Measurement of the nuclear modification factor for inclusive jets in Pb+Pb collisions at sNN=5.02 TeV with the ATLAS detector

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Measurement of the nuclear modification factor for inclusive jets in Pb+Pb collisions at sNN=5.02 TeV with the ATLAS detector

ATLAS Collaboration AKILLI, Ece (Collab.), et al.

Abstract

Measurements of the yield and nuclear modification factor, RAA , for inclusive jet production are performed using 0.49 nb −1 of Pb+Pb data at sNN=5.02TeV and 25 pb −1 of Pb+Pb data at s=5.02TeV with the ATLAS detector at the LHC. Jets are reconstructed with the anti-kt algorithm with radius parameter R=0.4 and are measured over the transverse momentum range of 40–1000 GeV in six rapidity intervals covering |y|

ATLAS Collaboration, AKILLI, Ece (Collab.), et al . Measurement of the nuclear modification factor for inclusive jets in Pb+Pb collisions at sNN=5.02 TeV with the ATLAS detector. Physics Letters. B , 2019, vol. 790, p. 108-128

DOI : 10.1016/j.physletb.2018.10.076

Available at:

http://archive-ouverte.unige.ch/unige:113947

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

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Contents lists available atScienceDirect

Physics Letters B

www.elsevier.com/locate/physletb

Measurement of the nuclear modification factor for inclusive jets in Pb + ^{Pb collisions at} √

s

= ⁵ . 02 TeV with the ATLAS detector

.TheATLAS Collaboration

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

Articlehistory:

Received16May2018

Receivedinrevisedform8October2018 Accepted13October2018

Availableonline2January2019 Editor: W.-D.Schlatter

Measurements of the yield and nuclear modification factor, RAA, for inclusive jet production are performedusing 0.49 nb⁻¹ ofPb+^{Pb dataat}√_s

NN=⁵.02 TeV and25 pb⁻¹ofPb+^{Pb data at}√ s= 5.02 TeV withthe ATLASdetector atthe LHC. Jetsare reconstructedwiththe anti-kt algorithmwith radiusparameter R=⁰.4 andare measuredoverthe transversemomentumrangeof40–1000 GeV in sixrapidityintervalscovering|^y|<2.8.ThemagnitudeofRAAincreaseswithincreasingjettransverse momentum,reachingavalueofapproximately 0.6at1 TeV inthemostcentralcollisions.Themagnitude ofRAAalsoincreasestowardsperipheralcollisions.ThevalueofRAAisindependentofrapidityatlowjet transversemomenta,butitisobservedtodecreasewithincreasingrapidityathightransversemomenta.

©^{2019TheAuthor.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense} (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP³.

1. Introduction

Heavy-ioncollisionsatultra-relativisticenergiesproduceahot, dense medium of strongly interactingnuclear matter understood tobecomposedofunscreenedcolourchargeswhichiscommonly called a quark–gluon plasma (QGP) [1–4]. Products of the hard scatteringofquarksandgluonsoccurringinthesecollisionsevolve aspartonshowersthatpropagatethroughthehotmedium.Parton showerconstituentsemit medium-inducedgluonradiationorsuf- fer from elastic scattering processes and as a consequence they lose energy, leading to the formation of lower-energy jets. This phenomenonistermed“jetquenching” [5–7].Ithasbeendirectly observedasthe suppressionof thejet yields inPb+Pb collisions comparedtojetyieldsinPb+Pb collisions [8–11],themodification ofjetinternal structure [12–15], anda significantmodification of the transverse energy balance in dijet [16–18] and multijet sys- tems [19].

