Transverse momentum spectra and nuclear modification factors of charged particles in Xe–Xe collisions at √sNN=5.44 TeV

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Contents lists available atScienceDirect

Physics Letters B

www.elsevier.com/locate/physletb

Transverse momentum spectra and nuclear modiﬁcation factors of charged particles in Xe–Xe collisions at √

s _NN = ⁵ . 44 TeV

.ALICE Collaboration

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

Articlehistory:

Received24May2018

Receivedinrevisedform26September 2018

Accepted24October2018 Availableonline31October2018 Editor: M.Doser

Transverse momentum (pT) spectraof charged particlesatmid-pseudorapidity inXe–Xe collisions at

√_s

NN=⁵.44 TeV measured withthe ALICE apparatusatthe LargeHadronCollider are reported. The kinematic range0.15<pT<50GeV/c and |η_|<0.8 iscovered. Resultsare presented innineclasses of collision centralityin the 0–80%range. For comparison, app reference at the collisionenergy of

√_s=⁵.44 TeV isobtainedbyinterpolatingbetweenexistingppmeasurementsat√_s=⁵.02 and7 TeV.

ThenuclearmodiﬁcationfactorsincentralXe–Xecollisions andPb–Pbcollisionsatasimilarcenter-of- mass energyof√_s

NN=⁵.02 TeV,andinadditionat2.76 TeV,atanalogousrangesofchargedparticle multiplicitydensity^dNch/dη^showâ^remarkable^similarityât^pT>10 GeV/c.Thecentralitydependence of the ratio of the average transverse momentum ^p^T ⁱⁿ ^Xe–Xe ^collisions ôver ^Pb–Pb ^collision ât

√s=⁵.02 TeV iscomparedtohydrodynamicalmodelcalculations.

©²⁰¹⁸^TheÂuthor.^Published^byÊlsevier^B.V.^Thisîsânôpenâccessârticleûnder^the^CC^BY^license (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP³.

1. Introduction

Transverse momentum (pT) spectra ofcharged particles carry essential information aboutthe high-density deconﬁned state of strongly-interacting matter commonly denoted as quark–gluon plasma,thatisformedinhigh-energynucleus–nucleus(A–A)collisions [1].Relativistichydrodynamicsisabletomodeltheevolution ofthismedium [2,3].

At low to intermediate p_T, typically in the range of up to 10 GeV/c, chargedparticle productionis governedby the collec- tive expansion of the system, which is observed in the shapes of single-particle transverse-momentum spectra [4,5] and multi- particle correlations [2]. However, there is presently an intense debate as to whether the strikingly similar signatures observed insmallcollision systems (ppandp–A) are alsoofhydrodynam- ical origin [6–14]. A key ingredient of calculations in relativistic hydrodynamics is the initial energy density [2,15,16]. The number of produced particles and the volume of the medium are approximatelyproportional to thenumberof nucleons Npart that participateinthecollision [17–19].Thus, theparticledensityper unit volume is roughly independent of Npart. As a consequence, particlespectraat smalltransversemomentum shouldbe similar innucleus–nucleuscollisions, independentlyofthemassnumber, whencomparedatsimilarvaluesofNpart [20].

At high p_T,typically above 10 GeV/c,particles originate from parton fragmentationand are sensitive to the amount of energy

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

loss that thepartons sufferwhenpropagating inthe medium. In a simpliﬁed model, the energy loss depends on the number of scattering centers, which is roughly proportional to the energy density,andonthepathlength thatthepartonpropagatesinthe medium [21].Forelasticcollisions,thedependenceislinear,while formediuminducedgluonradiation,itisquadratic [22].Adescrip- tionofexperimentaldataliesinbetweenthosetwo [23].

For hard processes, the production yield N_AA in nucleus–

nucleus (A–A) collisions is expected to scale with the average nuclear overlapfunction^TAA^when^compared ^to^the^production crosssection

σ

ppinppcollisions.Intheabsenceofnucleareffects, thenuclearmodiﬁcationfactor

R_AA

(

p_T

) =

¹

^TAA

·

^dN^AA

(

p_T

)/

dp_T

d

σ

pp

(

p_T

)/

dp_T (1)

equals unity. The average nuclear overlap function is deﬁned as the average number of binary nucleon-nucleon collisions Ncoll per inelasticnucleon-nucleoncrosssectionandisestimatedviaa Glaubermodelcalculation [24].AttheLargeHadronCollider(LHC), particleproductionisobservedtobestronglysuppressedinPb–Pb collisions by a factorofup to 7–8around p_T = ^{6–7 GeV}/c with alineardecreaseofthesuppressionfactorathigher p_T butstill a substantialsuppressionevenabove100 GeV/c [5,25].

The LHC produced for the ﬁrst time collisions of xenon nu- cleiatacenter-of-massenergyof√

s_NN=⁵.44 TeV duringa pilot run with 6 hours of stable beams in October 2017. This allows for studying the dependence of particle production on the colli- sionsystemsizewherexenonneatlybridgesthegapbetweendata https://doi.org/10.1016/j.physletb.2018.10.052

0370-2693/©²⁰¹⁸^TheÂuthor.^Published^byÊlsevier^B.V.^Thisîsânôpenâccessârticleûnder^the^CC^BY^license(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP³.

