Dilepton production and $m_T$-scaling at BEVALAC/SIS energies

Abstract

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 $ρ$-meson spectral functions that include the pion modifications in the nuclear medium as well as the polarization of the $ρ$-meson due to resonant $ρ- N$ scattering. We find that the experimental data from the DLS collaboration cannot be described within the $ρ$-meson spectral function approach. A dropping $η$-mass scenario leads to a good reproduction of the DLS dilepton data, however, violates the $m_T$-scaling of $π^0$ and $η$ spectra as observed by the TAPS collaboration as well as $η$ photoproduction on nuclei.