THE ROLE OF QUANTUM EFFECTS AND NONEQUILIBRIUM TRANSPORT-COEFFICIENTS FOR RELATIVISTIC HEAVY-ION COLLISIONS

Stopping power and thermalization in relativistic heavy ion collisions is investigated employing the quantum molecular dynamics approach. For heavy systems stopping of the incoming nuclei is predicted, independent of the energy. The influence of the quantum effects and their increasing importance at low energies, is demonstrated by inspection of the mean free path of the nucleons and the n-n collision number. Classical models, which neglect these effects, overestimate the stopping and the thermalization as well as the collective flow and squeeze out. The sensitivity of the transverse and longitudinal momentum transfer to the in-medium cross section and to the pressure is investigated. The usefulness of thermodynamic concepts, e.g. density, temperature and pressure, is discussed. Local equilibration can be defined only in a fluid picture. It is proven that the projectile and target nuclei do not penetrate into each other, as assumed in the two-fluid model. They both collide instead with a 'participant' component, which consists of those nucleons which have suffered at least one collision. Local equilibration can reach up to about 80% in each separated fluid. It is shown that the Stress tensor in a one-fluid model cannot be cast in the Newtonian form due to the non-isotropic structure dictated by the initial conditions in relativistic heavy ion collisions. In the 'three-fluid' picture the transverse and longitudinal viscosity coefficients have nearly the same magnitude. Thus, both one- and two-fluid viscous hydrodynamic models are not justified microscopically. The three-fluid model and anisotropic hydrodynamics are currently the only macroscopic models which are supported by the microscopic theory.