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Description
Heavy-flavour production represents a crucial probe for studying transport properties of the Quark-Gluon Plasma (QGP), with the spatial diffusion coefficient $D_s(T)$ encoding the interaction strength between heavy quarks (HQs) and the medium. Recent lattice QCD (lQCD) results with dynamical fermions show very low values, $2\pi T D_s \approx 1$ for charm quarks at $T = T_c$, much lower than quenched QCD and most phenomenological models ($2\pi T D_s \approx \, 3.5–5$). These values imply short thermalization times ($\tau_{th} \approx \, 1–1.5 fm/c$) for HQs, raising questions about their compatibility with experimental data such as the nuclear modification factor $R_{AA}$, and flow coefficients $v_2$, $v_3$ of D mesons and $\Lambda_c$ baryons; both in close agreement to the recent experimental data of ALICE and CMS. We study this aspect using an event-by-event Langevin transport model. In particular, we test different scenarios and show that low $D_s$ ($p \rightarrow 0$) values can match experimental data only if the thermalization time $\tau_{th}(p) = 1/A(p)$ depends strongly on momentum, as predicted by T-matrix approaches and the extended Quasi-Particle Model (QPMp). In contrast, assuming a constant $\tau_{th} = M_c D_s^{lQCD} / T$ does not reproduce the observed experimental trends. We also study the implications of a small thermalization time for both charm and bottom quarks. Moreover, fast thermalization makes final-state observables largely insensitive to the initial charm-quark momentum distribution up to $p_T\approx M_c$, suggesting a universal behavior driven by a dynamical attractor.
[1] M.L.Sambataro, V. Minissale, S. Plumari and V. Greco, Phys.Lett.B 849 (2024) 138480.
[2] M. L. Sambataro, V. Greco, G. Parisi and S. Plumari, Eur.Phys.J.C 84 (2024) 9, 881.
[3] M. L. Sambataro, V. Minissale, S. Plumari and V. Greco, e-print:2508.01024 (accepted by PLB).