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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
r
a
c
t
Article history: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 innineclassesof 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–Pbcollisionsatasimilar center-of-mass energyof√sNN=5.02 TeV,andinadditionat2.76 TeV,atanalogousrangesofchargedparticle
multiplicitydensitydNch/d
η
showaremarkablesimilarityatpT>10 GeV/c.Thecentralitydependenceof the ratio of the average transverse momentum pT in Xe–Xe collisions over Pb–Pb collision at √
s=5.02 TeV iscomparedtohydrodynamicalmodelcalculations.
©2018TheAuthor.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (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) colli-sions [1].Relativistichydrodynamicsisabletomodeltheevolution ofthismedium [2,3].
At low to intermediate pT, 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 alsoof hydrodynam-ical origin [6–14]. A key ingredient of calculations in relativistic hydrodynamics is the initial energy density [2,15,16]. The num-ber of produced particles and the volume of the medium are approximatelyproportional to thenumberof nucleons Npart thatparticipateinthecollision [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 pT,typically above 10 GeV
/
c,particles originate fromparton fragmentationand are sensitive to the amount of energy
E-mail address: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].A descrip-tionofexperimentaldataliesinbetweenthosetwo [23].
For hard processes, the production yield NAA in nucleus–
nucleus (A–A) collisions is expected to scale with the average nuclear overlapfunction
TAAwhencompared totheproduction crosssectionσ
ppinppcollisions.Intheabsenceofnucleareffects,thenuclearmodificationfactor
RAA
(
pT)
=
1 TAA·
dNAA(
pT)/
dpT dσ
pp(
pT)/
dpT (1)equals unity. The average nuclear overlap function is defined as the average number of binary nucleon-nucleon collisions
Ncollper inelasticnucleon-nucleoncrosssectionandisestimatedviaa Glaubermodelcalculation [24].AttheLargeHadronCollider(LHC), particleproductionisobservedtobestronglysuppressedinPb–Pb collisions by a factorofup to 7–8around pT
=
6–7 GeV/
c withalineardecreaseofthesuppressionfactorathigher pT butstill a
substantialsuppressionevenabove100 GeV
/
c [5,25].The LHC produced for the first time collisions of xenon nu-cleiatacenter-of-massenergyof
√
sNN=
5.
44 TeV duringa pilotrun 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/©2018TheAuthor.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP3.
Table 1
AveragedvaluesofdNch/dη,Npart,NcollandTAAforninecentralityclassesofXe–Xecollisions[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 forxenon,andA=
208 forleadwithhalf-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-chargedensitydistributionfor129Xearenotyetmeasuredbutwereextrapolatedfromneighboringisotopesandarethusless preciselyknownthanfor208Pb.While208Pbisasphericalnucleus,
129Xehasadeformationparameterof
β
2
= (
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 nineclassesofcollisioncentrality,coveringthemostcentral80%ofthe hadronic cross section. It is organized as follows: Section 2 de-scribestheexperimentalsetupanddataanalysis.Systematic uncer-taintiesarediscussedinSect.3.Resultsandcomparisontomodel calculationsarepresentedinSect.4.AsummaryisgiveninSect.5.
2. Experimentanddataanalysis
Collisionsofxenonnucleiwere recordedatanaverage instan-taneous luminosity of about 2
·
10−25cm−2s−1 and a hadronic interaction rateof80–150 s−1. A detaileddescription oftheAL-ICEexperimentalapparatuscanbefoundelsewhere [27].
2.1.Triggerandeventselection
Aminimum-biasinteractiontriggerwasoptimizedforhigh effi-ciencyonhadroniccollisions.Itrequiredsignalsfrombothforward scintillator arrays covering 2
.
8<
η
<
5.
1 (V0A) and−
3.
7<
η
<
−
1.
7 (V0C). Additionally, coincidence withsignalsfromtwo neu-tron Zero-Degree Calorimeters (ZDC), ZNA and ZNC, at|
η
|
>
8.
