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Ultrarelativistic Nuclear Collisions

Astrophysical objects and processes, both connected with very early and very late phenomena in the cosmological evolution of strongly interacting matter, present an enormous challenge to modern nuclear and particle physics: can we recreate -- in experiments carried out in the terrestrial laboratory -- the conditions prevailing during the first microseconds of the cosmological expansion, or during the late stages of a violent supernova stellar implosion? These investigations culminate, for the time being, in the CERN SPS which accelerates Pb nuclei to 158 GeV per nucleon to create extended "fireballs" of strongly interacting matter in head-on collisions of heavy nuclei. The systems created in these reactions reach energy densities close to the "critical" value of about 1.5 - 3 GeV/fm3, corresponding to temperatures of 150-190 MeV, where lattice QCD predicts the phase transition to a quark-gluon plasma as indicated in Figure [*]. Since its inception in 1986 at the AGS in Brookhaven and the SPS in CERN, the field of ultra-relativistic heavy ion physics has proceeded through three essential phases: The initial round of 'exploratory' experiments has shown that appropriate detectors and analysis procedures can cope with the extreme particle densities produced in heavy ion collisions. They have shown that an extended, interacting and very dense system has been formed that differs in many observables from the more elementary hadron-hadron reactions investigated in the past. Falling short of striking discoveries, this phase has nevertheless provided a 'principle proof of feasibility' and has substantiated the expectation that heavy ion collisions are an appropriate tool to create equilibrium hadron matter and eventually the quark-gluon plasma. The next phase was characterised by efforts to get a comprehensive and precise set of data and to come to a quantitative theoretical understanding of the experimental results. A close and very effective interaction between theory and experiment has led to significant progress in identifying relevant ingredients and important microscopic processes. For example, a consistent and very intriguing picture emerged describing all relevant data on production of J/$\chi$ taken with hadrons and light ions in terms of initial and final state interactions involving partons, nascent coloured hadrons and physical quarkonium states. The field is currently in its third, and most dramatic phase. The advent of a new generation of detectors, and most important, the availability of really heavy ion beams, has lead in the last 3 years to exciting new results. A number of major experiments have taken part in the AGS Au-program and, more recently, in the SPS Pb-program, which started in late 1994. Most of them are second generation experiments, either completely new or upgraded versions of detectors operating previously with lighter ions. Built on the experience gained with the lighter silicon and sulphur beams, these experiments cope well with the total hadron multiplicity of about 2500 in an average central Pb+Pb collision (600 at the lower energy Au+Au collisions at the AGS), either by extreme granularity in case of the large acceptance detectors or by extreme selectivity for the experiments focusing on very specific signals. They address the most crucial questions: Are there indications for deconfinement, indications for chiral symmetry restoration, indications for equilibrium hadronic matter? The tantalising answer today to each of these questions seems to be: yes! In the following, we will first describe progress in calculating the properties of the hadron-gas to QGP phase transition. Then we will turn to the current status of the experimental program and the understanding of the results. Finally, we give an outlook concerning the future of the field opening up with the 1999 startup of the BNL RHIC collider, and with the CERN LHC experiment ALICE in 2005. 
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