$CP$ asymmetries in $\tau\to K_S\pi\nu_\tau$ decays

TL;DR

This paper investigates CP asymmetries in tau decays to K_S pi nu_tau within the Standard Model and beyond, using effective field theories, and finds current bounds limit observable CP violation effects.

Contribution

It provides a model-independent analysis of CP violation in tau decays using LEFT and SMEFT frameworks, connecting decay asymmetries with EDM and meson mixing constraints.

Findings

Standard Model CP asymmetries arise from K0-ar{K}^0 mixing.

Beyond Standard Model effects are constrained by neutron EDM and D0-ar{D}^0 mixing bounds.

No significant CP violation effects are expected to be observable in current or near-future experiments.

Abstract

We present here the CP asymmetries in the decay rate and angular distributions of $\tau\to K_S\pi\nu_\tau$ decays in the Standard Model (SM) and beyond (BSM). The CP asymmetries in the SM are induced by the CP violation in $K^0-\bar{K}^0$ mixing. To investigate the BSM CP-violating (CPV) effects, a model-independent analysis is performed by using the low-energy effective field theory (LEFT) framework at $\mu=2$~GeV. If one further assumes the BSM physics to stem from above the electroweak scale, the LEFT shall then be matched onto the SM effective field theory (SMEFT), the operators of which contributing to $\tau\to K_S\pi\nu_\tau$ decays will also contribute to the neutron electric dipole moment (EDM) and $D^0-\bar{D}^0$ mixing. The stringent bounds from the latter suggest that no remarkable CPV effects can be observed in either the decay rate or the angular distributions. The prospects for future measurements of these observables are also mentioned.