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, $A'$, 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 $e^+e^- \to γA'$, providing sensitivity to the $A'$ 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 $ε^2$ as a function of the $A'$ mass. The experiment with 500~MeV beam will provide the data for $A'$ mass range 7-19 MeV. The projected sensitivity on $ε^2$ is $4\times10^{-8}$ at $m_{A'}=19$~MeV.