Observation of the radiative decays of ${\rm Y}(1S)$ to ${\chi }_{c1}$

Abstract

We report the first observation of the radiative decay of the ${\rm Y}(1S)$ into a charmonium state. The statistical significance of the observed signal of ${\rm Y}(1S)\to \gamma {\chi }_{c1}$ is 6.3 standard deviations including systematics. The branching fraction is calculated to be Br(${\rm Y}(1S)\to \gamma {\chi }_{c1}$) = (4.7^{+2.4}_{-1.8} (stat) ^{+0.4}_{-0.5} (sys)) * 10^{-5}. We also searched for ${\rm Y}(1S)$ radiative decays into ${\chi }_{c0,2}$ and ${\eta }_c(1S,2S)$ and set upper limits on their branching fractions. These results are obtained from a 24.9 fb^{-1} data sample collected with the Belle detector at the KEKB asymmetric-energy $e^{+}{e}^{-}$ collider at a center-of-mass energy equal to the ${\rm Y}(2S)$ mass using ${\rm Y}(1S)$ tagging by the ${\rm Y}(2S)\to {\rm Y}(1S){\pi }^{+}{\pi }^{-}$ transitions.

7pages, 3 figures