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

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