A multi-signature approach to low-scale sterile neutrino phenomenology

Since the discovery of non-zero neutrino masses, through the observation of neutrino flavour oscillations, we had a plethora of successful experiments which have made increasingly precise measurements of the mixing angles and mass-differences that drive the phenomena. In this thesis we highlight the fact that there is still significant room for new physics, however, when one removes the assumption of unitarity of the 3x3 neutrino mixing matrix, an assumption inherent in the 3ν paradigm. We refit all global data to show just how much non-unitarity is currently allowed. The canonical way that such a non-unitarity is introduced to the 3x3 neutrino mixing matrix is by the addition of additional neutral fermions, singlets under the Standard Model gauge group. These “Sterile Neutrinos” have a wide range of the- oretical and phenomenological implications. Alongside the sensitivity non-unitarity measurements have to sterile neutrinos, in this thesis we will study in detail two additional signatures of low-scale sterile neutrinos; the case of one or more light O(1eV) sterile neutrinos detected by their effect on neutrino flavour oscillations, and heavier O(100 MeV) detected via their subsequent decay to Standard Model particles. These two regimes have markedly different phenomenology, but are both measurable at terrestrial short-baseline experiments. We consistently use the Fermilab Short-Baseline Neutrino program as a concrete example which would produce world-leading bounds in both scenarios.