Observation of $\Upsilon \left(4S\right)\to {\eta }^{\prime }\Upsilon \left(1S\right)$

We report the first observation of the hadronic transition Υ(4S) → η'Υ(1S), using 496 fb^–1 data collected at the Υ(4S) resonance with the Belle detector at the KEKB asymmetric-energy e⁺e^– collider. We reconstruct the η' meson through its decays to ρ⁰γ and to π⁺π^–η, with η → γγ. We measure: B(Υ(4S) → η'Υ(1S)) = (3.43 ± 0.88(stat.) ± 0.21(syst.)) × 10^–5, with a significance of 5.7σ.