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Table 1

Averagedvaluesof^dNch/dη,^N^part^,^Ncollând^TÂA^for^nine^centrality^classesôf^Xe–Xe^collisions^{[18,19] at}

√sNN=5.44 TeV,anddNch/dηforPb–Pbcollisionsat√

sNN=5.02 TeV [30].Thevaluesfor dNch/dηare measuredintherange|η|<0.5.

Centrality (%) ^dNch/dηXe–Xe ^Npart ^Ncoll ^TAA(mb⁻¹) ^dNch/dηPb–Pb

0–5 1167±²⁶ ²³⁶±² ⁹⁴⁹±⁵³ ^13.9±^0.8 ¹⁹⁴³±⁵⁴

5–10 939±²⁴ ²⁰⁷±² ⁷³⁷±⁴⁶ ^10.8±^0.7 ¹⁵⁸⁶±⁴⁶

10–20 706±¹⁷ ¹⁶⁵±² ⁵¹¹±²⁶ ^7.5±^0.5 ¹¹⁸⁰±³¹

20–30 478±¹¹ ¹¹⁸±³ ³⁰³±²⁸ ^4.4±^0.4 ⁷⁸⁶±²⁰

30–40 315±8 82±3 171±19 2.5±0.3 512±15

40–50 198±5 55±3 92±11 1.3±0.2 318±12

50–60 118±3 34±2 46±6 0.7±0.1 183±8

60–70 65±² ²⁰±² ²²±³ ^0.32±^0.04 ⁹⁶±⁶

70–80 32±¹ ¹¹±¹ ¹⁰±¹ ^0.14±^0.02 ⁴⁵±³

frompp, p–PbandPb–Pbcollisions. Here,theatomicmass num- bersare A=^{129 for}^xenon,^and^A=^{208 for}^lead^withhalf-density radii of the nuclear-charge distribution of r=(5.36±⁰.1) fm and(6.62±⁰.06) fm,respectively [24,26]. The parameters ofthe nuclear-chargedensitydistributionfor¹²⁹Xearenotyetmeasured butwereextrapolatedfromneighboringisotopesandarethusless preciselyknownthanfor²⁰⁸Pb.While²⁰⁸Pbisasphericalnucleus, 129Xehasadeformationparameterofβ2=(0.18±⁰.02).

This article reports transverse momentum spectra of charged particles at mid-pseudorapidity in Xe–Xe collisions at √

sNN = 5.44 TeV measured with the ALICE apparatus atthe LHC in the kinematic range 0.15< pT<50 GeV/c and |

η

|<0.8 for nine classesofcollisioncentrality,coveringthemostcentral80%ofthe hadronic cross section. It is organized as follows: Section 2 de- scribestheexperimentalsetupanddataanalysis.Systematicuncer- taintiesarediscussedinSect.3.Resultsandcomparisontomodel calculationsarepresentedinSect.4.AsummaryisgiveninSect.5.

2. Experimentanddataanalysis

Collisionsofxenonnucleiwere recordedatanaverageinstan- taneous luminosity of about 2·¹⁰⁻²⁵^cm⁻²^s⁻¹ ^and ^a ^hadronic interaction rateof80–150 s⁻¹. A detaileddescription ofthe AL- ICEexperimentalapparatuscanbefoundelsewhere [27].

2.1.Triggerandeventselection

Aminimum-biasinteractiontriggerwasoptimizedforhigheﬃ- ciencyonhadroniccollisions.Itrequiredsignalsfrombothforward scintillator arrays covering 2.8<

η

<5.1 (V0A) and −³.7<

η

<

−¹.7 (V0C). Additionally, coincidence withsignalsfromtwoneu- tron Zero-Degree Calorimeters (ZDC), ZNA and ZNC, at |

η

|>8.7 wasrequiredinorderto removecontamination fromelectromag- netic processes. Here A and C denote opposite sides of the ex- perimentalongthe beamline.The oﬄineeventselectionwas op- timized to reject beam-induced background. Background events were eﬃciently rejected by exploiting the timing signals in the two V0 detectors. Parasitic collisions are removed by using the correlationbetweenthesumandthedifferenceinarrivaltimesas measuredineachoftheneutronZDCs.Intotal,1.1·¹⁰⁶^minimum- biascollisionspasstheeventselectionandwerefurtheranalyzed.

This analysis is based on tracking information from the In- ner Tracking System (ITS) [28] and the Time Projection Cham- ber (TPC) [29] which are located in the central barrel of ALICE.

A solenoidalmagnetprovides momentumdispersioninthedirec- tiontransversetothebeamaxis.Thenominalﬁeldstrengthinthe ALICEcentral barrelis0.5 T.However, inordertoextendparticle trackingandidentiﬁcationtothelowestpossiblemomenta,itwas reducedto0.2 TinXe–Xecollisions.

