Phase-space densities and effects of resonance decays in hydrodynamic approach to heavy ion collisions

The method that allows to analyse an overpopulation of phase-space in heavy ion collisions in model independent way is proposed within hydrodynamic approach. It makes possible to extract a chemical potential of thermal pions at freeze out irrespective of a form of freeze-out (isothermal) hypersurface in Minkowski space and transverse flows on it. The contributions of resonance (with masses up to 2 GeV) decays to spectra, interferometry volumes and phase-space densities are calculated and discussed in detail. The estimates of average phase-space densities and chemical potentials of the thermal pions are obtained for the SPS and RHIC energies. They demonstrate that multibosonic phenomena at those energies might be considered rather as a correction factor than as an significant physical effect. The analysis of evolution of the pion average 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 the freeze-out criterion and final thermodynamic parameters, being connected rather to the initial phase-space density formed in relativistic nucleus-nucleus collisions. It could explain its relative high value at BNL RHIC and shed light to the HBT puzzle. 1 Bogolyubov Institute for Theoretical Physics, Kiev 03143, Metrologichna 14b, Ukraine. PACS: 24.10.Nz, 24.10.Pa, 25.75.-q, 25.75.Gz, 25.75.Ld.