Local equilibrium in heavy ion collisions: Microscopic model versus statistical model analysis

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 $t>~10\hspace{0.5em}\mathrm{fm}/c$ in a central cell. At these times the equation of state may be approximated by a simple dependence $P\cong (0.12\mathrm{}-0.15)\varepsilon$. 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.