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Letters
B
www.elsevier.com/locate/physletb
+
c
production
in
Pb–Pb
collisions
at
√
s
NN=
5
.
02 TeV
.ALICE
Collaboration
a
r
t
i
c
l
e
i
n
f
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a
b
s
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r
a
c
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Articlehistory:
Received 12 October 2018
Received in revised form 26 March 2019 Accepted 17 April 2019
Available online 23 April 2019 Editor: L. Rolandi
Ameasurementoftheproductionofprompt
+
c baryonsinPb–Pbcollisionsat√sNN=5.02 TeVwith
theALICEdetectorattheLHCisreported.The
+
c and
−c werereconstructedatmidrapidity(|y|<0.5)
via the hadronicdecay channel
+c →pK0S (and chargeconjugate) in the transverse momentumand
centrality intervals6
<
pT<12 GeV/c and0–80%. The+c/D0 ratio,whichis sensitive to thecharm
quark hadronisationmechanisms in themedium, ismeasured and found to be largerthan the ratio measured in minimum-bias ppcollisions at√s=7 TeV and in p–Pb collisions at √sNN=5.02 TeV.
In particular, the values in p–Pb and Pb–Pb collisions differ by about two standard deviations of the combined statistical and systematic uncertainties in the common pT interval covered by the
measurementsinthetwocollisionsystems.The
+c/D0ratioisalsocomparedwithmodelcalculations
includingdifferentimplementationsofcharmquarkhadronisation.Themeasuredratioisreproducedby models implementingapurecoalescencescenario,whileaddingafragmentationcontributionleadsto an underestimation.The +c nuclearmodification factor,RAA,isalsopresented. Themeasured values
ofthe RAAof
+c,Ds+ and non-strangeDmesonsare compatiblewithinthecombined statisticaland
systematicuncertainties.Theyshow,however,ahintofahierarchy
(
RD0AA<R D+s
AA<R +c
AA),conceivablewith
acontributionfromcoalescencemechanismstocharmhadronformationinthemedium.
©2019EuropeanOrganizationforNuclearResearch.PublishedbyElsevierB.V.Thisisanopenaccess articleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3.
1. Introduction
Measurementsoftheproductionofopen-heavyflavourhadrons inheavy-ioncollisionsprovideimportantinformationonthe prop-erties of the Quark–Gluon Plasma (QGP), the state of strongly-interactingmatter formedat thevery hightemperaturesand en-ergy densities reachedin heavy-ioncollisions [1,2]. Several mea-surements of the production and elliptic flow of D mesons and leptons fromthe decay of heavy-flavour hadrons in Pb–Pb colli-sionsattheLHCandinAu–AucollisionsatRHIC [3,4] indicatethat charmquarksinteractstronglywiththemedium constituents. In-mediumenergylossisstudiedviathenuclearmodificationfactor,
RAA,definedastheratiooftheyieldinPb–Pbcollisionsandthat inpp collisions scaled bythe numberofbinary nucleon–nucleon collisions. A model [5,6] including a significant fraction of low and intermediate transverse momentum (pT) charm and beauty
quarks hadronising via coalescence (or recombination) withlight quarks from the medium better describes the experimental re-sults. This mechanism is expected to also affect the production of D+s given the strange-quark rich environment of the created
medium. At highertransverse momentum (pT
>
7 GeV/c at LHCE-mailaddress:alice-publications@cern.ch.
energies [7])hadronisation by vacuumfragmentation isexpected tobethedominantproductionmechanism.
Inthiscontext,the studyofcharm baryonsisessentialto un-derstand charm hadronisation. Models includingcoalescence pre-dict an enhanced baryon-to-mesonratioatlow andintermediate transversemomentumincomparisontothatexpectedinpp colli-sions. Thiseffectadds tothe hadron-massdependent transverse-momentum shiftdue tothe presenceof radial flowin heavy-ion collisions, that is able to explain the observed increase of the baryon-to-mesonratiointhelightsectoruptoabout2 GeV/c [8]. Thestudyofnon-strangeD-mesons,D+s and
+c couldhelpto
dis-entangle the role of coalescence andradial flow, because of the smallermassdifferencesthanforlight-flavourhadrons.
Fortheparticularcaseofcharmbaryons,thepossibleexistence of light di-quark bound states inthe QGP could further enhance the
+c/D0 ratiointhecoalescencemodel [9].Anenhancementof
the pT-integrated
+c/D0 ratio in the presence of a QGP is also
predicted by the statistical hadronisation model [10], where at LHC energiestherelative abundanceofhadronsdependsontheir masses,theirflavourcontentandthefreeze-outtemperatureofthe medium.Inaddition,anenhancementofcharm-baryonproduction in Pb–Pbcollisions wouldmake thecharm baryonsan important fractionofthetotalcharmproductioncrosssection.
The study ofa potential enhancement effect incharm-baryon production in relativistic heavy-ion collisions requires a baseline
https://doi.org/10.1016/j.physletb.2019.04.046
0370-2693/©2019 European Organization for Nuclear Research. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP3.
reference in smaller collision systems. The
+c-baryon produc-tionwas measured by theALICECollaborationin ppcollisions at
√
s
=
7 TeV in the transverse momentum andrapidity ( y) inter-vals1<
pT<
8 GeV/c and|
y|
<
0.
