Improved Limits on Lepton-Flavor-Violating Decays of Light Pseudoscalars via Spin-Dependent μ→ e Conversion in Nuclei

Lepton-flavor-violating decays of light pseudoscalars, = 0, , ′→ ⁢ , 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 → 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 → limits in titanium for the → ⁢ decays. We find that these indirect limits surpass direct ones by many orders of magnitude.