Measurement of the {eta}{sub b}(1S) mass and the branching fraction for {Upsilon}(3S){yields}{gamma}{eta}{sub b}(1S)

We report evidence for the ground state of bottomonium, {eta}{sub b}(1S), in the radiative decay {Upsilon}(3S){yields}{gamma}{eta}{sub b} in e{sup +}e{sup -} annihilation data taken with the CLEO III detector. Using 6x10{sup 6} {Upsilon}(3S) decays, and assuming {Gamma}({eta}{sub b})=10{+-}5 MeV/c{sup 2}, we obtain B({Upsilon}(3S){yields}{gamma}{eta}{sub b})=(7.1{+-}1.8{+-}1.3)x10{sup -4}, where the first error is statistical and the second is systematic. The statistical significance is {approx}4{sigma}. The mass is determined to be M({eta}{sub b})=9391.8{+-}6.6{+-}2.0 MeV/c{sup 2}, which corresponds to the hyperfine splitting {Delta}M{sub hf}(1S){sub b}=68.5{+-}6.6{+-}2.0 MeV/c{sup 2}. Using 9x10{sup 6} {Upsilon}(2S) decays, we place an upper limit on the corresponding {Upsilon}(2S) decay, B({Upsilon}(2S){yields}{gamma}{eta}{sub b})<8.4x10{sup -4} at 90% confidence level.