Scale-Invariant Three-Component Dark Matter and Gravitational Wave Signals
We study a classically scale-invariant extension of the Standard Model (SM) with four new fields in a hidden sector: a complex scalar $S$, two Dirac fermions $ψ^{1,2}$ and the gauge boson $V_μ$ of a new $U(1)_D$ symmetry. The scalar $S$ couples to the SM through the Higgs portal. We identify regions of parameter space in which $V_μ$, $ψ^1$ and $ψ^2$ are all stable, giving a three-component dark matter (DM) scenario. The model is classically scale invariant, and the electroweak symmetry breaks because of the loop effects. We impose the observed relic density and current direct detection bounds, and then study the finite temperature effective potential. For a benchmark point, we find a first-order phase transition and compute the resulting gravitational wave (GW) spectrum, which we compare with the projected sensitivities of space-based interferometers. Although the viable parameter space is small, it can be probed by future experiments.