We investigate dilepton production in transport-based approaches and show that the baryon couplings of the $rho$ meson represent the most important ingredient for understanding the measured dilepton spectra. At SIS energies, the baryon resonances nat
urally play a major role and affect already the vacuum spectra via Dalitz-like contributions, which can be captured well in transport simulations. Recent pion-beam measurements at GSI will help to constrain the properties of the involved resonances further.
We investigate dilepton production in transport-based approaches and show that the baryon couplings of the $rho$ meson represent the most important ingredient for understanding the measured dilepton spectra. At low energies (of a few GeV), the baryon
resonances naturally play a larger role and affect already the vacuum spectra via Dalitz-like contributions, which can be captured well in an on-shell-transport scheme. At higher energies, the baryons mostly affect the in-medium self energy of the $rho$, which is harder to tackle in transport models and requires advanced techniques.
We present dilepton spectra from p+p, d+p and p+Nb reactions at SIS energies, which were simulated with the GiBUU transport model in a resonance model approach. These spectra are compared to the data published by the HADES and DLS collaborations. It
is shown that the $rho$ spectral function includes non-trivial effects already in elementary reactions, due to production via baryon resonances, which can yield large contributions to the dilepton spectrum. Dilepton spectra from nuclear reactions in the energy range of the HADES experiment are thus found to be sensitive also to properties of nucleon resonances in the nuclear medium.