TheITS is comprisedof sixcylindricallayers of silicon detec- torswithradiibetween3.9and43.0 cm.Thetwoinnermostlayers,

withaverageradiiof3.9cmand7.6 cm,areequippedwithSilicon Pixel Detectors (SPD); the two intermediate layers, with average radiiof15.0 cmand23.9 cm,areequippedwithSiliconDriftDe- tectors (SDD) and the two outermost layers, with average radii of 38.0cm and43.0 cm, are equippedwith double-sided Silicon Strip Detectors (SSD). The large cylindrical TPC hasan active ra- dial range fromabout85 to250 cm andan overall length along the beam directionof 500 cm. It covers the full azimuth in the pseudo-rapidityrange |

η

_|<0.9 andprovidestrackreconstruction with up to 159 points along the trajectory of a charged particle aswell asparticle identiﬁcationvia the measurement ofspeciﬁc energyloss dE/dx.

The collision vertex is determined using reconstructed parti- cletrajectories inthe TPCincluding hitsin theITS. All collisions with a reconstructed vertex position within ±¹⁰ ^{cm along} ^the beam directionfrom the nominalinteraction point are accepted.

Thecollisioncentralityisdefinedasthepercentileofthehadronic crosssectioncorrespondingtothemeasuredchargedparticlemul- tiplicity.The centralitydetermination isbasedon thesumof the amplitudesoftheV0AandV0Csignals[18,19].Averagedquantities characterizing a centrality class such as the number of partici- pants Npart,thenumberofbinarycollisions Ncoll,andthenuclear overlapfunction TAA arecalculated astheaverageoverall events inthisclassbyfittingtheexperimentaldistributionwithaGlauber MonteCarlomodelthatemploysnegativebinomialdistributionsto modelmultiplicityproduction [18,19] (seeTable1).Theanalysisis restrictedtothe0–80%centralityrangeinordertoensurethatef- fectsoftriggerinefficiency andcontaminationby electromagnetic processesarenegligible.

2.2. Trackselection

Primarychargedparticleswithinthekinematicrange|

η

|<0.8 and0.15<pT<50 GeV/c are measured.Here, primaries arede- ﬁnedasallchargedparticleswithaproperlifetime

τ

largerthan 1 cm/c that are either produced directly in the primary beam- beaminteraction,orfromdecaysofparticles with

τ

smallerthan 1 cm/c,excludingparticles produced ininteractions withthede- tectormaterial [31].Thetrackselectionisoptimizedforbesttrack qualityandminimumcontaminationfromsecondaryparticles.The selection criteria are identical to those of the previous analysis ofPb–Pb collisionsat√

s_NN=⁵.02 TeV [5] except forthefollow- ingchangesintheparameterizationonthetransversemomentum dependence.Thegeometrical tracklength intheTPCﬁducialvol- ume [29] isL/(1 cm) >130−(p_T/(1 GeV/c))⁻⁰^.⁷,andthedistance ofclosestapproachtotheprimaryvertexinthetransverseplaneis

|^DCAxy|/(1 cm) <0.0119+⁰.049(pT/(1 GeV/c))⁻¹.Thesechanges reﬂectdifferencesinparticletrackingduetothereducedmagnetic ﬁeld.Inordertorejectfaketracksthatcontaminatethespectrum, especially athigh pT,another selection isintroduced:the uncertainty in the reconstructed p_T asestimated from the covariance

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matrix ofthe trackﬁt must be lessthan ten timesthe standard deviation,whenaveragedoveralltracksatthatmomentum.

2.3. Corrections

Thedoubly-differentialtransversemomentumspectrainXe–Xe collisionsarenormalizedbythenumberofeventsNevineachcen- tralityclass,andaregivenby

1 Nev

d²N_ch d

η

dpT

≡

^N

rec

ch

( η ,

pT

)

Nev

· η

pT

· δ

p_T

(

pT

)

α (

pT

) · ε (

pT

) ,

(2) whereN^rec_ch istherawyieldofreconstructedprimarychargedpar- ticlesin each interval ofpseudo-rapidity andtransverse momentum (

η

,pT). The symbols

α

(pT) and

ε

(pT) are the cor- rectionfactorsfordetectoracceptanceandtrackingefficiency,respectively.The correctiondueto thefinitetransverse-momentum resolution in the reconstruction of primary charged particles is denotedby δpT(pT).Theefficienciesfortrigger,eventvertexre- construction andtrackingare estimatedusingMonteCarlosimu- lationswithHIJING[32] astheeventgeneratorandGEANT3 [33]

forparticle propagationand simulation ofthe detectorresponse.

The triggerandvertexselectionsare fullyeﬃcient forthewhole centralityrangeusedintheanalysis.

Contaminationfromsecondarychargedparticles,i.e.fromweak decaysandinteractionsinthedetectormaterial,issubtractedfrom the raw spectrum by employing a data driven method [5]. Re- constructed trajectoriesofprimary chargedparticles point tothe collisionvertex,whilechargedparticlesfromweakdecaysandpar- ticlesgeneratedinthedetectormaterialpreferentially pointaway from it. In order to distinguish between primary and secondary particles, the distance ofclosest approach to the collision vertex inradialdirection, DCAxy,is used.Amulti-templatefunctionthat consistsoftemplatesforprimaryparticles,secondaryparticlespro- ducedfromweakdecaysandsecondaryparticlesfrominteractions inthedetectormaterialisﬁttedtotheDCA_xydistributionsineach pTinterval.

