Observation of the Radiative Decays of Υ ( 1 S ) to χ c 1

Abstract

We report the first observation of the radiative decay of the Υ ( 1 S ) into a charmonium state. The significance of the observed signal of Υ ( 1 S ) → γ χ c 1 is 6.3 standard deviations including systematics. The branching fraction is calculated to be B [ Υ ( 1 S ) → γ χ c 1 ] = [ 4.7 + 2.4 − 1.8 ( stat ) + 0.4 − 0.5 ( sys ) × 10 − 5 ] . We also searched for Υ ( 1 S ) radiative decays into χ c 0 , 2 and η c ( 1 S , 2 S ) , 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 Υ ( 2 S ) mass using Υ ( 1 S ) tagging by the Υ ( 2 S ) → Υ ( 1 S ) π + π − transitions.