$\tau$ dipole moments via radiative leptonic $\tau$ decays

TL;DR

This paper introduces a new method to measure the tau lepton's magnetic and electric dipole moments using radiative leptonic tau decays at high-luminosity B factories, with potential to improve bounds on the magnetic moment.

Contribution

The paper presents a model-independent approach using differential decay rates to probe tau dipole moments, including analytic expressions and a feasibility study for Belle II.

Findings

Potential to improve bounds on tau magnetic dipole moment.

Limited sensitivity to tau electric dipole moment.

Feasibility confirmed for Belle II data analysis.

Abstract

We propose a new method to probe the magnetic and electric dipole moments of the $\tau$ lepton using precise measurements of the differential rates of radiative leptonic $\tau$ 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 $\tau$ 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 $\tau$ decays, has the potential to improve the present experimental bound on the $\tau$ anomalous magnetic moment. On the contrary, its foreseen sensitivity is not expected to lower the current experimental limit on the $\tau$ electric dipole moment.