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In recent years there have been many proposals for new electron-positron colliders, such as the Circular Electron-Positron Collider, the International Linear Collider, and the Future Circular Collider in electron-positron mode. Much of the motivation for these colliders is precision measurements of the Higgs boson and searches for new electroweak states. Hence, many of these studies are focused on energies above the h Z threshold. However, there are proposals to run these colliders at the lower WW threshold and Z-pole energies. In this paper, we study a new search for Higgs physics accessible at lower energies: e+e− → h Zd, where Zdis a new light gauge boson such as a dark photon or dark-Z. Such searches can be conducted at the WW threshold, i.e. energies below the h Z threshold where exotic Higgs decays can be searched for in earnest. Additionally, due to very good angular and energy resolution at future electron-positron colliders, these searches will be sensitive to Zd masses below 1 GeV, which is lower than the current direct LHC searches. We will show that at √s = 160 GeV with 10 ab−1, a search for e+e− → h Zd is sensitive to h −Z −Zd couplings of δ ∼ 9 × 10−3and cross sections of ∼ 2 − 3 ab for Zd masses below 1 GeV. The results are similar at √s = 240 GeV with 5 ab−1.

A new beam dump experiment that utilizes the beam of future high energy electron-positron colliders could be an excellent avenue to search for dark sector particles due to its unprecedented high energy and intensity. We consider heavy neutral leptons (HNLs) as a specific example to demonstrate the sensitivity of searches for dark sector particles at future electron-positron collider beam dump experiments. This includes the study of the reach at the International Linear Collider (ILC), the Cool Copper Collider (C3), and the Compact Linear Collider (CLIC). We comprehensively examine the HNL production and detector acceptance at these electron beam dump experiments. We show that these experiments will probe regions of HNL parameter space, not yet probed by past experiments, as well as by future approved experiments. Our study also motivates a more detailed analysis of heavy meson productions in high-energy electron-nucleon collisions in thick targets.