The primary charged particle reconstruction eﬃciency is obtained from the Monte Carlo simulation. As discussed in detail in [5], this eﬃciencydepends on the relative abundances ofthe various primary particles species. These relative abundances are adjusted inthe simulation using a data-driven re-weighting pro- cedure. The particle composition in Xe–Xe collisions is not yet known.However,bulkparticleproductionscaleswiththeaverage chargedparticlemultiplicity density,^dNch/d

η

,independently of the collision system [34]. In Xe–Xe collisions, the weights from existing analyses [35–37,4,5] with Pb–Pb collisions at √

s_NN = 2.76 TeV atequivalentvaluesin^dNch/d

η

areapplied.

The acceptance times tracking eﬃciency for charged pions, chargedkaons and (anti-)protons for5% most central Xe–Xecol- lisions isshown inFig. 1asa function ofthe particletransverse momentumandcomparedto 10–20%Pb–Pbcollisions at√

sNN= 5.02 TeV.Thetwocentralityclasseshavesimilarmultiplicityden- sities. The particular shape witha dip at p_T∼⁰.4 GeV/c arises fromthe geometricallengthselectionthat isespeciallyvisiblefor pions.This dipcorresponds toparticles that crossthe TPCsector boundariesundersmall angles.The decreaseat low valuesof pT is dueto curling trajectoriesin the magnetic field which do not reach therequired minimumtracklength in theTPC anddueto energylossandabsorptioninthedetectormaterial.InPb–Pbcolli- sions,themagneticfieldwassetto B=⁰.5 T,whichresultsinthe dipbeingpositionedaround1 GeV/c.Atlarge pT,above7 GeV/c, thetrackingefficiencyisreducedby anincreasedlocaltrackden- sity, i.e.high pT particlesare preferentially produced within jets, leadingtoaslightdecreaseinthetrackfindingperformance.

Fig. 1. Transversemomentum dependenceofthe acceptancetimestrackingeﬃ- ciency for the5% mostcentral Xe–Xecollisions and comparisontothe 10–20%

centralityclassforPb–Pbcollisions.Thetwocentralityclasseshavesimilarmul- tiplicitydensities.

The transverse momentum ofprimary chargedparticles is reconstructed fromthe trackcurvatureasmeasured bytheITSand theTPC [38].Thefinitemomentumresolutionmodifiestherecon- structed charged-particlespectrumandisestimatedbythecorre- spondingcovariancematrixelementoftheKalmanfit.Therelative p_T resolution,

σ

(p_T)/p_T,dependsonthemomentumandamounts toapproximately4.5%atpT=⁰.15 GeV/c,itshowsaminimumof 1.5% around pT=¹.0 GeV/c,and increaseslinearlyforlarger pT, approaching 9.3% at50 GeV/c. The centrality dependenceof the relative p_Tresolutionisnegligible.Toaccountfortheﬁnitep_T resolution, correction factors to thespectra are determinedfrom an unfolding procedureasdescribed in[5],usingBayesian unfolding atlow pT andabin-by-bin correctionatlarge pT.The pT dependent correction factorsare applied to themeasured p_T spectrum anddepend slightlyon collisioncentrality becauseofthe change in the slope ofthe spectrum athigh pT.At transverse momenta below10 GeV/c, δpT deviatessigniﬁcantly fromunity onlyatthe lowest momentum interval of 0.15≤^pT<0.2 GeV/c where it amounts to 0.5% for all centrality classes, and by up to 3% (4%) in0–5%(70–80%)centralcollisionsabove10 GeV/c.

The statistical uncertainty ofthe spectra is dominated by the statisticaluncertainty inthe rawdata.It islargestat thehighest momentumintervalof40–50 GeV/candamountsto28%(38%)for the0–5%(30–40%)centralityclasswhilethecontributionfromthe MonteCarloeﬃciencyis2%(4%)orless.

2.4. ppreferenceat√

s=⁵.44TeV

The p_T-differential inelastic cross section in pp collisions at

√s=⁵.44 TeV is needed to measure the corresponding nuclear modiﬁcationfactor.Astherearenomeasurementsofppcollisions at this energy, a referenceis obtained by interpolating pp references as measured at √

s=⁵.02 TeV and √

s=7 TeV assuming a power-lawdependence ineach pT interval,d

σ

/dpT(√

s)∝√ sⁿ. The value of thefree parameter n variesbetween0.35 and 1.75, depending on pT.Thisapproach isa combinationofthe interpo- lationmethodthatwas usedoverthefull p_T rangein [6] andfor

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Fig. 2. Ratioof pT-differential inelasticcross sections inpp collisions at √ s= 5.44 TeV over5.02 TeV usingapowerlawinterpolationandtheeventgenerator PYTHIA 8.

p_T<5 GeV/c asusedin [39].Thestatisticaluncertaintyofthepp referenceisinterpolatedbetweenthereferencesat√

s=⁵.02 TeV and7 TeV assumingalsoapower-lawdependenceandisassigned totheinterpolatedreference.Itamountsto7.8%atthemomentum intervalof30–50 GeV/c.

