# Observation of $\Upsilon \left(2S\right)\to \gamma {\eta }_{b}\left(1S\right)$ Decay

04 December 2018

Abstract: We report the observation of $\Upsilon \left(2S\right)\to \gamma {\eta }_{b}\left(1S\right)$ decay based on an analysis of the inclusive photon spectrum of $24.7\text{}\text{}{\mathrm{fb}}^{-1}$ of ${e}^{+}{e}^{-}$ collisions at the $\Upsilon \left(2S\right)$ center-of-mass energy collected with the Belle detector at the KEKB asymmetric-energy ${e}^{+}{e}^{-}$ collider. We measure a branching fraction of $B\left[\Upsilon \left(2S\right)\to \gamma {\eta }_{b}\left(1S\right)\right]=\left(6.{1}_{-0.7-0.6}^{+0.6+0.9}\right)×{10}^{-4}$ and derive an ${\eta }_{b}\left(1S\right)$ mass of $9394.{8}_{-3.1-2.7}^{+2.7+4.5}\text{}\text{}\mathrm{MeV}/{c}^{2}$, where the uncertainties are statistical and systematic, respectively. The significance of our measurement is greater than 7 standard deviations, constituting the first observation of this decay mode.

Published in: Physical Review Letters 121 (2018)