Shinning Light on Sterile Neutrino Portal Dark Matter from Cosmology and Collider

Abstract

Provided the dark sector consisted of a dark scalar $\ddot{I}•$ and a dark fermion $\ddot{I}‡$ under an exact $Z_2$ symmetry, the sterile neutrino $N$ can act as the messenger between the dark sector and standard model via the Yukawa coupling $\hat{I}{»}_{ds}\bar{\ddot{I}}‡\ddot{I}•N$. In this paper, we focus on the specific scenario $m_N>m_{\ddot{I}}•+m_{\ddot{I}}‡$ with $\ddot{I}‡$ being a FIMP dark matter. The decay width of dark scalar $\ddot{I}•$ is doubly suppressed by the smallness of Yukawa coupling $\hat{I}{»}_{ds}$ and mixing angle $\hat{I}¸$. The delayed decay $\ddot{I}•\to \ddot{I}‡\hat{I}½$ 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 $N_{\mathrm{eff}}$ and the energetic neutrino flux. Meanwhile, the sterile neutrino can generate displaced vertex signature at colliders when $m_N<m_W$. The dark scalar $\ddot{I}•$ 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 $m_N<m_W$ is under the reach of future experiments.

18 pages,8 figures