Reaction dynamics in Pb + Pb at the CERN/SPS: From partonic degrees of freedom to freeze-out

We analyze the reaction dynamics of central Pb+Pb collisions at 160 GeV/nucleon. First we estimate the energy density e pile-up at mid-rapidity and calculate its excitation function: e 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 ⩾ 30GeV/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/γνcm can be viewed as nearly chemically equilibrated. This matter never exceeds energy densities of ∼ 0.4 GeV/fm3, i.e. a density above which the notion of separated hadrons loses its meaning. The final reaction stage is analyzed in terms of hadron ratios, freeze-out distributions and a source analysis for final state pions.