A mean-field potential version of the Ultra-relativistic Quantum Molecular
Dynamics (UrQMD) model is used to investigate the production of strange
baryons, especially the Λs and Λs, from heavy ion
collisions at SPS energies. It is found that, with the consideration of both
formed and pre-formed hadron potentials in UrQMD, the transverse mass and
longitudinal rapidity distributions of experimental data of both Λs and
Λs can be quantitatively explained fairly well. Our
investigation also shows that both the production mechanism and the
rescattering process of hadrons play important roles in the final yield of
strange baryons.Comment: 15 pages, 7 figure