Observation of the Bottomonium Ground State in the Decay ${\rm Y}(3S)\to \gamma {\eta }_b$

We report the results of a search for the bottomonium ground state ${\eta }_b(1S)$ in the photon energy spectrum with a sample of $(109\pm 1)$ million of ${\rm Y}(3S)$ recorded at the ${\rm Y}(3S)$ energy with the BABAR detector at the PEP-II $B$ factory at SLAC. We observe a peak in the photon energy spectrum at $E_{\gamma }={921.2}_{-2.8}^{+2.1}(\mathrm{stat})\pm 2.4(\mathrm{syst})\mathrm{MeV}$ with a significance of 10 standard deviations. We interpret the observed peak as being due to monochromatic photons from the radiative transition ${\rm Y}(3S)\to \gamma {\eta }_b(1S)$. This photon energy corresponds to an ${\eta }_b(1S)$ mass of ${9388.9}_{-2.3}^{+3.1}(\mathrm{stat})\pm 2.7(\mathrm{syst})\mathrm{MeV}/c^2$. The hyperfine ${\rm Y}(1S)$-${\eta }_b(1S)$ mass splitting is ${71.4}_{-3.1}^{+2.3}(\mathrm{stat})\pm 2.7(\mathrm{syst})\mathrm{MeV}/{\mathrm{c}}^2$. The branching fraction for this radiative ${\rm Y}(3S)$ decay is estimated to be $[4.8\pm 0.5(\mathrm{stat})\pm 1.2(\mathrm{syst})]\times {10}^{-4}$.

Figure 1

Inclusive photon energy spectrum in the below-${\rm Y}(4S)$ data, with the nonpeaking background subtracted.The peak at 1.03 GeV is from the ISR process $e^{+}{e}^{-}\to {\gamma }_{\mathrm{ISR}}{\rm Y}(1S)$. The superimposed histogram corresponds to a fit with a CB function.

Figure 2

(a) Inclusive photon spectrum in the region $0.50<E_{\gamma }<1.1\mathrm{GeV}$. The component PDFs determined from the fit are overlaid on the data points. A prominent ${\chi }_{bJ}(2P)$ peak is clearly seen. The dashed line corresponds to the nonpeaking background component. (b) Inclusive photon spectrum after subtracting the nonpeaking background, with PDFs for the ${\chi }_{bJ}(2P)$ peak (solid line), ISR ${\rm Y}(1S)$ (dotted line), ${\eta }_b$ signal (dashed line) and the sum of all three (solid line). (c) Inclusive photon spectrum after subtracting all components except the ${\eta }_b$ signal. The CB function shape describes the data points well.