Production of Λc + baryons in proton-proton and lead-lead collisions at √sNN=5.02 TeV

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Physics Letters B

www.elsevier.com/locate/physletb

Production of  ⁺ _c baryons in proton-proton and lead-lead collisions at

√ s

_NN

= ⁵ . 02 TeV

.The CMS Collaboration

^

CERN,Switzerland

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

Articlehistory:

Received7June2019

Receivedinrevisedform17February2020 Accepted18February2020

Availableonline25February2020 Editor: M.Doser

Keywords:

CMS Physics

Lambda(c)baryons Nuclearmodificationfactor Heavyflavor

The transverse momentum (pT) spectra of inclusively produced ⁺_c baryons are measured via the exclusivedecaychannel⁺_c →^pK−π⁺usingtheCMSdetectorattheLHC.Spectraaremeasuredasa functionoftransversemomentuminproton-proton(pp)and lead-lead(PbPb)collisionsatanucleon- nucleon center-of-mass energy of 5.02 TeV. The measurement is performed within the ⁺_c rapidity interval|^y|<1 inthe pT rangeof5–20 GeV/c inpp and10–20 GeV/c inPbPb collisions.The observed yields of⁺_c for pT of10–20 GeV/c suggesta suppression incentral PbPb collisionscompared topp collisions scaledbythenumber ofnucleon-nucleon(NN)interactions. The⁺_c/D⁰ productionratioin pp collisionsiscomparedtotheoreticalmodels.InPbPb collisions,thisratioisconsistentwiththeresult frompp collisionsintheircommonpTrange.

©^{2020TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense} (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP³.

1. Introduction

Measurements of heavy-quark production provide unique in- puts in understanding the parton energy loss and the degree of thermalization in the quark-gluon plasma (QGP) [1] formed in high energy heavy ion collisions. Compared to light quarks, dif- ferentenergy loss mechanisms [2] are expectedto dominatethe interactionbetweenheavyquarksandthemedium.Besidesthein- mediuminteractions,adetailedstudyofthehadronizationprocess iscriticalfortheinterpretation ofexperimental data.Inrelativis- ticheavy ion collisions, in addition tothe fragmentation process present in proton-proton (pp) collisions, hadron production can alsooccurviacoalescence,wherepartonscombinewitheachother whiletraversingtheQGPmediumoratthephaseboundary [3,4].

At high transverse momentum (pT^{6 GeV/c), the} probability of coalescence is reduced, and therefore the hadronization process is expected to be dominated by fragmentation. In the interme- diate p_T region (2^pT^{6 GeV/c), a significant} enhancement of thebaryon-to-meson ratioisobserved inheavy ioncollisions for hadronswithup,down,orstrangequarks [5,6].Thisenhancement, anditsdependenceoncentrality(i.e.,thedegreeofoverlapofthe two colliding nuclei) can be explained in a scenario with had- ronizationvia coalescence. Furthermore,elliptic flow, the second Fouriercomponent of the azimuthal distribution of emitted par- ticles, is found to roughly scale with the number of constituent

 E-mailaddress:cms-publication-committee-chair@cern.ch.

quarks in the pT range of 2–5 GeV/c at RHIC [7], an observation whichisalsoconsistentwiththeexpectationforcoalescence.

A significant contribution of coalescence to the hadronization of charm quarks fromthe QGP medium is supported by various measurementsofcharmoniumandopencharmproductionatRHIC andLHCenergies [8–16].Onesuchobservableisthenuclearmod- ification factor, RAA, which isthe ratio ofthe yield in heavy ion collisionstothatinpp collisionsscaledbythenumberofnucleon- nucleon (NN)interactions. At RHIC,the RAA forJ/ψ mesonswith pT≤7 GeV/c produced in AuAu collisions decreases significantly fromperipheraltocentralcollisions [8].Incontrast,inhigheren- ergy PbPb collisionsatthe LHC,the J/ψ R_AA has amuch smaller centralitydependence [9,10].ThedifferencebetweentheAuAu and PbPb resultscan beexplained bya larger coalescenceprobability inPbPb collisionsbecauseofthelargernumberofproducedcharm andanti-charmquarksatthehighercenter-of-massenergy.ForD⁰ mesonproductioninAuAu collisions, RAA isobservedtoincrease withpT upto1.5 GeV/c anddecreasewithpTfrom2to6 GeV/c,an effectthatcanbequalitativelyreproducedbymodelsinvolvingco- alescence [11,12].AttheLHC,themeasurementsofD⁰ R_AAandD⁰ azimuthalanisotropy [13–16] arewellexplainedbymodelsinvolv- ing coalescence. The relative coalescence contribution to baryon productionisexpectedtobemoresignificantthanformesonsbe- cause oftheir larger number of constituentquarks. In particular, modelsinvolvingcoalescenceofcharmandlight-flavorquarkspre- dictalargeenhancementinthe⁺_c/D⁰ productionratioinheavy ioncollisionsrelativetopp collisionsandalsopredictthattheen- hancement has a strong p_T dependence [17–20]. Comparison of

https://doi.org/10.1016/j.physletb.2020.135328

0370-2693/©^{2020TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense}(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP³.

