Displaced e+e- Signatures from an X17-Mediated Inelastic Dark-Sector Cascade

We investigate displaced electron-positron signatures from an inelastic dark sector mediated by a light vector boson associated with the X17 anomaly. A heavy neutral fermion decays into a lighter stable state and the mediator, whose subsequent decay produces a single visible vertex after two stages of neutral propagation. We calculate the decay width, displacement distribution, lepton kinematics, and production rate in electro-positron collisions, including vector polarization. At Belle II, stronger dark-sector couplings increase production but require finely adjusted masses to preserve a macroscopic decay length. We quantify off-shell contributions and assess the validity of the two-body approximation. We show that the electron-positron pair cannot satisfy simultaneously the moment and opening-angle cuts of an existing search proposal for the configuration studied. Less restrictive selections allow nonzero kinematic acceptance. Our results clarify the experimental requirements for searching these signatures at Belle II.