Hadronic and electromagnetic probes of hot and dense nuclear matter

We review the constraints imposed by the spontaneously broken chiral symmet ry of the QCD vacuum on the hadron properties at finite temperature T and b aryon 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 rh o(B) in various approaches. This suggests the hadrons to become approximate ly 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 hadron s - as produced in relativistic nucleus-nucleus collisions - by means of a covariant hadronic transport approach where scalar and vector hadron self-e nergies are taken into account explicitly which are modelled in terms of ef fective 'chiral' Lagrangians. Within this transport approach we investigate the reaction dynamics of relativistic heavy-ion collisions and analyse exp erimental data on pi, eta, K+, K-, rho, omega, phi, (p) over bar and charmo nium 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 substantiall y, antiprotons and antikaons do show sizeable attractive self-energies as c an be extracted from their experimental abundancies and spectra. The proper ties of the vector mesons rho, omega and phi at finite baryon density are i nvestigated by their dileptonic decay; the CERES and HELIOS-3 data at SPS e nergies are found to be incompatible with a 'bare' vector meson mass scenar io. Here, a description by 'dropping' rho and omega masses leads to a very good reproduction of the data, however, also approaches based on more conve ntional hadronic interactions as pion polarizations and meson-nucleon scatt ering amplitudes are compatible with the present dilepton spectra at SPS en ergies. Constraints from dilepton studies at BEVALAC/SIS energies are inves tigated in all decay schemes as well as a variety of further observables th at allow to disentangle the different scenarios experimentally. Furthermore , the charmonium production and suppression in proton-nucleus and nucleus-n ucleus collisions is investigated within the transport approach in order to probe a possible transition to a quark-gluon plasma (QGP) phase. We finall y 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.