⁺_c baryon productionin pp and lead-lead (PbPb)collisions can thus shed new light on understanding heavy-quark transport in the medium and heavy-quark hadronization via coalescence. All discussions of⁺_c andD⁰ alsoincludethe corresponding charge conjugatestates.

Recently, the productionof ⁺_c baryons fora variety ofcolli- sion configurations has been measured in a similar pT range by theLHC experimentsALICE andLHCbinthe central andforward rapidityregions,respectively [21–24].Bothexperimentsmeasured the⁺_c pT-differentialcrosssectionsinpp collisionsata center- of-massenergy of √

s=^{7TeV and compared them to} theoretical predictionsusingthenext-to-leadingorderGeneralizedMassVari- ableFlavorNumberScheme [25].TheLHCbresultsfortherapidity range2.0<y<4.5 werefoundtobecompatiblewiththeory [23], whiletheALICEvaluesfor|^y|<0.5 were largerthanthe predic- tions [21].TheALICEexperimentalsoreported⁺_c/D⁰ production ratios in7 TeV pp collisions, aswell asin proton-lead(pPb) and PbPb collisionsatanNNcenter-of-massenergyof√

s_NN=⁵.02TeV.

TheALICEratiosfrompp andpPbcollisions [21] werefoundtobe above the corresponding LHCb values [24] (however in different rapidity ranges), withthelatter agreeing withtheoretical predic- tions.The ALICE⁺_c/D⁰ productionratiofor6<pT<12 GeV/c in PbPb collisionswasmeasuredtobelargerthaninpp andpPbcol- lisions [22], and this difference can be described using a model involvingonlycoalescence inhadronization [20].The ALICEmea- surementsof the R_AA of ⁺_c baryonsinpPb andPbPb collisions were found to be compatiblewithunity andless thanunity, re- spectively, buthave limitedpower to constrain models owing to largeuncertainties [21,22].

Inthisletter,we reportmeasurementsofinclusive⁺_c baryon productionin pp and PbPb collisionsathigh pT where inclusive refers tobothprompt (directlyproduced incharmquark hadron- ization orfrom strong decays of excited charmed hadron states) andnonprompt(fromb hadrondecays)production.Thedatawere collectedat√

s_NN=⁵.02TeV in2015usingtheCMSdetector.The

⁺_c baryonsare reconstructed in thecentral region (|^y|<1) via thehadronicdecaychannel⁺_c →^pK−π⁺.The pT spectrumand

⁺_c/D⁰ productionratioaremeasuredinthe pT ranges5–20and 10–20 GeV/c in pp and PbPb collisions, respectively. The ⁺_c/D⁰ production ratios use the corresponding CMS measurements of promptD⁰ production [14].Centrality binsforPbPb collisionsare giveninpercentagerangesofthetotalinelastichadroniccrosssec- tion, with the 0–30% centrality bin corresponding to the 30% of collisionshavingthe largestoverlapofthetwo nuclei.The values of RAA areobtainedforthree centralityintervals:0–100%,0–30%, and30–100%.

2. TheCMSdetector

The central feature of the CMS apparatus is a superconduct- ing solenoidof 6 m internal diameter, providinga magnetic field of3.8 T. Withinthe solenoidvolume are a silicon tracker,a lead tungstatecrystalelectromagneticcalorimeter,andabrassandscin- tillator hadron calorimeter, each composed of a barrel and two endcapsections.Thetrackermeasureschargedparticleswithinthe pseudorapidity range |

η

_|<2.5 and the calorimeters record de- posited energy forparticles with |

η

|<3.0. Two forward hadron (HF) calorimeters use steel as an absorber and quartz fibers as thesensitivematerial.ThetwoHFcalorimetersarelocated11.2 m fromthe interaction region, one on each end, and together they extendthecalorimetercoveragefrom|

η

_|=3.0 to5.2.EachHFcal- orimeterconsistsof432readouttowers,containinglongandshort quartz fibersrunning parallelto thebeam, providing information on the shower energy and the relative contribution originating fromhadronsversuselectronsandphotons.Adetaileddescription oftheCMSexperimentcanbefoundinRef. [26].

3. Eventreconstructionandsimulatedsamples

The total transverseenergy depositedin bothHF calorimeters isusedtodeterminethecollisioncentralityinPbPb collisionsand was utilized by the triggers for both data sets included in this analysis [27]. Onetrigger selected minimum-bias(MB) eventsby requiring transverse energy deposits in one (both) HF calorime- tersaboveapproximately1 GeV forpp (PbPb)collisions.Asnotall MBeventscouldbesaved,anadditionaltriggerselectedthemore peripheralcentralityregionof30–100%forPbPb events.Theinte- gratedluminositiesofpp collisions,PbPb collisionswithcentrality 0–100%,andPbPb collisionswithcentrality30–100%are38 nb⁻¹, 44

μ

b⁻¹,and102

μ

b⁻¹,respectively.

The trackreconstruction algorithms usedin thisstudy for pp and PbPb collisions are described in Refs. [28] and [29], respec- tively.InPbPb collisions,minormodificationsaremadetothepp reconstructionalgorithminordertoaccommodatethemuchlarger trackmultiplicities.TracksarerequiredtohavearelativepTuncer- taintyoflessthan30%inPbPb collisionsand10%inpp collisions.

