The physics of ultra-relativistic heavy-ion collisions

The physics of ultra-relativistic heavy-ion collisions is presented, with particular emphasis on experimental results, the basic concepts used in interpreting data and some of the principal problems encountered in this new and rapidly evolving field. A number of selected results from experiments at CERN and BNL are reviewed and compared qualitatively with models based on hadronic scenarios, as well as with models including quark-gluon plasma (QGP) formation. No unambiguous evidence for this formation has emerged from the data so far, but some large and significant effects have been observed, which clearly show that nucleus-nucleus collisions cannot be described as a straightforward superposition of independent nucleon-nucleon reactions. Results from a number of independent observables (p perpendicular to distributions, strangeness abundances, J/ psi suppression, particle interferometry) indicate the existence of an extended and strongly interacting system. The first exploratory phase of experimentation seems therefore to confirm that ultra-relativistic heavy-ion collisions are a promising tool for studying the properties of bulk hadronic matter under extreme conditions.