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There are three observables related to neutrino mass, namely the kinematicmass 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 degenerateneutrinos there are very simple relations between those observables, and wecalculate corrections due to non-zero mass splitting. We discuss how thepossible non-unitarity of the lepton mixing matrix may modify these relationsand find in particular that corrections due to non-unitarity can exceed thecorrections due to mass splitting. We furthermore investigate constraints fromneutrino-less double beta decay on mass and mixing parameters of heavyneutrinos in the type I see-saw mechanism. There are constraints from assumingthat heavy neutrinos are exchanged, and constraints from assuming lightneutrino exchange, which arise from an exact see-saw relation. The latter hasits origin in the unitarity violation arising in see-saw scenarios. Weillustrate that the limits from the latter approach are much stronger. Thedrastic impact on inverse neutrino-less double beta decay (e e -> W W) isstudied. We furthermore discuss neutrino mixing in case there is one or morelight sterile neutrino. Neutrino oscillation probabilities for long baselineneutrino oscillation experiments are considered, and the analogy to generalnon-unitarity phenomenology, such as zero-distance effects, is pointed out.

On Non-Unitary Lepton Mixing and Neutrino Mass Observables