First observation of the hadronic transition $ \Upsilon(4S) \to \eta h_{b}(1P)$ and new measurement of the $h_b(1P)$ and $\eta_b(1S)$ parameters

Using a sample of 771.6×106 ϒϒ(4S) decays collected by the Belle experiment at the KEKB e+e- collider, we observe, for the first time, the transition ϒ(4S)→ηhb(1P) with the branching fraction B[ϒ(4S)→ηhb(1P)]=(2.18±0.11±0.18)×10-3 and we measure the hb(1P) mass Mhb(1P)=(9899.3±0.4±1.0)  MeV/c2, corresponding to the hyperfine (HF) splitting ΔMHF(1P)=(0.6±0.4±1.0)  MeV/c2. Using the transition hb(1P)→γηb(1S), we measure the ηb(1S) mass Mηb(1S)=(9400.7±1.7±1.6)  MeV/c2, corresponding to ΔMHF(1S)=(59.6±1.7±1.6)  MeV/c2, the ηb(1S) width Γηb(1S)=(8-5+6±5)  MeV/c2 and the branching fraction B[hb(1P)→γηb(1S)]=(56±8±4)%.