Charm Hadrons in pp collisions at LHC energy within a Coalescence plus Fragmentation approach
aa r X i v : . [ h e p - ph ] D ec Charm Hadrons in pp collisions at LHC energy within a Coalescence plusFragmentation approach
Vincenzo Minissale b , Salvatore Plumari a,b and Vincenzo Greco a,b a Department of Physics and Astronomy ’E. Majorana’,University of Catania, Via S. Sofia 64, 1-95125 Catania, Italy and b Laboratori Nazionali del Sud, INFN-LNS, Via S. Sofia 62, I-95123 Catania, Italy
The recent experimental measurements on pp collisions at √ s = 5 .
02 TeV have shown a very largeabundance of heavy baryon production corresponding to a ratio of Λ c /D ∼ .
6, about one order ofmagnitude larger than what measured in e + e − , ep collisions and even in pp collisions at LHC, butat forward rapidity. We apply for the first time a quark coalescence plus fragmentation approach,assuming the formation of Hot QCD matter at finite temperature. An approach that have predicteda Λ c /D ∼ O (1) in AA collisions at RHIC energy. We calculate the heavy baryon/meson ratio andthe p T spectra of charmed hadrons with and without strangeness content: D , D s , Λ + c , Ξ c and Ω c in pp collisions at top LHC energies, finding a satisfactory prediction of the measured Λ + c /D and theΞ c /D without any specific tuning of parameters to pp collisions. At variance with other approachesa coalescence approach predicts also a significant production of Ω c such that Ω c /D ∼ O (10 − ) . I. INTRODUCTION
The production of heavy flavored baryons in highenergy collisions has become accessible mainly in thelast decade in e + e − [1], e ± p [2, 3], and in pp colli-sions at forward rapidity at LHC energy [4]. Thehadronization and, in particular, the heavy flavourhadronization of HQs in pp collisions is usually de-scribed by the traditional fragmentation mechanism.An overall analysis on the charmed hadron produc-tion have indicated that the charm fragmentationfraction f ( c → D ) is about 0.6, while f ( c → D + ) ≃ .
25 with a feed-down from resonances such that f ( c → D ∗ ) ≃ .
25, while the strange charm mesonrepresents only about a 8 −
10% of the productionand for the heavy baryon is estimated a fraction of f ( c → Λ + c ) ≃ .
06 [5]. Such an heavy baryon pro-duction is similar to the one estimated also by MonteCarlo generators like PYTHIA.Very recent measurements in pp and pA collisionsat top LHC in the mid-rapidity region have shown asurprising large production of the Λ c baryons. Sucha production at low p T corresponds to Λ c /D ∼ . − .
6, at variance with some early observationin pp at large rapidity [4]. This is nearly an orderof magnitude larger than what expected with thetraditional fragmentation approach. Furthermore,it has been observed in preliminary results even asignificant Ξ c production with a Ξ c /D ∼ . − . c production, again withΛ c /D ∼ O (1), was predicted about ten years agoassuming an hadronization via quark coalescence [7],an approach that has been able also to correctlypredict several features of the produced hadrons forboth light and heavy hadrons [8–18]. Such a predic-tion was confirmed by the STAR data in Au + Au at 200 AGeV [19] and also by the successive measure-ments by ALICE in P b + P b at 5.02 ATeV [20, 21],with a similar but somewhat reduced Λ c /D ratio,as again correctly predicted by a coalescence plusfragmentation approach [12].In ultra-relativistic nucleus-nucleus collisionsthese results have been justified by the formation of adeconfined matter of quarks and gluons (QGP). Thegeneral expectation in elementary pp collision is thata QGP is not created, but above TeV energy havebeen observed several features similar to those in AA collisions: strangeness enhancement [22], the ridgeand large collectivity [23] and enhancement of thebaryon to meson ratios [24, 25]. Theoretical studiesof these phenomena in small collision systems haveobserved that hydrodynamics and transport calcu-lations are able to give reasonable description of the p T spectra and even two-particle correlations [26–29]. This would point