Description of collective flow in HI central collisions using a semi-analytical approach

Available as short communication only. The first studies of Au + Au collisions at 150 MeV/u using Phase I of the FOPI detector at GSI evidenced a midrapidity source of intermediate mass fragments for central collisions. The mean kinetic energy per nucleon for these fragments as a function of their atomic number suggests the presence of a nonthermal component. Preliminary results of a method to extract quantitative information on such a collective component are shown. The main ingredients of a formal model based on the physical image of a spherically symmetric expanding fireball characterized by thermally equilibrated local momentum distribution are presented. The aim of this model is to extract information and systematics on some quantities like compressibility, the number of participant nucleons, the shape of the compressed and hot zone and its dynamics by fitting the experimental data. Qualitative results obtained for a hard equation of state for the Au+Au system at 400 MeV/u are shown. (Author) 2 Figs., 10 Refs