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

B Aubert, Christopher Hawkes, Nitesh Soni, Alan Watson

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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(-2.8)(+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 Upsilon(3S)->gamma eta(b)(1S). This photon energy corresponds to an eta(b)(1S) mass of 9388.9(-2.3)(+3.1)(stat)+/- 2.7(syst) MeV/c(2). The hyperfine Upsilon(1S)-eta(b)(1S) mass splitting is 71.4(-3.1)(+2.3)(stat)+/- 2.7(syst) MeV/c(2). The branching fraction for this radiative Upsilon(3S) decay is estimated to be [4.8 +/- 0.5(stat)+/- 1.2(syst)]x10(-4).
Original languageEnglish
JournalPhysical Review Letters
Issue number7
Publication statusPublished - 1 Jan 2008


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