Observation of the radiative decays of $\Upsilon(1S)$ to $\chi_{c1}$

TL;DR

This paper reports the first observation of the radiative decay of the $5S$ into a charmonium state, specifically $c_{1}$, with a measured branching fraction, using data from the Belle detector at KEKB.

Contribution

First experimental observation of $5S$ radiative decay into $c_{1}$, providing new insights into bottomonium to charmonium transitions.

Findings

Observed $5S o c_{1}$ decay with 6.3 sigma significance.

Measured branching fraction: (4.7^{+2.4}_{-1.8} (stat) ^{+0.4}_{-0.5} (sys)) * 10^{-5}.

Set upper limits on other radiative decays of $5S$.

Abstract

We report the first observation of the radiative decay of the $\Upsilon(1S)$ into a charmonium state. The statistical significance of the observed signal of $\Upsilon(1S) \to \gamma \chi_{c1}$ is 6.3 standard deviations including systematics. The branching fraction is calculated to be Br($\Upsilon(1S) \to \gamma \chi_{c1}$) = (4.7^{+2.4}_{-1.8} (stat) ^{+0.4}_{-0.5} (sys)) * 10^{-5}. We also searched for $\Upsilon(1S)$ 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 $\Upsilon(2S)$ mass using $\Upsilon(1S)$ tagging by the $\Upsilon(2S) \to \Upsilon(1S) \pi^+\pi^-$ transitions.