Observation of Υ(2S)→γηb(1S) Decay

Abstract

We report the observation of Υ(2S)→γηb(1S) decay based on an analysis of the inclusive photon spectrum of 24.7 fb−1 of e+e− collisions at the Υ(2S) center-of-mass energy collected with the Belle detector at the KEKB asymmetric-energy e+e− collider. We measure a branching fraction of B[Υ(2S)→γηb(1S)]=(6.1+0.6+0.9−0.7−0.6)×10−4 and derive an ηb(1S) mass of 9394.8+2.7+4.5−3.1−2.7 MeV/c2, 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.