Lepton-flavor-violating decays of light pseudoscalars, $P={\pi }^0,\eta ,{\eta }^{\prime }\to \mu e$, are stringently suppressed in the standard model up to tiny contributions from neutrino oscillations, so that their observation would be a clear indication for physics beyond the standard model. However, in effective field theory such decays proceed via axial-vector, pseudoscalar, or gluonic operators, which are, at the same time, probed in spin-dependent $\mu \to e$ conversion in nuclei. We derive master formulas that connect both processes in a model-independent way in terms of Wilson coefficients and study the implications of current $\mu \to e$ limits in titanium for the $P\to \mu e$ decays. We find that these indirect limits surpass direct ones by many orders of magnitude.

• Received 14 April 2022
• Revised 23 June 2022
• Accepted 30 November 2022

DOI:https://doi.org/10.1103/PhysRevLett.130.131902

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