A first order dark $SU(2)_D$ phase transition with vector dark matter in the light of NANOGrav 12.5 yr data

TL;DR

This paper explores a dark $SU(2)_D$ gauge extension of the Standard Model that predicts a first order phase transition capable of generating gravitational waves consistent with NANOGrav data, while also providing a dark matter candidate.

Contribution

It introduces a conformally invariant dark $SU(2)_D$ model with a first order phase transition that explains gravitational wave signals and produces vector dark matter.

Findings

A strong first order phase transition can generate nano-Hz gravitational waves matching NANOGrav data.

Dark vector bosons from the phase transition can account for dark matter relic abundance.

Future gravitational wave detectors can probe the model's parameter space.

Abstract

We study a dark $SU(2)_D$ gauge extension of the standard model (SM) with the possibility of a strong first order phase transition (FOPT) taking place below the electroweak scale in the light of NANOGrav 12.5 yr data. As pointed out recently by the NANOGrav collaboration, gravitational waves (GW) from such a FOPT with appropriate strength and nucleation temperature can explain their 12.5 yr data. We impose a classical conformal invariance on the scalar potential of $SU(2)_D$ sector involving only a complex scalar doublet with negligible couplings with the SM Higgs. While a FOPT at sub-GeV temperatures can give rise to stochastic GW around nano-Hz frequencies being in agreement with NANOGrav findings, the $SU(2)_D$ vector bosons which acquire masses as a result of the FOPT in dark sector, can also serve as dark matter (DM) in the universe. The relic abundance of such vector DM can be generated in a non-thermal manner from the SM bath via scalar portal mixing. We also discuss future sensitivity of gravitational wave experiments to the model parameter space.