Search for a massless dark photon in ${\mathrm{\Lambda }}_c^{+}\to p{\gamma }^{\mathrm{\prime }}$ decay

A search for a massless dark photon ${\gamma }^{\mathrm{\prime }}$ is conducted using 4.5 ${\mathrm{f}\mathrm{b}}^{-1}$ of $e^{+}{e}^{-}$ collision data collected at center-of-mass energies between 4.600 and 4.699 $\mathrm{G}\mathrm{e}\mathrm{V}$ with the BESIII detector at BEPCII. No significant signal is observed, and the upper limit on the branching fraction $\mathcal{B}({\mathrm{\Lambda }}_c^{+}\to p{\gamma }^{\mathrm{\prime }})$ is determined to be $8.0\times {10}^{-5}$ at 90% confidence level.