Observation of $\Upsilon(4S)\to \eta' \Upsilon(1S)$

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 $ρ^0 γ$ 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 5.7$σ$.