Searching for the massless dark photon in c→ uγ'

In the effective field theory, the massless dark photon γ' can only couple with the Standard Model particle through operators of dimension higher than four, thereby offering a high sensitivity to the new physics energy scale. Using 7.9~fb⁻¹ of e⁺e⁻ collision data collected at √s=3.773 GeV with the BESIII detector at the BEPCII collider, we measure the effective flavor-changing neutral current coupling of cuγ' in D⁰→ωγ' and D⁰→γγ' processes to search for the massless dark photon. No significant signals are observed, and the upper limits at the 90% confidence level on the massless dark photon branching fraction are set to be 1.1×10⁻⁵ and 2.0×10⁻⁶ for D⁰→ωγ' and D⁰→γγ', respectively. These results provide the most stringent constraint on the new physics energy scale associated with cuγ' coupling in the world, with the new physics energy scale related parameter |C|²+|C₅|²<8.2×10⁻¹⁷~GeV⁻² at the 90% confidence level, playing a unique role in the dark sector search with the charm sector.