Electromagnetic Probes of Hot and Dense Nuclear Matter

Abstract

The primary motivation for studying high-energy heavy-ion collisions is to investigate nuclear matter under conditions of extremely high densities and temperatures. The response of the nuclear medium to changes of its temperature and density is described by the nuclear matter equation of state which is subject of quantitative discussions in the energy regime of 1 GeV per nucleon. Many new results from experiments adressing these questions at the Berkeley Bevalac and SIS in Darmstadt have been discussed during this meeting. Among the ultimate goals of ultrarelativistic nuclear collisions (E _proj ≳ 10 AGeV) is the formation and observation of a quark-gluon plasma (QGP) which has been predicted by QCD lattice calculations (for a recent review on this subject the reader is referred to Ref.¹). In such a novel state of matter quarks and gluons are deconfined over an extended volume and chiral symmetry may (partially) be restored. A vigorous experimental programme is now under way at the CERN-SPS and Brookhaven-AGS accelerators with additional preparations being carried out for Brookhaven’s Relativistic Heavy-Ion Collider (RHIC) and CERN’s Large Hadron Collider (LHC). The creation and observation of a QGP represents a considerable challenge, both in its experimental realization and also in the theoretical interpretation of the experimental results. The lifetimes involved are ofthe order of ≈ 10 fm/c and the detailed dynamics of the collision process may furthermore play an important role, because the subsequent hadronization tends to mask the signal from the QGP phase, thus complicating the extraction of a clear signal. Nevertheless, much progress has been made both in theory and experiment, and even in the absence of a genuine QGP the study of hot and dense hadronic systems is a fascinating subject from which a great deal can be learned².