5 [11].Theobtainedbaryon-to-mesonratioislargerthanpreviousmeasurementsatlower centre-of-massenergies andin different collision systems(see Ref. [11] andreferencestherein),andalsohigherthantheresultsreported by the LHCb Collaborationin pp collisions at
√
s=
7 TeV in the rapidity range 2.
0<
y<
4.
5 [12]. Expectations from perturba-tive Quantum Chromodynamics (pQCD) calculations and Monte Carloeventgenerators underpredictthe data, indicating that the fragmentation of charm quarks is not fully understood [11] and partially challenged by data collected so far at the LHC, as dis-cussedextensivelyinRef. [13].Theproductionof+c baryonswas
also measured by the ALICE Collaboration in p–Pb collisions at
√
sNN
=
5.
02 TeV in2<
pT<
12 GeV/c and−
0.
96<
y<
0.
04 [11],and a measurement in the same collision system by the LHCb Collaboration [14] is alsoavailable. The
+c nuclear modification
factorRpPbiscompatiblewithunitywithinstatisticaland
system-atic uncertainties. The baryon-to-meson ratios
+c/D0 measured
inpp andp–Pb collisions are compatible within uncertainties. A model [15,16] includinghadronisationviacoalescenceinthese col-lisionsystemshasbeenproposedtodescribethemeasurementsat LHCenergies.
This letter reports measurements of the production of the promptcharmbaryon
+c anditschargeconjugateinPb–Pb
colli-sionsat
√
sNN=
5.
02 TeV withtheALICEdetector [17] attheLHC.Hereafter,
c refers indistinctly toboth particleandanti-particle,
andallmentioned decaychannels referalso totheir charge con-jugates.The
+c correctedyield isobtainedastheaverageofthe
particleandtheanti-particleyield.Thenotation
+c isusedwhen referring to thisaverage, andthus to indicate physics quantities such as the
+c/D0 ratio. The measurement was performed in
the 0–80% centrality class in the transverse momentum and ra-pidity intervals 6
<
pT<
12 GeV/c and|
y|
<
0.
5. Only promptc-baryons were considered: the beauty-hadron feed-down was
subtracted,as described inthe next section. The D0-mesonyield wasobtainedinthesametransversemomentumandcentrality in-tervalasthe
c-baryon,followingtheanalysisproceduredescribed
inRef. [18].
2. Datasampleandanalysisstrategy
The measurement of the
c-baryon production was
per-formed by reconstructing the decays
+c
→
pK0S with abranch-ing ratio (BR) equal to
(
1.
58±
0.
08)
% and K0S→
π
+π
− with BR= (
69.
20±
0.
05)
% [19].The D0 mesons were reconstructedin thedecaychannelD0→
K−π
+withBR= (
3.
93±
0.
04)
% [19].Thec andD0 candidateswere reconstructedin thesametransverse
momentum, rapidity and centrality intervals. The analysis bene-fitsfromthetrackingandparticleidentificationcapabilitiesofthe ALICE central barrel detectors located within a large solenoidal magnetthatprovidesamagneticfieldof0.5 TparalleltotheLHC beamaxis.Acompletedescriptionofthe ALICEapparatusandits performancecanbefoundinRefs. [17,20].Themaindetectorsused inthis analysisinclude theInner TrackingSystem (ITS) [21], the TimeProjection Chamber (TPC) [22], the Time-Of-Flight detector (TOF) [23] and the V0detector [24] located insidethe solenoidal magnet,aswellastheZeroDegreeCalorimeters(ZDC) [17] located intheLHCtunnelatabout
±
112.
5 mfromthenominalinteraction pointandcomposedoftwoprotonandtwoneutroncalorimeters.The analysed data sample consists of about 83
×
106 Pb–Pbcollisions at
√
sNN=
5.
02 TeV,corresponding to an integratedlu-minosityof
L
int≈
13.
4 μb−1.Theinteractiontriggerwasprovidedby the coincident signals from the two arrays of the V0 detec-tor, covering the pseudorapidity intervals
−
3.
7<
η
<
−
1.
7 and 2.
8<
η
<
5.
