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Measurement of the eta_b(1S) mass and the branching fraction for Upsilon(3S) --> gamma eta_b(1S)

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arxiv 0909.5474 v2 pith:YB4WRIG6 submitted 2009-09-30 hep-ex

Measurement of the eta_b(1S) mass and the branching fraction for Upsilon(3S) --> gamma eta_b(1S)

classification hep-ex
keywords gammaupsilondecaydecaysmassmillionstatisticalannihilation
verification ladder T0 review T1 audit T2 compute T3 formal T4 reserved
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We report evidence for the ground state of bottomonium, eta_b(1S), in the radiative decay Upsilon(3S) --> gamma eta_b in e^+e^- annihilation data taken with the CLEO III detector. Using 6 million Upsilon(3S) decays, and assuming Gamma(eta_b) = 10 MeV/c^2, we obtain B(Upsilon(3S) --> gamma eta_b) = (7.1 +- 1.8 +- 1.1) X 10^{-4}, where the first error is statistical and the second is systematic. The statistical significance is about 4 sigma. The mass is determined to be M(eta_b) = 9391.8 +- 6.6 +- 2.0 MeV/c^2, which corresponds to the hyperfine splitting Delta M_{hf}(1S)_b = 68.5 +- 6.6 +- 2.0 MeV/c^2. Using 9 million Upsilon(2S) decays, we place an upper limit on the corresponding Y(2S) decay, B(Y(2S) --> gamma eta_b) < 8.4 X 10^{-4} at 90 % confidence level.

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