On Non-Unitary Lepton Mixing and Neutrino Mass Observables

TL;DR

This paper explores how non-unitarity in lepton mixing and neutrino mass splittings affect observable relations in neutrino physics, with implications for double beta decay and sterile neutrino scenarios.

Contribution

It analyzes the impact of non-unitarity and mass splitting corrections on neutrino mass observables and constraints from double beta decay within the see-saw framework.

Findings

Non-unitarity corrections can surpass mass splitting effects.

Stronger constraints from light neutrino exchange in see-saw models.

Impact on inverse neutrino-less double beta decay processes.

Abstract

There are three observables related to neutrino mass, namely the kinematic mass in direct searches, the effective mass in neutrino-less double beta decay, and the sum of neutrino masses in cosmology. In the limit of exactly degenerate neutrinos there are very simple relations between those observables, and we calculate corrections due to non-zero mass splitting. We discuss how the possible non-unitarity of the lepton mixing matrix may modify these relations and find in particular that corrections due to non-unitarity can exceed the corrections due to mass splitting. We furthermore investigate constraints from neutrino-less double beta decay on mass and mixing parameters of heavy neutrinos in the type I see-saw mechanism. There are constraints from assuming that heavy neutrinos are exchanged, and constraints from assuming light neutrino exchange, which arise from an exact see-saw relation. The latter has its origin in the unitarity violation arising in see-saw scenarios. We illustrate that the limits from the latter approach are much stronger. The drastic impact on inverse neutrino-less double beta decay (e e -> W W) is studied. We furthermore discuss neutrino mixing in case there is one or more light sterile neutrino. Neutrino oscillation probabilities for long baseline neutrino oscillation experiments are considered, and the analogy to general non-unitarity phenomenology, such as zero-distance effects, is pointed out.