1.Backgroundeventsfrombeam–gasinteractionswere removed intheofflineanalysisusingthetiming information pro-videdbytheV0andtheneutronZDC.Onlyeventswithaprimary vertexreconstructedwithin±
10 cmfromthecentreofthe detec-tor along thebeam linewere considered forthe analysis. Events were selected in the centrality class 0–80%, defined in terms of percentilesof thehadronicPb–Pb cross section,using the ampli-tudesofthesignalsintheV0arrays [25].The
c candidates were constructed by combining a proton
candidate track with a K0S candidate identified through its V-shapedneutraldecaytopology(V0).ThechargedtracksandtheK0S candidateswereselectedasdescribedinRef. [11] forppcollisions with additional requirements to reduce the larger combinatorial backgroundduetothehighercharged-trackmultiplicityinPb–Pb withrespecttoppcollisions.Inparticular,candidateprotontracks wererequiredtohaveahitintheinnermostITSlayerandtighter selectionson the K0S were applied: a maximumdistance of clos-estapproachbetweentheV0 decaytracksof0.4 cm,a minimum
cosine ofthe V0 pointingangle to theprimary vertexof0.9998, aminimum pT oftheKS0 candidates of1 GeV/c,andacut inthe
Armenteros-Podolanskispace [26] toremovecontributionsfrom
decays.Theidentificationofprotonswasbasedonthespecific ion-isationenergylossdE
/
dx intheTPCandonthetimeofflight mea-suredwiththeTOFdetector,usingasadiscriminatingvariable(nσ ) thedifferencebetweenthemeasuredvalueandtheexpectedvalue fortheprotonmasshypothesisdividedbythedetectorresolution. A|
nσ|
<
3 selectionwasappliedontheTPCdE/
dx andTOF time-of-flight measurements for tracks with pT<
3 GeV/c. For trackswith pT
>
3 GeV/c anasymmetric selectionwasusedtolimit the contamination frompions inthe TPCandfromkaonsin theTOF andtherequirements were−
3<
nTPCσ
<
2 and−
2<
nTOFσ<
3 fortheTPCandTOFsignals.TrackswithoutTOFinformationwere dis-carded.The
c candidateswereselectedrequiringacosineofthe
proton emission anglein the
c centre-of-mass systemwith
re-specttothe
c momentumdirectionsmallerthan0.5.Aselection
on thesignedtransverse impactparameter ofthe proton,i.e.the distanceofclosestapproachbetweentheprotontrackandthe pri-maryvertex,largerthan0.003 cmwasalsoapplied(thesignofthe impact parameteris definedaspositive whenthe anglebetween the
c flightlineandthemomentumvectorissmallerthan90◦).
The D0 candidates were reconstructed by combining pairs of tracks withthe proper charge sign combination and selected in theinterval6
<
pT<
12 GeV/c usingthesamecriteriadescribedin Ref. [18] fortheinterval6<
pT<
7 GeV/c inthe10%mostcentralPb–Pbcollisions.
After all selections, the acceptance in rapidity for
c and D0
candidates drops steeply to zero for
|
y|
>
0.
8 in the pT intervalusedfortheanalysis.Therefore,afiducialacceptancecut
|
y|
<
0.
8 wasappliedasdescribedinRefs. [11] and[18].The
c and D0 raw yields were extracted by fitting the
in-variantmassdistributions ofthe candidatespassing theselection criteria.ThefitfunctionsconsistofaGaussiantodescribethe sig-nalandan exponentialtodescribethebackground.Inthecaseof the
c,thewidthoftheGaussianwasfixedtothevalueobtained
from MonteCarlo simulations. The stability of the
c signal
ex-tractionwasverifiedbyfittingtheinvariantmassdistributionafter thesubtractionofthebackgroundevaluatedwithanevent-mixing technique and no discrepancy between the two approaches was observed.Forthe D0-mesonyield,the contributionofsignal can-didateswiththewrongK–
π
massassignment (reflections)to the invariant-massdistributionwastakenintoaccountbyincludingan additional term, parameterised from simulations with a double-Gaussianshape,inthefitfunction [27].Fig. 1. Invariant-mass
distributions for the
c(left) and D0(right) candidates in the momentum interval 6 <pT<12 GeV/c and for the 0–80% centrality class. The dashed curves represent the fit to the background, while the solid curves represent the total fit function.Theinvariantmassdistributionsoftheselected
c andD0
can-didatesareshowninFig.1.
Theprompt
+c (D0)productionyieldwascalculatedas
dN+c(D0) prompt dpT
|y|<0.5=
1 2 1 cy 1pT fprompt
·
Nraw||y|<0.8(
Acc×
ε
)
prompt·
BR·
Nevt,
(1)where Nraw is theraw yield(sum ofparticles andanti-particles)
inthetransversemomentumintervalofwidth
pT, fprompt isthe fractionofprompt
c (D0)intherawyield,
(
Acc×
ε
)
istheprod-uctofacceptanceandreconstructionefficiencyforprompt
c(D0),
BRisthebranching ratiooftheconsidereddecaymode andNevt
isthenumberofeventsconsideredfortheanalysis.Thecorrection factorfortherapidity coverage
c
y was computedastheratioofthegenerated
c (D0)yieldin
|
y|
<
0.
8 andthatin|
y|
<
0.
5.Thefactor1
/
2 takesintoaccountthattherawyieldisthesumof par-ticlesandanti-particles, whiletheproductionyieldisreportedas theiraverage.The correction for the detectoracceptance andreconstruction efficiency was determined by means of Monte Carlo (MC) sim-ulations. The underlying Pb–Pb events at
√
sNN=
5.
