Speaker
Description
We study a generic extension of the Type-I seesaw framework where the neutrino sector is connected to a dark sector containing either fermionic or scalar dark matter (DM) through a scalar mediator ($\Phi$). In this scenario, all beyond the Standard Model (BSM) fields are charged under a discrete symmetry (such as $\mathcal{Z}_4$) that forbids the usual renormalizable Type-I seesaw Yukawa interaction and instead allows an effective dimension-five operator $\bar{l^{\alpha}_L}\tilde{H}\Phi N_i$ involving the Standard Model lepton doublet $l^{\alpha}_L$, the SM Higgs doublet $H$, and right-handed neutrinos $N_i$. Once the scalar field $\Phi$ develops a vacuum expectation value, the discrete symmetry is spontaneously broken, generating the usual Type-I seesaw interaction ($\bar{l^{\alpha}_L}\tilde{H}N_i$) responsible for active neutrino mass generation after the electro-weak phase transition. Depending on the symmetry-breaking pattern, a residual discrete symmetry may survive that naturally stabilises the dark matter candidate, while in the absence of any residual symmetry, an additional symmetry ($CP$ or $\mathcal{Z}_N$) must be imposed to ensure DM stability. The same higher-dimensional operator can also modify the dynamics of leptogenesis, enabling the successful generation of the baryon asymmetry beyond the conventional Type-I scenario. Furthermore, the spontaneous breaking of the discrete symmetry may lead to the formation of cosmological domain walls whose subsequent annihilation, required for cosmological consistency, produces a stochastic background of gravitational waves. The resulting gravitational-wave signal is sensitive to the symmetry-breaking scale, which simultaneously governs neutrino mass generation, leptogenesis, and the viable dark matter parameter space, thereby providing a potential observational link between these phenomena.