Reviving $Z^\prime$ Portal Dark Matter with Conversion Mechanism

Abstract: In many new physics models with extended gauge symmetry, the new gauge boson $Z'$ could mediate the interactions between the dark matter and standard model particles. For the conventional $Z^\prime$ portal dark matter, the collider and the direct detection constraints typically pose a significant challenge. To address this pressing issue, we present in this paper a new benchmark model based on the gauged $U(1){B-L}$ symmetry, which introduces a Dirac dark fermion $\tildeχ_1$ and a heavier partner $\tildeχ_2$ with zero and nonzero $U(1){B-L}$ charge, respectively. Including the mass term $δm \bar{\tildeχ}1\tildeχ_2$ results in the dark fermions $χ_1$ and $χ_2$ in the mass eigenstate, where the lighter one $χ_1$ is regarded as the dark matter candidate. Various intriguing processes for the relic density arise with the compressed mass spectrum $m{χ1}\simeq m{χ_2}$, such as the coscattering $χ_2f\toχ_1f$, the conversion $χ_2χ_i\toχ_1χ_j$, and the coannihilation $χ_1χ_2\to f\bar{f}$ processes. Suppressed by the small mixing angle $θ$ between the dark fermions, the small effective gauge coupling of dark matter $χ_1$ to the gauge boson $Z'$ is one distinct feature of this model, rendering phenomenology in many aspects more promising. In this paper, we investigate the production of dark matter through new mechanisms within the frameworks of resonance and secluded scenarios. The impacts of phenomenological constraints from collider, dark matter, and cosmology are also taken into account. We report that the conversion mechanism is both favored by the resonance and secluded scenarios under current constraints.