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

Abstract

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 ⩽ 1.4 GeV is found to be dominated by the decays of the η, ϱ, ω and Φ mesons at all energies from 10–450 GeV. For nucleus-nucleus collisions, however, the dilepton yield shows an additional large contribution from π + π −, K + K − and πϱ 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 ⩽ M ⩽ 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.