Evidence for a New State of Matter: An Assessment of the Results from the CERN Lead Beam Programme

The year 1994 marked the beginning of the CERN lead beam programme. A beam of 33 TeV (or 160 GeV per nucleon) lead ions from the SPS now extends the CERN relativistic heavy ion programme, started in the mid eighties, to the heaviest naturally occurring nuclei. A run with lead beam of 40 GeV per nucleon in fall of 1999 complemented the program towards lower energies. Seven large experiments participate in the lead beam program, measuring many different aspects of lead-lead and lead-gold collision events: NA44, NA45/CERES, NA49, NA50, NA52/NEWMASS, WA97/NA57, and WA98. Some of these experiments use multipurpose detectors to measure simultaneously and correlate several of the more abundant observables. Others are dedicated experiments to detect rare signatures with high statistics. This coordinated effort using several complementing experiments has proven very successful. The present document summarizes the most important results from this program at the dawn of the RHIC era: soon the relativistic heavy ion collider at BNL will allow to study gold-gold collisions at 10 times higher collision energies. Physicists have long thought that a new state of matter could be reached if the short range repulsive forces between nucleons could be overcome and if squeezed nucleons would merge into one another. Present theoretical ideas provide a more precise picture for this new state of matter: it should be a quark-gluon plasma (QGP), in which quarks and gluons, the fundamental constituents of matter, are no longer confined within the dimensions of the nucleon, but free to move around over a volume in which a high enough temperature and/or density prevails. This plasma also exhibits the so-called “chiral symmetry” which in normal nuclear matter is spontaneously broken, resulting in effective quark masses which are much larger than the actual masses. For the transition temperature to this new state, lattice QCD calculations give values between 140 and 180 MeV, corresponding to an energy density in the neighborhood of 1 GeV/fm 3 , or seven times that of nuclear matter. Temperatures and energy densities above these values existed in the early universe during the first few microseconds after the Big Bang.