Dark photons in exotic Higgs boson decays at FCC-ee

A prospective search for dark photons decaying into collimated, displaced muon pairs is presented for the electron-positron stage of the Future Circular Collider, FCC-ee. Assuming the full luminosity of the Higgs run at 240 GeV, the study probes a broad range of dark photon masses and couplings. The signature comprises two jets from a Z boson decay and two muon pairs from dark photons produced in Higgs decays. A kinematics-based selection achieves zero background while retaining high signal efficiency. Expected limits are set on the Higgs boson branching ratio to dark photons as a function of the dark photon mass and coupling.