Constraints on sterile neutrino portal dark matter from cosmology and collider

Provided the dark sector consisted of a dark scalar and a dark fermion under an exact Z₂ symmetry, the sterile neutrino N can act as the messenger between the dark sector and standard model via the Yukawa coupling ₃ₒ. In this paper, we focus on the specific scenario m₍>m_+m_ with being a feebly interacting massive particle (FIMP) dark matter. The decay width of dark scalar is doubly suppressed by the smallness of Yukawa coupling ₃ₒ and mixing angle. The delayed decay will have a great impact on cosmological observables such as the big bang nucleosynthesis, the cosmic microwave background anisotropy power spectra, the effective number of relativistic neutrino species N₄₅₅ and the energetic neutrino flux. Meanwhile, the sterile neutrino can generate displaced vertex signature at colliders when m₍<mₖ. The dark scalar will also induce measurable Higgs invisible decay for relatively large quartic coupling. A comprehensive analysis of constraints from cosmology and collider is performed in this paper. We find that almost the whole parameter space with m₍<mₖ is under the reach of future experiments.