7 wasrequiredinorderto removecontamination from electromag-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 momentumdispersioninthe direc-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,areequippedwithSiliconDrift De-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 crosssectioncorrespondingtothemeasuredchargedparticle mul-tiplicity.The centralitydetermination isbasedon thesumof the amplitudesoftheV0AandV0Csignals[18,19].Averagedquantities characterizing a centrality class such as the number of partici-pants Npart,thenumberofbinarycollisions Ncoll,andthenuclearoverlapfunction TAA arecalculated astheaverageoverall events
inthisclassbyfittingtheexperimentaldistributionwithaGlauber MonteCarlomodelthatemploysnegativebinomialdistributionsto modelmultiplicityproduction [18,19] (seeTable1).Theanalysisis restrictedtothe0–80%centralityrangeinordertoensurethat ef-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 withτ
smallerthan 1 cm/c, excludingparticles produced ininteractions withthe de-tectormaterial [31].Thetrackselectionisoptimizedforbesttrack qualityandminimumcontaminationfromsecondaryparticles.The selection criteria are identical to those of the previous analysis ofPb–Pb collisionsat√
sNN=
5.
02 TeV [5] except forthefollow-ingchangesintheparameterizationonthetransversemomentum dependence.Thegeometrical tracklength intheTPCfiducial vol-ume [29] isL
/(
1 cm) >
130− (
pT/(
1 GeV/
c))
−0.7,andthedistanceofclosestapproachtotheprimaryvertexinthetransverseplaneis
|
DCAxy|/(1 cm) <
0.
0119+
0.
049(
pT/(
1 GeV/
c))
−1.Thesechangesreflectdifferencesinparticletrackingduetothereducedmagnetic field.Inordertorejectfaketracksthatcontaminatethespectrum, especially athigh pT,another selection isintroduced:the
matrix ofthe trackfit must be lessthan ten timesthe standard deviation,whenaveragedoveralltracksatthatmomentum.
2.3. Corrections
Thedoubly-differentialtransversemomentumspectrainXe–Xe collisionsarenormalizedbythenumberofeventsNevineach
cen-tralityclass,andaregivenby
1 Nev d2Nch d
η
dpT≡
Nrecch(
η
,
pT
)
Nev·
η
pT
·
δ
pT(
pT)
α
(
pT)
·
ε
(
pT)
,
(2)whereNrecch istherawyieldofreconstructedprimarycharged par-ticlesin each interval ofpseudo-rapidity andtransverse momen-tum
(
η
,
pT
)
. The symbolsα
(
pT)
andε
(
pT)
are thecor-rectionfactorsfordetectoracceptanceandtrackingefficiency, re-spectively.The correctiondueto thefinitetransverse-momentum resolution in the reconstruction of primary charged particles is denotedby
δ
pT(
pT)
.Theefficienciesfortrigger,eventvertex re-construction andtrackingare estimatedusingMonteCarlo simu-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,whilechargedparticlesfromweakdecaysand par-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,secondaryparticles pro-ducedfromweakdecaysandsecondaryparticlesfrominteractions inthedetectormaterialisfittedtotheDCAxydistributionsineach pTinterval.
The primary charged particle reconstruction efficiency is ob-tained 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/
dη,independently ofthe collision system [34]. In Xe–Xe collisions, the weights from existing analyses [35–37,4,5] with Pb–Pb collisions at
√
sNN=
2
.
76 TeV atequivalentvaluesindNch/
dηareapplied.The acceptance times tracking efficiency for charged pions, chargedkaons and (anti-)protons for5% most central Xe–Xe col-lisions isshown inFig. 1asa function ofthe particletransverse momentumandcomparedto 10–20%Pb–Pbcollisions at
√
sNN=
5
.
02 TeV.Thetwocentralityclasseshavesimilarmultiplicity den-sities. The particular shape witha dip at pT∼
0.
4 GeV/
c arisesfromthe 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–Pb colli-sions,themagneticfieldwassetto B
=
0.
5 T,whichresultsinthe dipbeingpositionedaround1 GeV/
c.Atlarge pT,above7 GeV/
c,thetrackingefficiencyisreducedby anincreasedlocaltrack den-sity, i.e.high pT particlesare preferentially produced within jets,
leadingtoaslightdecreaseinthetrackfindingperformance.
Fig. 1. Transversemomentum dependenceofthe acceptancetimestracking effi-ciency for the5% mostcentral Xe–Xecollisions and comparisontothe 10–20% centralityclassforPb–Pbcollisions.Thetwocentralityclasseshavesimilar mul-tiplicitydensities.
