Evidence for the ηb(1S) Meson in Radiative Υ(2S) Decay

We have performed a search for the ηb(1S) meson in the radiative decay of the Υ(2S) resonance using a sample of 91.6×106 Υ(2S) events recorded with the BABAR detector at the PEP-II B factory at the SLAC National Accelerator Laboratory. We observe a peak in the photon energy spectrum at Eγ=609.3-4.5+4.6(stat)±1.9(syst)  MeV, corresponding to an ηb(1S) mass of 9394.2-4.9+4.8(stat)±2.0(syst)  MeV/c2. The branching fraction for the decay Υ(2S)→γηb(1S) is determined to be [3.9±1.1(stat)-0.9+1.1(syst)]×10-4. We find the ratio of branching fractions B[Υ(2S)→γηb(1S)]/B[Υ(3S)→γηb(1S)]=0.82±0.24(stat)-0.19+0.20(syst).