On the description of non-unitary neutrino mixing

TL;DR

This paper develops a comprehensive formalism to describe deviations from unitarity in light neutrino mixing caused by heavy neutrinos, impacting oscillation experiments and providing model-independent constraints.

Contribution

It introduces a complete description of non-unitarity in neutrino mixing, including new parameters and CP phase, extending the standard oscillation framework.

Findings

Neutrino oscillations can be described with three new parameters and a CP phase.

Current experiments impose constraints on heavy neutrino couplings.

The formalism applies to electron and muon neutrino oscillations.

Abstract

Neutrino oscillations are well established and the relevant parameters determined with good precision, except for the CP phase, in terms of a unitary lepton mixing matrix. Seesaw extensions of the Standard Model predict unitarity deviations due to the admixture of heavy isosinglet neutrinos. We provide a complete description of the unitarity and universality deviations in the light neutrino sector. Neutrino oscillation experiments involving electron or muon neutrinos and anti-neutrinos are fully described in terms of just three new real parameters and a new CP phase, in addition to the ones describing oscillations with unitary mixing. Using this formalism we describe the implications of non-unitarity for neutrino oscillations and summarize the model-independent constraints on heavy neutrino couplings that arise from current experiments.