Dilepton production and mTm_T-scaling at BEVALAC/SIS energies

We present a dynamical study of $e^+e^-$ production in C + C and Ca + Ca collisions at BEVALAC/SIS energies on the basis of the covariant transport approach HSD employing momentum-dependent $\rho$-meson spectral functions that include the pion modifications in the nuclear medium as well as the polarization of the $\rho$-meson due to resonant $\rho - N$ scattering. We find that the experimental data from the DLS collaboration cannot be described within the $\rho$-meson spectral function approach. A dropping $\eta$-mass scenario leads to a good reproduction of the DLS dilepton data, however, violates the $m_T$-scaling of $\pi^0$ and $\eta$ spectra as observed by the TAPS collaboration as well as $\eta$ photoproduction on nuclei.Comment: 35 pages, ReVTeX, including 11 postscript figures, UGI-97-06, Nucl. Phys. A, in pres