Reopening the Z portal with semi-annihilations

TL;DR

This paper shows that multi-component dark matter models with a scalar doublet can evade direct detection bounds through semi-annihilations, expanding viable scenarios beyond traditional single-component assumptions.

Contribution

It introduces a multi-component dark matter model with a scalar doublet and singlet, demonstrating semi-annihilations can suppress relic abundance and satisfy direct detection constraints.

Findings

Semi-annihilations reduce the relic abundance of the doublet component.

Both dark matter components remain detectable in future experiments.

The model is consistent with current theoretical and experimental constraints.

Abstract

In one-component dark matter (DM) scenarios is commonly assumed that a scalar WIMP 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 multi-component scenarios with a dark scalar doublet exhibiting vector-like 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 $\phi$, both charged under a $Z_6$ symmetry. We find that semi-annihilation processes drastically reduce the relic abundance of the neutral component of the doublet, $H^0$, sufficiently attenuating the effects of its large $Z$-mediated elastic scattering cross-section with nucleons to satisfy the DD constraints. Although the contribution of $H^0$ to the total relic abundance is nearly negligible, with $\phi$ 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 non-degenerate mass spectrum at the electroweak scale.