Conversion-Driven Dark Matter in $U(1)_{B-L}$

Abstract

The new gauge boson $Z'$ in $U(1)_{B-L}$ is widely considered as the mediator of dark matter. In this paper, we propose the conversion-driven dark matter in $U(1)_{B-L}$. The dark sector contains two Dirac fermions $\tilde{\chi}_1$ and $\tilde{\chi}_2$ with $U(1)_{B-L}$ charge 0 and $-1$, respectively. A $Z_2$ symmetry is also introduced to ensure the stability of dark matter. The mass term $\delta m \bar{\tilde{\chi}}_1\tilde{\chi}_2$ induces the mixing of dark fermion. Then the lightest dark fermion $\chi_1$ becomes the dark matter candidate, whose coupling to $Z'$ is suppressed by the mixing angle $\theta$. Instead of freezing-out via pair annihilation, we show that the observed relic abundance can be obtained through the conversion processes. We then explore the feasible parameter space of conversion-driven dark matter in $U(1)_{B-L}$. Under various experimental constraints, the conversion-driven dark matter prefers the region with $3\times10^{-6}\lesssim g'\lesssim2\times10^{-4}$ and $0.02~\text{GeV}\lesssim m_{Z'}\lesssim10$~GeV, which is within the reach of future Belle II, FASER and SHiP.