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 us to produce an $A^{\prime }$ via nonresonant ($e^{+}+e^{-}\to \gamma +A^{\prime }$) and resonant ($e^{+}+e^{-}\to A^{\prime }$) 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 $A^{\prime }$-strahlung. This idea, applied to beam-dump experiments and active beam-dump experiments, pushes down the current limits by an order of magnitude.

Figure 1

Three different $A^{\prime }$ production modes in fixed target lepton beam experiments: (a) $A^{\prime }$-strahlung in $e^{-}/e^{+}$-nucleon scattering; (b) $A^{\prime }$-strahlung in $e^{+}{e}^{-}$ annihilation; (c) resonant $A^{\prime }$ production in $e^{+}{e}^{-}$ annihilation.

Figure 2

Differential track-length distribution as a function of $x=E/E_0$ for positrons produced in a thick target by an impinging $e^{+}/e^{-}$ beam, normalized to the radiation length $X_0$.

Figure 3

Left: Continuous lines show exclusion limits at 90% C.L. for electron BDE and ABDE due to resonant and nonresonant positron annihilation (only). Dashed lines show exclusion limits obtained by considering $A^{\prime }$-strahlung only. The combined exclusion region is shown as a filled area: Light gray indicates previous limits (including E137, BABAR [28], and Liquid Scintillator Neutrino Detector [29]); dark gray shows the effect of including positron annihilation on existing limits. Different colors correspond to the different experiments: E137 (blue), BDX (magenta), NA64 (yellow), and LDMX (brown). Limits are given for the parameter $y\equiv {\alpha }_D{ϵ}^2[m_{\chi }/(m_{A^{\prime }}){]}^4$ as a function of $m_{\chi }$. The prescription ${\alpha }_D=0.5$, $m_{A^{\prime }}=3m_{\chi }$ is adopted when applicable. Right: The same as in the left plot but for possible positron-beam BDE and ABDE. Exclusion limits are derived assuming the same (positron) charge and experimental efficiency quoted for the corresponding $e^{-}$-beam setup.