We consider a U(1)$_X$ gauge symmetry extension of the Standard Model (SM) with a $Z^\prime$-portal Majorana fermion dark matter that allows for a relatively light gauge boson $Z^\prime$ with mass of 10 MeV$-$ a few GeV and a much heavier dark matter through the freeze-in mechanism. In a second scenario the roles are reversed, and the dark matter mass, in the keV range or so, lies well below the $Z^\prime$ mass, say, $\sim 1$ GeV. We outline the parameter space that can be explored for these two scenarios at the future Lifetime Frontier experiments including Belle-II, FASER, LDMX and SHiP.

中文翻译：

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来自 U(1) 规范对称性的光 Z' 和暗物质

我们考虑标准模型 (SM) 的 U(1)$_X$ 规范对称扩展，它具有 $Z^\prime$-portal Majorana 费米子暗物质，允许相对较轻的规范玻色子 $Z^\prime$ 与质量10 MeV$-$ 几个 GeV 和通过冻结机制重得多的暗物质。在第二种情况下，角色颠倒了，暗物质质量在 keV 范围左右，远低于 $Z^\prime$ 质量，例如 $\sim 1$ GeV。我们概述了可以在未来 Lifetime Frontier 实验（包括 Belle-II、FASER、LDMX 和 SHiP）中为这两种场景探索的参数空间。