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

TL;DR

This paper provides the first evidence for the eta_b(1S) bottomonium ground state through radiative decay measurements, determining its mass and branching fraction with statistical significance, and setting an upper limit for related decay modes.

Contribution

The study reports the first evidence and precise measurement of the eta_b(1S) mass and branching fraction in Upsilon(3S) decays, using CLEO III data.

Findings

Eta_b(1S) mass measured at 9391.8 MeV/c^2

Branching fraction B(Upsilon(3S) --> gamma eta_b) = (7.1 +- 1.8 +- 1.1) x 10^{-4}

Statistical significance of about 4 sigma

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.