Asan alternative approach,the scaling ofthe measured cross section at√

s=⁵.02 TeV to √

s=⁵.44 TeV byusing theratioof spectraatthosetwoenergiesobtainedwiththePYTHIA 8(Monash tune)event generator [40] is studied. The ratio of the pp refer- encesat√

s=⁵.44 TeV from thepower-law interpolationandat

√s=⁵.02 TeV is shownin Fig.2 together with resultsobtained withthealternativemethod.Thespectrumisharderathighercol- lisionenergy,withasmallchangeinthetotalcrosssectionof4%

below1 GeV/c andan increase of about 10% at transverse mo- mentaabove10 GeV/c.

3. Systematicuncertainties

Forthetotalsystematicuncertainty,allcontributionsareadded inquadratureandaresummarizedinTable2.

The effectof the selection of events based on the vertexpo- sitionis studiedby comparing thefully corrected p_T spectra ob- tainedwithalternativevertexselectionscorresponding to±^{5 cm,}

and ±^{20 cm.} ^The ^difference ⁱⁿ ^the ^fully ^corrected ^pT spectra is lessthan0.3%forcentralcollisions andlessthan0.5%forperiph- eralcollisions.

In order to test the description of the detector response and thetrackreconstructioninthesimulation,allcriteriafortrackse- lectionare varied within theranges asdescribed in theprevious publication [5].Afull analysisisperformedby varyingone selec- tioncriterionatatime.Themaximumchangeinthecorrected p_T spectrum isthen considered assystematic uncertainty.The overall systematic uncertainty related to track selection is obtained fromsummingupallindividual contributionsquadraticallyandit amountsto0.6–3.0%,dependingon pTandcentrality.

Thesystematicuncertaintyonthe secondary-particlecontami- nation isestimatedbyvarying thefitmodelusingtwotemplates, i.e. forprimariesandsecondaries,orthreetemplates,i.e. primaries, secondaries from interactions in the detector material and secondaries from weak decays of K⁰_s and, aswell asvarying the fit ranges. The maximum difference between data and the two- component-template fit is summed in quadrature together with thedifferencebetweenresultsobtainedfromthetwo- andthree- component-template fits. The systematic uncertainty due to the contamination fromsecondariesisdecreasing withincreasing pT. Itdominatesatlowp_Twithvaluesupto4%andisnegligibleabove 2 GeV/c.

Thesystematicuncertaintyontheprimaryparticlecomposition istakenfrom [5].Anadditionaluncertaintyisestimatedbyassum- ing theparticlecomposition fromaneighboringdNch/d

η

range tothe matchedone inthePb–Pbanalysisandisaddedquadrati- cally.Thesumpeaksaround3 GeV/c withamaximumof5%(less than2%)forthe0–5%(70–80%)centralityclass.

In order to estimate the systematic uncertainty due to the tracking efficiency, the track matching betweenthe TPC andthe ITS informationin data andMonte Carlois compared after scaling the fraction of secondary particles obtained from the fits to the DCA_xy distributions [5]. The difference in the TPC-ITS track- matchingefficiencybetweendataandsimulationisassignedtothe corresponding systematicuncertainty(seeTable2). Itamounts to 2%incentralcollisions,andupto3.5%inperipheralcollisions.

ThematerialbudgetinALICEat

η

≈^{0 amounts}^to(11.4±⁰.5)% inradiationlengthsforprimary chargedparticlesthat havesuffi- cient tracklength inthe TPC [38].A difference intheamount of detectormaterialleadstodifferentamountsofsecondaryparticles thatareproduced.Afterthesubtractionofthecontributiondueto secondariesusingthethree-componentDCAxyfits,thedifferences onthesecondarycorrectionfactorisnegligible.Avariationofthe materialbudget withinabovelimitsleads toa pT dependent sys- tematicuncertaintyonthetrackingefficiencyof0.1–0.3%.

Theuncertaintyduetotheﬁnitep_T resolutionathighp_Tises- timated using the azimuthal dependence of the 1/pT spectra for

Table 2

Contributionstothesystematicuncertaintyinunitsofpercentforthe0–5%,30–40%,and70–80%centralityclassesinXe–

Xecollisions.ThenumbersareaveragedinthepTintervalsfrom0.2–0.5 GeV/c(left),1–2 GeV/c(middle)and40–50 GeV/c (right).ForthepT-dependentsum,contributionsareaddedinquadrature.

Centrality (%) 0–5 (%) 30–40 (%) 70–80 (%)

pTrange (GeV/c) 0.2–0.5/1–2/40–50 0.2–0.5/1–2/40–50 0.2–0.5/1–2/40–50

Source

Vertex selection 0.2/0.2/0.2 0.8/0.8/0.8 0.8/0.8/0.8

Track selection 1.6/0.9/1.2 0.9/0.6/0.8 0.9/0.5/1.0

Secondary particles 1.4/0.2/negl. 0.8/0.2/negl. 0.6/0.2/negl.

