Evidence for the ${\eta }_b(1S)$ Meson in Radiative ${\rm Y}(2S)$ Decay

We have performed a search for the ${\eta }_b(1S)$ meson in the radiative decay of the ${\rm Y}(2S)$ resonance using a sample of $91.6\times {10}^6$ ${\rm Y}(2S)$ 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 $E_{\gamma }={609.3}_{-4.5}^{+4.6}(\mathrm{stat})\pm 1.9(\mathrm{syst})\mathrm{MeV}$, corresponding to an ${\eta }_b(1S)$ mass of ${9394.2}_{-4.9}^{+4.8}(\mathrm{stat})\pm 2.0(\mathrm{syst})\mathrm{MeV}/c^2$. The branching fraction for the decay ${\rm Y}(2S)\to \gamma {\eta }_b(1S)$ is determined to be $[3.9\pm 1.1(\mathrm{stat}{)}_{-0.9}^{+1.1}(\mathrm{syst})]\times {10}^{-4}$. We find the ratio of branching fractions $\mathcal{B}[{\rm Y}(2S)\to \gamma {\eta }_b(1S)]/\mathcal{B}[{\rm Y}(3S)\to \gamma {\eta }_b(1S)]=0.82\pm 0.24(\mathrm{stat}{)}_{-0.19}^{+0.20}(\mathrm{syst})$.

Figure 1

(a) Inclusive photon spectrum in the region $0.27<E_{\gamma }<0.80\mathrm{GeV}$. The fit is overlaid on the data points. (b) (color online) Inclusive photon spectrum after subtracting the nonpeaking background, with the PDFs for ${\chi }_{bJ}(1P)$ peak (light solid line), ISR ${\rm Y}(1S)$ (dotted line), ${\eta }_b$ signal (dashed line) and the sum of all three (dark solid line).