First observation of the hadronic transition $\mathrm{{\rm Y}}(4S)\to \eta h_b(1P)$ and new measurement of the $h_b(1P)$ and ${\eta }_b(1S)$ parameters

Using a sample of $771.6\times {10}^6$ $\mathrm{{\rm Y}}(4S)$ decays collected by the Belle experiment at the KEKB $e^{+}{e}^{-}$ collider, we observe for the first time the transition $\mathrm{{\rm Y}}(4S)\to \eta h_b(1P)$ with the branching fraction ${\cal B}[\Upsilon(4S) \to \eta h_b(1P)]= (2.18 \pm 0.11 \pm 0.18) \times 10^{-3}$andwemeasurethe$h_b(1P)$mass$M_{h_{b}(1P)} = (9899.3 \pm 0.4 \pm 1.0)$MeV\mathrm{/}$c^{2}$,correspondingtothehyperfinesplitting$\Delta M_{\mathrm HF}(1P) = (0.6 \pm 0.4 \pm 1.0)$MeV\mathrm{/}$c^{2}$\mathrm{.}Usingthetransition$h_b(1P) \to \gamma \eta_b(1S)$,wemeasurethe$\eta_b(1S)$mass$M_{\eta_{b}(1S)} = (9400.7 \pm 1.7 \pm 1.6)$MeV\mathrm{/}$c^{2}$,correspondingto$\Delta M_{\mathrm HF}(1S) = (59.6 \pm 1.7 \pm 1.6)$MeV\mathrm{/}$c^{2}$,the$\eta_b(1S)$width$\Gamma_{\eta_{b}(1S)} = (8 ^{+6}_{-5} \pm 5)$MeV\mathrm{/}$c^{2}$ and the branching fraction $\mathcal{B}[h_b(1P)\to \gamma {\eta }_b(1S)]=(56\pm 8\pm 4)\mathrm{\%}$.