Coarse-Graining of Heavy-Ion Collisions at Fermi Energy*

Heavy-ion collisions aim at exploring the properties of hot and dense nuclear matter. Among the key questions are if and how the produced medium reaches (local) thermal equilibrium enabling the study of generic properties of strongly interacting matter. Here, we utilize a coarse graining method to investigate the kinetic properties of the fireball formed in nuclear collisions at Fermi energies. Based on the output from microscopic transport simulations for nucleon positions and momenta within the Constrained Molecular Dynamics (CoMD) model, we employ fit functions for the single-nucleon momentum distributions and extract the time evolution of local thermodynamic properties, focusing on central collisions of Ca-40 nuclei at 35 AMeV bombarding energy. It turns out that the transverse-momentum distributions are well described by Fermi distributions with a time-dependent temperature and chemical potential. In the longitudinal direction, off-equilibrium effects due to the primordial motion of the incoming nuclei are essential and are accounted for by introducing a two-centroid motion into the distributions. We find that the ``directed" kinetic energy associated with the centroid motion dissipates after about 150fm/c, at which point approximate local thermal equilibrium is reached at temperatures of around 6 MeV. We convert our results into a trajectory in the phase diagram and discuss future applications of calculating photon emission spectra.