Toward the Limits of Matter: Ultra-relativistic nuclear collisions at CERN

Strongly interacting matter as described by the thermodynamics of QCD undergoes a phase transition, from a low temperature hadronic medium, to a high temperature quark–gluon plasma state. In the early universe this transition occurred during the early microsecond era. It can be investigated in the laboratory, in collisions of nuclei at relativistic energy, which create “fireballs” of sufficient energy density to cross the QCD phase boundary. We describe three decades of work at CERN, devoted to the study of the QCD plasma and the phase transition. From modest beginnings at the SPS, ultrarelativistic heavy ion physics is today a central pillar of contemporary nuclear physics and forms a significant part of the LHC programme.