to the possible formation of ahot QCD matter at energy density larger than thepseudo-critical one with a lifetime τ ≈ / c.It has to be considered that signatures of anhadronization by recombination/coalescence hasbeen spotted even at FNAL in π − + A by the observa-tion that the D − /D + ratio at forward rapidity getsclose to unity against the fragmentation approachpredicting a vanishing ratio. This is also known as”leading particle effect” and is explained as a re-combination of the charm with the valence d quark[30]. This represents a signature that a highly densequark medium favor an hadronization by quark re-combination, in this case with valence quark.An attempt to explain the large charmed baryonproduction with respect to the charmed mesons ithas been proposed in [31] assuming independentfragmentation of charm quarks in D and Λ c baryonswith the inclusion of a large set of charm-baryonstates beyond the current listings of the ParticleData Group [32]. In such an approach the enhancedfeed-down from excited charm baryons can accountfor the large Λ c /D ratio at low p T as measuredby ALICE collaboration. However, the decay of ex-cited charm baryon states proposed have not beenseen in the e + + e − annihilation (Belle [33]). An-other approach able to give an enhancement of theΛ c production in pp collision is given by a color re-connection mechanism [34], as recently implementedin PYTHIA 8. It has been shown that this model isconsistent with the CMS result for the Λ c /D ratio[35], but still seems to fail to predict a significantproduction for higher charmed baryons like Ξ c .In this Letter, we present the prediction for charmhadron production in pp collisions at top LHC en-ergy assuming a coalescence plus fragmentation ap-proach occurring in a bulk matter according to vis-cous hydro simulations that have been applied tostudy the spectra and collectivity in pp collisions[26]. The approach is essentially the same as theone developed for AA collisions to study the spectraand the enhanced baryon over meson ratio for bothlight and heavy sector [9, 12].We provide a comprehensive predictions for theproduction as a function of the transverse momen-tum of D , D s , Λ + c , Ξ c and also the Ω c that will belikely been measured in the upcoming future. Wealso show the details of the feed-down from reso-nances. We find a quite satisfying description ofthe available data including an abundant produc-tion for Ξ c and Ω c differently to color reconnection inPYTHIA8 [34] and also to hadronization approachesincluding charm states according to the quark mod-els [31]. An advantage of such an approach is that itcould provide a unified description of charm hadronproduction at low and intermediate p T in pp , pA and AA collisions above the TeV energy scale. II. HYBRID HADRONIZATION BYCOALESCENCE AND FRAGMENTATION
The coalescence model was initially proposed asan hadronization mechanism in Heavy Ion collisionsat RHIC energy to explain the p T spectra andthe splitting of elliptic flow of light mesons andbaryons [8, 36–39]. Subsequent works have beendevoted to extended the model to include finitewidth to take into account for off-shell effects[40–42]. While recently it has been extendedto LHC energies describing the spectra of mainlight hadrons like π, K, p, φ, Λ and the baryon tomeson ratios at both RHIC and LHC energies[9]. Furthermore the HF hadron chemistry in AA collisions it has been investigated with the coales-cence model predicting a large Λ C /D [7, 12, 16, 17]. In this section we recall the basic elements of thecoalescence model developed in [8, 37, 38, 43] andbased on the Wigner formalism. The momentumspectrum of hadrons formed by coalescence of quarkscan be written as: dN H dyd P T = g H Z N q Y i =1 d p i (2 π ) E i p i · dσ i f q i ( x i , p i ) × f H ( x ...x N q , p ...p N q ) δ (2) P T − n X i =1 p T,i ! (1)with g H we indicate the statistical factor to forma colorless hadron from quarks and antiquarks withspin 1/2. The dσ i denotes an element of a space-like hypersurface, while f q i are the quark (anti-quark) phase-space distribution functions for i-thquark (anti-quark). Finally f H ( x ...x N q , p ...p N q ) isthe Wigner function which describes the spatial andmomentum distribution of quarks in a hadron. N q is the number of quarks that compound the hadronand for N q = 2 Eq. 1 describes meson formation,while for N q = 3 the baryon one. For D mesonsthe statistical factors g D = 1 /
