The dependence of K+ production on the nuclear equation of state is investigated in heavy-ion collisions. An increase of the excitation function of K+ multiplicities obtained in heavy (Au+Au) over light (C+C) systems when going far below threshold which has been observed by the KaoS Collaboration strongly favors a soft equation of state. This observation holds despite the influence of an in-medium kaon potential predicted by effective chiral models which is necessary to reproduce the experimental K+ yields.

The freeze-out conditions in the light (S+S) and heavy (Pb+Pb) colliding systems of heavy nuclei at 160 AGeV/c are analyzed within the microscopic Quark Gluon String Model (QGSM). We found that even for the most heavy systems particle emission takes place from the whole space-time domain available for the system evolution, but not from the thin "freeze-out hypersurface", adopted in fluid dynamical models. Pions are continuously emitted from the whole volume of the reaction and reflect the main trends of the system evolution. Nucleons in Pb+Pb collisions initially come from the surface region. For both systems there is a separation of the elastic and inelastic freeze-out. The mesons with large transverse momenta, p t , are predominantly produced at the early stages of the reaction. The low p t -component is populated by mesons coming mainly from the decay of resonances. This explains naturally the decreasing source sizes with increasing p t , observed in HBT interferometry. Comparison with S+S and Au+Au systems at 11.6 AGeV/c is also presented.PACS numbers: 24.10. Lx, 25.75.Gz, 25.75.Dw Typeset using REVT E X * Present address:

The assumption of local equilibrium in relativistic heavy ion collisions at energies from 10.7A GeV (AGS) up to 160A GeV (SPS) is checked in the microscopic transport model. Dynamical calculations performed for a central cell in the reaction are compared to the predictions of the thermal statistical model. We find that kinetic, thermal, and chemical equilibration of the expanding hadronic matter are nearly approached late in central collisions at AGS energy for t ≥ 10 fm/c in a central cell. At these times the equation of state may be approximated by a simple dependence P ∼ = (0.12 − 0.15) ε. Increasing deviations of the yields and the energy spectra of hadrons from statistical model values are observed for increasing energy, 40A GeV and 160A GeV. These violations of local equilibrium indicate that a fully equilibrated state is not reached, not even in the central cell of heavy ion collisions at energies above 10A GeV. The origin of these findings is traced to the multiparticle decays of strings and many-body decays of resonances.