Improved Limits on Lepton-Flavor-Violating Decays of Light Pseudoscalars via Spin-Dependent <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mi>μ</mml:mi><mml:mo stretchy="false">→</mml:mo><mml:mi>e</mml:mi></mml:mrow></mml:math> Conversion in Nuclei
Martin Hoferichter, J. Menéndez, Frederic Noël
Abstract
Lepton-flavor-violating decays of light pseudoscalars, $P={\ensuremath{\pi}}^{0},\ensuremath{\eta},{\ensuremath{\eta}}^{\ensuremath{'}}\ensuremath{\rightarrow}\ensuremath{\mu}e$, 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 $\ensuremath{\mu}\ensuremath{\rightarrow}e$ 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 $\ensuremath{\mu}\ensuremath{\rightarrow}e$ limits in titanium for the $P\ensuremath{\rightarrow}\ensuremath{\mu}e$ decays. We find that these indirect limits surpass direct ones by many orders of magnitude.