The preliminary data on hadron multiplicities measured in central Pb+Pb collisions at $158A$ $\mathrm{GeV}/c$ 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 pion enhancement due to different hard-core repulsion for pions and other hadrons and strangeness suppression because of incomplete chemical equilibrium. Each of these two modifications improves the results. The combined effect of these two mechanisms leads to 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.

• Received 5 October 1998

DOI:https://doi.org/10.1103/PhysRevC.59.2788