02 TeV weresimulatedusingtheHIJINGv1.383 [28] generatorandpromptand feed-down
c(D0)wereaddedusingthePYTHIAv6.421 [29]
gen-erator withPerugia 11 tune. The generatedparticles were trans-portedthroughtheALICEdetectorusingtheGEANT3 [30] package. Arealisticdetectorresponsewasintroducedinthesimulationsto reproduce the performance of the ALICE detector system during datataking.
The pT distributions of the
c and D0 in PYTHIA were
cor-rected in order to obtain more realistic distributions. The same
pT-dependentweightingfactor,calculatedastheratioofthe
mea-sured D0 pT distribution infiner pT bins [18] and the one sim-ulatedwithPYTHIA, was usedfor bothparticles. The
c andD0
reconstruction efficiency in the large centrality class 0–80% was obtainedastheweightedaverageoftheefficienciesinsmaller cen-tralityclassesto take into account the variationof the efficiency andthe scaling ofthe yields of the
c baryons and D0 mesons
withcentrality.Theappliedweightswerecalculatedastheproduct ofthe RAA oftheD0 andtheaveragenumberofnucleon–nucleon
collisions (
<
Ncoll>
) in the centrality class considered [18]. The(
Acc×
ε
)
valueisabout6%forpromptandabout9%forfeed-downc andabout8%forpromptandabout11%forfeed-downD0.
Theprompt
c (D0)fraction, fprompt,wascalculatedas
fprompt
=
1−
⎛
⎝
Nc(D 0) feed-down Nc(D0) prompt⎞
⎠ =
=
1−
TAA · d 2σ
d ydpT FONLL feed-down·
Rfeed-downAA·
(
Acc×
ε
)
feed-down·
cy·
pT·
BR·
NevtNraw
/
2.
(2)Thecontributionof
c(D0)frombeauty-hadrondecayswas
es-timated usingtheFONLL [31,32] beauty-productioncrosssections as described in detail in Ref. [33]. The fraction ofbeauty quarks that fragment tobeautyhadronsandsubsequentlydecayinto
c
baryons f
(
b→
c)
=
0.
073 wastakenfromRef. [34].Thebeauty-hadron decay kinematics were modeled using the EVTGEN [35] package. The
(
Acc×
ε
)
feed-down term for both particles wascal-culated from the Monte Carlo simulations described above. The averagenuclearoverlapfunction,
TAA,wasestimatedviaGlauber model calculations [36,37]. In thisformalism the nuclear modifi-cationfactor RAA isthentheratiooftheyieldinPb–Pbcollisions andtheproductioncrosssectioninppcollisionsscaledbyTAA.A hypothesis on the Rfeed-down
AA of feed-down
c and D0 is
used. For the D0, the hypothesis is the same as in other analy-ses (e.g. in Ref. [18]): the central value isobtained by assuming
Rfeed-down D0
AA
/
Rprompt D0
AA
=
2,justifiedbytheCMSmeasurementofJ/
ψ
fromB-mesondecays [38] andbytheALICEandCMS measure-ments ofDmesons[18,39] indicatingthatprompt charmmesons are more suppressed than non-prompt charm mesons. The ra-tio is varied in the interval 1<
RAAfeed-down D0/
Rprompt DAA 0<
3 to estimate the systematic uncertainty. Since no measurements of beauty-baryon productioninnucleus–nucleus collisions are avail-able,forthec thecentralhypothesiswastakenfrommodel
cal-culations which predict Rfeed-down+c
AA
/
Rprompt+c
AA
=
2 whencon-sidering c and b quark fragmentation and energy loss in the medium [40].TheratioRfeed-down+c
AA
/
Rprompt+c
AA wasdecomposed
intotwotermstoestimatetheuncertaintyontheassumption:
Rfeed-down+c AA Rprompt+c AA
=
Rfeed-down D 0 AA Rprompt DAA 0·
(+c/D0)PbPb,feed-down (+c/D0)pp,feed-down (+c/D0)PbPb,prompt (+c/D0)pp,prompt.
(3)ThefirsttermisthesameasfortheD0 andthusthesame hy-pothesisisadopted.Thesecondtermisvariedintherange0.5–1.5
Fig. 2.+c/D0ratio as a function of pTin 0–80% most central Pb–Pb collisions compared with the measurements in pp and p–Pb collisions [11] (left), and model calcula-tions [7] (right). Statistical and systematic uncertainties are presented as vertical bars and boxes, respectively.
Table 1
Systematic uncertainties on the corrected yields. When the uncertainty was found to be <1%, it was considered negligible (negl.
in the table). Uncertainty +c D0 Raw-yield extraction 8% 2% Tracking efficiency 3.6% 5% PID 5% negl. Cut variation 2% 5% MC pTshape 2% negl.
MC centrality weights 3% negl. Feed-down subtraction +−612% +−1213%
Branching ratio 5% 1%
to calculatethe systematicuncertainty. The upper limit is deter-mineda-posteriorisuchthat
R
feed-down+cAA
<
2 assuggestedbythefact that nobaryon RAA exceeds thisvalue. The uncertainties on the two terms are added in quadrature. The resulting values of
fpromptareabout0.93and0.81forthe
c andD0,respectively.
