Measurement of the Muonic Branching Fractions of the Narrow Upsilon Resonances

The decay branching fractions of the three narrow ${\rm Y}$ resonances to ${\mu }^{+}{\mu }^{-}$ have been measured by analyzing about $4.3{\mathrm{f}\mathrm{b}}^{-1}$ $e^{+}{e}^{-}$ data collected with the CLEO III detector. The branching fraction $\mathcal{B}\mathbf{(}{\rm Y}(1S)\to {\mu }^{+}{\mu }^{-}\mathbf{)}=(2.49\pm 0.02\pm 0.07)\%$ is consistent with the current world average, but $\mathcal{B}\mathbf{(}{\rm Y}(2S)\to {\mu }^{+}{\mu }^{-}\mathbf{)}=(2.03\pm 0.03\pm 0.08)\%$ and $\mathcal{B}\mathbf{(}{\rm Y}(3S)\to {\mu }^{+}{\mu }^{-}\mathbf{)}=(2.39\pm 0.07\pm 0.10)\%$ are significantly larger than prior results. These new muonic branching fractions imply a narrower total decay width for the ${\rm Y}(2S)$ and ${\rm Y}(3S)$ resonances and lower other branching fractions that rely on these decays in their determination.