Sensitivity of a Missing-Mass Search for a Light Dark Photon with a Positron Beam on a Hydrogen Target

Abstract

We present a Geant4-based simulation study of the sensitivity to a light dark photon, $A^{\prime }$, using a positron beam incident on a thin liquid-hydrogen target and a high-resolution forward electromagnetic calorimeter for final-state photon reconstruction. This study is motivated by ongoing research and development toward a low-energy positron facility at Jefferson Lab. The proposed search employs the missing-mass technique in the annihilation-in-flight process $e^{+}{e}^{-}\to \gamma A^{\prime }$, providing sensitivity to the $A^{\prime }$ independently of its decay mode. We evaluate the expected backgrounds from standard QED annihilation and bremsstrahlung processes and present the projected sensitivity to the kinetic-mixing parameter ${\epsilon }^2$ as a function of the $A^{\prime }$ mass. The experiment with 500~MeV beam will provide the data for $A^{\prime }$ mass range 7-19 MeV. The projected sensitivity on ${\epsilon }^2$ is $4\times 10^{-8}$ at $m_{A^{\prime }}=19$~MeV.