TheenergylossofpartonspropagatingthroughtheQGPresults in a reduction of the jet yield at a given transverse momentum (pT). This together withthe falling shapeof thejet pT spectrum lead to the observed suppression of jets in collisions of nuclei relative to Pb+Pb collisions. Centralheavy-ion collisions have an enhanced hard-scattering rate due to the larger geometric over- lapbetweenthecollidingnuclei,resultinginalargerper-collision nucleon–nucleon flux. To quantitatively assess the quenching ef- fects, the hard-scatteringrates measured in Pb+Pb collisions are normalisedby themean nuclearthicknessfunction, TAA, which

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

accounts forthisgeometric enhancement [20]. The magnitudeof theinclusivejetsuppressioninnuclearcollisionsrelativetoPb+^Pb isquantifiedbythenuclearmodificationfactor

R_AA= 1 N_evt

d²N_jet dp_Tdy cent

TAA^d2σjet

dpTdy pp

,

where N_jet and σjet are thejet yieldin Pb+Pb collisionsandthe jet cross-section inPb+Pb collisions,respectively, both measured as a function of transverse momentum, pT, andrapidity, y, and where Nevt isthetotalnumberofPb+Pb collisionswithinacho- sencentralityinterval.

A value of RAA≈⁰.5 in central collisions was reported in Pb+Pb measurementsat√

sNN=².76 TeV bytheATLASandCMS Collaborations for jet pT above 100 GeV [9,10]. These measure- mentsthereforeshowasuppressionofjetyieldsbyafactoroftwo incentralcollisions relativetothecorresponding Pb+^{Pb yieldsat} thesamecentre-of-massenergy.Alsoaclearcentralitydependence isobserved.Twounexpectedfeatures[21] alsoemergefromthose studies: R_AAincreasesonlyveryslowlywithincreasingjet p_T,and no dependenceof RAA onjet rapidity isobserved.Measurements by the ATLAS and CMS Collaborations can be complemented by the measurement by the ALICE Collaboration which reports RAA forjetsmeasured in p_T interval of30–120 GeVincentral Pb+^Pb collisions[22].

This Letter describesthe new measurements ofyields of R= 0.4 anti-kt jets [23] performedwith0.49nb⁻¹ ofPb+^{Pb datacol-}

https://doi.org/10.1016/j.physletb.2018.10.076

0370-2693/©^{2019TheAuthor.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense}(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP³.

lected at √

sNN=⁵.02 TeV in 2015and 25 pb⁻¹ ofPb+^{Pb data} collected at √

s=⁵.02 TeV in the same year. This new study closely follows the first measurement by the ATLAS Collabora- tion [9] performed using 0.14 nb⁻¹ of Pb+^{Pb data collected at}

√s_NN=².76 TeV in 2011and4.0 pb⁻¹ ofPb+^{Pb data collected} at√

s=².76 TeV in2013.Higher luminosity,increasedcentre-of- massenergy,andimprovedanalysistechniquesallowedtoextend themeasurement tomore thantwo times highertransversemo- menta, and to larger rapidities. This new measurement provides inputrelevanttoa detailedtheoretical descriptionofjetsuppres- sion,especiallyits dependenceon thecollision energy,centrality, jetpT,andrapidity.

2. Experimentalsetup

TheATLASexperiment [24] attheLHCfeaturesamultipurpose particle detectorwith a forward–backward symmetric cylindrical geometry and a nearly full coverage in solid angle.¹ The mea- surementspresentedherewereperformedusingtheATLAS inner detector,calorimeter,triggeranddataacquisitionsystems.

Theinner-detectorsystem(ID)isimmersedina2 Taxialmag- netic field and provides charged-particle trackingin the pseudo- rapidity range |η_|<2.5. The high-granularitysilicon pixel detec- torcoversthe vertexregionandtypicallyprovides fourmeasure- ments per track. It is followed by the silicon microstrip tracker (SCT)which comprisesfour cylindricallayers ofdouble-sided sil- icon strip detectors in the barrel region, and nine disks in each endcap. These silicon detectorsare complementedby the transi- tionradiationtracker,adrift-tube-baseddetector,whichsurrounds theSCTandhascoverageupto|η|=².0.

