Hadronic centrality dependence in nuclear collisions

Abstract

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 $ε$ and is found to exhibit a discontinuity around $ε$=1.3 GeV/fm$^3$. This suggests a higher degree of chemical equilibrium for $ε>$ 1.3 GeV/fm$^3$. It can also be interpreted as reflection of the same discontinuity, appearing in the chemical freeze out temperature (T) as a function of $ε$. The $N^{α\sim 1}$ 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 $ε>$1.3 GeV/fm$^3$.Assuming that the intermediate mass region (IMR) dimuon enhancement seen by NA50 is due to open charm ($D \bar{D}$), the following observation can be made: a) Charm is not equilibrated. b) $J/Ψ/D \bar{D}$ suppression -unlike $J/Ψ/DY$- appears also in S+A collisions, above $ε$ $\sim$1 GeV/fm$^3$. c) Both charm and strangeness show a discontinuity near the same $ε$. d) $J/Ψ$ could be formed mainly through $c \bar{c}$ 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 occuring near $ε\sim $1.3 GeV/fm$^3$.