New Constraints on Millicharged Particles from Cosmic-ray Production
  We study the production of exotic millicharged particles (MCPs) from cosmic
ray-atmosphere collisions which constitutes a permanent MCP production source
for all terrestrial experiments Our calculation of the MCP flux can be used to
reinterpret existing limits from experiments such as MACRO and Majorana on an
ambient flux of ionizing particles. Large-scale underground neutrino detectors
are particularly favorable targets for the resulting MCPs. Using available data
from the Super-K experiment, we set new limits on MCPs, which are the best in
sensitivity reach for the mass range $0.1 \lesssim m_{\chi} \lesssim 0.5$ GeV,
and which are competitive with accelerator-based searches for masses up to 1.5
GeV. Applying these constraints to models where a sub-dominant component of
dark matter (DM) is fractionally charged allows us to probe parts of the
parameter space that are challenging for conventional direct-detection DM
experiments, independently of any assumptions about the DM abundance. These
results can be further improved with the next generation of large-scale
neutrino detectors.