The calorimeter system consists of a sampling lead/liquid- argon(LAr)electromagnetic(EM)calorimetercovering|η_|<3.2,a steel/scintillatorsamplinghadroniccalorimetercovering |η_|<1.7, aLAr hadroniccalorimetercovering 1.5<|η|<3.2,andtwo LAr forwardcalorimeters (FCal)covering3.1<|η|<4.9.The hadronic calorimeterhasthreesamplinglayerslongitudinalinshowerdepth in |η_|<1.7 and four sampling layers in 1.5<|η_|<3.2, with a slightoverlap.TheEM calorimeterissegmentedlongitudinallyin shower depth into three compartments with an additional pre- samplerlayer.

Atwo-leveltrigger system [25] was usedto selectthePb+^Pb and Pb+Pb collisions analysed here. The first level (L1) is a hardware-basedtrigger stage which isimplemented withcustom electronics.Thesecondlevelisthesoftware-basedhigh-leveltrig- ger(HLT).TheeventswereselectedbytheHLTwhichwasseeded byaL1jettrigger,totalenergytrigger,orzero-degreecalorimeter (ZDC) trigger.The total energytrigger required atotal transverse energy measured in the calorimeter system to be greater than 5 GeV in Pb+Pb interactions and50 GeV in Pb+Pb interactions.

The ZDC trigger required a presence of at least one neutron on both sides ofZDC (|η_|>8.3). The HLT jet trigger used a jet re- constructionalgorithmsimilartothePb+^{Pb oneappliedinoffline} analyses.Itselectedeventscontainingjetswithtransverseenergies exceedingathreshold,usinga rangeofthresholdsup to 100 GeV inPb+Pb collisionsandupto85 GeV inPb+Pb collisions.Inboth

1 ATLAS uses a right-handed coordinate systemwith itsorigin at the nomi- nalinteractionpoint (IP)inthecentreofthedetector andthe z-axisalongthe beampipe.Thex-axispointsfromtheIPtothecentreoftheLHCring,andthe y-axispointsupward.Cylindricalcoordinates(r,φ)areusedinthetransverseplane, φ being the azimuthal anglearound the beam pipe.The pseudorapidity is de- finedintermsofthepolarangleθasη= −ln tan(θ/2).Rapidity yisdefinedas y=0.5ln[(E+pz)/(E−pz)]whereEandpz aretheenergyandthecomponent ofthemomentumalongthebeamdirection,respectively.Angulardistanceismea- suredinunitsofR≡

(η)²+(φ)².

Table 1

Themeannumber ofparticipants, ^{Npart, themeannuclearthickness function,} TAA,andtheiruncertainties(seeSection5)fordifferentcentralityintervals.

Centrality range Npart TAA[1/mb]

70–80% 15.4±1.0 0.22±0.02

60–70% 30.6±1.6 0.57±0.04

50–60% 53.9±^{1.9 1.27}±^0.07

40–50% 87.0±^{2.3 2.63}±^0.11

30–40% 131.4±^{2.6 4.94}±^0.15

20–30% 189.1±^{2.7 8.63}±^0.17

10–20% 264.0±2.8 14.33±0.17

0–10% 358.8±2.3 23.35±0.20

the Pb+^{Pb and Pb}+Pb collisions, the highest-threshold jet trig- gersampledthefulldeliveredluminositywhilealllowerthreshold triggerswereprescaled.

Inadditiontothejet trigger,twotriggerswereusedinPb+^Pb collisions toselectminimum-biasevents.Theminimum-biastrig- gerrequiredeithermorethan50 GeV transverseenergyrecorded inthewholecalorimetersystembyL1triggerorasignalfromthe ZDCtriggerandatrackidentifiedbytheHLT.

