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In rich dark sector models, dark photons heavier than tens of MeV can behaveas semi-visible particles: their decays contain both visible and invisiblefinal states. We present models containing multiple dark fermions which allowfor such decays and inscribe them in the context of inelastic dark matter andheavy neutral leptons scenarios. Our models represent a generalization of thetraditional inelastic dark matter model by means of a charge conjugationsymmetry. We revisit constraints on dark photons from $e^+e^-$ colliders andfixed target experiments, including the effect of analysis vetoes onsemi-visible decays, $A^\prime \to \psi_i (\psi_j \to \psi_k \ell^+\ell^-)$. Wefind that in some cases, the BaBar and NA64 experiments no longer exclude largekinetic mixing, $\varepsilon \sim 10^{-2}$, and, specifically, the relatedexplanation of the discrepancy in the muon $(g-2)$. This reopens an interestingwindow in parameter space for dark photons with exciting discovery prospects.We point out that a modified missing-energy search at NA64 can targetshort-lived $A^\prime$ decays and directly probe the newly-open parameterspace.

Semi-Visible Dark Photon Phenomenology at the GeV Scale