We present a consistent implementation of weak decays involving an axion or axionlike particle in the context of an effective chiral Lagrangian. We argue that previous treatments of such processes have used an incorrect representation of the flavor-changing quark currents in the chiral theory. As an application, we derive model-independent results for the decays $K^{-}\to {\pi }^{-}a$ and ${\pi }^{-}\to e^{-}{\overline{\nu }}_{e}a$ at leading order in the chiral expansion and for arbitrary axion couplings and mass. In particular, we find that the $K^{-}\to {\pi }^{-}a$ branching ratio is almost 40 times larger than previously estimated.

• Received 25 February 2021
• Accepted 20 July 2021

DOI:https://doi.org/10.1103/PhysRevLett.127.081803

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP³.

Published by the American Physical Society