We have studied the muon decay channel of the $\tau$ lepton with the presence of a photon in $e^{+}{e}^{-}$ annihilation data at $E_{\mathrm{c}.\mathrm{m}.\mathrm{}}=29\mathrm{}$ GeV from the Mark II detector. Included in this study is the first direct measurement of radiative $\tau$ decay. We find the ratio of the measured $\tau \to \mu \gamma \nu \overline{\nu }$ branching fraction to the expected value from QED to be 1.03±0.42.

• Received 4 December 1989

DOI:https://doi.org/10.1103/PhysRevD.41.2339