The photoproduction of $\omega$ mesons on nuclei has been investigated using the Crystal Barrel/TAPS experiment at the ELSA tagged photon facility in Bonn. The aim is to study possible in-medium modifications of the $\omega$ meson via the reaction $\gamma +A\to \omega +X\to {\pi }^0\gamma +X^{\prime }$. Results obtained for Nb are compared to a reference measurement on a ${\mathrm{LH}}_2$ target. While for recoiling, long-lived mesons (${\pi }^0$, $\eta$, and ${\eta }^{\prime }$), which decay outside of the nucleus, a difference in the line shape for the two data samples is not observed, we find a significant enhancement towards lower masses for $\omega$ mesons produced on the Nb target. For momenta less than $500\mathrm{MeV}/c$ an in-medium $\omega$ meson mass of $M_{\mathrm{medium}}=[{722}_{-4}^{+4}(\mathrm{stat}{)}_{-5}^{+35}(\mathrm{syst})]\mathrm{MeV}/c^2$ has been deduced at an estimated average nuclear density of $0.6{\rho }_0$.

• Received 15 February 2005

DOI:https://doi.org/10.1103/PhysRevLett.94.192303