Vector dark matter with non-abelian kinetic mixing

Abstract

An appealing framework for dark matter is provided by light hidden sectors, below the electroweak scale, feebly coupled to the Standard Model via light mediators. We consider a minimal, predictive model where both the dark matter and the mediator are vector bosons, and have the same mass. The portal between the dark sector and the Standard Model is provided by a kinetic mixing between the dark gauge symmetry, $SU(2)_X$, and the hypercharge, $U(1)_Y$, induced by a dimension-six operator. The dark-matter candidates, $X^\pm$, are charged under a custodial symmetry and therefore stable, while the mediator is a massive dark photon, $Z_D$, mixing with the photon and the $Z$. We show how the observed dark-matter abundance can be reproduced via freeze-out or freeze-in, through either the kinetic mixing or the dark gauge interaction. We also analyse dark 3-to-2 annihilations, that can become dominant in model variations with $Z_D$ heavier than $X^\pm$. We confront our relic-density predictions with current and projected experimental, astrophysical and cosmological bounds on the model parameter space, highlighting the correlation between the dark-photon and dark-matter phenomenologies.