Millicharged Cosmic Rays and Low Recoil Detectors
  We consider the production of a "fast flux" of hypothetical millicharged
particles (mCPs) in the interstellar medium (ISM). We consider two possible
sources induced by cosmic rays: (a) $pp\rightarrow$(meson)$\rightarrow$(mCP)
which adds to atmospheric production of mCPs, and (b) cosmic-ray up-scattering
on a millicharged component of dark matter. We notice that the galactic
magnetic fields retain mCPs for a long time, leading to an enhancement of the
fast flux by many orders of magnitude. In both scenarios, we calculate the
expected signal for direct dark matter detection aimed at electron recoil. We
observe that in Scenario (a) neutrino detectors (ArgoNeuT and Super-Kamiokande)
still provide superior sensitivity compared to dark matter detectors (XENON1T).
However, in scenarios with a boosted dark matter component, the dark matter
detectors perform better, given the enhancement of the upscattered flux at low
velocities. Given the uncertainties, both in the flux generation model and in
the actual atomic physics leading to electron recoil, it is still possible that
the XENON1T-reported excess may come from a fast mCP flux, which will be
decisively tested with future experiments.
