Λc+ production in Pb–Pb collisions at √sNN = 5.02 TeV

Contents lists available atScienceDirect

Physics 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 o a b s t ra c t

Articlehistory:

Received12October2018

Receivedinrevisedform26March2019 Accepted17April2019

Availableonline23April2019 Editor: L.Rolandi

Ameasurementoftheproductionofprompt⁺_c baryonsinPb–Pbcollisionsat√_s

NN=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 and 0–80%. 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} √_s

NN=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,D⁺_s and non-strangeDmesonsare compatiblewithinthecombined statisticaland systematicuncertainties.Theyshow,however,ahintofahierarchy(R^D_AA⁰<R^D_AA^+s <R_AA^+c),conceivablewith acontributionfromcoalescencemechanismstocharmhadronformationinthemedium.

©^2019EuropeanOrganizationforNuclearResearch.PublishedbyElsevierB.V.Thisisanopenaccess articleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP³.

1. Introduction

Measurementsoftheproductionofopen-heavyflavourhadrons inheavy-ioncollisionsprovideimportantinformationontheprop- erties of the Quark–Gluon Plasma (QGP), the state of strongly- interactingmatter formedat thevery hightemperaturesanden- ergy densities reachedin heavy-ioncollisions [1,2]. Severalmea- 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 (p_T) 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 atLHC

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

energies [7])hadronisation by vacuumfragmentation isexpected tobethedominantproductionmechanism.

Inthiscontext,the studyofcharm baryonsisessentialtoun- derstand charm hadronisation. Models includingcoalescence pre- dict an enhanced baryon-to-mesonratioatlow andintermediate transversemomentumincomparisontothatexpectedinppcolli- 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 couldhelptodis- 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 p_T-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/©^2019EuropeanOrganizationforNuclearResearch.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP³.

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<p_T<8 GeV/c and|^y|<0.5 [11].Theobtainedbaryon-to- mesonratioislargerthanpreviousmeasurementsatlowercentre- 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 factorR_pPbiscompatiblewithunitywithinstatisticalandsystem- atic uncertainties. The baryon-to-meson ratios ⁺_c/D⁰ measured inpp andp–Pb collisions are compatible within uncertainties. A model [15,16] includinghadronisationviacoalescenceinthesecol- lisionsystemshasbeenproposedtodescribethemeasurementsat LHCenergies.

This letter reports measurements of the production of the promptcharmbaryon⁺_c anditschargeconjugateinPb–Pbcolli- 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 prompt c-baryons were considered: the beauty-hadron feed-down was subtracted,as described inthe next section. The D⁰-mesonyield wasobtainedinthesametransversemomentumandcentralityin- tervalasthec-baryon,followingtheanalysisproceduredescribed inRef. [18].

2. Datasampleandanalysisstrategy

The measurement of the c-baryon production was per- formed by reconstructing the decays ⁺_c →^pK0_S ^{with a branch-} ing ratio (BR) equal to (1.58±⁰.08)% and K⁰_S→

π

⁺

π

⁻ with BR=(69.20±⁰.05)% [19].The D⁰ mesons were reconstructedin thedecaychannelD⁰→^K−

π

⁺withBR=(3.93±⁰.04)% [19].The c 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–Pb} collisions at√

sNN=⁵.02 TeV,corresponding to an integratedlu- minosityofLint≈¹³.4 μb⁻¹.Theinteractiontriggerwasprovided

by 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 informationpro- videdbytheV0andtheneutronZDC.Onlyeventswithaprimary vertexreconstructedwithin±^{10 cmfromthecentreofthedetec-} 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 theampli- tudesofthesignalsintheV0arrays [25].

