$τ$ dipole moments via radiative leptonic $τ$ decays

Abstract

We propose a new method to probe the magnetic and electric dipole moments of the $τ$ lepton using precise measurements of the differential rates of radiative leptonic $τ$ decays at high-luminosity $B$ factories. Possible deviations of these moments from the Standard Model values are analyzed in an effective Lagrangian approach, thus providing model-independent results. Analytic expressions for the relevant non-standard contributions to the differential decay rates are presented. Earlier proposals to probe the $τ$ dipole moments are examined. A detailed feasibility study of our method is performed in the conditions of the Belle and Belle II experiments at the KEKB and Super-KEKB colliders, respectively. This study shows that our approach, applied to the planned full set of Belle II data for radiative leptonic $τ$ decays, has the potential to improve the present experimental bound on the $τ$ anomalous magnetic moment. On the contrary, its foreseen sensitivity is not expected to lower the current experimental limit on the $τ$ electric dipole moment.