Charm breaks fragmentation universality

The study of heavy-flavour hadron production in proton–proton (pp) collisions provides an important test for quantum chromodynamics (QCD) calculations. Heavy-flavour hadron production is usually computed with perturbative–QCD (pQCD) calculations as the convolution of the parton distribution functions (PDFs) of the incoming protons, the partonic cross section and the fragmentation functions that describe the transition from charm quarks into charm hadrons. The latter are typically parametrised from measurements performed in e⁺e^– or ep collisions, under the assumption that the hadronisation of charm quarks into charm hadrons is a universal process that is independent of the colliding systems.

The assumption that charm-to-hadron fragmentation is universal is not valid

The large data samples collected during Run 2 of the LHC at √s = 5.02 TeV allowed the ALICE collaboration measure the vast majority of charm quarks produced in the pp collisions by reconstructing the decays of the ground-state charm hadrons, measuring all the charm-meson species and the most abundant charm baryons (Λ_c⁺, and Ξ_c^0,+) down to very low transverse momenta. The result was presented today at the European Physical Society conference on high-energy physics (EPS-HEP 2021).

Charm fragmentation fractions, f(c → H_c), represent the probability for a charm quark to hadronise into a given charm hadron. These have now been measured for the first time at the LHC in pp collisions at midrapidity, and, in the case of the Ξ_c⁰ , for the first time in any collision system (figure 1). The measured f(c → H_c) are observed to be different from those measured in e⁺e^– and ep collisions – evidence that the assumption that charm-to-hadron fragmentation is universal is not valid.

Charm quarks were found to hadronise into baryons almost 40% of the time – four times more often than at colliders with electron beams. Several models have been proposed to explain this “baryon enhancement”. The explanations feature various different assumptions, such as including hadronisation via coalescence, considering a set of as-yet-unobserved higher-mass charm-baryon states, and including string formation beyond the leading-colour approximation.

The cc̄ production cross section per unit of rapidity at midrapidity (dσ^cc̄/dy|_|y|<0.5) was calculated by summing the cross sections of all measured ground-state charm hadrons (D⁰, D⁺, D_s⁺ , Λ_c⁺ , and Ξ_c⁰). The contribution of the Ξ_c⁰ was multiplied by a factor of two, in order to account for the contribution of the Ξ_c⁺. The resulting cc̄ cross section per unit of rapidity at midrapidity is dσ^cc̄/dy|_|y|<0.5 = 1165 ± 44(stat) ⁺¹³⁴ _–101 (syst) μb. This measurement was obtained for the first time in hadronic collisions at the LHC including the charm-baryon states. The cc̄  cross section measured at the LHC lies at the upper edge of the theoretical pQCD calculations.

The measurements described above not only provide constraints to pQCD calculations, but also act as important references for investigating the interaction of charm quarks with the medium created in heavy-ion collisions. These measurements could be extended to include rarer baryons and studied as a function of the event multiplicity in pp and heavy-ion systems in future LHC runs.