Non-thermal distributions of charm and charmonium in relativistic heavy-ion collisions

Abstract

We employ the Boltzmann transport model to study the charmonium regeneration with non-thermal charm quarks in relativistic heavy-ion collisions. As heavy quarks do not reach kinetic thermalization in the quark-gluon plasma (QGP), the final transverse momentum distribution of regenerated charmonium depends on the degree of charm quark kinetic thermalization. When the charm momentum distribution becomes harder, more charm quarks are distributed in the middle and high $p_T$, where the production of regenerated charmonium also becomes larger. With non-thermal momentum distribution of charm quarks in their coalescence process, the nuclear modification factor $R_{AA}(p_T)$ of charmonium enhances at middle $p_T$. Besides, the elliptic flow $v_2(p_T)$ of charmonium also enhances at middle $p_T$ as more regenerated charmonium are distributed in this $p_T$ region. The theoretical calculations with non-thermal charm distribution explain well the $p_T$ dependence of charmonium $R_{AA}$ and $v_2$, which indicates that charm quarks do not reach complete kinetic thermalization in the QGP when charmonium are regenerated.