36 gives the probabil-ity that two random quarks have the right colour,spin, isospin matching the quantum number of theconsidered mesons. For Λ c the statistical factors is g Λ = 1 / f H ( ... ) = Q N q − i =1 A W exp (cid:16) − x ri σ ri − p ri σ ri (cid:17) where N q is thenumber of constituent quarks and A W is a normal-ization constant that it has been fixed to guaranteethat in the limit p → p → P totcoal = 1. The 4-vectors for the relative co-ordinates in space and momentum x ri and p ri arerelated to the quark coordinate in space and momen-tum by the Jacobian transformations. For mesonsthe relative coordinates ( x r , p r ) are given only by x r = x − x , p r = m p − m p m + m (2)while for baryon we have x r and p r and the otherstwo relative coordinates x r , p r given by x r = m x + m x m + m − x p r = m ( p + p ) − ( m + m ) p m + m + m . (3)The σ ri are the covariant widths, they can be relatedto the oscillator frequency ω by σ ri = 1 / √ µ i ω where µ i are the reduced masses µ = m m m + m , µ = ( m + m ) m m + m + m . (4)The widths of the Wigner function f H is related tothe size of the hadron and in particular to the rootmean square charge radius of the hadron, h r i ch = P Ni =1 Q i h ( x i − X cm ) i with N = 2 , h r i ch = 32 Q m + Q m ( m + m ) σ r (5)with Q i the charge of the i-th quark and thecenter-of-mass coordinate calculated as X cm = P i =1 m i x i / P i =1 m i . In a similar way to themesons, the oscillator frequency and the widths forbaryons can be related to the root mean squarecharge radius of the corresping baryons by h r i ch = 32 m Q + m Q ( m + m ) σ r (6)+ 32 m ( Q + Q ) + ( m + m ) Q ( m + m + m ) σ r In our approach the Wigner function for the heavymesons have only one parameter σ r that we fix in or-der to have their mean square charge radius. Whilethe Wigner function for Heavy baryons depends onthe two widts σ r and σ r as shown in Eq.6. How-ever, for baryons there is only one free parameter,because the two widths are related by the oscilla-tory frequency ω through the reduced masses by σ pi = σ − ri = 1 / √ µ i ω . The mean square charge ra-dius of mesons and baryons used in this work havebeen taken from quark model [44, 45]. The corre-sponding widths for heavy hadron are shown in Ta-ble I. Meson h r i ch σ p σ p D + = [ c ¯ d ] 0.184 0.282 — D + s = [¯ sc ] 0.083 0.404 —Baryon h r i ch σ p σ p Λ + c = [ udc ] 0.15 0.251 0.424Ξ + c = [ usc ] 0.2 0.242 0.406Ω c = [ ssc ] -0.12 0.337 0.53TABLE I: Mean square charge radius h r i ch in fm andthe widths paraemters σ pi in GeV . The mean squarecharge radius are taken quark model [44, 45].
The multi-dimensional integrals in the coales-cence formula are evaluated by using a Monte-Carlomethod, see [12] for more details. In these calcula-tions the partons are distribuited uniformly in thetransverse plane and rapidity y z .The hadron momentum spectra from the charm par-ton fragmentation is given by: dN had d p T dy = X Z dz dN fragm d p T dy D had/c ( z, Q ) z (7) D had/c ( z, Q ) is the fragmentation function and z = p had /p c is the momentum fraction of heavy quarkstrasfered to the final heavy hadron while Q =( p had / z ) is the momentum scale for the fragmen-tation process.In our calculations we have applied a com-monly used fragmentation function for heavy quarks,that is the Peterson fragmentation function [46] D had ( z ) ∝ / [ z [1 − z − − ǫ c (1 − z ) − ] ] where ǫ c is afree parameter that depends on the hadronic speciesand it is determined assuring that the shape of thefragmentation function agrees with the experimentaldata on p T distributions. For mesons the ǫ c param-eter it has been fixed to ǫ c = 0 . c wehave considered ǫ c = 0 .