The c candidates were constructed by combining a proton candidate track with a K⁰_S candidate identified through its V- shapedneutraldecaytopology(V⁰).ThechargedtracksandtheK⁰_S candidateswereselectedasdescribedinRef. [11] forppcollisions with additional requirements to reduce the larger combinatorial backgroundduetothehighercharged-trackmultiplicityinPb–Pb withrespecttoppcollisions.Inparticular,candidateprotontracks wererequiredtohaveahitintheinnermostITSlayerandtighter selectionson the K⁰_S were applied: a maximumdistance ofclos- estapproachbetweentheV⁰ decaytracksof0.4 cm,a minimum cosine ofthe V⁰ pointingangle to theprimary vertexof0.9998, aminimum p_T oftheK⁰_S candidates of1 GeV/c,andacut inthe Armenteros-Podolanskispace [26] toremovecontributionsfrom decays.Theidentificationofprotonswasbasedonthespecificion- isationenergylossdE/dxintheTPCandonthetimeofflightmea- suredwiththeTOFdetector,usingasadiscriminatingvariable(nσ) thedifferencebetweenthemeasuredvalueandtheexpectedvalue fortheprotonmasshypothesisdividedbythedetectorresolution.

A|ⁿσ|<3 selectionwasappliedontheTPCdE/dxandTOFtime- of-flight measurements for tracks with p_T<3 GeV/c. For tracks with pT>3 GeV/c anasymmetric selectionwasusedtolimit the contamination frompions inthe TPCandfromkaonsin theTOF andtherequirements were −³<n^TPCσ <2 and−²<n^TOFσ <3 for theTPCandTOFsignals.TrackswithoutTOFinformationweredis- carded.Thec candidateswereselectedrequiringacosineofthe proton emission anglein the c centre-of-mass systemwith re- specttothec momentumdirectionsmallerthan0.5.Aselection on thesignedtransverse impactparameter ofthe proton,i.e.the distanceofclosestapproachbetweentheprotontrackandthepri- maryvertex,largerthan0.003 cmwasalsoapplied(thesignofthe impact parameteris definedaspositive whenthe anglebetween thec flightlineandthemomentumvectorissmallerthan90^◦).

The D⁰ candidates were reconstructed by combining pairs of tracks withthe proper charge sign combination and selected in theinterval6<pT<12 GeV/cusingthesamecriteriadescribedin Ref. [18] fortheinterval6<p_T<7 GeV/c inthe10%mostcentral Pb–Pbcollisions.

After all selections, the acceptance in rapidity forc and D⁰ candidates drops steeply to zero for|^y|>0.8 in the p_T interval usedfortheanalysis.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.ThefitfunctionsconsistofaGaussiantodescribethesig- nalandan exponentialtodescribethebackground.Inthecaseof thec,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 contributionofsignalcan- didateswiththewrongK–

π

massassignment (reflections)to the invariant-massdistributionwastakenintoaccountbyincludingan additional term, parameterised from simulations with a double- Gaussianshape,inthefitfunction [27].

Fig. 1.Invariant-massdistributionsforthec(left)andD⁰(right)candidatesinthemomentuminterval6<pT<12 GeV/candforthe0–80%centralityclass.Thedashed curvesrepresentthefittothebackground,whilethesolidcurvesrepresentthetotalfitfunction.

Theinvariantmassdistributionsoftheselectedc andD⁰can- didatesareshowninFig.1.

Theprompt⁺_c (D⁰)productionyieldwascalculatedas

dN_prompt^+c(D0) dp_T

|^y|<0.5

=

¹ 2

1 c_y

1

p_T

f_prompt

·

^Nraw

|

|y|<0.8

(

Acc

× ε )

prompt

·

^BR

·

^Nevt

,

(1)

where N_raw is theraw yield(sum ofparticles andanti-particles) inthetransversemomentumintervalofwidthpT, fprompt isthe fractionofpromptc (D⁰)intherawyield,(Acc×

ε

)istheprod- uctofacceptanceandreconstructionefficiencyforpromptc(D⁰), BRisthebranching ratiooftheconsidereddecaymode andNevt isthenumberofeventsconsideredfortheanalysis.Thecorrection factorfortherapidity coveragec_y was computedastheratioof thegeneratedc (D⁰)yieldin|^y|<0.8 andthatin|^y|<0.5.The factor1/2 takesintoaccountthattherawyieldisthesumofpar- 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 were simulatedusingtheHIJINGv1.383 [28] generatorandpromptand feed-downc(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 p_T-dependentweightingfactor,calculatedastheratioofthemea- 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 obtainedastheweightedaverageoftheefficienciesinsmallercen- tralityclassesto take into account the variationof the efficiency andthe scaling ofthe yields of the c baryons and D⁰ mesons withcentrality.Theappliedweightswerecalculatedastheproduct ofthe R_AA oftheD⁰ andtheaveragenumberofnucleon–nucleon collisions (<Ncoll>) in the centrality class considered [18]. The (Acc×