The transverse momentum ofprimary chargedparticles is re-constructed fromthe trackcurvatureasmeasured bytheITSand theTPC [38].Thefinitemomentumresolutionmodifiesthe recon-structed charged-particlespectrumandisestimatedbythe corre-spondingcovariancematrixelementoftheKalmanfit.Therelative
pT resolution,
σ
(
pT)/
pT,dependsonthemomentumandamountstoapproximately4.5%atpT
=
0.
15 GeV/
c,itshowsaminimumof1.5% around pT
=
1.
0 GeV/
c,and increaseslinearlyforlarger pT,approaching 9.3% at50 GeV
/
c. The centrality dependenceof the relative pTresolutionisnegligible.ToaccountforthefinitepTres-olution, correction factors to thespectra are determinedfrom an unfolding procedureasdescribed in[5],usingBayesian unfolding atlow pT andabin-by-bin correctionatlarge pT.The pT
depen-dent correction factorsare applied to themeasured pT spectrum
anddepend slightlyon collisioncentrality becauseofthe change in the slope ofthe spectrum athigh pT.At transverse momenta
below10 GeV
/
c,δ
pT deviatessignificantly fromunity onlyatthe lowest momentum interval of 0.
15≤
pT<
0.
2 GeV/
c where itamounts 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
/
c andamountsto28%(38%)for the0–5%(30–40%)centralityclasswhilethecontributionfromthe MonteCarloefficiencyis2%(4%)orless.2.4. ppreferenceat
√
s=
5.
44 TeVThe pT-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 refer-ences as measured at√
s=
5.
02 TeV and√
s=
7 TeV assuming a power-lawdependence ineach pT interval,dσ
/
dpT(
√
s)
∝
√
sn.The value of thefree parameter n varies between0
.
35 and 1.75, depending on pT.Thisapproach isa combinationoftheFig. 2. Ratioof pT-differential inelasticcross sections inpp collisions at √s=
5.44 TeV over5.02 TeV usingapowerlawinterpolationandtheeventgenerator PYTHIA 8.
pT
<
5 GeV/
c asusedin [39].Thestatisticaluncertaintyoftheppreferenceisinterpolatedbetweenthereferencesat
√
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.Thespectrumisharderathigher col-lisionenergy,withasmallchangeinthetotalcrosssectionof4% below1 GeV/
c and an increase of about 10% at transverse mo-mentaabove10 GeV/
c.3. Systematicuncertainties
Forthetotalsystematicuncertainty,allcontributionsareadded inquadratureandaresummarizedinTable2.
The effectof the selection of events based on the vertex po-sitionis studiedby comparing thefully corrected pT spectra
ob-tainedwithalternativevertexselectionscorresponding to
±
5 cm,and
±
20 cm. The difference in the fully corrected pT spectra islessthan0.3%forcentralcollisions andlessthan0.5%for periph-eralcollisions.
In order to test the description of the detector response and thetrackreconstructioninthesimulation,allcriteriafortrack se-lectionare varied within theranges asdescribed in theprevious publication [5].Afull analysisisperformedby varyingone selec-tioncriterionatatime.Themaximumchangeinthecorrected pT
spectrum isthen considered assystematic uncertainty.The over-all systematic uncertainty related to track selection is obtained fromsummingupallindividual contributionsquadraticallyandit amountsto0.6–3.0%,dependingon pTandcentrality.
Thesystematicuncertaintyonthe secondary-particle contami-nation isestimatedbyvarying thefitmodelusingtwotemplates, i.e. forprimariesandsecondaries,orthreetemplates,i.e. primaries, secondaries from interactions in the detector material and sec-ondaries from weak decays of K0s and
, aswell asvarying the fit ranges. The maximum difference between data and the two-component-template fit is summed in quadrature together with thedifferencebetweenresultsobtainedfromthetwo- and three-component-template fits. The systematic uncertainty due to the contamination fromsecondariesisdecreasing withincreasing pT.
