Terrestrial Probes of Electromagnetically Interacting Dark Radiation

We study the possibility that dark radiation, sourced through the decay of dark matter in the late Universe, carries electromagnetic interactions. The relativistic flux of particles induces recoil signals in direct detection and neutrino experiments through its interaction with millicharge, electric/magnetic dipole moments, or anapole moment/charge radius. Taking the DM lifetime as 35 times the age of the Universe, as currently cosmologically allowed, we show that direct detection (neutrino) experiments have complementary sensitivity down to $\epsilon\sim 10^{-11}$ $(10^{-12})$, $d_\chi/\mu_\chi \sim 10^{-9}\,\mu_B$ $(10^{-13}\mu_B)$, and $a_\chi/b_\chi \sim 10^{-2}\,{\rm GeV}^{-2}$ $(10^{-8}\,{\rm GeV}^{-2})$ on the respective couplings. Finally, we show that such dark radiation can lead to a satisfactory explanation of the recently observed XENON1T excess in the electron recoil signal without being in conflict with other bounds.