$Z^{\prime }$ Portal Dark Matter with Observable $\hat{I}”N_{\mathrm{eff}}$

Abstract

In the conventional $Z^{\prime }$ portal dark matter scenario, the prediction of detectable dark matter $\ddot{I}‡$ typically relies on the collider sensitivities of $Z^{\prime }$ and direct detection, where the Majorana type right-handed neutrinos are usually assumed. However, if the right-handed neutrinos $\hat{I}{½}_R$ are Dirac type, they will contribute to the additional effective number of relativistic species $\hat{I}”N_{\mathrm{eff}}$, which brings different detectable predictions for $Z^{\prime }$ portal dark matter. In light of the great improvement of $\hat{I}”N_{\mathrm{eff}}$ for the upcoming experiments, we investigate the $Z^{\prime }$ portal dark matter with Dirac type $\hat{I}{½}_R$. Under the $U(1{)}_{B-L}$ symmetry, this model includes $\hat{I}{½}_R$ with $U(1{)}_{B-L}$ charge $Q_{\hat{I}{½}_R}=-1$ and $\ddot{I}‡$ with arbitrary $Q_{\ddot{I}}‡$ beyond the SM. Based on the relation in the production of $\ddot{I}‡$ and $\hat{I}{½}_R$, both the WIMP and FIMP dark matter through the $Z^{\prime }$ portal scenario are considered. We perform a comprehensive exploration of the viable parameter space under the constraints from $\hat{I}”N_{\mathrm{eff}}$ induced by thermal and non-thermal $\hat{I}{½}_R$, perturbative limit, dark matter direct and indirect detection, and collider searches of $Z^{\prime }$.

25 pages, 7 figures