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

TL;DR

This paper compares Standard Model predictions with recent experimental measurements of radiative leptonic tau and muon decays, highlighting discrepancies and proposing tests for tau dipole moments at high-luminosity B factories.

Contribution

It provides updated SM predictions for radiative leptonic decays and discusses potential tests of tau dipole moments using high-precision measurements.

Findings

The measured branching ratio for $ au o e ar{ u} u \gamma$ differs from SM prediction by 3.5 standard deviations.

Predictions align with the MEG collaboration's muon decay measurements.

Proposes testing tau dipole moments through precise decay measurements at B factories.

Abstract

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.