New physics models with sterile neutrinos at different energy scales

Neutrino mass requires new physics beyond the Standard Model: sterile neutrinos are one such example. In general the mass scale of such particles is unknown, so that different model building scenarios arise. The aim of this thesis is to accommodate light sterile neutrinos in working models and study the phenomenological consequences. Indeed, sterile neutrinos at the eV scale could explain observed short-baseline anomalies, whereas keV-scale warm dark matter particles could resolve tensions in the standard cosmological model. Different A4 flavour symmetry models are modified to include sterile neutrinos, with the Froggatt-Nielsen mechanism controlling their mass spectrum and higher-order effects explicitly taken into account. The resulting signatures in neutrinoless double beta decay are focussed on. In addition, the connection between that process and neutrino mass is studied in the left-right symmetric model: the multitude of contributions are categorised and various special cases are studied numerically. The role of mixed helicity diagrams is emphasised, and the inverse neutrinoless double beta decay process at a linear collider is analysed in detail. These and other experimental signatures will be crucial in deciphering the nature and origin of neutrino mass