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 2–200 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. In Pb+Pb collisions at 160 GeV/nucleon the energy density reaches up to 4 GeV/fm3, 95% of which are contained in partonic degrees of freedom.