Intermediate mass excess of dilepton production in heavy ion collisions at BEVALAC energies

Abstract

Dielectron mass spectra are examined for various nuclear reactions recently measured by the DLS collaboration. A detailed description is given of all dilepton channels included in the transport model UrQMD 1.0, i.e. Dalitz decays of $π^0,η,ω,η'$ mesons and of the $Δ(1232)$ resonance, direct decays of vector mesons and $pn$ bremsstrahlung. The microscopic calculations reproduce data for light systems fairly well, but tend to underestimate the data in $pp$ at high energies and in $pd$ at low energies. These conventional sources, however, cannot explain the recently reported enhancement for nucleus-nucleus collisions in the mass region 0.15 GeV<$M_{ee}$<0.6 GeV. Chiral scaling and $ω$ meson broadening in the medium are investigated as a source of this mass excess. They also cannot explain the recent DLS data.