Observation of the decay η→μ+μ−γ

The results of studying the mass spectrum of μ+μ−γ systems produced in πp collisions at 25 GeVc are presented. A wide-aperture magnetic spectrometer with a hodoscope γ-detector was used to identify muon pairs and photons. There is a clear peak in the mass spectrum which corresponds to the decay η→μ+μ−γ previously not observed. The branching ratio for the decay is estimated to be 1.5 × 10−4.