3. DataandMonteCarlosamples,andeventselection

The impact of detector effects on the measurement was de- termined using a simulated detector response evaluated by run- ningMonteCarlo(MC) samplesthroughaGeant4-baseddetector simulation package [26,27]. Two MC samples were used in this study. In the first one, multi-jet processes were simulated with Powheg-Boxv2[28–30] interfacedtothePythia8.186[31,32] par- tonshower model.TheCT10PDFset [33] wasusedinthematrix elementwhiletheA14setoftunedparameters[34] wasusedto- getherwiththeNNPDF2.3LOPDFset[35] forthemodellingofthe non-perturbativeeffects.TheEvtGen1.2.0program[36] was used for the propertiesof b- and c-hadrondecays. In total, 2.9×¹⁰⁷ hard-scattering events at √

s=⁵.02 TeV were simulated at the NLO precision, spanning a rangeof jet transversemomenta from 20to1300 GeV.Thesecond MCsample consistsofthesamesig- nal events as those used in the first sample but embedded into minimum-biasPb+^{Pb dataevents.This}minimum-biassamplewas combinedwiththesignalfromPowheg+^{Pythia8simulationatthe} digitisationstage,andthenreconstructedasacombinedevent.So- called “truth jets” are defined by applying the anti-kt algorithm withradiusparameter R=⁰.4 tostableparticlesintheMCevent generator’soutput,definedasthosewithaproperlifetimegreater than10 ps,butexcludingmuonsandneutrinos,whichdonotleave significantenergydepositsinthecalorimeter.

Thelevel ofoverall eventactivityorcentralityinPb+^{Pb colli-} sionsischaracterisedusingthesumofthetotaltransverseenergy inthe forwardcalorimeter,E_T^FCal,atthe electromagneticenergy scale.TheE^FCal_T distributionisdividedintopercentilesofthetotal inelastic cross-section forPb+^{Pbcollisions with0–10% centrality} interval classifyingthe mostcentral collisions.The minimum-bias trigger andeventselection areestimated tosample 84.5% of the total inelasticcross-section,withan uncertaintyof1%. AGlauber modelanalysisoftheE^FCal_T distributionisusedtoevaluate^TAA andthenumberofnucleonsparticipatinginthecollision,^Npart^, ineachcentralityinterval [20,37,38].Thecentralityintervalsused inthismeasurementareindicatedinTable1along withtheir re- spectivecalculationsof^Npart^andTAA^.

Jetsusedinthisanalysisarereconstructedeitherinminimum- bias events or in events selected by inclusive jet triggers in the regionofjet pT forwhichthetriggerefficienciesaregreater than 99%. Events are required to have a reconstructed vertex within 150 mm of the nominal interaction point along the beam axis.

Fig. 1.TheleftpanelshowstheJESasafunctionofp^truth_T andtherightpanelshowstheJERasafunctionofp^truth_T inMCsamples.Bothareforjetswith|^y|<2.8.The curvesintherightpanelshowfitstoEq. (1) forPb+^Pb,andPb+^{Pb ineightcentralityintervals(0–10%,10–20%,20–30%,30–40%,40–50%,50–60%,60–70%,and70–80%).}

Only events taken during stable beam conditions and satisfying detectoranddata-qualityrequirements,whichincludetheID and calorimeters beingin nominaloperation, are considered. The av- eragenumber ofPb+Pb inelastic interactions per bunch crossing was μ <1.4.InPb+Pb collisions, μwassmallerthan10⁻⁴.

4. Jetreconstructionandanalysisprocedure

The reconstruction of jets in Pb+^{Pb and Pb}+Pb collisions closelyfollows theprocedures described inRefs. [8,39] including theunderlyingevent(UE)subtractionprocedure.Abriefsummary is given here. Jets are reconstructed using the anti-kt algorithm, which is implemented in the FastJet software package [40].

