  If dark matter has mass lower than around 1 GeV, it will not impart enough
energy to cause detectable nuclear recoils in many direct-detection
experiments. However, if dark matter is upscattered to high energy by
collisions with cosmic rays, it may be detectable in both direct-detection
experiments and neutrino experiments. We report the results of a dedicated
search for boosted dark matter upscattered by cosmic rays using the PROSPECT
reactor antineutrino experiment. We show that such a flux of upscattered dark
matter would display characteristic diurnal sidereal modulation, and use this
to set new experimental constraints on sub-GeV dark matter exhibiting large
interaction cross-sections.
