Charm-strange mesons probe hadronisation

The ALICE collaboration has reported a new measurement of the production of D_s⁺ mesons, which contain a charm and an anti-strange quark, in Pb–Pb collisions collected in 2018 at a centre- of-mass energy per nucleon pair of 5.02 TeV. The large data sample and the use of machine-learning techniques for the selection of particle candidates led to increased precision on this important quantity. 

D-meson measurements probe the interaction between charm quarks and the quark–gluon plasma (QGP) formed in ultra-relativistic heavy-ion collisions. Charm quarks are produced in the early stages of the nucleus–nucleus collision and thus experience the whole system evolution, losing part of their energy via scattering processes and gluon radiation. The presence of the QGP medium also affects the charm-quark hadronisation and, in addition to the fragmentation mechanism, a competing process based on charm–quark recombination with light quarks of the medium might occur. Given that strange quark–antiquark pairs are abundantly produced in the QGP, the recombination mechanism could enhance the yield of D_s⁺ mesons in Pb–Pb collisions with respect to that of D⁰ mesons, which do not contain strange quarks. 

ALICE investigated this possibility using the ratio of the yields of D_s⁺ and D⁰ mesons. The figure displays the D_s⁺/D⁰ yield ratio in central (0–10%) Pb–Pb collisions divided by the ratio in pp collisions, showing that the values of the ratio in the 2 < p_T< 8 GeV/c interval are higher in central Pb–Pb collisions by about 2.3σ. The measured D_s⁺ /D⁰ double ratio also hints at a peak for p_T ≃ 5–6 GeV/c. Its origin could be related to the different D-meson masses and to the collective radial expansion of the system with a common flow-velocity profile. In addition, the hadronisation via fragmentation becomes dominant at high transverse momenta, and consequently, the values of the D_s⁺/D⁰ ratio become similar between Pb–Pb and pp collisions.

The measurement was compared with theoretical calculations based on charm–quark transport in a hydrodynamically expanding QGP (LGR, TAMU, Catania and PHSD), which implement the strangeness enhancement and the hadronisation of charm quarks via recombination in addition to the fragmentation in the vacuum. The Catania and PHSD models predict a ratio almost flat in p_T, while TAMU and LGR describe the peak at p_T ≃  3–5 GeV/c. 

Complementary information was obtained by comparing the elliptic flow coefficient v₂ of D_s⁺ and non-strange D mesons (D⁰, D⁺ and D^*+) in semi-central (30–50%) Pb–Pb collisions. The D_s⁺– meson v₂ is positive in the 2 < p_T < 8 GeV/c interval with a significance of 6.4σ, and is compatible within uncertainties with that of non-strange D mesons. These features of the data are described by model calculations that include recombination of charm and strange quarks.

The freshly-completed upgrade of the detectors and the harvest of Pb–Pb collision data expected in Run 3 will allow the ALICE collaboration to further improve the measurements, deepening our understanding of the heavy-quark interaction and hadronisation in the QGP.