Measurement of the branching fraction of the radiative decay τ− → μ−νμντγ

Abstract The branching ratio of the radiative τ decay τ − → μ − ν μ ν τ γ has been measured using data collected with the OPAL detector at LEP. The result is B(τ − → μ − ν μ ν τ γ) = (3.0 ± 0.4 ± 0.5) × 10 −3 for photon energies above 20 MeV measured in the τ rest frame. The result is in good agreement with the Standard Model expectation.