A no-go theorem for non-standard explanations of the $\tau\to K_S\pi\nu_\tau$ CP asymmetry

TL;DR

This paper proves that non-standard explanations for the observed CP asymmetry in tau decays are highly constrained and unlikely, especially those involving tensor interactions, due to fundamental interference suppression and existing experimental bounds.

Contribution

It establishes a no-go theorem showing that most non-standard CP violation mechanisms cannot account for the observed asymmetry in tau decays.

Findings

Interference between vector and tensor phases is suppressed by Watson's theorem.

Tensor interactions are strongly constrained by neutron EDM and D-meson mixing bounds.

Explains the difficulty in attributing the CP asymmetry to physics beyond the Standard Model.

Abstract

The CP asymmetry in $\tau\to K_S\pi\nu_\tau$, as measured by the BaBar collaboration, differs from the Standard Model prediction by $2.8\sigma$. Most non-standard interactions do not allow for the required strong phase needed to produce a non-vanishing CP asymmetry, leaving only new tensor interactions as a possible mechanism. We demonstrate that, contrary to previous assumptions in the literature, the crucial interference between vector and tensor phases is suppressed by at least two orders of magnitude due to Watson's final-state-interaction theorem. Furthermore, we find that the strength of the relevant CP-violating tensor interaction is strongly constrained by bounds from the neutron electric dipole moment and $D$-$\bar{D}$ mixing. These observations together imply that it is extremely difficult to explain the current $\tau\to K_S\pi\nu_\tau$ measurement in terms of physics beyond the Standard Model originating in the ultraviolet.