Recent results on in-medium properties of the omega meson*

Data on the photo production of $\omega $ mesons on nuclei have been re-analyzed. For incident photon energies of 900 -- 2200 MeV, $\omega$ mesons have been identified via the $\pi ^{0}\gamma $ channel using the CBELSA/TAPS detector. A new procedure has been developed which allows a model-independent background determination in shape and absolute magnitude directly from the data. Applying this method, an earlier claim of an in-medium lowering of the $\omega $ mass [1] can not be confirmed. Because of the strong in-medium broadening of the $\omega $ meson, deduced from a transparency ratio measurement [2], the fraction of in-medium $\omega \quad \to \quad \pi ^{0}\gamma $ decays is correspondingly reduced and the experiment becomes less sensitive to in-medium mass shifts. A higher sensitivity is expected for incident energies close to the production threshold [3]. A measurement at incident photon energies of 800-1400 MeV has been performed. Results of this experiment, including an analysis of the $\omega$ excitation function, will be presented \\[4pt] [1] D. Trnka et al., PRL 94 (2005)192303.\\[0pt] [2] M. Kotulla et al., PRL 100 (2008) 192302.\\[0pt] [3] K. Gallmeister et al., Prog. Part. Nucl. Phys. 61 (2008) 283.