Constraining hadronization processes with charmed baryons in pp and p–Pb collisions with ALICE

. The latest measurements of D 0 , D + and D s + mesons with the final measurements of Λ + c , Ξ 0 , + c , Σ 0 , ++ c , and the first measurement of Ω 0c baryons performed with the ALICE detector at midrapidity in pp collisions at √ s = 5 . 02 TeV and √ s = 13 TeV are presented. Furthermore, the recent Λ + c / D 0 yield ratio, measured down to p T = 0 in p–Pb collisions are discussed. Finally, the first measurements of charm fragmentation fractions and charm production cross section at midrapidity per unit of rapidity are shown for both pp and p–Pb collisions using all measured charm ground state hadrons.


Introduction
The production cross sections of heavy flavor mesons are well described by theoretical models based on perturbative QCD calculations that assume fragmentation functions of charm to be universal among different collision systems.Recent measurements of charm-baryon production at midrapidity in pp and p-Pb collisions systems show a baryon-to-meson ratio significantly higher than that in e + e − collisions [1,2], suggesting that the fragmentation of charm is not universal.Therefore, measurements of charm-baryon production are crucial to study the charm quark hadronization in a partonic-rich environment like the one produced in pp collisions at LHC energies.Thanks to the excellent particle identification capabilities, and optimal vertex resolution, ALICE is well suited to measure various charmed hadrons.These measurements provide information about the hadronization process of heavy quarks.c /D 0 ∼ 0.11 [1,2] measured in e + e − and ep collisions, as well as strong p T dependence not expected by models, which by assuming the fragmentation functions in pp collisions to be the same as those extracted from e + e − and ep collisions values.The p T dependence of charmed baryon-to-meson ratio may suggest that the assumption of universality of charm fragmentation among collision systems is not valid.The production of Λ + c in p-Pb collisions is also measured down to p T = 0.The Λ + c /D 0 ratio measured in p-Pb collisions is higher than the one measured in pp collisions for p T > 3 GeV/c, and lower for p T < 2 GeV/c.This could be interpreted as a consequence of radial flow [6] or different hadronization processes between pp and p-Pb collisions [7].The p T integrated yields of Λ + c /D 0 ratio in both collision systems are compatible within uncertainties [4].

Charmed baryon measurements
ALICE measured Σ 0,++ c to investigate further the charmed baryon production in pp collisions at √ s = 13 TeV.The ratio shown in Fig. 1 (right) presents an enhancement in the measured p T region with respect to the e + e − predictions of models based on e + e − data and assuming hadronization to be unaltered in pp collisions.The PYTHIA8 Monash [8] uses the Lund string model and the fragmentation functions tuned to reproduce e + e − data.The PYTHIA8 Mode 0,2,3 tunes implement the new Color Reconnection topologies beyond leading color approximation (CR-BLC) [9], including junctions.The junctions (three-leg topology) increase the chance to connect partons from different multi-parton interactions, enhancing the formation of baryons.Therefore, the PYTHIA 8 with CR-BLC can describe the enhancement of Λ + c and Σ 0,++ c .The Catania model [10] assumes that the hadronization occurs via vacuum fragmentation and coalescence of charm quarks with light quarks in a hot QCD matter in pp collisions.A blast wave parametrization is used for the light quark spectra, and the FONLL calculation [11,12] is used for the heavy quark spectra.The Statistical Hadrobnization Model (SHM) [13], which assumes that the yields of the various hadron species are determined by thermal weights depending mainly on the system temperature and hadron masses, can reproduce the Λ + c /D 0 and Σ 0,++ c /D 0 ratios by assuming the existence of exciting charm baryon states which are not yet observed.All three CR modes and the SHM describe the observed p T dependence, while the PYTHIA8 Monash tune significantly underestimates the measurement by a factor of 15-20 at low p T and a factor of 5 at high p T [5].

Ξ 0 c and Ω 0 c measurements
The charm-strange baryons, Ξ 0,+ c and Ω 0 c , were measured by ALICE in pp collisions at √ s = 13 TeV.The Ξ 0,+ c /D 0 ratios [14] are shown in the left panel of Fig. 2, and the Ω 0 c /D 0 [15] ratio is shown in the right panel.The branching ratio (BR) of this Ω 0 c decay is not yet measured, and a theoretical calculation [16] is used to scale the model predictions.Most models significantly underestimate the measurements, except for Catania model, which is close to the data in the measured p T interval, both for the Ξ 0,+ c /D 0 and the Ω 0 c /D 0 ratios.The charm fragmentation fractions in pp [17] and p-Pb collisions at √ s = 5.02 TeV are calculated summing the production cross section of all all single charm hadron ground states measured by ALICE.Thanks to measurements of D 0 [18,19], D + , and Λ + c [3] performed down to p T = 0 in pp and p-Pb collisions, and the measurement of D + s and Ξ 0 c down to very low p T (extrapolated to p T = 0 with theoretical predictions), ALICE was could determine the charm fragmentation fractions in pp and p-Pb collisions for the first time.
The Ω 0 c was not measured in pp and p-Pb collisions at √ s = 5.02 TeV .Its contribution is treated as a systematic uncertainty, taking into consideration the measurement performed in pp collisions at √ s = 13 TeV.Charm fragmentation fractions in pp and p-Pb collisions are presented in the left panel of Fig. 3, showing significant baryon enhancement with respect to the fragmentation fractions measured in e + e − and ep collisions.
The total charm production cross section per unit of rapidity at midrapidity measured in pp [17] and p-Pb collisions at √ s = 5.02 TeV are shown in Fig. 3 (right).They are compatible within uncertainties.Using the new charm fragmentation fractions [17], the charm production cross section in pp collisions at √ s = 2.76 TeV [20] and 7 TeV [21] are about 40% higher than the previously published results.The updated charm production cross section lies on the upper edge of the pQCD calculation uncertainty bands [11].

Conclusions
ALICE measured the production cross section of all charm hadron ground states in pp collisions.The productions of Λ + c was measured down to p T = 0 in pp and p-Pb collisions.A large enhancement of all charm baryon-to-meson ratios is observed in pp collisions with respect to e + e − collisions.None of the models describes the enhancement for all charmbaryons.Reproducing the measurements of charm baryons with a strange quark is particularly challenging.The charm fragmentation fractions measured in pp and p-Pb collisions at √ s = 5.02 TeV show that the assumption of the universality of charm fragmentation fractions across colliding systems is not valid already from e + e − to pp collisions.

Figure 1 .
Figure 1.Left panel: Λ + c /D 0 as a function of p T in pp and p-Pb collisions at √ s = 5.02 TeV .Right panel: Σ 0,++ c /D 0 ratio as a function of p T at √ s = 13 TeV.

2 EPJALIFigure 2 .Figure 3 .
Figure 2. Left panel: Ξ 0,+ c /D 0 as a function of p T in pp collisions at √ s = 13 TeV.Right panel: BR × Ω 0 c /D 0 ratio as a function of p T in pp collisions at √ s = 13 TeV.