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

Abstract

We report the results of a search for the bottomonium ground state η b ( 1 S ) in the photon energy spectrum with a sample of ( 109 ± 1 ) million of Υ ( 3 S ) recorded at the Υ ( 3 S ) energy with the BABAR detector at the PEP-II B factory at SLAC. We observe a peak in the photon energy spectrum at E γ = 921.2 + 2.1 − 2.8 ( 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 ( 3 S ) → γ η b ( 1 S ) . This photon energy corresponds to an η b ( 1 S ) mass of 9388.9 + 3.1 − 2.3 ( stat ) ± 2.7 ( syst ) MeV / c 2 . The hyperfine Υ ( 1 S ) - η b ( 1 S ) mass splitting is 71.4 + 2.3 − 3.1 ( stat ) ± 2.7 ( syst ) MeV / c 2 . The branching fraction for this radiative Υ ( 3 S ) decay is estimated to be [ 4.8 ± 0.5 ( stat ) ± 1.2 ( syst ) ] × 10 − 4 .