Asummaryofthesystematicuncertaintiesonthecorrected
+c
andD0 yieldsisshowninTable1.TheD0 systematicuncertainties ontheparticle identification(PID),trackingandcut variation are takenfromRef. [18] andarenotdiscussedinthefollowing.
Thesystematicuncertainty onthe raw-yieldextractionfor
c
andD0 wasestimatedby repeating thefits severaltimesvarying (i)thelower andupperlimitsofthefitrange,(ii)thebackground fit function and(iii) only in the caseof the
c, considering the
Gaussianmeanandwidthasfreeparametersinthefit.Inaddition, the signal yield was obtained by integrating the invariant-mass distributionaftersubtractingthebackgroundestimatedfromafit tothesidebands.
Forthe
c,thesystematicuncertaintyonthetrackingefficiency
wasevaluatedbycomparingtheprobabilityofmatchingtracks re-constructedinthe TPCto ITShitsindataandsimulation andby varyingthe quality cutsto selectthetracks used intheanalysis. Thecontributionduetothevariationofthequalitycutswas eval-uatedusingprotonsfrom
decaysandaninclusiveK0
Ssampleand
bycalculatingtheratioofthecorrectedyieldsobtainedusing dif-ferentselectioncriteria. The uncertaintyontheITS-TPC matching efficiencyisdefinedastherelativedifference ofthematching ef-ficiencyindataandsimulationsafterweightingtherelative abun-dances ofprimary and secondary particles in the simulations to matchthoseindata.Thelatterwereestimatedviafitstothetrack impact-parameter distributions. The values calculated as a func-tion of track momentum were propagated to the pT-differential uncertaintyofthe
c usinga MonteCarlosimulation. A3%
sys-tematicuncertaintyon the ITS-TPC matching efficiencyofproton
trackswasassignedwhilefortheK0
S thematchingisnotrequired.
Theuncertaintyresultingfromthesestudieswasaddedin quadra-turetotheuncertaintyonthetrackselection.
The systematicuncertainty onthe
c PID efficiencywas
eval-uated using protons from the decay of
baryons. The ratio of the
yieldmeasuredwithPIDtothatmeasuredwithoutPIDwas calculated inboth dataandMC andtheir difference was usedto estimatethesystematicuncertainty.
Systematicuncertainties ontheefficienciescanalsoarisefrom possible differencesin the distributions and resolutions of selec-tionvariables betweendataandsimulation.The systematiceffect induced by these imperfections was estimated by repeating the analysisvaryingthemainselectioncriteriaforthecandidates.The efficiencies determined from the simulations depend also on the generated
pT
distributionsofthec andtheD0.Thecentralvalues
ofthecorrectionfactorswereobtainedbyre-weightingthe
cand
D0 distributionsgeneratedby PYTHIAasdescribed above.Forthe D0,theefficienciescalculatedwithandwithoutthe
p
Tweightsare
compatibleandthereforenouncertaintywasassigned.Forthe
c,
the systematic uncertainty was defined by considering the vari-ation of the efficiencies determined with different generated pT
shapes. The new
c pT shape was calculated by multiplying the
measured D0 pT distributionwiththe
+c
/
D0 ratios predictedby themodels [6] and[41].Finally, the efficiencies in the centrality class 0–80% depend on thecentrality weightsused to combinetheefficiencies in the smaller centrality classes.The stability of the efficiencies against thevariationofthecentralityweightswas testedby recalculating theefficiencies withoutweighting for
Ncolland, forthec,
us-ing asan alternative centralityweight theproduct
/
K0S·
Ncoll, wheretheratio/
K0SistakenfromRef. [8].
The systematic uncertainty on the subtraction of feed-down from beauty-hadron decays was estimated by varying (i) the
pT-differential cross section of feed-down
c (D0) from FONLL
calculationswithin the theoreticaluncertainties (see Ref. [11] for details on the
c andRef. [33] for the D0) and (ii) the ratio of
promptandfeed-downRAAasdescribedabove.
Theproductionyields of
c andD0 alsohaveaglobal
system-aticuncertaintyduetothebranchingratio. 3. Results
Theyieldofprompt
+c baryonsmeasuredinPb–Pbcollisions at
√
sNN=
5.
02 TeV in the0–80%centralityclass in|
y|
<
0.
5 and6
<
pT<
12 GeV/
c is N +c
= (
2.
1±
0.
4(
stat.)
+0.3−0.4
(
syst.))
×
10−2.The measured
+c
/
D0 ratiois showninFig.2.The systematic uncertainty ofthe+c-baryon productionarising fromthe
track-Fig. 3. RAAof prompt +c compared with model calculations [7,15,16] (left), and the non-strange D mesons, D+s, and charged particle RAAin 0–10% most central Pb–Pb collisions for pT>1 GeV/c [18,42] (right). Statistical, systematic and normalisation uncertainties are presented as vertical bars, empty boxes and shaded boxes around unity, respectively.
ing efficiency was treated as fully correlated to that of the D0
meson. The contributionto the feed-downuncertainty relatedto heavy-quarkenergylossandthat originatingfromthe FONLL un-certaintyonthefeed-down
+c andD0 crosssectionsweretreated
as fully correlated when propagated to the ratio. All the other sourcesofuncertaintywereconsideredasuncorrelated.Intheleft panel of Fig. 2, the
c+
/
D0 ratio measured in Pb–Pb collisions is compared with the results obtained by the ALICE Collabora-tionin minimum-biaspp andp–Pbcollisions at√
s=
7 TeV and√
sNN
=
5.
