Reopening the Z portal with semi-annihilations

In one-component dark matter (DM) scenarios is commonly assumed that a scalar weakly interacting massive particle must either be part of an SU(2)L multiplet with zero hypercharge or have suppressed vector interactions with the Z-gauge boson to circumvent stringent direct detection (DD) bounds. In this work, we demonstrate that multicomponent scenarios with a dark scalar doublet exhibiting vectorlike interactions with the Z boson are also compatible with bounds arising from DD searches. Specifically, we consider a simple extension of the Standard Model wherein the dark sector comprises a doublet and a complex singlet ϕ, both charged under a Z6 symmetry. We find that semi-annihilation processes drastically reduce the relic abundance of the neutral component of the doublet, H0, sufficiently attenuating the effects of its large Z-mediated elastic scattering cross-section with nucleons to satisfy the DD constraints. Although the contribution of H0 to the total relic abundance is nearly negligible, with ϕ dominating, both dark matter components are expected to be detectable in ongoing and future DD experiments. The viability of the model is tested against several theoretical and experimental constraints, resulting in a parameter space featuring a nondegenerate mass spectrum at the electroweak scale. Published by the American Physical Society 2024