INTERMEDIATE-MASS EXCESS OF DILEPTON PRODUCTION IN HEAVY-ION COLLISIONS AT RELATIVISTIC ENERGIES

Dielectron mass spectra are examined for various nuclear reactions rec ently measured by the DLS Collaboration. A detailed description is giv en of all dilepton channels included in the transport model UrQMD 1.0, i.e., Dalitz decays of pi(0), eta, omega, eta' mesons and of the Delt a(1232) resonance, direct decays of vector mesons and pn bremsstrahlun g. The microscopic calculations reproduce data for light systems fairl y well, but tend to underestimate the data in pp at high energies and in pd at low energies. These conventional sources, however, cannot exp lain the recently reported enhancement for nucleus-nucleus collisions in the mass region 0.15 GeV less than or equal to Me+e- less than or e qual to 0.6 GeV. Chiral scaling and w meson broadening in the medium a re investigated asa Source of this mass excess. They also cannot expla in the recent DLS data.