ε

)valueisabout6%forpromptandabout9%forfeed-down c andabout8%forpromptandabout11%forfeed-downD⁰.

Thepromptc (D⁰)fraction, f_prompt,wascalculatedas

fprompt

=

1

−

⎛

⎝

^Nc(D

0) feed-down

N_prompt^c(D0)

⎞

⎠ =

=

1

−

TAA

·

^d2

σ

dydpT

^FONLL

feed-down

·

R^feed-down_AA

· (

Acc

× ε )

feed-down

·

^cy

·

p_T

·

^BR

·

^Nevt

N_raw

/

2

.

(2)

Thecontributionofc(D⁰)frombeauty-hadrondecayswases- timated usingtheFONLL [31,32] beauty-productioncrosssections as described in detail in Ref. [33]. The fraction ofbeauty quarks that fragment tobeautyhadronsandsubsequentlydecayintoc 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 was cal- 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 R^feed-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 /R^{prompt D}_AA ⁰=^{2,justifiedbytheCMS}measurementof J/ψ fromB-mesondecays [38] andbytheALICEandCMSmeasure- ments ofDmesons[18,39] indicatingthatprompt charmmesons are more suppressed than non-prompt charm mesons. The ra- tio is varied in the interval 1< Rfeed-down D⁰

AA /R^{prompt D}_AA ⁰ <3 to estimate the systematic uncertainty. Since no measurements of beauty-baryon productioninnucleus–nucleus collisions areavail- able,forthec thecentralhypothesiswastakenfrommodelcal- culations which predict R^feed-down_AA ^+c/R^prompt_AA ^+c =^{2 when con-} sidering c and b quark fragmentation and energy loss in the medium [40].TheratioR^feed-down_AA ^+c/R^prompt_AA ^+c wasdecomposed intotwotermstoestimatetheuncertaintyontheassumption:

R^feed-down_AA ^+c R^prompt_AA ^+c

=

^Rfeed-down D⁰ AA

R^{prompt D}_AA ⁰

·

(⁺c/D⁰)PbPb,feed-down

(⁺c/D⁰)pp,feed-down

(⁺_c/D⁰)_PbPb,prompt (⁺c/D⁰)pp,prompt

.

(3)

ThefirsttermisthesameasfortheD⁰ andthusthesamehy- pothesisisadopted.Thesecondtermisvariedintherange0.5–1.5

Fig. 2.⁺_c/D⁰ratioasafunctionofpTin0–80%mostcentralPb–Pbcollisionscomparedwiththemeasurementsinppandp–Pbcollisions [11] (left),andmodelcalcula- tions [7] (right).Statisticalandsystematicuncertaintiesarepresentedasverticalbarsandboxes,respectively.

Table 1

Systematic uncertainties on the corrected yields.Whentheuncertaintywasfoundto be<1%,itwasconsiderednegligible(negl.

inthetable).

Uncertainty ⁺_c D⁰

Raw-yield extraction 8% 2%

Tracking efficiency 3.6% 5%

PID 5% negl.

Cut variation 2% 5%

MCpTshape 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-posteriorisuchthatR^feed-down_AA ^+c <2 assuggestedbythe fact that nobaryon RAA exceeds thisvalue. The uncertainties on the two terms are added in quadrature. The resulting values of

f_promptareabout0.93and0.81forthec andD⁰,respectively.

Asummaryofthesystematicuncertaintiesonthecorrected⁺_c andD⁰ yieldsisshowninTable1.TheD⁰ systematicuncertainties ontheparticle identification(PID),trackingandcut variation are takenfromRef. [18] andarenotdiscussedinthefollowing.