ItdominatesatlowpTwithvaluesupto4%andisnegligibleabove
2 GeV
/
c.Thesystematicuncertaintyontheprimaryparticlecomposition istakenfrom [5].Anadditionaluncertaintyisestimatedby assum-ing theparticlecomposition fromaneighboring
dNch/
dηrangetothe matchedone inthePb–Pbanalysisandisadded quadrati-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 scal-ing the fraction of secondary particles obtained from the fits to the DCAxy distributions [5]. The difference in the TPC-ITS
track-matchingefficiencybetweendataandsimulationisassignedtothe corresponding systematicuncertainty(seeTable2). Itamounts to 2%incentralcollisions,andupto3.5%inperipheralcollisions.
ThematerialbudgetinALICEat
η
≈
0 amountsto(
11.
4±
0.
5)
% inradiationlengthsforprimary chargedparticlesthat have suffi-cient tracklength inthe TPC [38].A difference intheamount of detectormaterialleadstodifferentamountsofsecondaryparticles thatareproduced.Afterthesubtractionofthecontributiondueto secondariesusingthethree-componentDCAxyfits,thedifferencesonthesecondarycorrectionfactorisnegligible.Avariationofthe materialbudget withinabovelimitsleads toa pT dependent
sys-tematicuncertaintyonthetrackingefficiencyof0.1–0.3%.
TheuncertaintyduetothefinitepT resolutionathighpTis
es-timated using the azimuthal dependence of the 1/pT spectra for
Table 2
Contributionstothesystematicuncertaintyinunitsofpercentforthe0–5%,30–40%,and70–80%centralityclassesinXe– Xecollisions.Thenumbersareaveragedinthe
p
Tintervalsfrom0.2–0.5 GeV/c (left), 1–2 GeV/c (middle) and40–50 GeV/c(right).Forthe
p
T-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
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 pT resolutionis
sig-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 the re-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 eachpT interval, the systematic uncertainty of the pp references at
√
s
=
5.
02 TeV and√
s=
7 TeV areinterpolatedto√
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 cen-trality classes together with the interpolated pp reference spec-trumat
√
s=
5.
44 TeV. The latteris obtained fromthe interpo-lated pT-differential cross section by dividing it by theinterpo-lated inelastic nucleon-nucleon cross section of
(
68.
4±
0.
5)
mb at√
s=
5.
44 TeV [24]. In the most-peripheralcollisions, the pTspectrum issimilar to that ofpp collisions andexhibitsa power law behavior that is characteristic of hard-parton scattering and vacuumfragmentation. Withincreasingcollisioncentrality,the pT
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/
c withavalueof1.4%(2.1%)forthe0–5%(70–80%)centralityclass. Inordertodeterminethenuclear modificationfactor RAA,the
interpolated pT-differential pp cross section is scaled by the
av-eragenuclear overlap function
TAA.The resulting nuclear mod-ification factor as a function of transverse momentum is shown inFig.4 forninecentrality classesandcomparedto resultsfrom Pb–Pb collisions [5]. The overall normalization uncertainties forRAA 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
rela-tive uncertainty in
TAA[18,19]. The nuclear modification factor exhibitsa strong centrality dependencewith a minimumaroundpT
=
6–7 GeV/
c and an almost linearrise above.In particular, inthe 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 inninecentralityclassestogetherwiththeinterpolatedpp
refer-encespectrumat√s=5.44 TeV (toppanel)andsystematicuncertainties(bottom panel).
0.6 at the highest measured transverse-momentum interval of 30–50 GeV
/
c.Forcomparison,thenuclearmodificationfactorRAAinPb–Pbcollisionsat
√
sNN=
5.
02 TeV isshowninFig.4asopencircles forthe same centralityclassesas Xe–Xe.In both collision systems,asimilarcharacteristicpTdependenceofRAAisobserved.
InPb–Pbcollisions,thesuppressionofhigh-momentumparticlesis apparentlystrongerforthesamecentralityclassbutstillin agree-mentwithXe–Xecollisionswithinuncertainties.