02 TeV [11], respectively. The ratio measured in Pb–Pbcollisionsishigherthanthat measuredinppandp–Pbcollisions. In particular, the values in p–Pb and Pb–Pb collisions differ by abouttwostandarddeviationsofthecombinedstatisticaland sys-tematicuncertaintiesin6
<
pT<
12 GeV/
c.The
+c
/
D0 ratio in Pb–Pb collisions is compared withtheo-retical model calculations in the right panel of Fig. 2. The Cata-niamodel [7] providestwodifferenttreatments ofhadronisation. Inone case, charm quarkshadronise via coalescence only. Inthe othercase,acoalescenceplusvacuumfragmentationmodellingof hadronisationisconsidered:atincreasing
p
Tthecoalescenceprob-abilitydecreasesandeventuallyvacuumfragmentationtakesover. ForD0 mesons,the shapeof thefragmentationfunction istuned assuring thatthe experimental resultson D-mesonproduction in ppcollisions are welldescribed bya fragmentationhadronisation mechanism.Datafrome+e− collisionsareusedtofixtheshapeof the fragmentationfunctions for
+c. The coalescence mechanism istreatedasa three-quarkprocess andimplementedthroughthe Wigner formalism. The momentum spectrum of hadrons formed by coalescence is obtained from the quark phase-space distribu-tionsandthehadronwavefunction.The widthparameters ofthe hadronwavefunctionsarecalculatedfromthechargeradiusofthe hadronsaccordingtothe quarkmodel.Thehadronwave function normalisationisdeterminedbyrequiringatotalcoalescence prob-abilityforcharmquarksequaltounityforzero-momentumheavy quarks.Moreover,thecontributionsfromthefirstexcitedstatesfor Dand
c hadronswere included inthecalculations. The
experi-mental results are described by the model calculation including coalescenceonly.Thecurveobtainedbymodellingcharm hadroni-sationviavacuumfragmentationpluscoalescence,whichdescribes the
+c
/
D0ratiomeasuredinAu–AucollisionsatRHICenergy [43],significantly underestimatesthe measurement inPb–Pb collisions attheLHC. Inthe Shao-Songmodel [15,16], coalescenceinvolves quarks which are close in momentum space, and it takes place mainly forthe quark with a given fractionof the momentum of thehadron.ItdoesnotconsidertheWignerformalismtodescribe thespatial andmomentumdistribution ofquarks ina hadron. It cannotdirectlypredicttheabsolutemagnitudeofthe
+c
/
D0ra-tio because the relative production of single-charm baryons and single-charm mesonsRBM istreatedasaparameter ofthemodel.
ThecurveobtainedbyconsideringRBM
=
0.
425,whichisthevalueneededtodescribetheresultsinppandp–Pbcollisions, underesti-matesthe
+c
/
D0ratiomeasuredinPb–Pbcollisions.AnRBM=
1.
2is neededtoachieve a better descriptionofthe experimental re-sults in Pb–Pb collisions. However, the hadronisation mechanism via quarkcoalescence includedinthemodelisresponsibleofthe
pT dependence of the
+c
/
D0 ratio, which needs to be verifiedby comparingto ameasurementatlower pT.The RAA ofprompt
+c was obtainedby considering asreference the
+c cross
sec-tion measured in p–Pb collisions at
√
sNN=
5.
02 TeV [11] scaledby1
/
A ( A=
208)andcorrectedforthedifferentrapiditycoverage ofthep–Pbmeasurement.Thecrosssectionmeasuredinp–Pbwas scaledineach pTintervalto|
y|
<
0.
5 usingacorrectionfactor ob-tained withFONLLcalculations [31,32]. Thecorrection factorwas determined fromthe ratios of the cross sectionscalculated with FONLL in the rapidity intervals|
y|
<
0.
5 and−
0.
96<
y<
0.
04. Since FONLL does not provide predictions for+c baryons, the average of the correction factors obtained for D0, D+ and bare
charm quarks, which was found to be 1
.
024±
0.
008, was used. The choice of using the p–Pb cross section to obtain the refer-enceforthe RAA was motivatedbythefactthat itwasmeasuredup to pT
=
12 GeV/
c, whilethe measurement inpp collisions at√
s
=
7 TeV in|
y|
<
0.
5 only reaches pT=
8 GeV/
c. In addition,the
+c nuclear modification factor measured in p–Pb collisions isconsistent withunityfor pT
>
2 GeV/
c [11].The+c reference
crosssectionin6
<
pT<
12 GeV/
c was obtainedbycombiningthe results in the transverse momentum intervals 6<
pT<
8 GeV/
cand8
<
pT<
12 GeV/
c. The uncertainties werepropagatedtreat-ing the statistical and the systematic uncertainties on the yield extractionasuncorrelatedandtheothersourcesofsystematic un-certaintyascorrelatedinpT.The
+c RAA alsohasa3.75%
uncer-tainty dueto thenormalisation of the
+c p–Pbcross section at
√
sNN
=
5.
