June 16, 2015 15:44 60 Years of CERN Experiments and Discoveries – 9.75in x 6.5in b2114-ch03 page 61 Toward the Limits of Matter: Ultra-relativistic Nuclear Collisions at CERN Jurgen Schukraft1 and Reinhard Stock2 1CERN, CH-1211 Geneva 23, Switzerland
[email protected] 2FIAS, 60438 Frankfurt am Main, Germany
[email protected] 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, ultra- relativistic heavy ion physics is today a central pillar of contemporary nuclear physics and forms a significant part of the LHC programme. 1. Strongly Interacting Matter We recall here the development of a novel research field at CERN, devoted to the phases, and phase structure of matter governed by the strong fundamental force. Its proper field theory was discovered around 1970: Quantum Chromodynamics (QCD) addresses the fundamental interactions of elementary quarks, as mediated by gluons. Importantly,the gluons carry strong charges themselves, unlike the uncharged photons that mediate the QED interaction. Thus QCD is a much more complicated theory, mathematically. Both these field theories constitute a part of the modern Standard Model of elementary interactions.