We report the first observation of the hadronic transition $\mathrm{\Upsilon }(4S)\to {\eta }^{\prime }\mathrm{\Upsilon }(1S)$, using $496{\mathrm{fb}}^{-1}$ data collected at the $\mathrm{\Upsilon }(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^{+}{e}^{-}$ collider. We reconstruct the ${\eta }^{\prime }$ meson through its decays to ${\rho }^0\gamma$ and to ${\pi }^{+}{\pi }^{-}\eta$, with $\eta \to \gamma \gamma$. We measure $\mathcal{B}(\mathrm{\Upsilon }(4S)\to {\eta }^{\prime }\mathrm{\Upsilon }(1S))=[3.43\pm 0.88(\mathrm{stat})\pm 0.21(\mathrm{syst})]\times {10}^{-5}$, with a significance of $5.7\sigma$.

• Received 27 March 2018
• Revised 11 July 2018

DOI:https://doi.org/10.1103/PhysRevLett.121.062001

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Published by the American Physical Society