Gravitational waves from a dark U (1)D phase transition in light of NANOGrav 12.5 yr data GRAVITATIONAL WAVES from A DARK U (1)D PHASE ... BORAH DEBASISH, DASGUPTA ARNAB, and KANG SIN KYU

We study a possibility of a strong first-order phase transition taking place below the electroweak scale in the context of a U(1)D gauge extension of the standard model. As pointed out recently by the NANOGrav Collaboration, gravitational waves from such a phase transition with appropriate strength and nucleation temperature can explain their 12.5 yr data. We first find the parameter space of this minimal model consistent with NANOGrav findings by considering only a complex singlet scalar and U(1)D vector boson. The existence of a singlet fermion charged under U(1)D can give rise to dark matter in this model, preferably of nonthermal type, while incorporating additional fields can also generate light neutrino masses through typical low-scale seesaw mechanisms like a radiative or inverse seesaw.