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.
