Charm baryons constrain hadronisation

Understanding the mechanisms of hadron formation represents one of the most interesting open questions in particle physics. Hadronisation is a non-perturbative process that is not calculable in quantum chromodynamics and is typically described with phenomenological models, such as the Lund string model. Ultrarelativistic nuclear collisions, where a high-density plasma of deconfined quarks and gluons, the quark–gluon plasma (QGP), is created, provide an ideal setup to test the limits of this description. In these conditions, hadrons may be formed via a combination of deconfined quarks close in phase space. This process can lead, for example, to increased production of baryons with respect to mesons in momentum ranges up to 10 GeV/c. The ALICE and CMS experiments at the LHC, and PHENIX and STAR at RHIC, have indeed observed substantial modifications of the event hadro-chemistry in heavy-ion collisions compared to proton–proton and e⁺e^– collisions. In particular, the total abundances of light and strange hadrons were found to follow, quite remarkably, the “thermal’’ expectations for a deconfined medium close to equilibrium. 

Measurements of heavy-flavour hadron production play a unique role in such studies. Heavy quarks are mostly produced in hard scatterings at the early stages of the collisions, well before the QGP is formed. Furthermore, their thermal production is negligible since their masses are larger than the typical QGP temperature. Due to the much better theoretical control on their production and propagation in the medium, heavy quarks provide unique constraints on the QGP properties and the nature of hadronisation mechanisms, compared to light quarks. Heavy-flavour measurements in heavy-ion collisions also test whether the transverse momenta (p_T) integrated yields of charm hadrons are consistent with the hypothesis of statistical models, in which charm quarks are expected to reach an almost complete thermalisation in the QGP, despite being initially very far from equilibrium.

ALICE has recently made an improvement towards a quantitative understanding of hadron formation from a QGP

The ALICE experiment has recently made an improvement towards a quantitative understanding of hadron formation from a QGP by performing the first measurement of the charm baryon-to-meson ratio Λ⁺_c/D⁰ in central (head-on) Pb–Pb collisions at √s_NN = 5.02 TeV. By exploiting its unique tracking and particle-identification capabilities, and using machine-learning techniques, ALICE has measured the ratio down to very low p_T (less than 1 GeV/c), where hadronisation mechanisms via a combination of quarks are expected to dominate (figure 1, left). The measured production ratio of Λ⁺_c/D⁰ in central Pb–Pb collisions is found to be larger than in pp collisions at p_T of 4–8 GeV/c (figure 1, right). On the other hand, the p_T-integrated ratio was found to be compatible with the result of pp collisions within one standard deviation. 

A comparison with theoretical calculations confirms the discrimination power of this measurement. The experimental data are well described by transport models that include mechanisms of the combination of quarks from the deconfined medium (TAMU and Catania). Given the current uncertainties, a conclusive answer on the agreement with statistical models (SHMc) cannot yet be reached. This motivates future high-precision and more differential measurements with the upgraded ALICE detector during the upcoming LHC Run-3 Pb–Pb runs. Thanks to the increased rate-capabilities of the new readout systems of the time projection chamber and the new inner tracking system, ALICE will increase its acquisition rate by up to a factor of about 50 in Pb–Pb collisions and will benefit from a much higher tracking resolution (by a factor 3–6 for low-p_T tracks). High-accuracy measurements performed in Runs 3 and 4 will therefore provide significant discrimination power on theoretical calculations and strong constraints on the mechanisms underlying the hadronisation of charm quarks from the QGP.