Hadronic and electromagnetic probes of hot and dense nuclear matter

We review the constraints imposed by the spontaneously broken chiral symmetry of the QCD vacuum on the hadron properties at finite temperature T and baryon density rho(B). A restoration of chiral symmetry is indicated by the dropping of the scalar quark condensate [(q) over bar q] at finite T and rho(B) in various approaches. This suggests the hadrons to become approximately massless in hot and dense nuclear matter or the vector and axial vector currents to become equal. In this respect we study the properties of hadrons - as produced in relativistic nucleus-nucleus collisions - by means of a covariant hadronic transport approach where scalar and vector hadron self-energies are taken into account explicitly which are modelled in terms of effective 'chiral' Lagrangians. Within this transport approach we investigate the reaction dynamics of relativistic heavy-ion collisions and analyse experimental data on pi, eta, K+, K-, rho, omega, phi, (p) over bar and charmonium production for proton-nucleus and nucleus-nucleus collisions from SIS to SPS energies (1-200A GeV). Whereas pi, eta and to some extent K+ mesons are found not to change their properties in the nuclear medium substantially, antiprotons and antikaons do show sizeable attractive self-energies as can be extracted from their experimental abundancies and spectra. The properties of the vector mesons rho, omega and phi at finite baryon density are investigated by their dileptonic decay; the CERES and HELIOS-3 data at SPS energies are found to be incompatible with a 'bare' vector meson mass scenario. Here, a description by 'dropping' rho and omega masses leads to a very good reproduction of the data, however, also approaches based on more conventional hadronic interactions as pion polarizations and meson-nucleon scattering amplitudes are compatible with the present dilepton spectra at SPS energies. Constraints from dilepton studies at BEVALAC/SIS energies are investigated in all decay schemes as well as a variety of further observables that allow to disentangle the different scenarios experimentally. Furthermore, the charmonium production and suppression in proton-nucleus and nucleus-nucleus collisions is investigated within the transport approach in order to probe a possible transition to a quark-gluon plasma (QGP) phase. We finally discuss 'optimized' observables for an experimental investigation of the restoration of chiral symmetry and/or the phase transition to a quark-gluon plasma. (C) 1999 Elsevier Science B.V. All rights reserved.