Completely general bounds on Non-Unitary leptonic mixing

TL;DR

This paper derives comprehensive, model-independent bounds on non-unitary leptonic mixing due to heavy neutrinos, highlighting current and future experimental sensitivities, especially from muon-related processes.

Contribution

It provides the first complete set of model-independent constraints on heavy neutrino mixing, incorporating both LFV and non-LFV observables in a global fit.

Findings

Current strongest bound from $ ext{BR}( ext{mu} o e ext{gamma})$

Future sensitivity dominated by $ ext{mu-e}$ conversion in nuclei

Model-independent bounds derived without LFV observables

Abstract

We derive constraints on the mixing of heavy right-handed neutrinos with the SM fields in the most general Seesaw scenario where the heavy neutrinos are integrated out. Among the electroweak and flavour observables included in the global fit, $\mu\rightarrow e\gamma$ sets the present strongest bound on the additional neutrino mixing, while in the future it will be dominated by $\mu-e$ conversion in nuclei. Increasing its sensitivity in future experiments could probe Non-Unitarity in Lepton Flavour Violating processes. Nevertheless, in order to determine completely model-independent constraints, we provide a second set of bounds derived through a global fit that does not include LFV observables. These indirect constraints on the off-diagonal elements come from the diagonal bounds through the Schwarz inequality.