In relativistic collisions of lead nuclei at 160 centre A GeV, measurements at CERN suggest that a system has been created with an energy density approaching 3 GeV per fm~3. Duet o 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 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.
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