In PbPb collisions,tracksmustalsohaveatleast11hitsandsat- isfyastringentfitqualityrequirement,specificallythatthe

χ

² per degree offreedom belessthan 0.15timesthenumberoftracker layerswithahit.

Fortheoffline analysis,eventsmustpassselection criteriade- signed to rejectevents frombackgroundprocesses (beam-gasin- teractions and nonhadronic collisions), as described in Ref. [29].

Events are required to have at least one reconstructed primary interactionvertex [28] withadistancefromthecenterofthenom- inalinteractionregion oflessthan 15 cm alongthebeamaxis.In addition,inPbPb collisions,theshapesoftheclustersinthepixel detectorhavetobecompatiblewiththoseexpectedfromparticles produced at the primary vertex location [30]. The PbPb collision eventsare alsorequiredto haveatleastthreetowers ineach HF detectorwithenergydepositsofmorethan3GeVpertower.These criteriaselect(99±²)% ofinelastichadronicPbPb collisions.Frac- tions above 100%reflect thepossiblepresence ofultra-peripheral (nonhadronic)collisionsintheselectedeventsample.

MonteCarlo(MC)simulatedeventsamplesareusedtooptimize theselectioncriteria,calculatetheacceptancetimesefficiency,and estimate thesystematicuncertainties.Proton-protoncollisions are generated with pythia 8.212 [31] tune CUETP8M1 [32], hereafter referred toas pythia 8, andincludesbothpromptandnonprompt

⁺_c baryonevents.ForthePbPb MCsamples,each pythia 8event containing a ⁺_c baryon isembedded intoa PbPb collisionevent generatedwithhydjet1.8 [33],whichistunedtoreproduceglobal eventpropertiessuchasthecharged-hadronp_Tspectrumandpar- ticlemultiplicity.The⁺_c →^pK−π⁺decayisperformedbyevtgen 1.3.0 [34] throughfoursub-channels:⁺_c →^pK∗(892)⁰→^pK−π⁺,

⁺_c → (¹²³²)⁺⁺K⁻→^pK−π⁺, ⁺_c → (¹⁵²⁰)π⁺→^pK−π⁺, and ⁺_c →^pK−π⁺ (nonresonant), with no modeling of inter- ference between the sub-channels. All particles are propagated throughtheCMSdetectorusingthe Geant4package [35].

4. Signalextraction

The ⁺_c →^pK−π⁺ candidates are reconstructed by selecting three charged tracks with |

η

_|<1.2 and a net charge of +^{1. All} tracks must have p_T>0.7(1.0)GeV/c for pp (PbPb) events.Dur- ing the invariant mass reconstruction, both possibilities for the mass assignments of the same-sign tracks are considered, while thekaonmassisassignedtotheopposite-signedtrack.Usingsim- ulated events, the incorrectassignment was found to produce a broad distributioninthe invariant mass(about30 timesthesig- nal width) andis indistinguishable fromthe combinatorialback- ground.

Fig. 1. Invariantmassdistributionof⁺_c candidateswith pT=5–6 GeV/c (left),10–20 GeV/c (middle)inpp collisions,and pT=10–20 GeV/c inPbPb collisionswithinthe centralityrange0–100%(right).Thesolidlinerepresentsthefullfitandthedashedlinerepresentsthebackgroundcomponent.

Astheeventmultiplicities forpp andPbPb collisionsare sub- stantiallydifferent,theselectioncriteriawereoptimizedseparately.

Intheoptimization,simulatedeventsinwhichareconstructed⁺_c candidateis matchedto a generated ⁺_c baryon are used asthe signal sample, and data events from the mass sideband region are used as the background sample. Requirements are made on three topological and three kinematic variables. The three topo- logical criteria are: the

χ

² probability of the vertex fit to the threechargedtracks makingup the ⁺_c candidate,the anglebe- tween the ⁺_c candidate momentum and the vector connecting the production anddecay vertices in radians (

α

), and the sepa- ration between the two vertices. While more than one collision per bunch crossing is rare in PbPb collisions, it is common in pp collisions. Therefore, two-dimensional variables in the trans- verseplanewithrespecttothebeamlineareusedfor

α

anddecay lengthinpp collisions,whilethree-dimensionalvariableswithre- spectto the primary vertexare used for PbPb collisions. Forthe PbPb events,thetopologicalrequirementsare

χ

²probabilityabove 20%,

α

<0.1,anddecaylengthgreaterthan3.75

σ

,where

σ

isthe uncertaintyintheseparation.Forpp events,thecorrespondingre- quirementsare

χ

²probabilityabove8%,

α

<0.4,anddecaylength greaterthan2.25

σ

.Thekinematicrequirementsarekaon(proton) pTdividedbythe⁺_c candidatepT greaterthan0.14(0.28)forall eventsandpion p_T divided bythe⁺_c candidatep_T greaterthan 0.12forPbPb events.