25; these values coupled toFONLL p T distribution correctly describe the high p T tail dominated by fragmentation, see Fig. 2. Ina similar way done in Refs.[12] for AA collisions weassume that charm quarks that do not hadronizevia coalescence are converted to hadrons by frag-mentation. Therefore we can introduce a fragmenta-tion probability given by P frag ( p T ) = 1 − P totcoal ( p T ),where P totcoal is the total coalescence probability. Thefragmentation fraction that gives the probabilitythat a charm quark fragment in a specific heavyhadron is evaluated in agreement with the e + + e − analysis presented in [5]. III. FIREBALL AND PARTONDISTRIBUTION
The charm pair production is described by hardprocess and it is described by perturbative QCD(pQCD) at NNLO. Therefore, the starting point tocompute the initial heavy quarks spectra in pp col-lisions at LHC collision energy of √ s = 5 . T eV isby pQCD calculation. In our calculation the charmquark spectrum have been taken in accordance withthe charm distribution in p + p collisions within theFixed Order + Next-to-Leading Log (FONLL), asgiven in Ref. [47, 48]. In the recent years, we areobserving that hydrodynamics models can also beextended even to extreme situations, for example insmall systems like pA collisions, but also even in pp collisions giving reasonable descriptions of the mea-sured two-particle correlations [26, 27], suggesting alife time of the fireball, at these collision energies,of about τ ≈ f m/c . On the other hand heavyquarks have a thermalization time that is about τ th ≈ − f m/c which is more than two timeslarger than the lifetime estimated for the fireball cre-ated in these collisions. It is reasonable to assumethat the modification of the spectrum due to the jetquenching mechanism could be negligible, and in-deed even in pA measurements show an R pA ≈ u, d, s quarks and anti-quarks. The longitudinal momen-tum distribution is assumed to be boost-invariant inthe range y ∈ ( − . , +0 . β T ( r T ) = β max r T R , where R is the transverse radius of the fire-ball. Partons at low transverse momentum, p T < dN q, ¯ q d r T d p T = g q, ¯ q τ m T (2 π ) exp (cid:18) − γ T ( m T − p T · β T ) T (cid:19) (8)where m T is the transverse mass with m T = q p T + m q, ¯ q . The factors g q = g ¯ q = 6 are thespin-color degeneracy. The presence of gluons in thequark-gluon plasma is taken into account by convert-ing them to quarks and anti-quark pairs accordingto the flavour compositions, as assumed in [8, 49].For the bulk properties we fix the parameter ac-cording to hydro-dynamical simulations [26] whith τ = 2 . /c , R = 2 fm and the temperature of thebulk is T C = 165 MeV For partons at high transversemomentum, p T > . IV. HEAVY HADRON TRANSVERSEMOMENTUM SPECTRA AND RATIO
In this section, we discuss the coalescence proba-bility and will be shown the results for the transversemomentum spectra of D , D s mesons and for Λ c us-ing the model described in previous sections for pp collisions at √ s = 5 TeV.The presence of resonance decay has a significantimpact because it gives an important contribution tothe ground-state spectra. In this study we includeground state hadrons as well as the first excited res-onances listed in table II, which include resonanceof D , Λ c , Ξ c and Ω c baryons as given by the Parti-cle Data Group [32]. Recent experimental analysistechniques have unveiled information about the Σ c spectra and their contribution to the total Λ c yield,which offer a unique possibility to test the hadroniza-tion models in detail.In Fig. 1 is shown the coalescence probabilities P coal for a charm quarks to hadronize via coales-cence into a specific hadron, as a function of thecharm transverse momentum. As shown P coal isa decreasing function of p T which means that, atlow momentum, charm quarks are more probable tohadronize via coalescence with light partons fromthe thermalized medium, in particular in our modelat p T ≈ Meson Mass(MeV) I (J) Decay modes B.R. D + = ¯ dc (0) D = ¯ uc (0) D + s = ¯ sc D ∗ + (1) D π + ; D + X D ∗ (1) D π ; D γ D ∗ + s D + s X + c = udc )Ξ + c = usc ( )Ξ c = dsc ( )Ω c = ssc )ResonancesΛ + c ) Λ + c π + π − + c ) Λ + c π + π − + c ) Λ + c π + c ) Λ + c π ′ + , c ( ) Ξ + , c γ + c ( ) Ξ + c π − , 100%Ξ + c ( ) Ξ ′ c π , 100%Ξ + c ( ) Ξ ′ c π , 100%Ω c ) Ω c γ , 100%TABLE II: We report the ground states and the firstexited states including their decay modes with their cor-responding branching ratios as given in Particle DataGroup [32, 52]. coalescence. In our modelization a charm quarkthat cannot hadronize by coalescence hadronizesby fragmentation with a fragmentation probabilitygiven by P fragm = 1 − P coal . Therefore at high p T the fragmentation becomes to be the dominantcharm hadronization mechanism and a charm willhadronize according to the different fragmentationfraction