Excitation function of energy density and partonic degrees of freedom in relativistic heavy ion collisions

We estimate the energy density ε pile-up at mid-rapidity in central Pb+Pb collisions from 1-200 GeV/nucleon. ε is decomposed into hadronic and partonic contributions. A detailed analysis of the collision dynamics in the framework of a microscopic transport model shows the importance of partonic degrees of freedom and rescattering of leading (di)quarks in the early phase of the reaction for Elab≥30 GeV/nucleon. The energy density reaches up to 4 GeV/fm3, 95% of which are contained in partonic degrees of freedom. It is shown that cells of hadronic matter, after t∼2R/γ, can be viewed as nearly chemically equilibrated. This matter never exceeds energy densities of ∝1 GeV/fm3, i.e. a density above which the notion of separated hadrons loses its meaning. (orig.)