Freeze-Out Parameters in Central158AGeVP208b+PbCollisions

Neutral pion production in central $158A\mathrm{GeV}$ ${}^{208}\mathrm{Pb}+{}^{208}\mathrm{Pb}$ 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.