Millicharged cosmic rays and low recoil detectors

We consider the production of a "fast flux" of hypothetical millicharged particles (mCPs) in the interstellar medium. We consider two possible sources induced by cosmic rays: (a) p p →(meson )→(mCP ) , which adds to atmospheric production of mCPs, and (b) cosmic-ray upscattering 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.