The ⁺_c baryon yields in each pT interval are obtained from unbinnedmaximumlikelihoodfits totheinvariantmassdistribu- tionintherangeof2.11–2.45 GeV/c².The signalshapeismodeled by the sumof two Gaussian functionswiththe same mean,but differentwidthsthat are fixed onthe basis ofthesimulated sig- nalsample.Onefitparameterscalesbothwidthstoaccommodate a potential difference inthe mass resolution betweensimulation anddata,withtheexceptionofthelowest pTregion(5–6 GeV/c)in thepp data,wherethisparameterwas found tocauseinstability inthefitandtheunmodifiedmassresolutionfromthesimulation was used.The background ismodeled witha third-order Cheby- shevpolynomial.Representativeinvariantmassdistributionsinpp andPbPb collisionsareshowninFig.1.

The⁺_c baryondifferentialcrosssectioninpp collisionsisde- finedas:

d

σ

pp^+c

dp_T

 



|^y|<¹

=

¹

2

L

^pT

B

N^_pp^+c

|

|y|<1

A

^,

⁽¹⁾

where N^_pp^+c||y|<1 is the ⁺_c yield extractedin each p_T bin,L ^is theintegratedluminosity, pT isthe widthofeach pT bin,B^is thebranchingfractionofthedecay,and A

istheproductofthe acceptanceandefficiency.Thefactorof1/2accountsforaveraging

Table 1

Summaryofthe^Ncoll^,^TAA^,and^Npart^{valuesforthree}PbPb centralityranges.

Centrality ^TAA[^mb−1] ^Npart ^Ncoll 0–30% 15.41⁺₋⁰₀^._.³³₄₇ 270.7⁺₋³₃^._.²₄ 1079⁺₋⁷⁴₇₈ 30–100% 1.41⁺₋⁰₀^._.⁰⁹₀₆ 46.8⁺₋²₁^._.⁴₂ 98⁺₋⁸₆ 0–100% 5.61⁺₋⁰₀^._.¹⁶₁₉ 114.0⁺₋²₂^._.⁶₆ 393⁺₋²⁶₂₈

theparticleandantiparticlecontributions.Thenormalized⁺_c p_T spectruminPbPb collisionsisdefinedas:

1



^TAA



dN^_PbPb^+c

dpT

 



_|

y|<1

=

¹



^TAA



1 2Nevents



pT

B

N^_PbPb^+c

|

|y|<1

A

^,

⁽²⁾

where Nevents isthe number of MB eventsused for the analysis (correctedbythe99%selectionefficiency)and^TAA^isthenuclear overlapfunction,whichisequaltotheaveragenumberofNNbi- nary collisions (^Ncoll^{) divided by theNNinelastic crosssection,} andcan be interpreted asthe NN-equivalentintegrated luminos- ity per heavy ion collision. The values of ^TAA^, ^Ncoll^{, and the} average numberof participatingnucleons (^Npart^{) are calculated} usingaMonteCarloGlaubermodel [36],inwhichtheNNinelastic crosssection(70 mb)isusedasaninput parameter.Theaverages ofthese quantitiesover theevents inthe givencentralityranges arelistedinTable1.

ThenuclearmodificationfactorRAAiscomputedas:

R_AA

(

p_T

) =

¹



^TAA



dN^_PbPb^+c

dp_T



d

σ

_pp^+c

dp_T

.

(3)

The values of A

are obtained from MC simulation as a frac- tion in which the denominator is the number of generated ⁺_c baryons with |^y|<1 and the numerator is the number of re- constructed ⁺_c candidates that pass the selection criteria and are matched to a generated ⁺_c baryon. The simulation includes both prompt andnonprompt ⁺_c baryonsestimated from pythia 8 andcontains an appropriately weighted combinationof decays in the four known sub-channels. For the pp simulation, the pT spectrum of the generated ⁺_c baryons is weighted to match a fit to the observed data (iterating until convergence is reached).

For pp collisions, A

increases from 7 to 19% as pT increases.

As the PbPb results are given for just one p_T range, an alter- native method is used to correct the pT spectra in simulation.

Under the transverse mass scaling hypothesis (mT scaling) [37], the ⁺_c baryon p_T spectrum is obtained for the 0–100% cen- tralityregion fromthe D⁰ measurements [14] usingthefunction m²(⁺_c)+^p2_T(⁺_c)=^m2(D⁰)+^p2_T(D⁰).ForthePbPb data set,the

centrality distribution in simulation is reweighted to match the data.Thereisoneadditionalcorrectionappliedto A

forthePbPb data set. Previous CMS results have found more suppression for promptthannonpromptD⁰mesons [14,38],whichcanbequanti- fiedfor10<p_T<20 GeV/c asR^nonprompt_AA /R^prompt_AA =¹.66±⁰.38.As nonpromptbaryonstendtohavegreaterpTanddecayfartherfrom thecollision point than promptbaryons,the requirementforthe decaylengthsignificanceresultsinavalueof A

thatislargerfor nonpromptbaryons. Changingthenonprompt fractionto account forthedifferentsuppression increases A

by 15%. After applying thecorrections, A

=^{5% for}PbPb collisions.

5. Systematicuncertainties

Systematic uncertainties arise from the extraction of the raw signal yield, the ability of the MC simulation to reproduce the combinedacceptanceandefficiency,thebranching fractionofthe decaymode,andtheintegratedluminosity.Unlessotherwiseindi- cated,systematicuncertainties are combined by addingthe indi- vidualcontributionsinquadrature.

