A Visible Coupling Benchmark for Massive Dark Photon Searches in D0→γA′→γe+e−

Rare charm decays offer a clean arena for visible massive dark photon searches through narrow dielectron resonances. In the process D0→γA′→γe+e−, the observable signal rate is controlled simultaneously by the flavor changing charm transition, the radiative D0→γ hadronic matrix element, the visible decay probability of A′, and the experimental response of the reconstructed mass spectrum. We introduce a visible coupling formulation that combines these ingredients into a single search parameter while keeping the form factor and the A′→e+e− branching fraction as explicit external inputs. This provides a direct bridge between branching fraction sensitivity and coupling reach, without assuming a specific experimental background model. The central novelty of this approach is that it separates the particle physics information entering the signal rate from the analysis-dependent ingredients that determine the observable mass spectrum. We apply this construction to the D0→γe+e− spectrum, including effective exposure, reconstruction efficiency, mass resolution, and the treatment of the ρ/ω and ϕ regions. The ρ/ω and ϕ dominated intervals are kept outside the coupling definition and left to experiment specific resonance treatment. This formulation gives a practical and reusable basis for comparing visible massive dark photon sensitivity across current and future charm data sets.