Observation of the eta. --> mu. /sup +/. mu. /sup -/. gamma. decay

The mass spectrum of ..mu../sup +/..mu../sup -/..gamma.. systems produced in ..pi../sup -/p collisions at momentum 25 GeV/c is investigated. The muon pairs and ..gamma.. rays were recorded with a wide-aperture magnetic spectrometer with a hodoscopic ..gamma.. detector. A distinct peak, corresponding to a hitherto unobserved eta..--> mu../sup +/..mu../sup -/..gamma.. decay, is separated in the mass spectrum. The relative probability of the observed decay was found to be 1.5 x 10/sup -4/.