Charting Dark Matter down to the neutrino floor/fog in the 2HD+a scenario

Abstract

Next-generation direct detection experiments will probe dark matter (DM) scattering cross-sections deep into the neutrino fog, the regime where coherent neutrino scattering becomes an irreducible background. We investigate whether thermally produced weakly interacting massive particles (WIMPs) can naturally populate this regime while satisfying relic density and indirect detection constraints. Adopting as a case study the 2HD+a model, we have performed comprehensive parameter scans over the Type-I and Type-II Yukawa configurations. We have included the limit of strongly suppressed singlet--doublet {\it A-a} mixing sin$θ\! \to \! 0$ and we show that annihilation into {\it ha} and ${\it Ha}$ final states sustains the correct relic density while loop-induced direct detection cross-sections naturally land inside the neutrino fog; in the same limit the light pseudoscalar boson becomes long-lived, featuring displaced-vertex signatures when produced at colliders. Finally, in the case of zero mixing, we have considered a new possibility for DM phenomenology as the $a$ state becomes cosmologically stable and, consequently, an additional DM component. We map all the viable parameter space against current LZ and FERMI-LAT bounds and projected XLZD and CTA sensitivities. We find that the single component setup lies naturally below the neutrino floor for DM masses above 100 GeV while, on the contrary, most of the parameter space of the two component DM scenario is strongly disfavored already considering present limit. The parameter space of both single and two component DM scenario can be nevertheless broadened by considering specific relations among the model parameters to suppress the coupling between the 125 GeV bosons and two $a$ states. Our results represent in any case a motivation to fully exploit future tonne-scale detectors.