A novel way to search for light dark matter in lepton beam-dump experiments

A novel mechanism to produce and detect Light Dark Matter in experiments making use of GeV electrons (and positrons) impinging on a thick target (beam-dump) is proposed. The positron-rich environment produced by the electromagnetic shower allows to produce an $A^{\mathrm{\prime }}$ via non-resonant ($e^+ + e^- \to \gamma + A^\prime$)andresonant($e^+ + e^- \to A^\prime$) annihilation on atomic electrons. The latter mechanism, for some selected kinematics, results in a larger sensitivity with respect to limits derived by the commonly used $A^{\mathrm{\prime }}-strahlung$. This idea, applied to Beam Dump Experiments and {\it active} Beam Dump Experiments pushes down the current limits by an order of magnitude.