The universal freeze-out criterion at SPS and RHIC

We formulate a freeze-out criterion for ultra-relativistic heavy ion collisions in terms of the pion escape probability from the collision region. We find that the increase in pion phase-space density from SPS to RHIC reported at this conference has a small influence on particle freeze-out because of the small pi-pi cross-section. Our treatment takes into account dynamical expansion, chemical composition and momentum of the escaping particle for particle freeze-out. It supports a freeze-out at rather low temperature - below 100 MeV - and earlier decoupling of high p_t particles.Comment: proceedings of Quark Matter 2002, 4 pages, 2 figure

Possible Origin of RHIC R_{out}/R_{sid} HBT Results

The effects of opacity of the nuclei together with a blackbody type of emission along the system history are considered as a means to explain the ratio $R_{out}/R_{sid}$ observed by STAR and PHENIX collaborations at RHIC. Within our model, no flow is required to explain the data trend of this ratio for large surface emissivities.Comment: 4 pages, 2 Postscript figures, uses espcrc1.sty. Talk presented at Quark Matter 2002, Nantes, France, July 18-24, 2002; to appear in the proceedings (Nucl. Phys. A