into specific final charmed hadron channels,as in Ref. [5]. By comparing the different coales-cence probabilities in Fig. 1 we notice that , at lowmomenta, the coalescence probability for Λ c and Ξ c are similar and larger than the one for D which is aquite peculiar feature of the coalescence mechanism,we expect that this particular characteristic leadsto an enhancement of the Λ c /D and Ξ c /D ratios.The coalescence probability for the Ω c baryon has adifferent slope respect to the other baryons, so weexpect that this behaviour can be reflected on theΩ c /D ratio. In Fig. 2 we show the p T spectra of D (left panel), D s (mid panel) and Λ c (right panel) for pp collisions at mid-rapidity. The black dot-dashedline and the red dashed line refer to the hadron spec-tra obtained by the contribution from pure coales-cence and pure fragmentation respectively. We ob- −2 −1
0 1 2 3 4 5 6 7 8 9 10 P c o a l p T [GeV] Prob tot c−−>D c−−>D s c−−> L c c−−> X c c−−> W c FIG. 1: (Color online) The charm quarks coalescenceprobabilities as a function of the charm quark p T for pp collisions at LHC. The different lines are the coa-lescence probabilities to produce the different hadronspecies. Black solid line is the total coalescence prob-ability. serve that the contribution of fragmentation is thedominant mechanism for the production of D in allthe p T range explored and coalescence gives only afew percent of contribution to the total spectrum,while in AA the contribution is significantly largerand comparable to the fragmentation one [12]. Forthe D + s spectrum the contribution of both mecha-nism becomes similar due to the fact that the frag-mentation fraction for D + s is quite small, about 8%of the total heavy hadrons produced, according toRef. [5]. The inclusion of both hadronization mech-anisms provide a quite good comparison with the −2 −1
0 1 2 3 4 5 6 7 8 9 10 D LHC: pp @ 5.02 TeV d s / ( d p T d y ) ( m b G e V − c ) p T [GeV] coal.+fragm.fragm.coal. 0 1 2 3 4 5 6 7 8 9 10 D s LHC: pp @ 5.02 TeV p T [GeV]
0 1 2 3 4 5 6 7 8 9 10 L c LHC: pp @ 5.02 TeV p T [GeV] FIG. 2: (Color online) Transverse momentum spectrafor D , D s mesons and Λ c baryon at mid-rapidity for pp collisions at √ s = 5 TeV. Black dot-dashed and reddashed lines refer to the spectra from only coalescenceand only fragmentation respectively, the green solid lineis the sum of fragmentation and coalescence. Experi-mental data from [24, 25, 53]. experimental data and the coalescence leads to anenhancement of the D + s production.As shown in the last panel on the right of Fig. 2the coalescence mechanism is the dominant mecha-nism for the Λ + c production for p T < ∼ + c is about the 6% of the total produced heavyhadrons [5], ii) the coalescence contribution in thebaryon case is dominant with respect to the mesonscase (see [9, 12]) because the coalescence mechanismtakes quarks that are already present abundantly inthe dense medium created at very high energy evenin pp collisions.In Fig. 3 we show the results for the Λ + c /D and D + s /D ratio in comparison with the LHC experi-mental data for pp collisions at √ s = 5 .
02 TeV, thedashed lines shown the ratio that comes from theonly fragmentation, this ratio is fixed by the frag-mentation ratio measured experimentally in e + e − .The hybrid approach of coalescence plus fragmenta-tion (solid lines) give a quite good description of theexperimental data.The coalescence mechanism plays a dominant rolein the enhancement of the Λ + c giving a Λ + c /D ≈ . p T . However, Λ + c /D ratio in AA collisionshows a rise and fall behaviour while in pp colli-sion the same approach predicta decreasing functionof p T , similarly to the experimental data. The in-crease of Λ c /D at very low momenta is driven bythe increase of the ratio in the fragmentation func-tion which is due to the tail in the low x regionof the Peterson fragmentation function with for Λ c ;also notice that at p T < ∼ p T spectra, Fig.2, underestimate the absolute LHC: pp @ 5.02 TeV R a ti o p T [GeV] L c /D : ALICED s /D : ALICE L c /D coal.+fragm L c /D fragmD s /D coal.+fragm.D s /D fragm.0.00.10.20.30.40.50.60.70.80.9 0 1 2 3 4 5 6 7 8 9 10 LHC: pp @ 5.02 TeV R a ti o p T [GeV] L c /D : ALICED s /D : ALICE L c /D coal.+fragm L c /D fragmD s /D coal.+fragm.D s /D fragm. FIG. 3: (Color online) Λ + c /D (blue) and D + s /D (red)ratios as a function of p T and at mid-rapidity for pp collisions at √ s = 5 TeV. Experimental data taken from[24, 25, 53]. Solid and dashed lines refer to the cases withboth coalescence and fragmentation and to the case withonly fragmentation respectively. T (GeV)00,10,20,30,40,5 R a ti o X c0 /D coal.+fragm. T (GeV) W c /D coal.+fragm. LHC: pp @ 5.02 TeV
FIG. 4: (Color online) Ξ , + c /D (left) and Ω c /D (right)ratios as a function of p T and at mid-rapidity for pp collisions at √ s = 5 .
