Freeze-out parameters in central 158A (GeVPb)-Pb-208+Pbcollisions

Neutral pion production in central 158A GeV Pb-208 + Pb-208 collisions has been studied in the WA98 experiment at the CERN Super Proton Synchrotron. The pi(0) transverse mass spectrum has been analyzed in terms of a thermal model with hydrodynamic expansion. The high accuracy and large kinematic coverage of the measurement allow one to limit previously noted ambiguities in the extracted freeze-out parameters. The results are shown to be sensitive to the shape of the velocity distribution at freeze-out.