02 TeV [11] and a2.4% uncertainty onthe averagenu-clearoverlap function
TAA,which wereadded inquadrature.In the leftpanel ofFig. 3,the RAA ofprompt+c iscomparedwith
Catania model calculations [7]. The threecurves are obtained by considering differenttreatments ofthehadronisation mechanisms inppandPb–Pbcollisions.Theshort-dashedcurverepresentsthe
+c RAA asobtainedbyincludingbothvacuumfragmentationand quarkcoalescenceforcharmhadronisationinPb–Pbandonly frag-mentation in pp collisions. The long-dashed curve includes only coalescence in Pb–Pb and fragmentation plus coalescence in pp collisions.Thesolidcurveisobtainedbyconsideringfragmentation plus coalescence in both collision systems. The limitedprecision andthe large pT intervalofthisfirst measurementprevent usto
draw a firm conclusion on which combination of the hadronisa-tionmechanismsinthetwocollisionsystemsbetterdescribesthe result. Moreover, thecomparisonbetween thedifferent scenarios obtainedfromtheCataniamodeldemonstratesthatitiscrucialto alsounderstandthe
+c productionmechanisminppcollisionsto interpret the RAA measurement. The rightpanel of Fig. 3 shows the RAA of prompt
+c baryons measured in the 0–80%
central-ity class (that is dominated by the 0–10% production given the scalingofthe yields with Ncoll
·
RAA) comparedwith theaverage nuclearmodificationfactorsofnon-strangeDmesons,D+s mesons, andchargedparticlesmeasuredinthe0–10%centralityclass [18]. The RAA ofchargedparticles issmallerthanthatofDmesonsby more than 2σ
of the combinedstatistical and systematic uncer-taintiesup topT
=
8 GeV/
c, whiletheyarecompatiblewithin1σ
forpT
>
10 GeV/
c. The RAA valuesofD+s mesonsare largerthan those of non-strange D mesons, but the two measurements are compatiblewithinone standarddeviationofthecombined uncer-tainties [18].Ahintofalarger+c RAAwithrespecttonon-strange Dmesons isobserved,althoughthe resultsarecompared for dif-ferentcentralityclasses.AD0 R
AA
=
0.
27±
0.
01(
stat.)
±
0.
04(
syst.)
wasmeasuredin6
<
pT<
12 GeV/
c in the0–80%centralityclass. TheD0 RAAhasalsoa3.5%uncertaintyarisingfromthe normalisa-tionofthecrosssectionmeasuredinppcollisions at√
s=
7 TeV, and a 2.4% uncertainty on the average nuclear overlap function TAA.ThepT
-differentialcrosssectionofpromptD0 mesonswith|
y|
<
0.
5 in pp collisions at√
s=
5.
02 TeV, used as reference for the nuclear modification factor, was obtained by scaling the measurementat√
s=
7 TeV [44] to√
s=
5.
02 TeVusing FONLL calculations [31,32]. Thescaling was appliedto theD0 crosssec-tion obtainedin 6
<
pT<
12 GeV/
c by combining the resultsin the pT intervalsofthe measurementat√
s=
7 TeV.The statisti-cal andthe systematicuncertainties onthe yieldextraction were propagatedasuncorrelated.Theothercontributionstothe system-aticuncertaintywereconsideredasfullycorrelatedamongthe pTintervals. A difference of about 1.7
σ
is obtained when compar-ing the+c RAA with that of the D0 in 6
<
pT<
12 GeV/
c and0–80%centralityinterval.Thisobservationisqualitativelyin agree-mentwithascenariowhereasignificantfractionofcharmquarks hadronise via coalescence with light quarks from the medium leadingtoanenhanced baryonproductionwithrespecttothatof mesons.
4. Summary
The measurement of the production of prompt
+c baryons
in the 0–80% most central Pb–Pb collisions at
√
sNN=
5.
02 TeVwas presented.The resultwas obtainedatmidrapidity,
|
y|
<
0.
5, in the 6<
pT<
12 GeV/
c transverse momentum interval. The+c
/
D0 ratio is larger than the ratio measured in pp and p–Pbcollisions at
√
s=
7 TeV and√
sNN=
5.
02 TeV [11], respectively.The
+c
/
D0 ratiomeasured in Pb–Pb collisions is described by a modelcalculation implementing only charm quark hadronisation via quark coalescence and it is underestimated when also vac-uumfragmentationisincluded.Thecomparisonofthe+c nuclear modification factor with non-strange D and D+s meson results, which were measured in 0–10% most central Pb–Pb collisions, suggests a hint of a hierarchy, conceivable in a scenario where charm quark hadronisation can occur via coalescence processes, thusenhancingthe
+c-baryonandD+s-mesonproductionwith
re-specttonon-strangeDmesons.However, thelimitedprecisionof thisfirst measurement preventsus from drawing a firm conclu-sion.