Particle composition 0.3/1.7/0.7 0.4/1.9/1.0 0.7/0.6/0.6

Tracking eﬃciency 1.9/1.2/0.4 2.2/1.2/0.4 2.2/1.4/0.6

Material budget 0.3/0.3/0.1 0.3/0.3/0.1 0.3/0.3/0.1

pTresolution negl./negl./0.5 negl./negl./0.7 negl./negl./0.9

Sum,pTdependent: 3.1/2.4/1.5 2.8/2.5/1.8 2.8/1.9/2.1

Centrality selection 0.1 0.8 3.2

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positivelyandnegativelychargedparticles.Therelativeshiftofthe spectraforoppositelychargedparticlesalong1/pT determinesthe size of uncertainty fora givenangle. The RMS of the 1/pT shift asdistributed over the full azimuthis used as an additional in- creaseofthe pT resolution.Forthelowest pT bintheuncertainty isestimatedfromtheunfoldingprocedureappliedtoMonteCarlo simulations.Theuncertaintyduetotheﬁnite p_T resolutionissig- niﬁcantonlyatthelowestandhighestmomentabinsandamounts to0.5%atthelowst pT binforallcentralities and0.5% (0.9%)for the0–5%(70–80%)centralityclass.

The uncertainty due to the centrality determination is esti- matedbychangingthefractionofthevisiblecrosssection(90.0± 0.5)%. The uncertainty is estimatedfromthe variation of there- sulting pT spectra and amounts to ∼^0.1% ^and∼^3.2% ^for ^central (0–5%)andperipheral(70–80%)collisions,respectively.

Thesystematicuncertaintyoftheppreferenceat√

s=5.44 TeV has two contributions, which are added quadratically. For each pT interval, the systematic uncertainty of the pp references at

√s=⁵.02 TeV and√

s=^{7 TeV are}interpolatedto√

s=⁵.44 TeV by usinga power-law. Thiscorresponds tointerpolating between theupperandlowerboundariesoftheexperimentaldatapointsas givenby theirsystematicuncertainties.Itassumesfullcorrelation ofsystematicuncertaintiesatbothenergies.

Thedifferencebetweentheinterpolatedreferenceandtheone usingthePYTHIA 8eventgenerator isassignedastheother con- tributiontothesystematicuncertaintyintheppreference,ineach pT interval. The systematicuncertaintyin thepp referencehasa minimum of 2.2% around 1 GeV/c and reaches its maximum of 7.7%atthehighestmomentumbin.

4. Results

Thetransverse momentumspectra ofchargedparticlesin Xe–

Xe collisions are shownin the top panel of Fig. 3 for nine centrality classes together with the interpolated pp reference spec- trumat √

s=⁵.44 TeV. The latteris obtained fromthe interpolated pT-differential cross section by dividing it by the interpolated inelastic nucleon-nucleon cross section of (68.4±⁰.5) mb at √

s=⁵.44 TeV [24]. In the most-peripheralcollisions, the p_T spectrum issimilar to that ofpp collisions andexhibitsa power law behavior that is characteristic of hard-parton scattering and vacuumfragmentation. Withincreasingcollisioncentrality,the p_T differentialcrosssectionisprogressivelydepletedabove5 GeV/c.

Systematic uncertainties are shown in the bottom panel. At momenta between0.4 and10 GeV/c,the systematic uncertainty is dominated by the contribution from tracking and amounts to about2–3%.ItisalmostindependentofpTabove10 GeV/cwitha valueof1.4%(2.1%)forthe0–5%(70–80%)centralityclass.

Inordertodeterminethenuclear modificationfactor R_AA,the interpolated pT-differential pp cross section is scaled by the av- eragenuclear overlap function ^TAA^.^The ^resulting ^nuclear ^modification factor as a function of transverse momentum is shown inFig.4 forninecentrality classesandcomparedto resultsfrom Pb–Pb collisions [5]. The overall normalization uncertainties for R_AA are indicated by vertical bars around unity. The uncertain- tiesoftheppreferenceandthecentralitydeterminationareadded in quadrature. The latter is larger for Xe–Xe collisions than for Pb–Pbbecauseofthelesspreciselyknownnuclear-charge-density distribution ofthe deformed ¹²⁹Xe andthe resulting larger relative uncertainty in^TAA^[18,19]. ^The ^nuclear modification factor exhibitsa strong centrality dependencewith a minimumaround p_T=^{6–7 GeV}/c andan almost linearrise above.In particular, in the 5% mostcentral Xe–Xe collisions, atthe minimum,the yield is suppressed by a factor of about 6 with respect to the scaled pp reference. The nuclear modification factor reaches a value of

Fig. 3.TransversemomentumspectraofchargedparticlesinXe–Xecollisionsat

√sNN=5.44 TeV inninecentralityclassestogetherwiththeinterpolatedpprefer- encespectrumat√

s=⁵.44 TeV (toppanel)andsystematicuncertainties(bottom panel).

