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Evidence for the ηb(1S) Meson in Radiative Υ(2S) Decay
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We have performed a search for the η b ( 1 S ) meson in the radiative decay of the Υ ( 2 S ) resonance using a sample of 91.6 × 10 6 Υ ( 2 S ) events recorded with the BABAR detector at the PEP-II B factory at the SLAC National Accelerator Laboratory. We observe a peak in the photon energy spectrum at E γ = 609.3 + 4.6 − 4.5 ( stat ) ± 1.9 ( syst ) MeV , corresponding to an η b ( 1 S ) mass of 9394.2 + 4.8 − 4.9 ( stat ) ± 2.0 ( syst ) MeV / c 2 . The branching fraction for the decay Υ ( 2 S ) → γ η b ( 1 S ) is determined to be [ 3.9 ± 1.1 ( stat ) + 1.1 − 0.9 ( syst ) ] × 10 − 4 . We find the ratio of branching fractions B [ Υ ( 2 S ) → γ η b ( 1 S ) ] / B [ Υ ( 3 S ) → γ η b ( 1 S ) ] = 0.82 ± 0.24 ( stat ) + 0.20 − 0.19 ( syst ) .
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GARRA TICÓ, Jordi, GRAUGÉS POUS, Eugeni and BABAR Collaboration. Evidence for the ηb(1S) Meson in Radiative Υ(2S) Decay. Physical Review Letters. 2009. Vol. 103, num. 16, pags. 161801. ISSN 0031-9007. [consulted: 13 of August of 2026]. Available at: https://hdl.handle.net/2445/138338