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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 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.
©2015CERNforthebenefitoftheATLASCollaboration.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].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 in d +Au collisions thanin pp collisions [4,5]. These
E-mailaddress: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
0370-2693/©2015CERNforthebenefitoftheATLASCollaboration.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense
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 energiesin d +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∗,andtransversemomentum, pT,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 d2σpp
jet/dpTd y∗
, (1)
foragivencentralityselection“cent”,whered2σpp
jet/dpTd y∗is
de-terminedusingthe jetcross-section measured in pp collisions at √
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 thicknessinthegivenclassof p +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≡
Ncollcent/ Npericoll. The RCPratioissensitivetorelativedeviationsinthejetratefrom
thegeometric expectationbetween the p +Pb event centralities. The RpPb and RCP measurements are presented as a function of
inclusivejet y∗and pT.
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 ATLASusesaright-handedcoordinatesystemwithitsoriginatthenominal
in-teractionpoint(IP)inthecentreofthedetectorandthez-axisalongthebeampipe. Thex-axispointsfromtheIPtothecentreoftheLHCring,andthe y-axispoints upward.Cylindricalcoordinates(r,φ)areusedinthetransverseplane,φbeingthe
azimuthalanglearoundthebeampipe.Thepseudorapidityisdefinedinlaboratory
coordinatesintermsofthepolarangleθas η= −ln tan(θ/2).During2013p+Pb
data-taking,thebeamdirectionswerereversedapproximatelyhalf-waythroughthe
runningperiod,butinpresentingresultsthedirectionoftheprotonbeamisalways
chosentopointtopositive η.
2 Thejetrapidityy∗isdefinedasy∗=0.5lnE+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,hasa η× 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-tionedat z= ±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].The p +Pb eventsusedforthisanalysiswererequiredtohave
3 Anexceptionisthethird(outermost)samplinglayer,whichhasasegmentation
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 EPbT intheFCal mod-uleonthePb-goingside.The EPbT 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+−00..2129.
Centrality Rcoll TpA[mb−1] 0–90% – 0.107+−00..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+−00..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+−00..4921 0.172+ 0.007 −0.003 0–10% 4.89+−00..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 EPbT on Npart wasused
todefine Npart-dependentgammadistributionsfor EPbT ,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 EPbT 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.TheresultingRcolland TpA 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 differentbeamdirectionstotakeintoaccountany z-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
sep-arate9-million-event sample of fullysimulated 2.76 TeV PYTHIA pp hard scatteringevents(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 groupedinto η× 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 thepresenceofadditional pp interactions inthesamecrossing (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,afinalmultiplicative in situ calibration wasappliedtoaccount fordifferencesbetweenthesimulateddetectorresponseanddata. Themeasured pTofjetsrecoilingagainstobjectswithan
indepen-dentlycalibratedenergyscale–suchas Z bosons, photons,orjets inadifferentregionofthedetector–wasinvestigated.The in situ 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, pgenT ,andgeneratorjetrapidity y∗,andinthesix p +Pb event centralityintervals.
The reconstruction efficiency for jets having pgenT >25 GeV was found to be greater than 99%. The performance was quan-tified by the means and standard deviations of the pT/pT
=precoT /pgenT −1distributions,referredtoasthejetenergyscale closureandjetenergyresolutionrespectively.Theclosurein p +Pb events was less than 2% for pgenT >25 GeV jets and was better than1%for pgenT >100 GeV jets.Atlow p
gen
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-biasandjettriggered p +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-creasing pTthreshold.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 d2Njet dpTd y∗= 1 Nevt Njet pTy∗ , (3)
where Nevtisthetotal(unprescaled)numberofMB p +Pb events
sampled, Njet istheyield ofjetscorrectedforall detectoreffects andtheinstantaneoustriggerprescaleduringdata-taking,and pT
and y∗ are the widths of the pT and y∗ bins. The
centrality-dependentyields wereconstructedby restrictingNevt and Njet to
come from p +Pb events within a givencentrality interval. The double-differential cross-section in pp collisions was constructed via d2σ dpTd y∗= 1 Lint Njet pTy∗ , (4)
where Lintisthetotalintegratedluminosityofthejettriggerused
in thegiven pT bin.The pT binning inthe pp cross-section was
chosen such that the xT=2pT/
√
s binning between the p +Pb and pp datasets isthesame.
