Observation of In-Medium Modifications of the<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>ω</mml:mi></mml:math>Meson

The photoproduction of $\ensuremath{\omega}$ 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 $\ensuremath{\omega}$ meson via the reaction $\ensuremath{\gamma}+A\ensuremath{\rightarrow}\ensuremath{\omega}+X\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}\ensuremath{\gamma}+{X}^{\ensuremath{'}}$. Results obtained for Nb are compared to a reference measurement on a ${\mathrm{LH}}_{2}$ target. While for recoiling, long-lived mesons (${\ensuremath{\pi}}^{0}$, $\ensuremath{\eta}$, and ${\ensuremath{\eta}}^{\ensuremath{'}}$), 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 $\ensuremath{\omega}$ mesons produced on the Nb target. For momenta less than $500\text{ }\text{ }\mathrm{MeV}/c$ an in-medium $\ensuremath{\omega}$ meson mass of ${M}_{\mathrm{medium}}=[{722}_{\ensuremath{-}4}^{+4}(\mathrm{stat}{)}_{\ensuremath{-}5}^{+35}(\mathrm{syst})]\text{ }\text{ }\mathrm{MeV}/{c}^{2}$ has been deduced at an estimated average nuclear density of $0.6{\ensuremath{\rho}}_{0}$.