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

TL;DR

This paper establishes a model-independent connection between lepton-flavor-violating decays of light pseudoscalars and spin-dependent muon-to-electron conversion in nuclei, showing that current muon conversion limits greatly surpass direct decay searches.

Contribution

It derives master formulae linking pseudoscalar decays and muon conversion processes via Wilson coefficients, providing new indirect limits on rare decays.

Findings

Current muon conversion limits exceed direct decay limits by many orders of magnitude.

Derived model-independent formulae connect pseudoscalar decays and muon conversion.

Implications for new physics constraints from existing experimental data.

Abstract

Lepton-flavor-violating decays of light pseudoscalars, $P=\pi^0,\eta,\eta'\to\mu e$, 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 formulae 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.