Observation of the Bottomonium Ground State, eta_b(1S), in the Decay Y(3S) --> gamma eta_b in BaBar -- Véronique Ziegler (SLAC)

We report the results of a search for the bottomonium ground state eta_b(1S) in the photon energy spectrum with a sample of (109 ± 1) million of Upsilon(3S) recorded at the Upsilon(3S) energy with the BABAR detector at the PEP-II B factory at SLAC. We observe a peak in the photon energy spectrum at E_gamma = 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 Upsilon(3S) -> gamma eta_b(1S). This photon energy corresponds to an eta_b(1S) mass of 9388.9 +3.1 -2.3 (stat) ± 2.7(syst) MeV/c2. The hyperfine Upsilon(1S)-eta_b(1S) mass splitting is 71.4 +2.3 -3.1 (stat) ± 2.7(syst) MeV/c2. The branching fraction for this radiative Upsilon(3S) decay is estimated to be (4.8 ± 0.5(stat) ± 1.2 (syst)) x 10^(-4).