Observation of the Radiative Decay ${\eta }^{\prime }\to \omega \gamma$ in the Reaction ${\pi }^{-}p\to {\pi }^{+}{\pi }^{-}3\gamma n$

We have obtained 68±9 events of the previously unseen radiative decay ${\eta }^{\prime }\to \omega \gamma$ ($\omega \to {\pi }^{+}{\pi }^{-}{\pi }^0$, ${\pi }^0\to \gamma \gamma$) in a charged- and neutral-particle spectrometer. Simultaneously, 1505 events of the decay ${\eta }^{\prime }\to \eta {\pi }^{+}{\pi }^{-}$ ($\eta \to \gamma \gamma$) were collected in the same apparatus, allowing a direct measurement of this branching ratio. We find $\frac{({\eta }^{\prime }\to \omega \gamma )}{({\eta }^{\prime }\to \eta {\pi }^{+}{\pi }^{-})}=(6.8\mathrm{}\pm 1.3)\%$ and use published data to derive $\frac{({\eta }^{\prime }\to \rho \gamma )}{({\eta }^{\prime }\to \omega \gamma )}=9.9\mathrm{}\pm 2.0$, in excellent agreement with predictions based on vector meson dominance.