Nuclear modificationfactors from Xe–Xe andPb–Pb collisions andtheirratiosatsimilarrangesof
dNch/
dηareshowninFig.5.In5% mostcentralXe–Xecollisions,the nuclearmodification fac-tor is remarkably well matched by 10–20% central Pb–Pb colli-sions over the entire pT range. ln the 30–40% Xe–Xe (40–50%
Pb–Pb) centrality class, again agreement is found within uncer-tainties.Thesefindingsofmatchingnuclearmodificationfactorsat similarrangesof
dNch/
dηareinagreementwithresultsfromthestudyoffractionalmomentumlossofhigh-pTpartonsatRHICand
LHCenergies [41].
A comparison of the nuclear modification factors as a func-tion of
dNch/
dη in Xe–Xe and Pb–Pb collisions for threedif-ferent regions of pT (low, medium, andhigh)is shownin Fig.6.
A remarkable similarity in RAA is observed between Xe–Xe
col-lision at
√
sNN=
5.
44 TeV and Pb–Pb collisions at√
sNN=
5.
02and 2.76 TeV when compared at identical ranges in
dNch/
dη,for
dNch/
dη>
400. This holds both at low momentum wherethehydrodynamicalexpansionofthemediumdominatesthe spec-trumandathighmomentum,wherepartonenergylossinsidethe
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) forsimilarrangesindNch/dηforthe 0–5%
(left)and30–40%(right)Xe–Xecentralityclasses.Theverticallines(brackets) rep-resentthestatistical(systematic)uncertainties.
medium drives the spectral shape. At
dNch/
dη<
400, theval-ues of RAA still agree within rather large uncertainties although
nodefinitiveconclusioncanbedrawnbecause,inparticular,event selectionandgeometrybiasescouldaffectthespectrumin periph-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 energydensitycan beestimatedfromtheaverage charged-particle multiplicity density [45] per transverse area,
ε
∝
dNch/
dη/
AT. In central collisions, the initialtrans-verse area AT is related to the radius r of the colliding nuclei, AT
=
π
·
r2 [22]. Therefore,the comparisonof themeasured RAAvaluesinthetwocollidingsystemscouldenableatestofthepath lengthdependenceofmedium-inducedpartonenergyloss [46].
Tofurtheraddressbulkproduction,theaveragetransverse mo-mentum
pTintherangefrom0–10 GeV/
c isderived.The spec-tra are extrapolated downto pT=
0 by fittinga Hagedornfunc-tion [47] in the range 0
.
15 GeV/
c<
pT<
1 GeV/
c. The relativefractionoftheextrapolatedparticleyieldamountsto8%(11%)for the0–5%(70–80%)centralityclass.Statisticaluncertaintiesin
pTare 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,andbychangingtheinterpolationrangeto0–0.2 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 ofpTwith centralityisvisiblein bothcol-Fig. 6. ComparisonofthenuclearmodificationfactorinXe–Xeand Pb–Pb colli-sionsintegratedoveridenticalregionsin
p
TasafunctionofdNch/dη.Theverticalbracketsindicatethequadraticsumofthetotalsystematicuncertaintyinthe mea-surementandtheoverallnormalizationuncertaintyinTAA.Thehorizontalbars
reflecttheRMSofthedistributionineachbin.Thedashedlinesshowresultsfrom power-lawfitstothedataandaredrawntoguidetheeye.
lisionsystemsandisattributedtotheincreasingtransverseradial flow. Thebottompanel ofFig.7 showsthe ratiosof
pTinboth 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 betweenboth collision systems in ordertostudythesystemsizedependence.Thepredictedtrendof alargerpTin5%mostcentralXe–Xecollisionandcontinuously lower values towards the 40–50% centrality class are consistent withthedata.5. Summary
Transverse momentum spectra and nuclear modification fac-tors of chargedparticles in Xe–Xe collisions at
√
sNN=
5.
44 TeVin the kinematic range 0
.
15<
pT<
50 GeV/
c and|
η
|
<
0.
8 arereportedfornine centralityclasses,inthe0–80%range.App ref-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 parti-cle multiplicity densities, a remarkable similarity between cen-tral Xe–Xe collisions andPb–Pb collisions at a similar center-of-massenergyof√
sNN=
5.
