0 O ct 2 00 3 Phase - space densities , their evolution and HBT puzzle in A + A collisions

The approach that allows to analyse an overpopulation of phase-space in heavy ion collisions in model independent way is proposed. It gives the possibility to extract a chemical potential of thermal pi-ons at freeze out irrespective of a form of freeze-out hypersurface in Minkovsky space and transverse flows on it. The correspondent estimates , that operate correctly with contributions to phase-space density from resonance decays, are obtained for SPS and RHIC energies. They demonstrate that multibosonic effects at those energies could be considered rather as correction factor than as important physical phenomena. The analysis of evolution of the average pion phase-space density in chemically frozen hadron systems shows that it is almost constant or slightly increases with time while the particle density and phase-space density at each space point drops down rapidly during the system expansion. We found, that, unlike to the particle density, the average phase-space density has no direct link to a final (freeze-out) conditions, being connected rather to the initial phase-space density formed in relativistic nucleus-nucleus collisions. It could explain its relative high value at RHIC and HBT puzzle.