Dark photon production through positron annihilation in beam-dump experiments

High energy positron annihilation is a viable mechanism to produce dark photons ($A^prime$). This reaction plays a significant role in beam-dump experiments using experiments using multi-GeV electron-beams on thick targets by enhancing the sensitivity to $A^prime$ production. The positrons produced by the electromagnetic shower can produce an $A^prime$ via non-resonant ($e^+ + e^- to gamma + A^prime$) and resonant ($e^+ + e^- to A^prime$) annihilation on atomic electrons. For visible decays, the contribution of resonant annihilation results in a larger sensitivity with respect to limits derived by the commonly used $A^prime$-strahlung in certain kinematic regions. When included in the evaluation of the E137 beam-dump experiment reach, positron annihilation pushes the current limit on $varepsilon$ downwards by a factor of two in the range 33 MeV/c$^2<m_{A^prime}<120$ MeV/c$^2$.