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

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 ${\pi }^0,\eta ,\omega ,{\eta }^{\prime }$ mesons and of the $\Delta \mathrm{}\left(1232\right)\mathrm{}$ resonance, direct decays of vector mesons and $\mathrm{pn}$ bremsstrahlung. The microscopic calculations reproduce data for light systems fairly well, but tend to underestimate the data in $\mathrm{pp}$ at high energies and in $\mathrm{pd}$ at low energies. These conventional sources, however, cannot explain the recently reported enhancement for nucleus-nucleus collisions in the mass region $0.15\mathrm{\hspace{0.5em}}\mathrm{GeV}<~M_{e^{+}{e}^{-}}<~0.6\mathrm{\hspace{0.5em}}\mathrm{GeV}.$ Chiral scaling and $\omega$ meson broadening in the medium are investigated as a source of this mass excess. They also cannot explain the recent DLS data.