The systematicuncertainty inthe signal yields is obtainedby varying the modeling functionsthat are used for the signal and background contributions. The background function is changed from the default third- to second- and fourth-order Chebyshev polynomials,withthemaximumdifferenceinyieldbetweenthese twoalternativefunctionsandthedefaultfitfunctiontakenasthe systematicuncertainty. Thisamounts to 4–10% and7–9% forpp indifferent pT binsandPbPb collisionsinthreecentralityclasses, respectively.The defaultsignal modelfunctionis thesumoftwo Gaussian functions with parameters chosen as described in Sec- tion 4.Forthe pp (PbPb) collision data,the alternative modelis a triple (single) Gaussian function with similar procedures used fortheparameters. Asthe signal widthisfixed foreventsinthe lowest⁺_c pTbinforpp collisions,anadditionalsystematicuncer- taintyisassessedbyvaryingthewidthby±^40%,correspondingto themaximumdeviationswithrespecttothesimulationobserved inother pT binsinpp andPbPb collisions.Theuncertaintydueto themodelingofthesignalis3–28%forpp collisionsand2–4%for PbPb collisions.

FivesourcesofsystematicuncertaintiesassociatedwiththeMC modelingofthedataareevaluated.Thefirstuncertaintymeasures the effect of the selection criteria variation. We define a double ratioas:

DR =

^{NData(varied)} N_Data(nominal)



N_MC(varied)

N_MC(nominal)

,

(4)

where NData(nominal) and NData(varied) are the yields obtained fromdata using thedefault andalternative selection criteria, re- spectively,andN_MC(nominal) and N_MC(varied) arethecorrespond- ing yields fromthe simulatedevents.Foreach of thetopological selection criteria, the doubleratio is evaluated atmany different values of the selection criterion. The specific ranges for pp col- lision eventsare >1.5

σ

to >6

σ

, >5% to >45%, and <0.1 tono cut for decay length, vertex fit probability, and

α

, respectively.

The correspondingranges forPbPb collision eventsare >2.5

σ

to

>8

σ

,>5% to>45%, and<0.05 to <0.2.Forallbutthe

α

cut in PbPb collisions,DR^{isplottedasafunctionoftheselectionvalue} and fit to a linear function. The systematic uncertainty is taken as the difference between unity and the value ofthe fitted line atthepoint wherenoselectionisapplied.Forthe

α

requirement inPbPb collisions,thesystematicuncertaintyisobtainedfromthe biggestdifferencesbetweenunityandthevalueofDR fromallof thealternativeselectionvalues.Combiningtheresultsofthethree topologicalselectioncriteriasystematicuncertaintiesinquadrature resultsinuncertaintiesof6% forthe pp datasetand19% forthe PbPb datasets.

The second uncertainty arises from a potential mismodeling of the pT distribution of⁺_c baryonsbecause A

isstrongly de- pendent on the⁺_c pT. In pp collisions,the default pT shape is derived from the data. ForPbPb collisions, the default pT shape is obtained from mT scaling of the measured D⁰ pT spectrum.

For each dataset, two alternative pT spectra,one from pythia 8 andonefrom pythia 8withcolorreconnection(describedinSec- tion6)areconsideredandthemaximumdeviationin A

istaken asthesystematicuncertainty.Theresultingsystematicuncertainty is0–3%forpp collisionsand5.2%forPbPb collisions.

The third uncertainty arisesfrom impreciseknowledge of the resonantsubstructureofthepK⁻π⁺decaymode [39].Thecalcula- tionofA

usestheappropriatelyweightedsumofthefourknown sub-channelsandthesystematicuncertaintyassociatedwiththisis evaluated by determining A

foreach sub-channelandrandomly adjustingtheweightsbytheuncertaintiesofeachbranchingfrac- tion. Theindividual values of A

vary by about±30% relative to theaverage.Thesystematicuncertaintyisobtainedfromthestan- darddeviationofaGaussianfittothedifferentaverage A

values andis8%forbothpp andPbPb events.

ThefourthuncertaintyassociatedwiththeMCmodelingofthe dataisthetrackreconstructionefficiency,whichis4%forpp colli- sions [14] and5%forPbPb collisions [40].Astherearethreetracks inthe⁺_c decay,thecorrespondinguncertaintiesonthemeasured pT spectra are 12 and 15% for pp and PbPb, respectively, while for the ⁺_c/D⁰ productionratio, the uncertainties are 4 and 5%, respectively.

