Origin and smallness of neutrino masses

Determine the origin of neutrino masses and ascertain the mechanism responsible for their smallness relative to other standard model fermions, as motivated by neutrino oscillation observations.

Background

Within the standard model, neutrinos were originally treated as massless. However, neutrino oscillation measurements have established that neutrinos do have non-zero mass. Despite extensive theoretical work, including seesaw mechanisms and heavy neutral lepton extensions, the underlying origin of neutrino masses and the reason for their remarkable smallness remain unresolved.

This open question motivates the searches reviewed in the paper, particularly those targeting heavy neutral leptons that could participate in mechanisms such as Type I or Type III seesaw, thereby potentially explaining neutrino mass generation and hierarchy.