From Thermal History to Multi-Messenger Signatures in $\mathbb{Z}_3$ Symmetric Dark Sector

Abstract

We investigate a $\mathbb{Z}_3$-symmetric extension of the Standard Model consisting of a right-handed neutrino ($N_R$), a dark fermion ($\chi$) that dominates the relic density and a dark complex scalar ($S$) that facilitates a strong first-order electroweak phase transition (SFOEWPT). The observed relic abundance, is achieved through the combined effects of annihilation, semi-annihilation, and dark-sector conversion processes over a broad region of parameter space consistent with a SFOEWPT. We further investigate the resulting multi-messenger signatures, including loop-induced direct detection, indirect detection through gamma-ray observations, and gravitational wave signals, with the latter lying within the projected sensitivities of future space-based detectors such as LISA, BBO, and DECIGO.