Thermal Freeze-out and Longitudinally Non-uniform Collective Expansion   Flow in Relativistic Heavy Ion Collisions
  The non-uniform longitudinal flow model (NUFM) propsed recently is extended to include also transverse flow. The resulting longitudinally non-uniform collective expansion model (NUCEM) is applied to the calculation of rapidity distribution of kaons, lambdas and protons in relativistic heavy ion collisions at CERN-SPS energies. The model results are compared with the 200 A GeV/$c$ S-S and 158 A GeV/$c$ Pb-Pb collision data. The central dips observed in experiments are reproduced in a natural way. 
FENG Shengqin, LIU Feng and LIU Lianshou