Search for Dimuon Decays of a Light Scalar Boson in Radiative Transitions ${\rm Y}\to \gamma A^0$

We search for evidence of a light scalar boson in the radiative decays of the $\mathit{{\rm Y}}(2S)$ and $\mathit{{\rm Y}}(3S)$ resonances: $\mathit{{\rm Y}}(2S,3S)\to \gamma A^0$, $A^0\to {\mu }^{+}{\mu }^{-}$. Such a particle appears in extensions of the standard model, where a light $CP$-odd Higgs boson naturally couples strongly to $b$ quarks. We find no evidence for such processes in the mass range $0.212\le m_{A^0}\le 9.3\mathrm{GeV}$ in the samples of $99\times {10}^6$ $\mathit{{\rm Y}}(2S)$ and $122\times {10}^6$ $\mathit{{\rm Y}}(3S)$ decays collected by the BABAR detector at the SLAC PEP-II $B$ factory and set stringent upper limits on the effective coupling of the $b$ quark to the $A^0$. We also limit the dimuon branching fraction of the ${\eta }_b$ meson: $\mathcal{B}({\eta }_b\to {\mu }^{+}{\mu }^{-})<0.9\%$ at 90% confidence level.

Figure 1

Distribution of the log-likelihood variable $\mathcal{S}$ with both statistical and systematic uncertainties included for a (a) $\mathit{{\rm Y}}(2S)$ fit, (b) $\mathit{{\rm Y}}(3S)$ fit, and (c) combination of $\mathit{{\rm Y}}(2S)$ and $\mathit{{\rm Y}}(3S)$ data. There are no points outside of the displayed region of $\mathcal{S}$. The solid curve is the standard normal distribution.

Figure 2

90% C.L. upper limits on (a) $\mathcal{B}\mathbf{(}\mathit{{\rm Y}}(2S)\to \gamma A^0\mathbf{)}\times {\mathcal{B}}_{\mu \mu }$, (b) $\mathcal{B}\mathbf{(}\mathit{{\rm Y}}(3S)\to \gamma A^0\mathbf{)}\times {\mathcal{B}}_{\mu \mu }$, and (c) effective coupling $f_{\mathit{{\rm Y}}}^2\times {\mathcal{B}}_{\mu \mu }$ as a function of $m_{A^0}$. The shaded areas show the regions around the $J/\psi$ and $\psi (2S)$ resonances excluded from the search.