02 TeV and at2.76 TeV isobserved forFig. 7. Averagetransversemomentuminthe
p
T-range0–10 GeV/c for Xe–Xecolli-sionsat√s=5.44 TeV (squares)andPb–Pbcollisionsat√s=5.02 TeV (diamonds) forninecentralityclasses(top)andtheirratios(bottom).Theverticalbrackets indi-catesystematicuncertainties.Thehashedareasshowresultsfromhydrodynamical calculationsbyGiacaloneetal. [43].
dNch/
dη>
400.Thecentralitydependenceoftheratiooftheav-erage transversemomentum
pT inXe–Xe collisions over Pb–Pb collisions isflatwithin uncertaintiesbutallows forrelative varia-tionsofuptotwopercent.Predictionsfromhydrodynamical calcu-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 andtechniciansfortheirinvaluablecontributionstothe construc-tionoftheexperimentandtheCERNacceleratorteamsforthe out-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 Committeeof Sci-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
(HIP),Finland;Commissariatàl’EnergieAtomique(CEA)and Insti-tut Nationalde Physique Nucléaire etde Physique des Particules (IN2P3)andCentre National de laRecherche Scientifique (CNRS), France; Bundesministerium für Bildung, Wissenschaft, Forschung und Technologie (BMBF) and GSI Helmholtzzentrum für Schw-erionenforschung GmbH, Germany; General Secretariat for Re-search andTechnology,Ministry ofEducation, Researchand Reli-gions,Greece;NationalResearch,DevelopmentandInnovation Of-fice,Hungary;DepartmentofAtomicEnergy,GovernmentofIndia (DAE),DepartmentofScienceandTechnology,GovernmentofIndia (DST),University Grants Commission, Governmentof India(UGC) andCouncil ofScientificandIndustrialResearch(CSIR), India; In-donesian Institute of Science, Indonesia; Centro Fermi - Museo StoricodellaFisicaeCentroStudieRicercheEnricoFermiand Isti-tutoNazionalediFisicaNucleare(INFN),Italy;Institutefor Innova-tiveScienceandTechnology,NagasakiInstituteofAppliedScience (IIST),Japan Societyforthe PromotionofScience(JSPS)KAKENHI andJapanese Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan; Consejo Nacional de Ciencia (CONA-CYT)yTecnología,throughFondodeCooperaciónInternacionalen CienciayTecnología (FONCICYT)andDirección Generalde Asun-tosdelPersonalAcademico(DGAPA),Mexico;Nederlandse Organ-isatievoorWetenschappelijk Onderzoek(NWO),Netherlands; The ResearchCouncil ofNorway, Norway;CommissiononScienceand TechnologyforSustainableDevelopmentintheSouth(COMSATS), Pakistan;PontificiaUniversidadCatólicadelPerú,Peru;Ministryof ScienceandHigherEducationandNationalScienceCentre,Poland; KoreaInstituteofScienceandTechnologyInformationandNational ResearchFoundationofKorea(NRF),RepublicofKorea;Ministryof EducationandScientificResearch,Institute ofAtomicPhysicsand RomanianNationalAgencyforScience,TechnologyandInnovation, Romania; Joint Institute for Nuclear Research (JINR), Ministry of EducationandScienceoftheRussianFederationandNational Re-search Centre Kurchatov Institute, Russia; Ministry of Education, Science,Researchand SportoftheSlovak Republic, Slovakia; Na-tionalResearchFoundationofSouthAfrica,SouthAfrica;Centrode AplicacionesTecnológicasyDesarrolloNuclear(CEADEN), Cubaen-ergía,CubaandCentrodeInvestigacionesEnergéticas, Medioambi-entalesyTecnológicas(CIEMAT),Spain;SwedishResearchCouncil (VR)andKnut&AliceWallenbergFoundation(KAW),Sweden; Eu-ropean Organization for Nuclear Research, Switzerland; National Scienceand Technology DevelopmentAgency (NSDTA), Suranaree University of Technology (SUT) and Office of the Higher Educa-tionCommissionunderNRUprojectofThailand,Thailand;Turkish Atomic Energy Agency (TAEK), Turkey; NationalAcademy of Sci-encesofUkraine,Ukraine;ScienceandTechnologyFacilities Coun-cil (STFC), United Kingdom; National Science Foundation of the UnitedStatesofAmerica(NSF)andU.S.DepartmentofEnergy, Of-ficeofNuclearPhysics(DOENP),UnitedStatesofAmerica.
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