Hadronic centrality dependence in nuclear collisions

Abstract

The kaon number density in nucleus + nucleus and p + p reactionsis investigated for the first time as a function of the initial energydensity ϵ and is found to exhibit a discontinuity aroundϵ = 1.3 GeV fm-3. This suggests a higher degree of chemicalequilibrium for ϵ > 1.3 GeV fm -3. It can also be interpretedas a reflection of the same discontinuity, appearing inthe chemical freeze-out temperature (T) as a functionof ϵ. The Nα~1 dependence of (u,d, s) hadrons, with N the number of participatingnucleons, also indicates a high degree of chemicalequilibrium and T saturation, reached at ϵ> 1.3 GeV fm-3. Assuming that the intermediatemass region (IMR) dimuon enhancement seen by NA50 is dueto open charm , the followingobservation can be made.(a) Charm is not equilibrated.(b) J/Ψ/ suppression,unlike J/Ψ/DY,appears also in S + A collisions, above ϵ1 ~ GeV fm-3.(c) Both charm and strangeness show a discontinuity near the same ϵ.(d) J/Ψ could be formedmainly through coalescence.(e) The enhancement factorsof hadrons with u, d, s, c quarks may be connected in a simple way to themass gain of these particles if they are produced out of a quark-gluonplasma (QGP).We discuss these results as possible evidence for the QCDphase transition occurring near ϵ ~ 1.3 GeV fm-3.