Local thermal and chemical equilibration and the equation of state in relativistic heavy ion collisions

Thermodynamical variables and their time evolution are studied for central relativistic heavy ion collisions from 10.7 to 160 A GeV in the microscopic ultrarelativistic quantum molecular dynamics model (UrQMD). The UrQMD model exhibits drastic deviations from equilibrium during the early high density phase of the collision. Local thermal and chemical equilibration of the hadronic matter seems to be established only at later stages of the quasi-isentropic expansion in the central reaction cell with volume 125 fm(3). Baryon energy spectra in this cell are reproduced by Boltzmann distributions at all collision energies for t greater than or equal to 10 fm/c with a unique rapidly dropping temperature. At these times the equation of state has a simple form: P congruent to (0.12-0.15)epsilon. At SPS energies the strong deviation from chemical equilibrium is found for mesons, especially pions, even at the late stage of the reaction. The final enhancement of pions is supported by experimental data.