Improved limits on lepton-flavor-violating decays of light pseudoscalars via spin-dependent $μ\to e$ conversion in nuclei

Abstract

Lepton-flavor-violating decays of light pseudoscalars, P = π 0 , η, η → µe, 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 µ → e conversion in nuclei. We derive master formulae that connect both processes in a model-independent way in terms of Wilson coefficients, and study the implications of current µ → e limits in titanium for the P → µe decays. We find that these indirect limits surpass direct ones by many orders of magnitude.

Introduction to Charged Lepton Flavor Violation

“…The sensitivities of µ → 3e, µ → eγ and µN → eN to Wilson coefficients at m W in the low energy EFT has been extensively studied [119][120][121][122][123]. Spin-dependent µ → e conversion in nuclei [121,122,124,125], although less constraining than the spin-independent searches, allows us to probe different combinations of coefficients and, thus, reduce the number of flat directions.…”