Phenomenology of Majorana and Pseudo-Dirac Neutrinos

In this talk I will outline a generalised inverse seesaw (ISS) mechanism and its related constraints. In the ISS we introduce both right-handed neutrinos and a set of SM singlet fermions with opposite CP parities. The attractive feature of this scenario is that the size of lepton number violation (LNV) is proportional to a small mass parameter directly related to the mass splitting between the sterile states. This parameter therefore determines whether they behave like Majorana or pseudo-Dirac neutrinos. For clarity I will first examine the interplay between the mixings and Majorana phases between a single active state and two sterile states. Various constraints apply to the active-sterile mixing over the mass range 1 eV to 10 TeV - for different values of the splitting and the phases I will compare the limits from neutrinoless double beta decay, radiative corrections and collider searches. I will also review future sensitivities to the active-sterile mixing and the subsequent implications for leptogenesis and keV-scale dark matter.