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.

References

The origin of the neutrino masses and the reason for their smallness are both unknown, posing further challenges to our understanding of particle physics.

— Review of searches for vector-like quarks, vector-like leptons, and heavy neutral leptons in proton-proton collisions at $\sqrt{s}$ = 13 TeV at the CMS experiment  (2405.17605 - Collaboration, 2024) in Section 1 (Introduction)

The discovery of neutrino oscillations established that neutrinos are massive, while the origin of their small masses remains an open question.

— Generalized Chiral $U(1)_{B-L}$: Towards a Less Constrained $Z'$ and Dark Matter  (2608.23521 - Kumar et al., 24 Aug 2026) in Section 1, Introduction

Oscillation data requires at least two massive neutrinos and global analyses have substantially improved the precision with which the mixing angles and mass-squared differences are known, yet the mechanism responsible for the small neutrino mass scale remains unresolved.

— Neutrino mass, scalar dark matter, and collider signatures in a radiative doublet-triplet model  (2609.08808 - Bamwidhi et al., 8 Sep 2026) in Section 1, Introduction