Measurement of the Branching Ratio for the Decay Η→μ+μ−

The branching ratio (B) for \ensuremath{\eta}\ensuremath{\rightarrow}${\mathrm{\ensuremath{\mu}}}^{+}$${\mathrm{\ensuremath{\mu}}}^{\mathrm{\ensuremath{-}}}$ has been measured at the SATURNE tagged-\ensuremath{\eta} facility using the reaction pd${\ensuremath{\rightarrow}}^{3}$He \ensuremath{\eta} as a source of \ensuremath{\sim}${10}^{8}$ tagged \ensuremath{\eta}'s per day. A sample of 100 \ensuremath{\eta}\ensuremath{\rightarrow}${\mathrm{\ensuremath{\mu}}}^{+}$${\mathrm{\ensuremath{\mu}}}^{\mathrm{\ensuremath{-}}}$ decays on a background of 8 events yield B(\ensuremath{\eta}\ensuremath{\rightarrow}${\mathrm{\ensuremath{\mu}}}^{+}$${\mathrm{\ensuremath{\mu}}}^{\mathrm{\ensuremath{-}}}$) =[5.${6}_{\mathrm{\ensuremath{-}}0.7}^{+0.6}$(stat)\ifmmode\pm\else\textpm\fi{}0.5(syst)]\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}6}$. This value is 1.3\ifmmode\pm\else\textpm\fi{}0.2 times the unitarity (lower) bound of 4.3\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}6}$, in agreement with most quark and vector meson dominance model predictions.