Thesystematicuncertainty onthe raw-yieldextractionforc 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.

Forthec,thesystematicuncertaintyonthetrackingefficiency wasevaluatedbycomparingtheprobabilityofmatchingtracksre- constructedinthe TPCto ITShitsindataandsimulation andby varyingthe quality cutsto selectthetracks used intheanalysis.

Thecontributionduetothevariationofthequalitycutswaseval- uatedusingprotonsfromdecaysandaninclusiveK⁰_Ssampleand bycalculatingtheratioofthecorrectedyieldsobtainedusingdif- ferentselectioncriteria. The uncertaintyontheITS-TPC matching efficiencyisdefinedastherelativedifference ofthematchingef- ficiencyindataandsimulationsafterweightingtherelativeabun- 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.

Theuncertaintyresultingfromthesestudieswasaddedinquadra- turetotheuncertaintyonthetrackselection.

The systematicuncertainty onthec PID efficiencywas eval- uated using protons from the decay of baryons. The ratio of theyieldmeasuredwithPIDtothatmeasuredwithoutPIDwas calculated inboth dataandMC andtheir difference was usedto estimatethesystematicuncertainty.

Systematicuncertainties ontheefficienciescanalsoarisefrom possible differencesin the distributions and resolutions ofselec- tionvariables betweendataandsimulation.The systematiceffect induced by these imperfections was estimated by repeating the analysisvaryingthemainselectioncriteriaforthecandidates.The efficiencies determined from the simulations depend also on the generatedpTdistributionsofthec andtheD⁰.Thecentralvalues ofthecorrectionfactorswereobtainedbyre-weightingthecand D⁰ distributionsgeneratedby PYTHIAasdescribed above.Forthe D⁰,theefficienciescalculatedwithandwithoutthep_Tweightsare compatibleandthereforenouncertaintywasassigned.Forthec, the systematic uncertainty was defined by considering the vari- ation of the efficiencies determined with different generated p_T shapes. The newc 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^Ncoll^{and, forthe}c,us- ing asan alternative centralityweight theproduct /K⁰_S· ^Ncoll, wheretheratio/K⁰_SistakenfromRef. [8].

The systematic uncertainty on the subtraction of feed-down from beauty-hadron decays was estimated by varying (i) the p_T-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-downR_AAasdescribedabove.

Theproductionyields ofc andD⁰ alsohaveaglobalsystem- aticuncertaintyduetothebranchingratio.

3. Results

Theyieldofprompt⁺_c baryonsmeasuredinPb–Pbcollisions at√

sNN=⁵.02 TeV in the0–80%centralityclass in|^y|<0.5 and 6<p_T<12 GeV/cisN^+c =(2.1±⁰.4(stat.)⁺₋⁰₀^._.³₄(syst.))×^10−2.

The measured ⁺_c/D⁰ ratiois showninFig.2.The systematic uncertainty ofthe ⁺_c-baryon productionarising fromthe track-

Fig. 3.RAAofprompt⁺_c comparedwithmodelcalculations [7,15,16] (left),andthenon-strangeDmesons,D⁺_s,andchargedparticle RAAin0–10%mostcentralPb–Pb collisionsforpT>1 GeV/c[18,42] (right).Statistical,systematicandnormalisationuncertaintiesarepresentedasverticalbars,emptyboxesandshadedboxesaroundunity, respectively.

ing efficiency was treated as fully correlated to that of the D⁰ meson. The contributionto the feed-downuncertainty relatedto heavy-quarkenergylossandthat originatingfromthe FONLLun- 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–Pb collisionsishigherthanthat measuredinppandp–Pbcollisions.

In particular, the values in p–Pb and Pb–Pb collisions differ by abouttwostandarddeviationsofthecombinedstatisticalandsys- tematicuncertaintiesin6<pT<12 GeV/c.

