Search for the Decay $\mathit{\Upsilon }\left(1\mathit{S}\right)\to {\mathit{\gamma }\mathit{\eta }}^{\prime }$

We report on a search for the radiative decay $\Upsilon \left(1S\right)\to \gamma {\eta }^{\prime }$ in $61.3{\mathrm{pb}}^{-1\mathrm{}}$ of data taken with the CLEO II detector at the Cornell Electron Storage Ring. Three decay chains were investigated, all involving ${\eta }^{\prime }\to {\pi }^{+}{\pi }^{-}\eta$, followed by $\eta \to \gamma \gamma$, $\eta \to {\pi }^0{\pi }^0{\pi }^0$, or $\eta \to {\pi }^{+}{\pi }^{-}{\pi }^0$. We find no candidate events in any of the three cases and set a combined upper limit of $1.6\times {10}^{-5\mathrm{}}$ at $90\%$ C.L., significantly smaller than the previous limit. We compare our result to other radiative $\Upsilon$ decays, to radiative $J/\psi$ decays, and to theoretical predictions.