Observation of the Bottomonium Ground State in the Decay Υ(3S)→γηb

We report the results of a search for the bottomonium ground state ηb(1S) in the photon energy spectrum with a sample of (109±1) million of Υ(3S) recorded at the Υ(3S) energy with the BABAR detector at the PEP-II B factory at SLAC. We observe a peak in the photon energy spectrum at Eγ=921.2-2.8+2.1(stat)±2.4(syst)  MeV with a significance of 10 standard deviations. We interpret the observed peak as being due to monochromatic photons from the radiative transition Υ(3S)→γηb(1S). This photon energy corresponds to an ηb(1S) mass of 9388.9-2.3+3.1(stat)±2.7(syst)  MeV/c2. The hyperfine Υ(1S)-ηb(1S) mass splitting is 71.4-3.1+2.3(stat)±2.7(syst)  MeV/c2. The branching fraction for this radiative Υ(3S) decay is estimated to be [4.8±0.5(stat)±1.2(syst)]×10-4.