Coherent Photoproduction of Dileptons on Light Nuclei: a New Means to Learn about Vector Mesons

In the last years much work has been done to learn about the properties of the $\rho$-meson in nuclear matter. In a calculation of the in-medium spectralfunction of the $\rho$ we find that it is strongly modified by baryonic resonances, especially the $D_{13}(1520)$, through its coupling to resonance-hole states. In order to test the predictions of a model for the in-medium properties of the $\rho$-meson so far mostly heavy-ion collision have been used. We argue that the coherent photoproduction of $\rho$-mesons yields additional information about the nature of the medium modification. It can be shown that the production amplitude is sensitive on the momentum dependence of the $\rho$-selfenergy. This can be used to distinguish experimentally between various models for the $\rho$ in matter