The fifth uncertaintyarises frompossible mismodeling of the nonpromptcomponent, namely⁺_c fromb hadrondecays,inthe inclusive ⁺_c sample. The inclusive A

is the weighted sum of prompt and nonprompt A

according to the prompt and non- promptfractions.Asfoundusingthestandard pythia 8MCsample, the nonprompt A

isgenerally3–4timeslargerthan theprompt A

and so an incorrect nonprompt fraction in pythia 8 will re- sultin an incorrect A

for theinclusivesample. To evaluatethis systematic uncertainty, an alternative method is used to obtain the final resultthat doesnot rely onthe pythia 8 predictionfor thenonpromptfraction.Agenerator-only pythia 8sampleofnon- prompt ⁺_c events is reweighted to matchthe p_T-differential b hadroncrosssectionfromafixed-orderplusnext-to-leadingloga- rithm (FONLL) calculation [41].The resulting pT-differentialcross section for nonprompt ⁺_c baryons is multiplied by the appro- priate luminosity,branchingfractions,and A

fornonprompt⁺_c eventstoobtainanestimateofthenumberofreconstructednon- prompt⁺_c baryonsineachpTbin.Subtractingthisvaluefromthe measurednumberofreconstructed⁺_c baryonsgivesthenumber ofreconstructedprompt⁺_c baryons.Thesereconstructedprompt yields are then corrected using the prompt A

aswell aslumi- nosityandbranchingfractionstoestimatethe p_T-differentialcross sectionforprompt⁺_c baryons.Finally,thetwocrosssectionsgive an alternativeestimate ofthenonpromptfractionineach pT bin, and therefore an alternative estimate of the weighted inclusive A

value. The systematic uncertainty is taken as the difference between the nominal and alternative A

values.The nonprompt fractionforeventspassingthepp selectioncriteriaisfoundtobe 28–34%forthenominalscenario(pythia 8only)and4–7%forthe alternative method,withhighervaluesassociatedwithlarger ⁺_c pT. The resultingsystematicuncertaintyvaries byonly ±1% as a function of pT soan averagevalue of18%isusedforall pT bins.

The samemethodisapplied tothePbPb dataset,wherethesys- tematic uncertainty is found to be 25% as a result of the more stringent selectioncriteria. For PbPb collisions, an additionalsys- tematic uncertaintyis assessed by takingthe difference between applying and not applying the correction for different values of R_AA fornonpromptandprompt ⁺_c baryonsasdiscussedin Sec- tion4,raisingthesystematicuncertaintyto29%.

The overall ⁺_c →^pK−π⁺ branching fraction uncertainty is 5.3% [39]. The uncertainties in the integrated luminosity in pp collisions and the MB selection efficiency in PbPb collisions are 2.3% [42] and2.0% [29],respectively.The uncertaintiesin TAA are listedinTable1.

Forthemeasurementofthe p_Tspectra,theuncertainties asso- ciatedwiththe⁺_c →^pK−π⁺branchingfractionandsubresonant contributions,theluminosityandMBselectionefficiency,andthe nonprompt fraction contribute only to the overall normalization andarelabeledglobaluncertainties.Addingthesecontributionsin quadratureyields globaluncertaintiesof21% (31%)forpp (PbPb) collisions. In measuring the nuclear modification factor RAA, the uncertaintiesassociated withthebranching fractionandsubreso- nantcontributionscancelandthenonpromptfractionuncertainty partially cancels. In calculating the ⁺_c/D⁰ production ratio, the uncertaintiesassociated withD⁰ fromtheyield extraction,selec- tioncriteriaefficiency, and pT shape are obtainedfromRef. [14], while the uncertainties in the integrated luminosity in pp colli- sionsandtheMBselectionefficiencyinPbPb collisionscancel.

6. Resultsanddiscussion

Fig.2 shows the pT-differential crosssection ofinclusive ⁺_c baryon production in pp collisions for the range of 5<pT<

20 GeV/c andtheTAA-scaledyieldsinPbPb collisionsfortherange of10<p_T<20 GeV/c, forthree centrality classes.The 21% (31%) normalizationuncertaintyforthepp (PbPb)resultsisnotincluded in the boxes representing the systematic uncertainties for each datapoint.Whiletheshapeofthe p_T distributioninpp collisions isconsistentwiththeinclusiveproductioncalculationfrom pythia 8 using tune CUETP8M1 and activating the “SoftQCD:nondiffrac- tive”processes,thedataaresystematicallyhigher.Thehadroniza- tionin pythia 8 can be modified by addinga colorreconnection (CR)mechanisminwhichthefinalpartonsinthestringfragmen- tation are considered to be color connected in such a way that thetotalstringlengthbecomesasshortaspossible [43]. Thecal- culationsusingthe recommendedcolorreconnectionmodelfrom Ref. [43] are consistent with our p_T-differential cross section in pp collisions.The pT-differential cross section inpp collisions is alsocomparedtotheGM-VFNSperturbativeQCDcalculations [44], whichincludesonlyprompt⁺_c baryonproduction.TheGM-VFNS predictionissignificantlybelowourdataforpT<10 GeV/c,similar tothedifferencefoundbyALICE [21]. pythia 8predictsthat8–15%

of generated ⁺_c baryons arise from b hadrons, with the low (high)valuecorrespondingtothe⁺_c pTinterval5<pT<6 GeV/c (10<pT<20 GeV/c).Therefore,accountingfortheeffectsofnon- prompt⁺_c productionwill onlymarginally reduce thedisagree- mentwiththeGM-VFNSprediction.

Thenuclear modification factor RAA forinclusive ⁺_c baryons in the p_T range 10–20 GeV/c is shown in Fig. 3 as a function of the numberof participating nucleons ^Npart ^for PbPb collisions.

