Resonant production of dark photons in positron beam dump experiments

Abstract

Positrons beam dump experiments have unique features to search for very narrow resonances coupled superweakly to $e^{+}{e}^{-}$ pairs. Due to the continue loss of energy from soft photon bremsstrahlung, in the first few radiation lengths of the dump a positron beam can continuously scan for resonant production of new resonances via $e^{+}$ annihilation off an atomic $e^{-}$ in the target. In the case of a dark photon $A^{\prime }$ kinetically mixed with the photon, this production mode is of first order in the electromagnetic coupling $\alpha$, and thus parametrically enhanced with respect to the $O({\alpha }^2)$ $e^{+}{e}^{-}\to \gamma A^{\prime }$ production mode and to the $O({\alpha }^3)$ $A^{\prime }$ bremsstrahlung in $e^{-}-$nucleon scattering so far considered. If the lifetime is sufficiently long to allow the $A^{\prime }$ to exit the dump, $A^{\prime }\to e^{+}{e}^{-}$ decays could be easily detected and distinguished from backgrounds. We explore the foreseeable sensitivity of the Frascati PADME experiment in searching with this technique for the $17$MeV dark photon invoked to explain the ${}^8$Be anomaly in nuclear transitions.

Comments: 11 pages, 4 figures, added 3 references and some clarifications. Version accepted for publication in Phys. Rev. D