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

TL;DR

This paper introduces a novel dark matter scenario in a $U(1)_{B-L}$ gauge model where dark matter relic abundance is achieved through conversion processes rather than traditional annihilation, with viable parameter space accessible to future experiments.

Contribution

It proposes a new conversion-driven dark matter mechanism within $U(1)_{B-L}$ models, expanding the understanding of dark matter relic abundance without relying on pair annihilation.

Findings

Conversion-driven dark matter can account for observed relic abundance.

Parameter space is within reach of future experiments like Belle II, FASER, and SHiP.

Preferred coupling and mass ranges are identified under experimental constraints.

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.