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

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