Complex $\tau$ Electric Dipole Moment from GeV-Scale New Physics

TL;DR

This paper investigates the $ au$ lepton's electric dipole moment, especially its imaginary part, and how future experiments like STCF and Belle II could detect new physics effects such as axion-like couplings.

Contribution

It analyzes the momentum-transfer dependence of $d_ au(q^2)$ and highlights the potential for future experiments to observe sizable EDMs induced by new physics.

Findings

Im[$d_ au$] constrains new physics models.

Future experiments can probe EDM values near current sensitivities.

Axion-like couplings can induce measurable $ au$ EDMs.

Abstract

Among the charged leptons, the $\tau$ electric dipole moment~($d_\tau$) is the least constrained. We show that the Im[$d_\tau$] imposes strong constraints on new physics that have yet to be discussed. Motivated in particular by the Super Tau-Charm Facility (STCF), which will provide a uniquely clean environment for precision $\tau$-physics, we study the momentum-transfer dependence of $d_\tau(q^2)$ and compare the projected sensitivities of STCF and Belle II. Our analysis shows that an axion-like coupling of the $\tau$ lepton can induce sizable real and imaginary components of the EDM. The predicted EDM values may approach the present experimental sensitivities, making them accessible to future measurements at Belle II and the STCF.