Evidence for the ηb(1S) Meson in Radiative Y(2S) Decay

We have performed a search for the eta(b)(1S) meson in the radiative decay of the Y(2S) resonance using a sample of 91.6 x 10(6) Y(2S) events recorded with the BABAR detector at the PEP-II B factory at the SLAC National Accelerator Laboratory. We observe a peak in the photon energy spectrum at E-gamma = 609.3(-4.5)(+4.6)(stat) +/- 1.9(syst) MeV, corresponding to an eta(b)(1S) mass of 9394.2(-4.9)(+4.8)(stat) +/- 2.0(syst) MeV/c(2). The branching fraction for the decay Y(2S) --> gamma eta(b)(1S) is determined to be 3.9 +/- 1.1(stat)(-0.9)(+1.1)(syst)] x 10(-4). We find the ratio of branching fractions B[Y(2S) --> gamma eta(b)(1S)]/B[Y(3S) --> gamma eta(b)(1S)] = 0.82 +/- 0.24(stat)(-0.19)(+0.20)(syst).