ALICE investigates charm-quark hadronisation

In two publications submitted to the Journal of High Energy Physics and Physics Letters B in December, the ALICE collaboration reports new production cross-section measurements of the charmed baryons Λ⁺_c and Ξ⁰_c in proton–proton collisions at an energy of 7 TeV and in proton–lead collisions at a collision energy of 5.02 TeV per nucleon–nucleon pair. The Λ⁺_c were reconstructed in the hadronic decay modes Λ⁺_c → pK⁻ π⁺ and Λ⁺_c → p K⁰_S, and in the semileptonic channel Λ⁺_c → e⁺ ν_e Λ (and charge conjugates). For the Ξ⁰_c analysis, the semi-leptonic channel Ξ⁰_c → e⁺ ν_e Ξ^– was used.

The comparison of charm baryon and meson cross-sections provides information on c-quark hadronisation. Surprisingly, the measured values of the Λ⁺_c/D⁰ baryon-to-meson ratio were significantly larger than those previously measured in other experiments in collisions involving electron beams at different centre-of-mass energies, rapidity and p_T intervals.

The results (see figure) are compared with the expectations obtained from perturbative QCD calculations and Monte Carlo event generators. None of the models reproduce the data, indicating that the fragmentation of charm quarks is not well understood. A similar pattern is seen when comparing the Ξ⁰_c/D⁰ baryon-to-meson ratio with predicted values (see figure, right), where the latter have a sizable uncertainty due to the unknown branching ratio of the decay.

These two results suggest that charmed baryon formation might not be universal, and that the baryon/meson ratio depends on the collision system. Hints of non-universality of the fragmentation functions are also seen when comparing beauty-baryon production measurements at the Tevatron and LHC with those at LEP. The ratios measured in pPb collisions are similar to the result in pp collisions.

The statistical precision of the Λ⁺_c and Ξ⁰_c measurements is expected to be improved with data collected during the LHC Run 2, and with data from Run 3 and Run 4 following a major upgrade of the ALICE apparatus. This set of measurements also provides a reference for future investigation of Λ⁺_c and Ξ⁰_c production in lead–lead collisions, where the formation and kinematic properties of charm baryons are expected to be affected by the presence of the quark–gluon plasma.