Equilibrium Strikes Out

Abstract

In relativistic collisions of lead nuclei at 160-A·GeV, measurements at CERN suggest that a system has been created with an energy density approaching 3 GeV per fm³. Due to the relativistic contraction of incoming nuclei and the semi-transparent nature of the stopping, the thermalized system is initially pancake-shaped and rapidly expanding along the beam axis. During the first five fm/c, the energy density falls to of the order of 500 MeV/fm³, approximately the energy density of the interior of a single hadron, cooling from a range where quarks and gluons are appropriate degrees of freedom to conditions where a hadronic vocabulary is justified.