Both the per-event yields in p +Pb collisions and the cross-sectionin pp collisions wererestrictedtothe pTrangewherethe
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 yields werecorrectedforjet en-ergy resolution and residual distortions of the jet energy scale which result in pT-bin migration. For each rapidity interval, the
yieldwascorrectedbytheuseof pT-dependent(and,inthe p +Pb
case,centrality-dependent)bin-by-bin correctionfactors C(pT, y∗)
obtained fromthe ratioof the reconstructed to the truth jet pT
C(pT,y∗)=
Njettruth(pT,y∗)
Njetreco(pT,y∗)
, (5)
where Njettruth (Njetreco)isthenumberoftruthjetsinthegiven ptruthT
(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 yields were at most30%, and were typically ≤10% forjetswithpT>100 GeV.Thesecorrectionswereapplied
tothedetector-levelyield Nrecojet togivetheparticle-levelyieldvia
Njet=C(pT,y∗)Njetreco. (6)
A√s=5.02 TeV pp referencejetcross-sectionwasconstructed throughtheuseofthecorrected 2.76 TeV pp cross-section anda previousATLASmeasurementofthe xT-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 wasevaluatedatfixed xT,
ρ(xT;y∗)= 2.76 TeV 7 TeV 3d2σ2.76 TeV/dp Td y∗ d2σ7 TeV/dp Td y∗ , (7) where d2σ√s/dp
Td 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-sectionmeasuredattheequivalent xT accordingto
d2σ5.02 TeV dpTd y∗ = ρ(xT;y∗)−0.643 2.76 TeV 5.02 TeV 3d2σ2.76 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 jet cross-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
and RpPbmeasurements, RpPbisalsosubjecttouncertaintiesfrom the xT-interpolationofthe
√
s=2.76 TeV pp cross-section tothe √
s=5.02 TeV centre-of-massenergyandfromtheintegrated lu-minosityofthe pp dataset.
Uncertainties in the jet energy scale and resolution influence thecorrectionofthe p +Pb andpp jet spectra.Theuncertaintyin thescalewastakenfromstudiesofthe in situ calorimeter response 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 inthemeasured pTrangewas4% 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. Theuncertaintyintheresolutionwastakenfrom in situ 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 response matrices 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.Theresulting δRpPbvaluesweresomewhatlargerthan
the δRCP values dueto therelative centre-of-mass shiftbetween
the p +Pb and pp collision systems.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.Theresulting δRpPband δRCPfromthe
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
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.
The xT-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) atfixed x
T
isrelatedtotheuncertaintyin ρ(δρ)via (δσ5.02 TeV/σ5.02 TeV) =
0.643(δρ/ρ),where δρwastakenfromRef.[34].Thevaluesof δρ 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-rapidityorathigh pT,theyare2%,
risingtoapproximately12% atlow pT inforwardrapidities.Thus,
inmostofthekinematicregionstudied,thedominantuncertainty on RCPisfromthegeometricfactors Rcoll.
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.
Thejetnuclearmodificationfactor RpPbfor0–90% 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
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,adependenceon xp 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. 6wherethe RCPvaluesareplottedversus
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,thesingletrendin RCPversus pT×cosh(y∗)at
forwardrapiditiesappearstoarisefromoppositetrendsinthe cen-tralandperipheral RpPb,bothasinglefunctionof pT×cosh(y∗).
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-tralitydependenceofjetproductionin p +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.Thisenhancementhasaweak pTandrapidity
dependenceandisgenerallyconsistent withpredictions fromthe modification of the parton distribution functions in the nucleus, which is small in the kinematicregion probed by this measure-ment.
Theresultsofthe RCPmeasurementindicateastrong
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 reportedasafunctionof pT×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
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,
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. Bocchetta80,A. Bocci45, C. Bock99, C.R. Boddy119,
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,
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. Doan5,D. Dobos30,C. Doglioni49, T. Doherty53,
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,
