Confrontation of Theoretical Approaches and Experimental Data on High Energy Heavy Ion Collisions

Abstract

We give an overview of high energy heavy ion collisions. The merits and drawbacks of macroscopic and microscopic theoretical approaches (Fluid Dynamics, TDHF, Cascade, Vlasov-Uehling-Uhlenbeck, Classical and Quantum Molecular Dynamics) are discussed. The importance of nonequilibrium transport properties (viscosity, mean free path, effective in-medium cross sections) and of the nuclear potential (equation of state) is pointed out. The liquid-vapour phase transition and multifragmentation have been studied. The possibility of meassuring Machshock fragments in inverse kinematics experiments is also pointed out. It is demonstrated that the projectile and target are stopped at YCM if central collisions are studied. The stopping is only sensitive to σeff. The predicted bounce-off of the rather cold fragments in the reaction plane and the predicted accompanying squeeze-out of the hot participant baryons perpendicular to the reaction plane are experimentally discovered. These effects are sensitive both to the viscosity (σeff(ρ,E,Ω)) and to the generalized equation of state (optical potential U(ρ, E)). The data clearly ask for a repulsive potential interaction. We conclude that nuclear matter produced in relativistic collisions is a hot, dense, viscous and rather incompresible fluid, with important quantum properties.