0.6 at the highest measured transverse-momentum interval of 30–50 GeV/c.Forcomparison,thenuclearmodiﬁcationfactorRAA inPb–Pbcollisionsat√

sNN=⁵.02 TeV isshowninFig.4asopen circles forthe same centralityclassesas Xe–Xe.In both collision systems,asimilarcharacteristicp_TdependenceofR_AAisobserved.

InPb–Pbcollisions,thesuppressionofhigh-momentumparticlesis apparentlystrongerforthesamecentralityclassbutstillinagree- mentwithXe–Xecollisionswithinuncertainties.

Nuclear modiﬁcationfactors from Xe–Xe andPb–Pb collisions andtheirratiosatsimilarrangesof^dNch/d

η

areshowninFig.5.

In5% mostcentralXe–Xecollisions,the nuclearmodiﬁcationfac- tor is remarkably well matched by 10–20% central Pb–Pb collisions over the entire pT range. ln the 30–40% Xe–Xe (40–50%

Pb–Pb) centrality class, again agreement is found within uncertainties.Theseﬁndingsofmatchingnuclearmodiﬁcationfactorsat similarrangesof^dNch/d

η

^areⁱⁿ^agreement^with^results^from^the studyoffractionalmomentumlossofhigh-pTpartonsatRHICand LHCenergies [41].

A comparison of the nuclear modiﬁcation factors as a function of ^dNch/d

η

ⁱⁿ ^Xe–Xe ^and ^Pb–Pb ^collisions ^for ^three ^dif- ferent regions of pT (low, medium, andhigh)is shownin Fig.6.

A remarkable similarity in RAA is observed between Xe–Xe collision at √

s_NN=⁵.44 TeV and Pb–Pb collisions at √

s_NN=⁵.02 and 2.76 TeV when compared at identical ranges in ^dNch/d

η

^, for ^dNch/d

η

>400. This holds both at low momentum where thehydrodynamicalexpansionofthemediumdominatesthespec- trumandathighmomentum,wherepartonenergylossinsidethe

(6)

Fig. 4.NuclearmodiﬁcationfactorinXe–Xeat√

sNN=5.44 TeV (ﬁlledcircles)andPb–Pbcollisions [5] at√

sNN=5.02 TeV (opencircles)forninecentralityclasses.The verticallines(brackets)representthestatistical(systematic)uncertainties.Theoverallnormalizationuncertaintyisshownasaﬁlledboxaroundunity.

Fig. 5.ComparisonofnuclearmodiﬁcationfactorsinXe–Xecollisions(ﬁlledcircles) andPb–Pbcollisions(opencircles) forsimilarrangesin^dNch/dη^for^the ^0–5%

(left)and30–40%(right)Xe–Xecentralityclasses.Theverticallines(brackets)representthestatistical(systematic)uncertainties.

medium drives the spectral shape. At dNch/d

η

<400, the values of R_AA still agree within rather large uncertainties although

nodeﬁnitiveconclusioncanbedrawnbecause,inparticular,event selectionandgeometrybiasescouldaffectthespectruminperiph- eralA–Acollisions [42].

In a simpliﬁed radiative energy loss scenario when assum- ingidenticalthermalizationtimes [43,44],theaverageenergyloss E ^is proportional to the density of scattering centers in the medium,whichinturnisproportionaltotheenergydensity

ε

,and to thesquare ofthe pathlength L oftheparton inthemedium, ^E∝

ε

·^L² ^[22].^The ^energy^density^can ^be^estimated^from^the average charged-particle multiplicity density [45] per transverse area,

ε

_∝dNch/d

η

/AT. In central collisions, the initial transverse area AT is related to the radius r of the colliding nuclei, A_T=

π

·^r² ^[22]. ^Therefore,^the ^comparison^of ^the^measured ^RAA valuesinthetwocollidingsystemscouldenableatestofthepath lengthdependenceofmedium-inducedpartonenergyloss [46].

Tofurtheraddressbulkproduction,theaveragetransversemo- mentum^pTⁱⁿ^the^range^from^{0–10 GeV}/c isderived.Thespec- tra are extrapolated downto p_T=^{0 by} ^fittingâ ^Hagedorn ^function [47] in the range 0.15 GeV/c<pT<1 GeV/c. The relative fractionoftheextrapolatedparticleyieldamountsto8%(11%)for the0–5%(70–80%)centralityclass.Statisticaluncertaintiesin^pT are negligible. Systematic uncertainties are estimated by varying each source of systematic uncertainty in the spectra at a time, by varying thefitrange to0.15GeV/c<pT<0.5 GeV/c,andby changingtheinterpolationrangeto0–0.2 GeV/c.Allcontributions arethenaddedquadratically.Therelativesystematicuncertaintyis 1.8%(1.3%)forthe0–5%(70–80%)centralityclass.

