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

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.