Search for a massless dark photon in $ c\to u{\gamma }^{\prime }$ decays

Using $ 7.9\mathrm{f}{\mathrm{b}}^{-1}$ of $ {e}^{+}{e}^{-}$ collision data collected at $ \sqrt{s}=3.773\text{}\text{}\mathrm{GeV}$ with the BESIII detector at the BEPCII collider, we search for the massless dark photon with the flavor-changing neutral current processes $ {D}^{0}\to \omega {\gamma }^{\prime }$ and $ {D}^{0}\to \gamma {\gamma }^{\prime }$ for the first time. 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\times {10}^{-5}$ and $ 2.0\times {10}^{-6}$ for $ {D}^{0}\to \omega {\gamma }^{\prime }$ and $ {D}^{0}\to \gamma {\gamma }^{\prime }$, respectively. These results provide the most stringent constraint on the new physics energy scale associated with $ cu{\gamma }^{\prime }$ coupling in the world, with the new physics energy scale related parameter $ |\mathbb{C}{|}^{2}+|{\mathbb{C}}_{5}{|}^{2}<8.2\times {10}^{-17}\text{}\text{}{\mathrm{GeV}}^{-2}$ at the 90% confidence level.