EXCITATION-FUNCTION OF ENERGY DENSITY AND PARTONIC DEGREES OF FREEDOMIN RELATIVISTIC HEAVY-ION COLLISIONS

We estimate the energy density epsilon pile-up at mid-rapidity in cent ral Pb+Pb collisions from 1 - 200 GeV/nucleon. epsilon is decomposed i nto hadronic and partonic contributions. A detailed analysis of the co llision dynamics in the framework of a microscopic transport model sho ws the importance of partonic degrees of freedom and rescattering of l eading (di)quarks in the early phase of the reaction for E-lab greater than or equal to 30 GeV/nucleon. The energy density reaches up to 4 G eV/fm(3), 95% of which are contained in partonic degrees of freedom. I t is shown that cells of hadronic matter, after t approximate to 2R/ga mma, can be viewed as nearly chemically equilibrated. This matter neve r exceeds energy densities of similar to 1 GeV/fm(-3), i.e. a density above which the notion of separated hadrons loses its meaning.