Observation of $\hat{I}\yen (2S)\to {\hat{I}}^3\hat{I}{\cdot }_b(1S)$ decay

Abstract

We report the observation of $\hat{I}\yen (2S)\to {\hat{I}}^3\hat{I}{\cdot }_b(1S)$ decay based on analysis of the inclusive photon spectrum of $24.7$ fb${}^{-1}$ of $e^{+}{e}^{-}$ collisions at the $\hat{I}\yen (2S)$ center-of-mass energy collected with the Belle detector at the KEKB asymmetric-energy $e^{+}{e}^{-}$ collider. We measure a branching fraction of $\mathcal{B}(\hat{I}\yen (2S)\to {\hat{I}}^3\hat{I}{\cdot }_b(1S))=(6.1_{-0.7-0.6}^{+0.6+0.9})\times 10^{-4}$, and derive an $\hat{I}{\cdot }_b(1S)$ mass of $9394.8_{-3.1-2.7}^{+2.7+4.5}$ 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.