Observation of $\Upsilon(2S)\to\gamma \eta_{b}(1S)$ decay

TL;DR

This paper reports the first observation of the decay of $Upsilon(2S)$ into a photon and $eta_b(1S)$, measuring its branching fraction and $eta_b(1S)$ mass with high significance using data from the Belle detector.

Contribution

The study provides the first experimental observation and measurement of the $Upsilon(2S) ogammaeta_b(1S)$ decay mode, including its branching fraction and $eta_b(1S)$ mass.

Findings

Branching fraction measured as (6.1^{+0.6+0.9}_{-0.7-0.6})×10^{-4}

$eta_b(1S)$ mass determined to be 9394.8^{+2.7+4.5}_{-3.1-2.7} MeV/c^2

Decay observed with significance > 7 standard deviations

Abstract

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