Precision tests via radiative $\mu$ and $\tau$ leptonic decays

Understand

The branching fractions of radiative leptonic $\tau$ decays $(\tau \to l \nu \bar{\nu} \gamma$, $l=e,\mu)$ were recently measured by the Babar collaboration with a relative error of about 3%.

• The measurement of the branching ratio $\mathcal{B} (\tau \to e \bar{\nu} \nu \gamma)$, for a minimum photon energy of 10 MeV in the $\tau$ rest frame, differs from our recent SM prediction by 3.5 standard deviations, whereas our result agrees with Babar's value for $\mathcal{B} (\tau \to \mu \bar{\nu} \nu \gamma)$.
• Our predictions also agree with the measurement of $\mathcal{B} (\mu \to e \bar{\nu} \nu \gamma)$ by the MEG collaboration.
• We also report on a recent proposal to test the $\tau$ dipole moments via precise measurements of radiative leptonic $\tau$ decays at high-luminosity $B$ factories.