Lepton-flavor changing decays and non-unitarity in the inverse seesaw mechanism (2409.03952v2)

Abstract: The pursuit for the genuine fundamental description, governing nature at some high-energy scale, must invariably consider the yet-unknown mechanism behind the generation of neutrino mass. Lepton-flavor violating decays $l_\alpha\to l_\beta\,\gamma$, allowed in the presence of neutrino mass and mixing, provide a mean to look for physics beyond the Standard Model. In the present work we consider the inverse seesaw mechanism and then revisit the calculation of its contributions to the branching ratios of the aforementioned decay processes. Our analytic results are consistent, as they are gauge invariant, gauge independent, ultraviolet finite, and of decoupling nature. Among the decays $l_\alpha\to l_\beta\gamma$, we find $\mu\to\gamma\,e$ to be the most promising, in the light of current bounds by the MEG Collaboration. Deviations from unitarity in the mixing of light neutrinos are related to the branching ratios ${\rm Br}\big( l_\alpha\to\gamma\,l_\beta \big)$ in a simple manner, which we address, then finding that the constraints on non-unitarity effects from MEG data will be improved by the upcoming MEG II update by a factor $\sim\frac{1}{3}$.