The analysis of particle multiplicities in Pb+Pb collisions at CERN SPS within hadron gas models

The preliminary data on total hadron multiplicities measured by NA49 Collaboration in central Pb+Pb collisions at 158A GeV/c are analyzed. The ideal hadron gas model fails to give a reasonable explanation to the Pb+Pb data sets. We study the possible effects of strangeness suppression because of incomplete chemical equilibrium and pion enhancement due to different hard-core repulsion for pions and other hadrons. Each of these two modifications significantly improves the results. The combined effect of these two mechanisms results in an extremely good agreement with the data. An interpretation of the obtained results in terms of the possible quark-gluon plasma formation at the early stage of the collision is also discussed.