The average transverse momentum is presented in the top panelofFig.7forXe–Xecollisionsat√

s=⁵.44 TeV (squares)and Pb–Pb collisions at √

s=⁵.02 TeV (diamonds)for nine centrality classes.An increase of^pT^with ^centrality^is^visibleⁱⁿ ^both^col-

(7)

Fig. 6.ComparisonofthenuclearmodificationfactorinXe–Xeand Pb–Pbcolli- sionsintegratedoveridenticalregionsinpTasafunctionof^dNch/dη^.^The^vertical bracketsindicatethequadraticsumofthetotalsystematicuncertaintyinthemea- surementandtheoverallnormalizationuncertaintyinTAA.Thehorizontalbars reflecttheRMSofthedistributionineachbin.Thedashedlinesshowresultsfrom power-lawfitstothedataandaredrawntoguidetheeye.

lisionsystemsandisattributedtotheincreasingtransverseradial flow. Thebottompanel ofFig.7 showsthe ratiosof^pTⁱⁿ^both collisionsystems.The ratioisflat withinuncertainties butallows forrelativevariations ofuptotwopercent.Comparisontoresults fromhydrodynamical calculations [43] are shown by the hashed areasforpions,kaonsandprotons.Whilethecalculationsarenot abletopredict absoluteparticlespectra,predictions aremadefor the relative difference in ^pT ^between^both ^collision ^systems ⁱⁿ ordertostudythesystemsizedependence.Thepredictedtrendof alarger^pTⁱⁿ^5%^most^central^Xe–Xe^collisionândcontinuously lower values towards the 40–50% centrality class are consistent withthedata.

5. Summary

Transverse momentum spectra and nuclear modiﬁcation factors of chargedparticles in Xe–Xe collisions at √

s_NN=⁵.44 TeV in the kinematic range 0.15<pT<50 GeV/c and |

η

|<0.8 are reportedfornine centralityclasses,inthe0–80%range.Appref- erence at√

s=⁵.44 TeV is obtained by the interpolation ofthe existingspectraat√

s=⁵.02 and7 TeV.Whencomparingnuclear modiﬁcation factors at similar ranges of averaged charged particle multiplicity densities, a remarkable similarity between central Xe–Xe collisions andPb–Pb collisions at a similar center-of- massenergyof√

s_NN=⁵.02 TeV and at2.76 TeV isobserved for

Fig. 7.AveragetransversemomentuminthepT-range0–10 GeV/cforXe–Xecolli- sionsat√

s=5.44 TeV (squares)andPb–Pbcollisionsat√

s=5.02 TeV (diamonds) forninecentralityclasses(top)andtheirratios(bottom).Theverticalbracketsindi- catesystematicuncertainties.Thehashedareasshowresultsfromhydrodynamical calculationsbyGiacaloneetal. [43].

^dNch/d

η

>400.Thecentralitydependenceoftheratiooftheav- erage transversemomentum ^pT ⁱⁿ^Xe–Xe ^collisions ôver ^Pb–Pb collisions isflatwithin uncertaintiesbutallows forrelative varia- tionsofuptotwopercent.Predictionsfromhydrodynamicalcalcu- lationsthattakeintoaccountthesignificantlydifferentgeometries ofbothcollisionsystemsareconsistentwiththedata.

Acknowledgements

The ALICE collaboration would like to thank G. Giacalone, J.

Noronha-Hostler, M. Luzum, and J.-Y. Ollitrault for providing the resultsoftheirhydrodynamicalcalculationspriortopublication.

The ALICECollaboration would like to thank all its engineers andtechniciansfortheirinvaluablecontributionstotheconstruc- tionoftheexperimentandtheCERNacceleratorteamsfortheout- standingperformanceoftheLHCcomplex.TheALICECollaboration gratefully acknowledges the resources and support provided by all Grid centresandthe WorldwideLHC ComputingGrid (WLCG) collaboration. The ALICE Collaboration acknowledges the follow- ingfundingagenciesfortheirsupportinbuildingandrunningthe ALICEdetector:A.I.AlikhanyanNationalScienceLaboratory(Yere- vanPhysics Institute)Foundation (ANSL),State CommitteeofSci- enceandWorld FederationofScientists(WFS), Armenia;Austrian AcademyofSciencesandNationalstiftungfürForschung,Technolo- gie und Entwicklung, Austria; Ministry of Communications and High Technologies, National Nuclear Research Center, Azerbaijan;

Conselho Nacional de Desenvolvimento Cientíﬁco e Tecnológico (CNPq), Universidade Federal do Rio Grande do Sul (UFRGS), Fi- nanciadoradeEstudoseProjetos(Finep)andFundaçãodeAmparo à Pesquisa do Estado de São Paulo (FAPESP), Brazil; Ministry of Science & Technology of China (MSTC), National Natural Science Foundation of China (NSFC) and Ministry of Education of China (MOEC), China;Ministry of Science and Education, Croatia; Min- istryofEducation,Youth andSportsofthe CzechRepublic, Czech Republic; The Danish Council for Independent Research | Natu- ral Sciences, the Carlsberg Foundation and Danish National Re- search Foundation (DNRF), Denmark; Helsinki Institute ofPhysics