The jets are formed by clustering η×φ=⁰.1×π/32 log- ical “towers” that are constructed using energy deposits in en- closedcalorimetercells.Abackgroundsubtractionprocedurebased on the UE average transverse energy density, ρ(η,φ), which is calorimeter-layer dependent, was applied. The φ dependence is duetoglobalazimuthal correlationsbetweentheproducedparti- cles(typicallyreferred toas“flow”).Thesecorrelationsarise from thehydrodynamicresponseofthemediumtothegeometryofthe initialcollision. Theflow contributiontothetransverse energyof towerscanbedescribedbythemagnitude(vn)andphase(n)of theFouriercomponentsoftheazimuthalangledistributionsas:

d²E_T dηdφ =^dET

dη

1+²

n

v_ncos(n(φ−n))

,

whereφistheazimuthalangleofthetowerandnindicatestheor- deroftheflowharmonic.Themodulationisdominatedbyv2 and v3 [41]. In thisanalysis, the second, third and fourthharmonics areusedtofurtherimprovetheUEestimation.Aniterativeproce- dureisusedtoremovetheeffectsofjetson ρ andthe vn values.

Inthe initialestimate of ρ andv_n,theseareestimatedfromthe transverseenergy ofcalorimetercells within |η|<3.2.The back- groundissubtracted from calorimeter-layer-dependenttransverse energieswithin towers associatedwiththejet toobtain thesub- tractedjet kinematics. Then ρ and vn valuesare recalculated by excluding towers within R=⁰.4 of seedjets. Seed jetsare de- finedascalorimeterjetswithsubtracted pT>25 GeV,whichare reconstructed with radius parameter R=⁰.2, and R=⁰.4 track jets with p_T>10 GeV, which are reconstructed from charged-

particletracksrecordedinthe ID.Thesenew ρ² andv_n valuesare then usedtoevaluate anewsubtractedenergyusingtheoriginal towers,andthenewjetkinematicvariablesarecalculated.Afinal correction depending on rapidity and pT isapplied toobtain the correct hadronic energy scale for the reconstructed jets. Jets are calibratedusinganMC-based procedurewhichisthesameasfor the “EM+^{JES” jetsused in the analysisof Pb}+Pb collisions [42].

This calibration isfollowed by a “cross-calibration”which relates the jet energy scale (JES) of Pb+^{Pb jets to the JES of Pb}+^Pb jets [43].

Theperformanceofthejetreconstructionwascharacterisedby evaluating theJESandjet energyresolution(JER), which arecor- respondinglythemeanandwidthofthejetresponse(p^rec_T /p^truth_T ) intheMCsimulation.Here p^rec_T andp^truth_T arethetransversemo- mentaofthereconstructedjetandtruthjet,respectively.Theper- formanceofthejetreconstructioninthesimulationissummarised inFig.1,wheretheleftandrightpanelsshowtheJESandJER,re- spectively.TheJESisshownasafunctionofp^truth_T intheleftpanel of Fig.1.It deviatesfromunityby lessthan1% in thekinematic region ofthemeasurement. Norapidity dependenceofthe JESis observed.AweakcentralitydependenceoftheJESiscorrectedby theunfoldingproceduredescribedlaterinthissection.Toexpress the differentcontributions, theJERisparameterised bya quadra- turesumofthreeterms,

σ p^rec_T

p^truth_T

= ^a p^truth_T

⊕ ^b

p^truth_T ⊕^c. (1)

Thefirstparameter(a)andthirdparameter(c)inEq. (1) aresen- sitivetothedetectorresponseandareexpectedtobeindependent of centrality,while thesecond parameter (b) iscentrality depen- dent and it is driven by UE fluctuations uncorrelated with the jet pT.TheJERfordifferentcentralityintervalsandforPb+^{Pb col-} lisions isshownintherightpanelofFig.1.FitsusingEq. (1) are indicatedwithdashedlines.TheJERislargestinthemorecentral collisions,asexpectedfromstrongerfluctuationsofthetransverse energyintheUE. TheJERisabout16%for p_T=^{100 GeV incen-} tral collisions and decreases with increasing pT to 5–6% forjets with pT greaterthan 500 GeV.Theparameters a andc in thefit arefoundtobeindependentofcentralitywhilethevaluesofb are consistentwiththeexpectedmagnitudeofUEfluctuations.Thefit

2 Theaverageρ is≈^{270 GeVand}≈^{10 GeVin0–10%and70–80%Pb}+^Pbcolli- sions,respectively.