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

Raymond Mountain, Department of Physics, Syracuse University, Syracuse, NY
M. Artuso, Department of Physics, Syracuse University, Syracuse, NY
S. Blusk, Department of Physics, Syracuse University, Syracuse, NY
S. Khalil, Department of Physics, Syracuse University, Syracuse, NY
First authors listed from Syracuse University, 88 additional authors not listed

10 pages, 3 figures, accepted by Phyical Review D Rapid Communications More information at http://arxiv.org/abs/0909.5474

Description/Abstract

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.