. Fabbri20a,20b, G. Facini31, R.M. Fakhrutdinov129, S. Falciano133a,R.J. Falla77, J. Faltova128, Y. Fang33a, M. Fanti90a,90b, A. Farbin8, A. Farilla135a, T. Farooque12,S. Farrell15,S.M. Farrington171, P. Farthouat30,F. Fassi136e, P. Fassnacht30,D. Fassouliotis9,A. Favareto50a,50b,L. Fayard116,P. Federic145a,O.L. Fedin122,l, W. Fedorko169,M. Fehling-Kaschek48,S. Feigl30, L. Feligioni84,C. Feng33d,E.J. Feng6, H. Feng88, A.B. Fenyuk129,S. Fernandez Perez30,S. Ferrag53,J. Ferrando53, A. Ferrari167, P. Ferrari106,

R. Ferrari120a, D.E. Ferreira de Lima53, A. Ferrer168, D. Ferrere49, C. Ferretti88, A. Ferretto Parodi50a,50b, M. Fiascaris31,F. Fiedler82, A. Filipˇciˇc74, M. Filipuzzi42, F. Filthaut105, M. Fincke-Keeler170,

K.D. Finelli151,M.C.N. Fiolhais125a,125c, L. Fiorini168,A. Firan40,A. Fischer2,J. Fischer176,W.C. Fisher89, E.A. Fitzgerald23,M. Flechl48, I. Fleck142,P. Fleischmann88,S. Fleischmann176,G.T. Fletcher140,

G. Fletcher75,T. Flick176,A. Floderus80,L.R. Flores Castillo174,m, A.C. Florez Bustos160b,

M.J. Flowerdew100,A. Formica137,A. Forti83,D. Fortin160a,D. Fournier116,H. Fox71, S. Fracchia12, P. Francavilla79, M. Franchini20a,20b, S. Franchino30, D. Francis30, L. Franconi118, M. Franklin57, S. Franz61,M. Fraternali120a,120b,S.T. French28,C. Friedrich42, F. Friedrich44,D. Froidevaux30, J.A. Frost28,C. Fukunaga157, E. Fullana Torregrosa82, B.G. Fulsom144, J. Fuster168, C. Gabaldon55, O. Gabizon173,A. Gabrielli20a,20b,A. Gabrielli133a,133b, S. Gadatsch106,S. Gadomski49,

G. Gagliardi50a,50b,P. Gagnon60,C. Galea105, B. Galhardo125a,125c, E.J. Gallas119,V. Gallo17, B.J. Gallop130, P. Gallus127,G. Galster36,K.K. Gan110,R.P. Gandrajula62,J. Gao33b,Y.S. Gao144,f, F.M. Garay Walls46,F. Garberson177,C. García168, J.E. García Navarro168,M. Garcia-Sciveres15, R.W. Gardner31, N. Garelli144,V. Garonne30,C. Gatti47,G. Gaudio120a,B. Gaur142,L. Gauthier94, P. Gauzzi133a,133b,I.L. Gavrilenko95, C. Gay169,G. Gaycken21,E.N. Gazis10,P. Ge33d,Z. Gecse169, C.N.P. Gee130,D.A.A. Geerts106,Ch. Geich-Gimbel21, K. Gellerstedt147a,147b,C. Gemme50a, A. Gemmell53, M.H. Genest55, S. Gentile133a,133b, M. George54, S. George76,D. Gerbaudo164,

A. Gershon154,H. Ghazlane136b,N. Ghodbane34, B. Giacobbe20a, S. Giagu133a,133b,V. Giangiobbe12, P. Giannetti123a,123b,F. Gianotti30, B. Gibbard25,S.M. Gibson76,M. Gilchriese15,T.P.S. Gillam28,