Observation of the bottomonium ground state in the decay Upsilon(3S) -> gamma etab

الأصل · EN

We report the results of a search for the bottomonium ground state etab(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 Egamma = 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 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 Upsilon(3S) decay is estimated to be (4.8 +/- 0.5(stat) +/- 1.2 (syst)) x 10(-4).