Bounds on lepton non-unitarity and heavy neutrino mixing

TL;DR

This paper provides an updated global fit analysis to constrain lepton non-unitarity and heavy neutrino mixing using recent experimental data, considering various seesaw scenarios and deviations from unitarity.

Contribution

It introduces a comprehensive analysis considering full correlations among observables and calibrates the test statistic, improving bounds on neutrino mixing and unitarity deviations.

Findings

Stronger bounds on heavy neutrino mixing parameters.

Evidence that generic unitarity deviations can better fit the data.

Challenges in modeling unitarity deviations without additional neutrinos.

Abstract

We present an updated and improved global fit analysis of current flavor and electroweak precision observables to derive bounds on unitarity deviations of the leptonic mixing matrix and on the mixing of heavy neutrinos with the active flavours. This new analysis is motivated by new and updated experimental results on key observables such as $V_{ud}$, the invisible decay width of the $Z$ boson and the $W$ boson mass. It also improves upon previous studies by considering the full correlations among the different observables and explicitly calibrating the test statistic, which may present significant deviations from a $\chi^2$ distribution. The results are provided for three different Type-I seesaw scenarios: the minimal scenario with only two additional right-handed neutrinos, the next to minimal one with three extra neutrinos, and the most general one with an arbitrary number of heavy neutrinos that we parametrize via a generic deviation from a unitary leptonic mixing matrix. Additionally, we also analyze the case of generic deviations from unitarity of the leptonic mixing matrix, not necessarily induced by the presence of additional neutrinos. This last case relaxes some correlations among the parameters and is able to provide a better fit to the data. Nevertheless, inducing only leptonic unitarity deviations avoiding both the correlations implied by the right-handed neutrino extension as well as more strongly constrained operators is challenging and would imply significantly more complex UV completions.