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A statistical model of the early stage of central nucleus--nucleus (A+A) collisions is developed. We suggest a description of the confined state with several free parameters fitted to a compilation of A+A data at the AGS. For the deconfined state a simple Bag model equation of state is assumed. The model leads to the conclusion that a Quark Gluon Plasma is created in central nucleus--nucleus collisions at the SPS. This result is in quantitative agreement with existing SPS data on pion and strangeness production and gives a natural explanation for their scaling behaviour. The localization and the properties of the transition region are discussed. It is shown that the deconfinement transition can be detected by observation of the characteristic energy dependence of pion and strangeness multiplicities, and by an increase of the event--by--event fluctuations. An attempt to understand the data on J/psi production in Pb+Pb collisions at the SPS within the same approach is presented.
A validity of a recent estimate of an upper limit of charm production in central Pb+Pb collisions at 158 AGeV is critically discussed. Within a simple model we study properties of the background subtraction procedure used for an extraction of the charm signal from the analysis of dilepton spectra. We demonstrate that a production asymmetry between positively and negatively charged background muons and a large multiplicity of signal pairs leads to biased results. Therefore the applicability of this procedure for the analysis of nucleus-nucleus data should be reconsidered before final conclusions on the upper limit estimate of charm production could be drawn.
The hypothesis of statistical production of J/psi mesons at hadronization is formulated and checked against experimental data. It explains in the natural way the observed scaling behavior of the J/psi to pion ratio at the CERN SPS energies. Using the multiplicities of J/psi and eta mesons the hadronization temperature T_H = 175 MeV is found, which agrees with the previous estimates of the temperature parameter based on the analysis of the hadron yield systematics.
The pion multiplicity per participating nucleon in central nucleus-nucleus collisions at the energies 2-15 A GeV is significantly smaller than in nucleon-nucleon interactions at the same collision energy. This effect of pion suppression is argued to appear due to the evolution of the system produced at the early stage of heavy-ion collisions towards a local thermodynamic equilibrium and further isentropic expansion.