Hadronic centrality dependence in nuclear collisions

The kaon number density in nucleus+nucleus and p+p reactions is investigated for the first time as a function of the initial energy density ∈ and is found to exhibit a discontinuity around ∈ = 1.3 GeV fm^-3. This suggests a higher degree of chemical equilibrium for ∈ > 1.3 GeV fm^-3. It can also be interpreted as a reflection of the same discontinuity, appearing in the chemical freeze-out temperature (T) as a function of ∈. The N^α∼1 dependence of (u, d, s) hadrons, with N the number of participating nucleons, also indicates a high degree of chemical equilibrium and T saturation, reached at ∈ > 1.3 GeV fm^-3. Assuming that the intermediate mass region (IMR) dimuon enhancement seen by NA50 is due to open charm (D̄), the following observation can be made. (a) Charm is not equilibrated. (b) J / Ψ / DD̄ suppression, unlike J / Ψ / DY, appears also in S + A collisions, above ∈ ∼ 1 GeV fm^-3. (c) Both charm and strangeness show a discontinuity near the same ∈. (d) J / Ψ could be formed mainly through cc̄ coalescence. (e) The enhancement factors of hadrons with u, d, s, c quarks may be connected in a simple way to the mass gain of these particles if they are produced out of a quark-gluon plasma (QGP). We discuss these results as possible evidence for the QCD phase transition occurring near ∈ ∼ 1.3 GeV fm^-3.