Constraining light dark matter upscattered by ultrahigh-energy cosmic
  rays
  Light halo dark matter (DM) particles upscattered by high-energy cosmic rays
(CRs) can be energetic, and become detectable by conventional direct detection
experiments. The current constraints derived from space-based direct CR
measurements can reach $\mathcal{O}(10^{-31})\text{ cm}^{2}$ for a constant
DM-nucleon scattering cross section. We show that if the CR energy spectrum
follows a power law of type $\sim E^{-3}$, the derived constraints on the
scattering cross section will be highly insensitive to DM particle mass. This
suggests that ultrahigh-energy CRs (UHECRs) indirectly measured by ground-based
detectors can be used to place constraints on ultralight DM particles, as
$E^{-3}$ is a very good approximation of the UHECR energy spectrum up to energy
$\sim10^{20}\text{ eV}$. Using the recent UHECR flux data, we show that the
current constraints derived from space-based CR measurements can in principle
be extended to ultralight DM particles far below eV scale.
