Illuminating the Nature of Neutrino Masses with Gravitational Waves

In this talk, I will explore how gravitational-wave (GW) signals from the early Universe can probe the fundamental nature of neutrino masses--whether they are Majorana, pure Dirac, or pseudo-Dirac. Working within the minimal $B-L$ gauge extension of the Standard Model, I will show that each neutrino mass realization is associated with a distinct symmetry-breaking pattern (scale) and a characteristic GW signature. High-scale $B-L$ breaking that generates Majorana neutrino masses leads to the formation of cosmic string networks, producing nearly flat GW spectra over a broad frequency range. In contrast, low-scale $B-L$ breaking in a pure Dirac neutrino scenario, can give rise to a strong first-order phase transition, resulting in a peaked GW spectrum. Finally, in the pseudo-Dirac case, the realization of neutrino masses require an additional discrete symmetry, whose spontaneous breaking leads to domain wall formation; the subsequent annihilation of these domain walls imprints characteristic kink-like features in the GW spectrum. These results demonstrate that GW observations provide a powerful and complementary avenue to discriminate between different origins of neutrino masses.