Probes for the early reaction dynamics of heavy ion collisions at AGS and SPS

The early evolution of ultra-relativistic heavy ion collisions is discussed within a multi-fluid dynamical model. In particular, it is shown that due to the finite mean-free path of the particles compression shock waves are smeared out considerably as compared to the one-fluid limit. Also, the maximal energy density of the baryons is much lower. The time scale of kinetic equilibration of the baryons in the central region and its relevance for directed flow is discussed. Finally, thermal emission of direct photons from the fluid of produced particles is calculated within the three-fluid model and two other simple expansion models. It is shown that the transverse momentum and rapidity spectra of photons give clue to the cooling law and the early rapidity distribution of the photon source. (author)