Universal Freeze-Out Criterion and Phase-Space Density in Ultrarelativistic Heavy-Ion Collisions

We explore the consequences of a freeze-out criterion for ultrarelativistic heavy-ion collisions, based on pion escape probabilities from the hot and dense but rapidly expanding collision region. We study to what extent the recently observed increase in pion phase space density from SPS to RHIC can be understood by the smaller nucleon contribution to the pion scattering rate. We find that the dependence of the pion scattering rate on the momentum of the escaping particle can contribute significantly to the transverse momentum slope of transverse HBT radius parameters, and that its temperature dependence favours rather low freeze-out temperatures of 100 MeV at most.