Missing energy signatures of inelastic magnetic dipole DM at NA64e

Some extensions of the Standard Model consider inelastic dark matter (iDM) as an attractive candidate for sub-GeV DM of thermal origin that could be detected at modern accelerators. In the present paper, we calculate the production rate of iDM pairs $\chi_{1} \bar{\chi}_0$ interacting with the ordinary photon via dipole magnetic moment in the reaction of high-energy electron scattering on nuclei, $e^- N \to e^- N \chi_{1} \bar{\chi}_0$, in the NA64e experiment at the CERN SPS. We derive the projected sensitivity of NA64e to such particles assuming of $\simeq10^{13}$ 100 GeV electrons on target. We also show, that incorporating heavy vector meson decays, $\gamma^* N \to N V (\to \chi_1 \bar{\chi}_0)$, alongside bremsstrahlung-like emission of inelastic dark matter pairs, $e^- N \to e^- N \gamma^* (\to \chi_1 \bar{\chi}_0)$, will allow NA64e to probe previously unexplored regions of the iDM parameter space, in particular for modest mass splittings, $\Delta \simeq 5 \times 10^{-2}$, and relatively light masses, $m_{\chi_0} \lesssim 100~\mbox{MeV}$.