Collective Expansion in High Energy Heavy Ion Collisions

Transverse mass spectra of pions, kaons, and protons from the symmetric heavy-ion collisions 200 $A$ GeV $\mathrm{S}+\mathrm{S}$ and 158 $A$ GeV $\mathrm{Pb}+\mathrm{Pb}$, measured in the NA44 focusing spectrometer at CERN, are presented. The mass dependence of the slope parameters provides evidence of collective transverse flow from expansion of the system in heavy-ion induced central collisions.