Measurement of the branching ratio for the decay η→${\mathrm{\mu }}^{+}$${\mathrm{\mu }}^{\mathrm{-}}$

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