Observation of the Radiative Decay η′→ωγ in the Reaction π−p→π+π−3γn

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