Dark Higgs Dark Matter

A new $U(1)$ dark gauge group coupled to the Standard Model (SM) via the kinetic mixing portal provides a natural dark matter candidate in the form of the Higgs field, $h_d$, responsible for generating the mass of the dark photon, $\gamma_d$. We show that the condition $m_{h_d}\leq m_{\gamma_d}$, together with smallness of the kinetic mixing parameter, $\epsilon$, and/or dark gauge coupling, $g_d$, leads the dark Higgs to be sufficiently metastable to constitute dark matter. We analyze the Universe's thermal history and show that both freeze-in, ${\rm SM}\to \{\gamma_d, h_d\}$, and freeze-out, $\{\gamma_d, h_d\} \to {\rm SM}$, processes can lead to viable dark Higgs dark matter with a sub-GeV mass and a kinetic mixing parameter in the range $10^{-13}\lesssim\epsilon\lesssim10^{-6}$. Observable signals in astrophysics and cosmology include modifications to primordial elemental abundances, altered energetics of supernovae explosions, dark Higgs decays in the late Universe, and dark matter self-interactions.