In rich dark sector models, dark photons heavier than tens of MeV can behave
as semi-visible particles: their decays contain both visible and invisible
final states. We present models containing multiple dark fermions which allow
for such decays and inscribe them in the context of inelastic dark matter and
heavy neutral leptons scenarios. Our models represent a generalization of the
traditional inelastic dark matter model by means of a charge conjugation
symmetry. We revisit constraints on dark photons from e+eβ colliders and
fixed target experiments, including the effect of analysis vetoes on
semi-visible decays, Aβ²βΟiβ(ΟjββΟkββ+ββ). We
find that in some cases, the BaBar and NA64 experiments no longer exclude large
kinetic mixing, Ξ΅βΌ10β2, and, specifically, the related
explanation of the discrepancy in the muon (gβ2). This reopens an interesting
window in parameter space for dark photons with exciting discovery prospects.
We point out that a modified missing-energy search at NA64 can target
short-lived Aβ² decays and directly probe the newly-open parameter
space.Comment: 41 pages, 22 figures, version published in PR