Dilepton production from ACS to SPS energies within a relativistic transport approach

We present a non-perturbative dynamical study of e(+)e(-) production in proton-nucleus and nucleus-nucleus collisions from AGS to SPS energies on the basis of the covariant transport approach HSD. For p+Be reactions the dilepton yield for invariant masses M less than or equal to 1.4 GeV is found to be dominated by the decays of the eta, rho, omega and Phi mesons at all energies from 10-450 GeV. For nucleus-nucleus collisions, however, the dilepton yield shows an additional large contribution from pi(+)pi(-), K+K- and pi rho channels. Systematic studies are presented for the ''free'' meson mass scenario in comparison to a ''dropping'' meson mass scenario at finite baryon density. We find that for ''dropping'' meson masses the invariant dilepton mass range 0.35 GeV less than or equal to M less than or equal to 0.65 GeV is increased in comparison to the ''free'' meson mass scenario and that the data of the CERES Collaboration for nucleus-nucleus collisions can be described much better within the ''dropping'' mass scheme. We study in detail the contributions from the various dilepton channels as a function of the transverse momentum and rapidity of the lepton pair as well as a function of the charged particle multiplicity. Furthermore, various direct photon channels for S+Au at 200 GeV/u are computed and found to be well below the upper bounds measured by the WA80 Collaboration (C) 1997 Elsevier Science B.V.