Dark photon production through positron annihilation in beam-dump experiments

TL;DR

This paper investigates how positron annihilation can produce dark photons in beam-dump experiments, significantly improving sensitivity and tightening existing limits on dark photon parameters in certain mass ranges.

Contribution

It introduces positron annihilation as a key production mechanism for dark photons, enhancing experimental sensitivity and refining existing constraints.

Findings

Positron annihilation increases dark photon production sensitivity.

Resonant annihilation dominates in certain kinematic regions.

Limits on kinetic mixing parameter epsilon are improved by a factor of two.

Abstract

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$^2$.