The ⁺_c/D⁰ ratio in Pb–Pb collisions is compared with theo- 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-spacedistribu- tionsandthehadronwavefunction.The widthparameters ofthe hadronwavefunctionsarecalculatedfromthechargeradiusofthe hadronsaccordingtothe quarkmodel.Thehadronwave function normalisationisdeterminedbyrequiringatotalcoalescenceprob- abilityforcharmquarksequaltounityforzero-momentumheavy quarks.Moreover,thecontributionsfromthefirstexcitedstatesfor Dandc hadronswere included inthecalculations. The experi- mental results are described by the model calculation including coalescenceonly.Thecurveobtainedbymodellingcharmhadroni- 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/D⁰ ra-

tio because the relative production of single-charm baryons and single-charm mesonsRBM istreatedasaparameter ofthemodel.

ThecurveobtainedbyconsideringRBM=0.425,whichisthevalue neededtodescribetheresultsinppandp–Pbcollisions,underesti- matesthe⁺_c/D⁰ratiomeasuredinPb–Pbcollisions.AnRBM=¹.2 is 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 verified by 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] scaled by1/A(A=^{208)andcorrectedforthedifferentrapiditycoverage} ofthep–Pbmeasurement.Thecrosssectionmeasuredinp–Pbwas scaledineach pTintervalto|^y|<0.5 usingacorrectionfactorob- 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 R_AA was motivatedbythefactthat itwasmeasured up to pT=^{12 GeV}/c, whilethe measurement inpp collisions at

√s=^{7 TeV in} |^y|<0.5 only reaches p_T=^{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/cwasobtainedbycombiningthe results in the transverse momentum intervals 6<pT<8 GeV/c and8<p_T<12 GeV/c.The uncertainties werepropagated treat- ing the statistical and the systematic uncertainties on the yield extractionasuncorrelatedandtheothersourcesofsystematicun- 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 in}quadrature.In the leftpanel ofFig. 3,the R_AA ofprompt ⁺_c iscomparedwith Catania model calculations [7]. The threecurves are obtained by considering differenttreatments ofthehadronisation mechanisms inppandPb–Pbcollisions.Theshort-dashedcurverepresentsthe ⁺_c RAA asobtainedbyincludingbothvacuumfragmentationand quarkcoalescenceforcharmhadronisationinPb–Pbandonlyfrag- 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 p_T 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 R_AA 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 2

σ

of the combinedstatistical and systematicuncer- taintiesup topT=^{8 GeV}/c,whiletheyarecompatiblewithin1

σ

forpT>10 GeV/c.The RAA valuesofD⁺_s mesonsare largerthan those of non-strange D mesons, but the two measurements are compatiblewithinone standarddeviationofthecombineduncer- tainties [18].Ahintofalarger⁺_c RAAwithrespecttonon-strange Dmesons isobserved,althoughthe resultsarecompared fordif- ferentcentralityclasses.AD⁰ R_AA=⁰.27±⁰.01(stat.)±⁰.04(syst.) wasmeasuredin6<pT<12 GeV/c inthe0–80%centralityclass.

TheD⁰ RAAhasalsoa3.5%uncertaintyarisingfromthenormalisa- tionofthecrosssectionmeasuredinppcollisions at√

s=^{7 TeV,} and a 2.4% uncertainty on the average nuclear overlap function ^TAA^{.The p}T-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.Theothercontributionstothesystem- aticuncertaintywereconsideredasfullycorrelatedamongthe pT intervals. A difference of about 1.7

σ

is obtained when compar- ing the ⁺_c R_AA with that of the D⁰ in 6<p_T<12 GeV/c and 0–80%centralityinterval.Thisobservationisqualitativelyinagree- 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 TeV was 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–Pb collisions 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-mesonproductionwithre- 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 totheconstruc- tionoftheexperimentandtheCERNacceleratorteamsfortheout- 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 ofSci- 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;MinistryofSci- ence&TechnologyofChina(MSTC),NationalNaturalScienceFoun- 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) andInstitutNationaldePhysiqueNucléaireetdePhysiquedesPar- 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,ResearchandRe- ligions, Greece; National Research, Development and Innovation Office,Hungary;DepartmentofAtomicEnergyGovernmentofIn- 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