Observation of $\mathrm{{\rm Y}}(4S)\to {\eta }^{\mathrm{\prime }}\mathrm{{\rm Y}}(1S)$

We report the first observation of the hadronic transition $\mathrm{{\rm Y}}(4S)\to {\eta }^{\mathrm{\prime }}\mathrm{{\rm Y}}(1S)$, using 496 fb${}^{-1}$ data collected at the $\mathrm{{\rm Y}}(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^{+}{e}^{-}$ collider. We reconstruct the ${\eta }^{\mathrm{\prime }}$ meson through its decays to ${\rho }^0\gamma$ and to ${\pi }^{+}{\pi }^{-}\eta$, with $\eta \to \gamma \gamma$. We measure: ${\cal B}(\Upsilon(4S)\to\eta’\Upsilon(1S))=(3.43\pm 0.88 {\rm(stat.)} \pm 0.21 {\rm(syst.)})\times10^{-5}$,withasignificanceof5.7$\sigma$.