The resultssuggest that ⁺_c is suppressed in PbPb collisions for p_T>10 GeV/c,butnoconclusioncanbedrawnbecauseofthelarge uncertainties.ThedifferenceinRAAvaluesbetweenthe0–30%and 30–100%centralityrangesisconsistentwithanenhancedsuppres- sioninthemorecentralPbPb collisions.

Fig.4showsthe⁺_c/D⁰productionratioasafunctionofpTfor pp collisions andPbPb collisions inthe centrality range0–100%.

Theproductionratiofound frompp collisionsissimilar inshape versus pT butaboutthreetimeslargerinmagnitudecomparedto the calculation from pythia 8.212tune CUETP8M1. Results using theMonash2013 [45] tunearefoundtobe consistentwiththose fromtheCUETP8M1tune.Besidesprovidingareasonabledescrip- tionof⁺_c baryon pT-differentialcrosssections,Fig.4showsthat calculationsusinga colorreconnection modelareconsistentwith ourresultsforthe⁺_c/D⁰ productionratioinpp collisions.

Fig. 2. The pT-differentialcrosssectionsforinclusive⁺c productioninpp colli- sionsandtheTAA-scaledyieldsforthreecentralityregionsofPbPb collisions.The boxesanderrorbarsrepresentthesystematicandstatisticaluncertainties,respec- tively.ThePbPb datapointsareshiftedinthehorizontalaxisforclarity.Predictions forpp collisionsaredisplayedfor pythia 8withtheCUETP8M1tune(opencrosses), pythia8withcolorreconnection [43] (openstars),andGM-VFNS [44] (opencircles labeled“JHEP12(2017)021”)alongwithratiostothedatainthelowertwopanels.

The pythia 8(GM-VFNS)predictionsareforinclusive(prompt)⁺_c production.The errorbarsontheGM-VFNSpredictionaccountforthescalevariationuncertainty.

Thelowerpanelsshowthedata-to-predictionratioforpp collisionswithinnerand outererrorbarscorrespondingtothestatisticalandtotaluncertaintyinthedata, respectively,andtheshadedboxatunityindicatingthe21%normalizationuncer- tainty.TheshadedboxesinthebottompanelrepresenttheGM-VFNSuncertainty.

Fig. 3. ThenuclearmodificationfactorRAAversus^Npart^forinclusive⁺_c produc- tion.TheerrorbarsrepresentthePbPb yieldstatisticaluncertainties.Theboxesat eachpointincludethePbPb systematicuncertaintiesassociatedwiththesignalex- traction,pTspectrum,selectioncriteria,trackreconstruction,andTAA.Thebandat unitylabeledpp uncertaintyincludesthesesameuncertaintiesforthepp data(ex- ceptforTAA)plustheuncertaintiesinpp yieldandluminosity.Thebandatunity labeledPbPb includestheuncertaintyfromthenonpromptfraction(accountingfor apartialcancelationbetweenpp andPbPb)andMBselectionefficiency.

Fig. 4. Theratiooftheproduction crosssectionsofinclusive⁺_c topromptD⁰ versuspTfrompp collisionsaswellas0–100%centralityPbPb collisions.Theboxes anderrorbarsrepresentthe systematicand statisticaluncertainties,respectively.

ThePbPb datapointisshiftedinthe horizontalaxisfor clarity.The20and31%

normalizationuncertaintiesinpp andPbPb collisions,respectively,arenotincluded intheboxesrepresentingthesystematicuncertaintiesforeachdatapoint.Theopen crossesandopenstarsrepresentthepredictionsof pythia 8withtheCUETP8M1 tuneandwithcolorreconnection [43],respectively.Thesolidanddashedlinesare thecalculationsforprompt⁺_c overpromptD⁰productionratiofromRef. [20] and Ref. [46],respectively.Allpredictionsareforpp collisions.

Thepp dataarealsocomparedwithtwopredictionswhichare for the prompt ⁺_c over D⁰ production ratio.Calculations using a modelthat includes both coalescenceandfragmentation inpp collisions [20] areshowninFig.4by thesolid line. Comparedto the data, this model predicts a stronger dependence on pT and underestimatesthe measurements.Anotherrecentmodel [46] at- tempts to usea statistical hadronization approachto explain the large ⁺_c/D⁰ production ratio as arising from ⁺_c baryons that are produced fromthe decayof excited charm baryon statesnot includedinRef. [39] andarethereforenotincludedinthehadron- izationsimulation in pythia 8.The predictionofthismodel, also showninFig.4bythedashedline,providesareasonabledescrip- tionofthedatafor pT<10 GeV/c.

WhiletheALICEresultsindicateanenhancementinthe⁺_c/D⁰ productionratiointhepTrangeof6–12 GeV/c forPbPb [22] com- paredtopPbandpp collisions,theCMSPbPb measurementinthe pT range 10–20 GeV/c is consistent with the pp result.This lack ofan enhancement maysuggest that there isno significant con- tribution fromthe coalescence process for p_T>10 GeV/c in PbPb collisions.

