Heavy quark mass and potential in QGP
We study the thermal production of charm quarks in relativistic heavy-ion collisions, using the off-shell Parton-Hadron-String Dynamics (PHSD) transport approach. The production cross sections of thermal charm quarks are calculated within the effective Dynamical Quasi-Particle Model (DQPM), which reproduces the spatial diffusion coefficients of heavy quarks from lQCD, while charm quark annihilation is realized by detailed balance. We find that the number of thermally produced charm quark pairs strongly depends on the charm quark mass in the QGP. Compared to the experimental data on the R AA of D mesons in heavy ion collisions at RHIC and LHC energies, it is more favorable for charm quarks in the QGP to gain additional mass due to thermal effects than to have a low bare mass. This conclusion is further supported by the continuity of charm number densities at T c , obtained by comparing the hadron resonance gas model in the hadronic phase and the quasi-particle model in the QGP. The heavy quark mass in QGP is related to the heavy quark potential at a large distance. We test three different heavy quark potentials, namely the free and internal energies of the heavy quark pair in QGP, and the unscreened potential recently proposed by the HotQCD Collaboration through the thermal production of charm quarks in heavy ion collisions. We find that the free energy potential overestimates the charm production in heavy-ion collisions at the LHC, while the unscreened potential produces results closest to the experimental data from the ALICE collaboration among the three potentials.