Observation of η′ → μ+μ−γ decay

Abstract The mass spectrum for μ + μ − γ systems produced in π − p interactions at 25 and 33 GeV/ c has been studied. To detect muon pairs and photons a wide-aperture magnetic spectrometer and a hodoscope γ-detector were used. In the mass spectrum a clear peak is seen corresponding to the decay η ′ → μ + μ − γ not previously observed. The branching ratio for this decay, 8 × 10 −5 , is in agreement with the η′ form-factor model, based on vector dominance.