7. Summary

ThepT-differentialcrosssectionsof⁺_c baryons,includingboth promptandnonpromptcontributions, havebeenmeasuredinpp andPbPb collisionsatanucleon-nucleoncenter-of-massenergyof 5.02 TeV.Theshape ofthe pT distribution inpp collisionsiswell described by the pythia 8eventgenerator. Ahintofsuppression of⁺_c productionfor10<p_T<20 GeV/c isobservedinPbPb when comparedto pp data,withcentral PbPb eventsshowingstronger suppression. This is consistent with the suppression observed in D⁰ meson measurements, which is understoodto originate from the strong interaction betweenthe charm quark andthe quark- gluon plasma. The ⁺_c/D⁰ production ratios in pp collisions are consistentwithamodelobtainedbyaddingcolorreconnectionin hadronization to pythia 8, and also with a model that includes enhancedcontributions fromthedecayofexcited charmbaryons.

The⁺_c/D⁰ productionratios inpp andPbPb collisions for pT=

10–20 GeV/c arefoundtobeconsistentwitheachother.Thesetwo observations may suggest that the coalescence process doesnot playasignificantrolein⁺_c baryonproductioninthispT range.

Acknowledgements

WethankV.Grecoforprovidingthetheoreticalcalculationsof the⁺_c/D⁰ productionratiosusedforcomparisonswithourmea- surementsinpp collisions.

WecongratulateourcolleaguesintheCERNacceleratordepart- ments for the excellent performance of the LHC and thank the technical andadministrativestaffs atCERNand atother CMSin- stitutes for their contributions to the success of the CMS effort.

Inaddition,wegratefullyacknowledgethecomputingcentresand personneloftheWorldwideLHCComputingGridfordeliveringso effectively thecomputinginfrastructure essentialto our analyses.

Finally, we acknowledge the enduring support for the construc- tion andoperationofthe LHCandtheCMSdetectorprovided by the followingfundingagencies: BMBWFandFWF(Austria);FNRS and FWO (Belgium); CNPq, CAPES,FAPERJ, FAPERGS, andFAPESP (Brazil); MES (Bulgaria); CERN; CAS, MOST, and NSFC (China);

COLCIENCIAS (Colombia); MSES and CSF (Croatia); RPF (Cyprus);

SENESCYT (Ecuador); MoER, ERC IUT, PUT and ERDF (Estonia);

AcademyofFinland,MEC,andHIP(Finland);CEAandCNRS/IN2P3 (France); BMBF, DFG, and HGF (Germany); GSRT (Greece); NK- FIA (Hungary); DAE and DST (India); IPM (Iran); SFI (Ireland);

INFN (Italy);MSIPandNRF(RepublicofKorea);MES(Latvia);LAS (Lithuania);MOEandUM(Malaysia);BUAP,CINVESTAV,CONACYT, LNS,SEP,andUASLP-FAI(Mexico);MOS(Montenegro);MBIE(New Zealand); PAEC (Pakistan); MSHE and NSC (Poland); FCT (Portu- gal);JINR(Dubna);MON,ROSATOM, RAS,RFBR,andNRCKI (Rus- sia);MESTD(Serbia);SEIDI,CPAN,PCTI,andFEDER(Spain);MoSTR (Sri Lanka); Swiss Funding Agencies (Switzerland); MST (Taipei);

ThEPCenter,IPST,STAR, andNSTDA(Thailand);TUBITAKandTAEK (Turkey);NASU andSFFR (Ukraine);STFC(United Kingdom);DOE andNSF(USA).

Individuals have received support from the Marie-Curie pro- gramme and the European Research Council and Horizon 2020 Grant, contract Nos. 675440, 752730, and 765710 (European Union); the Leventis Foundation; the A.P. Sloan Foundation; the Alexander vonHumboldt Foundation;theBelgianFederalScience Policy Office; the Fonds pour la Formation à la Recherche dans l’Industrie et dans l’Agriculture (FRIA-Belgium); the Agentschap voor Innovatie door Wetenschap en Technologie (IWT-Belgium);

the F.R.S.-FNRS andFWO(Belgium) under the“Excellence ofSci- ence – EOS” – be.h project n. 30820817; the Beijing Municipal Science & Technology Commission, No. Z181100004218003; the Ministry ofEducation,Youth andSports (MEYS)of theCzech Re- public; the Lendület (“Momentum”) Programme and the János Bolyai Research Scholarship of the Hungarian Academy of Sci- ences, the New National Excellence Program ÚNKP, the NKFIA researchgrants123842,123959,124845,124850,125105,128713, 128786, and 129058 (Hungary); the Council of Science and In- dustrial Research, India; the HOMING PLUS programme of the Foundation for Polish Science, cofinanced from European Union, Regional Development Fund,the MobilityPlus programme ofthe Ministry of Science and Higher Education, the National Science Center (Poland), contracts Harmonia 2014/14/M/ST2/00428, Opus 2014/13/B/ST2/02543, 2014/15/B/ST2/03998, and 2015/19/B/ST2/

02861,Sonata-bis2012/07/E/ST2/01406;theNationalPrioritiesRe- search Programby Qatar NationalResearch Fund;theMinistryof Science and Education, grant no. 3.2989.2017 (Russia); the Pro- gramaEstatalde Fomento de laInvestigaciónCientífica yTécnica de Excelencia María de Maeztu, grant MDM-2015-0509 and the ProgramaSeveroOchoadelPrincipadodeAsturias;theThalisand Aristeia programmes cofinanced by EU-ESF and the Greek NSRF;