Λ_c⁺-baryon probes charm-quark hadronisation

The first measurement of Λ_c⁺-baryon production in lead–lead (Pb–Pb) collisions at an energy of 5.02 TeV per colliding nucleon pair was presented by the ALICE collaboration at the International Conference on Hard and Electromagnetic Probes of High-Energy Nuclear Collisions, held at Aix-Les-Bains from 30 September to 5 October. This measurement is essential to understand how charm-quark hadronisation is affected by the presence of the quark–gluon plasma (QGP) created in high-energy heavy-ion collisions.

Charm quarks are produced early in the collision, interact with the plasma as they propagate through it, and eventually hadronise. It has been suggested that the presence of many quarks in the final state of a heavy-ion collision may affect the hadronisation process: charm quarks could form hadrons by recombining with light quarks that happen to be nearby. In high-energy proton–proton (pp) collisions, the main hadronisation mechanism is through the formation of light quarks in a parton shower, known as “fragmentation”.

Λ_c⁺ → pK⁰_s decays, and their charge conjugates, were reconstructed by ALICE in Pb–Pb collisions at mid-rapidity (|y| <0.5) in the transverse momentum interval 6 < p_T < 12 GeV/c and within 0–80% centrality range. The ratio of the production yields of Λ_c⁺ baryons (which consist of a charm quark and two light quarks) and D⁰ mesons (which contain a charm quark and a single, light antiquark) was measured. The Λ_c⁺/D⁰ ratio in Pb–Pb collisions is larger than those measured in minimum-bias pp collisions at 7 TeV and in p–Pb collisions at 5.02 TeV. The difference between the results in Pb–Pb and p–Pb collisions is about two times the standard deviation of the combined statistical and systematic uncertainties. The measured ratio in Pb–Pb collisions is also compatible with the Λ_c⁺/D⁰ ratio measured in gold–gold collisions at the Relativistic Heavy-Ion Collider at Brookhaven in the US. The measurement was compared with model calculations including different implementations of charm-quark hadronisation. The calculation with a pure coalescence scenario describes the experimental result, while adding a fragmentation contribution leads to a ratio that is smaller than that observed.

For this first measurement of Λ_c⁺-baryon production in Pb–Pb collisions, the uncertainties are still large and it is therefore not possible to draw a firm conclusion about the relative importance of recombination and fragmentation for charm-quark hadronisation. Moreover, it remains crucial to understand the charm-baryon production mechanisms in pp and p–Pb collisions, in particular, whether the assumptions made on the basis of e⁺e^– results also hold for fragmentation in hadronic collisions (CERN Courier March 2018 p12). The baryon-to-meson ratio has now been studied with light-flavour, strange and charm hadrons. All baryon-to-meson ratios in pp and p–Pb collisions show a characteristic p_T dependence with an enhancement at intermediate p_T values up to around 4 GeV/c, which still needs further investigation. 

Future datasets, to be collected during the heavy-ion run in 2018 and during LHC Run 3 and 4 after a major upgrade of the ALICE detector, will improve the Λ_c⁺-baryon production measurement. With a higher precision and a finer granularity in p_T and centrality, these measurements are fundamental in determining the role of recombination for charm-quark hadronisation.