Measurement of the branching ratio for the decay η→μ+μ−

Abstract

A new measurement of the branching ratio for the decay \ensuremath{\eta}\ensuremath{\rightarrow}${\mathrm{\ensuremath{\mu}}}^{+}$${\mathrm{\ensuremath{\mu}}}^{\mathrm{\ensuremath{-}}}$, made at the SATURNE II proton synchrotron, resulted in \ensuremath{\Gamma}(\ensuremath{\eta}\ensuremath{\rightarrow}${\mathrm{\ensuremath{\mu}}}^{+}$${\mathrm{\ensuremath{\mu}}}^{\mathrm{\ensuremath{-}}}$)/\ensuremath{\Gamma}(\ensuremath{\eta}\ensuremath{\rightarrow}all)= [5.7\ifmmode\pm\else\textpm\fi{}0.7(stat.)\ifmmode\pm\else\textpm\fi{}0.5(syst.)]\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}6}$. The reaction pd${\ensuremath{\rightarrow}}^{3}$He\ensuremath{\eta} close to threshold yielded 800 ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ tagged \ensuremath{\eta}'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 \ensuremath{\eta}\ensuremath{\rightarrow}${\mathrm{\ensuremath{\mu}}}^{+}$${\mathrm{\ensuremath{\mu}}}^{\mathrm{\ensuremath{-}}}$ on a background of 14 events. The new value for the branching ratio is 1.3\ifmmode\pm\else\textpm\fi{}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.

Measurement of the branching ratio for the decay η→μ+μ−

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Abstract

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