A higherprecision for a
+c-baryon production measurement withfinergranularity in pT andcentralitywill be achieved with futuredatasetstobecollectedduringLHCRun 2and,inparticular,
during theLHC Run 3and4,following themajor upgradeof the ALICEapparatus [45,46].
Acknowledgements
The ALICE Collaboration would like to thank all its engineers andtechniciansfortheir invaluablecontributions tothe construc-tionoftheexperimentandtheCERNacceleratorteamsforthe out-standingperformanceoftheLHCcomplex.TheALICECollaboration gratefully acknowledges the resources and support provided by all Gridcentres andtheWorldwide LHC ComputingGrid (WLCG) collaboration. The ALICE Collaboration acknowledges the follow-ingfundingagenciesfortheirsupportinbuildingandrunningthe ALICEdetector:A.I.AlikhanyanNationalScienceLaboratory (Yere-vanPhysicsInstitute) Foundation (ANSL),StateCommittee of Sci-enceandWorldFederationofScientists(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 àPesquisadoEstadodeSãoPaulo(FAPESP),Brazil;Ministryof Sci-ence&TechnologyofChina(MSTC),NationalNaturalScience Foun-dationofChina(NSFC)andMinistryofEducationofChina(MOEC), China;MinistryofScienceandEducation,Croatia;Centrode Apli-cacionesTecnológicasyDesarrolloNuclear(CEADEN),Cubaenergía, Cuba; Ministry of Education, Youth andSports of the Czech Re-public, Czech Republic; The Danish Council for Independent Re-search | Natural Sciences, the Carlsberg Foundation and Danish NationalResearchFoundation (DNRF),Denmark;HelsinkiInstitute ofPhysics(HIP),Finland;Commissariatàl’EnergieAtomique(CEA) andInstitutNationaldePhysiqueNucléaireetdePhysiquedes Par-ticules (IN2P3) and Centre National de la Recherche Scientifique (CNRS), France; Bundesministerium für Bildung, Wissenschaft, ForschungundTechnologie(BMBF)andGSIHelmholtzzentrumfür Schwerionenforschung GmbH, Germany; General Secretariat for ResearchandTechnology,MinistryofEducation,Researchand Re-ligions, Greece; National Research, Development and Innovation Office,Hungary;DepartmentofAtomicEnergyGovernmentof In-dia(DAE), DepartmentofScienceandTechnology,Governmentof India (DST), University Grants Commission, Government of India (UGC) and Council of Scientific and Industrial Research (CSIR), India; Indonesian Institute of Science, Indonesia; Centro Fermi -MuseoStorico dellaFisicae CentroStudi eRicerche EnricoFermi andIstitutoNazionalediFisicaNucleare(INFN),Italy;Institute for Innovative Science and Technology, Nagasaki Institute of Applied Science (IIST), Japan Society for the Promotion of Science (JSPS) KAKENHIandJapaneseMinistryofEducation,Culture, Sports, Sci-enceandTechnology (MEXT), Japan;Consejo Nacional de Ciencia (CONACYT) y Tecnología, through Fondo de Cooperación Interna-cional enCienciay Tecnología(FONCICYT)andDirección General deAsuntosdelPersonalAcademico(DGAPA),Mexico;Nederlandse OrganisatievoorWetenschappelijkOnderzoek(NWO),Netherlands; TheResearchCouncilofNorway,Norway;CommissiononScience andTechnology forSustainable Developmentin theSouth (COM-SATS),Pakistan;PontificiaUniversidadCatólicadelPerú,Peru; Min-istryofScienceandHigherEducationandNationalScienceCentre, Poland;KoreaInstituteofScienceandTechnologyInformationand National Research Foundation of Korea (NRF), Republic of Korea; Ministry ofEducation andScientific Research,Institute of Atomic PhysicsandRomanianNationalAgencyforScience,Technologyand Innovation, Romania; Joint Institute for Nuclear Research (JINR), Ministry ofEducation andScience of the Russian Federationand National Research Centre Kurchatov Institute, Russia; Ministry of
Education,Science,ResearchandSportoftheSlovakRepublic, Slo-vakia;NationalResearchFoundationofSouthAfrica,SouthAfrica; SwedishResearchCouncil(VR)andKnut&AliceWallenberg Foun-dation (KAW), Sweden; European Organization for Nuclear Re-search,Switzerland;NationalScienceandTechnologyDevelopment Agency(NSDTA),SuranareeUniversityofTechnology(SUT)and Of-fice of the Higher Education Commission under NRU project of Thailand,Thailand;TurkishAtomicEnergyAgency(TAEK),Turkey; National Academy of Sciences of Ukraine, Ukraine; Science and TechnologyFacilitiesCouncil(STFC),UnitedKingdom;National Sci-enceFoundationoftheUnitedStatesofAmerica(NSF)andUnited StatesDepartmentof Energy,Office ofNuclear Physics (DOE NP), UnitedStatesofAmerica.
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