02 TeV. Solid lines refer to the caseswith both coalescence and fragmentation. yield for D and to some extent also for Λ c , hencethe rise up may not be a stable physical result.The D + s /D ratio is almost flat in the p T rangeexplored. Comparing the red solid and dashed linesthe different relative contribution of coalescence andfragmentation for D s w.r.t. D leads to an enhance-ment of the ratio D s /D of about 20% in all therange of transverse momentum explored.We have extended this analysis to other single-charm baryons with content of strangeness, such asΞ c whose ratios to D and Λ c have been very re-cently presented with preliminary results from AL-ICE collaboration [6] showing interesting behaviourin pp collision at top LHC energies and, furthermore,we provide some predictions for the Ω c baryon thatwill likely be accessible to experiments in the nextyears. Recently the ALICE collaboration has de-tected an abundant Ξ c production up to 20% of D ,much larger than the expectations in the fragmenta-tion approach even including color reconnection [6].In Fig. 4, we show the Ξ c and Ω c ratios to D in pp collisions at √ s = 5 . T eV at mid-rapidity. Itis interesting that the contribution from fragmenta-tion is nearly negligible for Ξ c and Ω c in these ratios,therefore in our model the main contribution comesfrom a pure coalescence mechanism, so this ratiocan carry some more relevant information about thehadronization process. There are other interestingparticle ratios available lately, thanks to the refine-ment of particle identification technique. In Fig. 5are shown the ratios between the Λ c particles fromthe decay of the three Σ c states and the total Λ c pro-duced (black solid line) and with D (orange dashedline). The green dashed-dot line is the ratio betweenthe direct Σ c and Ξ c baryons. The (Σ c → Λ + c ) / Λ c ratio is different from the fragmentation observed in LHC:[email protected] R a ti o p T [GeV] L c (<−− S c0,+,++ )/D L c (<−− S c0,+,++ )/ L c S c0,+,++ / X c+,0 FIG. 5: (Color online) Ratios of Λ c that comes fromthe decay of Σ c , (Σ c → Λ + c ) /D (orange dashed line)and (Σ c → Λ + c ) / Λ c (black solid line) as a function of p T and at mid-rapidity for pp collisions at √ s = 5 .
02 TeV.The green dot-dashed line is the Σ , + , ++ c / Ξ + , c ratio. e + e − for Σ c that is only 10% of the Λ c fragmentation[6]. Since these ratios give us information about thecontributions from Σ c on the total yield of Λ c theycan represents a quite solid test for our approach.Hence, it is interesting to note how the hybrid ap-proach of hadronization is able to describe the p T dependence as well as the absolute values for theseratios shown in [6]. V. CONCLUSION
In this paper we have studied the formation ofcharmed hadrons, calculating the baryon and mesonspectra and ratios, extending a model developed inAA collisions to investigate the production of parti-cles observed in pp collisions at top LHC energy.The large baryon over meson ratios with magnitudesmuch larger with respect to the fragmentation frac-tions ratio measured in e + e − collisions, clearly indi-cate a strong violation of the universality of the frag-mentation function. Using our coalescence plus frag-mentation model, we have found a good descriptionof the meson and baryon spectra and their ratios. Inparticular, in the transition from AA collisions to pp collisions our results naturally predicts the reshap-ing of the baryon over meson ratios as a functionof the transverse momentum and the enhancementobserved in recent experimental data for Ξ c /D [6]ratio along with a prediction for an Ω c /D ≈ . < r > ch of the quark model,hence there are no free parameters specifically tunedto reproduce the charm hadron production in pp collisions. The results obtained seems to suggestthat the presence of a hot and dense QCD matterin small collision systems permits the recombina-tion of quarks that significantly modify the charmbaryon production. In particular, the Ξ c /D [6] andΩ c /D ratio may allow to discriminate between acoalescence mechanism and other approaches, cur-rently under development, in a fragmentation func-tion framework like the color reconnection mecha-nism [34] or the large feed-down from higher baryoncharm states[31]. Acknowledgments
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