Type-I Seesaw with eV-Scale Neutrinos

Understand

We consider seesaw type-I models including at least one (mostly-)sterile neutrino with mass at the eV scale.

• Three distinct situations are found, where the presence of light extra neutrinos is naturally justified by an approximately conserved lepton number symmetry.
• To analyse these scenarios consistently, it is crucial to employ an exact parametrisation of the full mixing matrix.
• We provide additional exact results, including generalised versions of the seesaw relation and of the Casas-Ibarra parametrisation, valid for every scale of seesaw.