Indication of a Co-Existing Phase in Nucleus - Nucleus Collisions at CERN-SPS Energies

The variation of average transverse mass of identified hadrons with charge multiplicity have been studied for AGS, SPS and RHIC energies. The observation of a plateau in the average transverse mass for multiplicities corresponding to SPS energies is attributed to the formation of a co-existence phase of quark gluon plasma and hadrons. There is also an indication of a subsequent rise for RHIC energies. Several possibilities which can affect the average transverse mass are discussed. Constraints on the initial temperature and thermalization time have been put from the various experimental data available at SPS energies.