Direct emission of multiple strange baryons in ultrarelativistic heavy-ion collisions from the phase boundary

We discuss a model for the space-time evolution of ultrarelativistic heavy-ion collisions which employs relativistic hydrodynamics within one region of the forward light-cone, and microscopic transport theory (i.e. UrQMD) in the complement. Our initial condition consists of a quark-gluon plasma which expands hydrodynamically and hadronizes. After hadronization the solution eventually changes from expansion in local equilibrium to free streaming, as determined self-consistently by the interaction rates between the hadrons and the local expansion rate. We show that in such a scenario the inverse slopes of the m(T)-spectra of multiple strange baryons (Xi, Omega) are practically unaffected by the purely hadronic stage of the reaction, while the flow of p's and Lambda's increases. Moreover, we find that the rather "soft" transverse expansion at RHIC energies (due to a first-order phase transition) is not washed out by strong rescattering in the hadronic stage. The earlier kinetic freeze-out as compared to SPS-energies results in similar inverse slopes (of the m(T)-spectra of the hadrons in the final state) at RHIC and SPS energies. (C) 1999 Published by Elsevier Science B.V. All rights reserved.