# 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.

## Key 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.