0 The ρ Spectral Function in a Relativistic Resonance Model

We calculate the spectral function A ρ of the ρ meson in nuclear matter. The calculation is performed in the low density approximation , where the in-medium self energy Σ med is completely determined by the vacuum ρ N forward scattering amplitude. This amplitude is derived from a relativistic resonance model. In comparison to previous non-relativistic calculations we find a much weaker momentum dependence of Σ med , especially in the transverse channel. Special attention is paid to uncertainties in the model. Thus, we compare the impact of different coupling schemes for the RN ρ interaction on the results and discuss various resonance parameter sets.