Observation of the bottomonium ground state, etab, at BaBar
Abstract
We present the first observation of the bottomonium ground state etab(1S) in the photon energy spectrum using a sample of 109+/-1 million of Upsilon(3S) events recorded at the Upsilon(3S) energy with the BaBar detector at the PEP-II B factory at SLAC. A peak at Egamma = 921.2 +2.1-2.8(stat) +/- 2.4(syst) MeV observed with a significance of 10 standard deviations in the photon energy spectrum is interpretated as being due to the radiative transition Upsilon(3S) -> gamma etab(1S). This photon energy corresponds to an etab(1S) mass of 9388.9 +3.1-2.3(stat) +/- 2.7(syst) MeV/c2. The hyperfine Upsilon(1S)-etab(1S) mass splitting is 71.4 +2.3-3.1(stat) +/- 2.7(syst) MeV/c2. The branching fraction for this radiative (3S) decay is obtained as (4.8 +/- 0.5(stat) +/- 1.2 (syst)) x 10(-4).