Muon radiative decay and limits on non-(V-A) weak interaction

Using the PIBETA detector in its original form and, more recently, as configured for the PEN experiment, we have recorded the world largest sample of radiative muon decay events $\mu^+ \to {\rm e}^+ \nu_e \overline{\nu}_\mu \gamma$, resulting from secondary muons produced by a stopped pion beam. Theoretical predictions of the muon radiative decay branching ratio depend on the Michel parameters $\rho$ and $\overline{\eta}$ which, along with other muon decay parameters, can be used to set limits on the possible extensions of the V-A form of the electroweak interactions. We will report the branching ratio for this process in a wide kinematic region of phase space, and a new, improved value of the parameter $<$eta-bar$>$, which, coupled with other results, provide comprehensive limits on non-standard contributions to the electoweak interaction.