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

A new measurement of the branching ratio for the decay η→${\mathrm{\mu }}^{+}$${\mathrm{\mu }}^{\mathrm{-}}$, made at the SATURNE II proton synchrotron, resulted in Γ(η→${\mathrm{\mu }}^{+}$${\mathrm{\mu }}^{\mathrm{-}}$)/Γ(η→all)= [5.7±0.7(stat.)±0.5(syst.)]×${10}^{\mathrm{-}6}$. The reaction pd${\to }^3$Heη close to threshold yielded 800 ${\mathrm{s}}^{\mathrm{-}1}$ tagged η’s in a narrow momentum band around 257 MeV/c. Muon pairs were detected in two range telescopes. The data obtained consist of 114 events η→${\mathrm{\mu }}^{+}$${\mathrm{\mu }}^{\mathrm{-}}$ on a background of 14 events. The new value for the branching ratio is 1.3±0.2 times the unitarity lower limit, consistent with most quark and vector meson dominance models which describe the decay as an electromagnetic transition with a two-photon intermediate state. The result resolves the discrepancy between the two previous measurements of this branching ratio.