Exposing dark sector with future $Z$-factories

We investigate the prospects of searching dark sector models via exotic $Z$-boson decay at future $e^+e^−$ colliders with Giga $Z$ and Tera $Z$ options. Four general categories of dark sector models: Higgs portal dark matter, vector portal dark matter, inelastic dark matter and axion-like particles, are considered. Focusing on channels motivated by the dark sector models, we carry out a model independent study of the sensitivities of $Z$-factories in probing exotic decays. The limits on branching ratios of the exotic Z decay are typically 𝒪($10^{−6} − 10^{−8.5}$) for the Giga $Z$ and 𝒪($10^{−7.5} − 10^{−11}$) for the Tera $Z$, and they are compared with the projection for the high luminosity LHC. We demonstrate that future $Z$-factories can provide its unique and leading sensitivity, and highlight the complementarity with other experiments, including the indirect and direct dark matter search limits, and the existing collider limits. Future $Z$ factories will play a leading role to uncover the hidden sector of the universe in the future.