Constraints and Projections for Millicharged Dark Matter in the Sun with Water Cherenkov Neutrino Detectors

Abstract

Millicharged particles are well-motivated dark matter candidates that have been extensively investigated in terrestrial experiments. Recent studies proposed using the IceCube Neutrino Observatory to search for high-energy neutrinos produced by the capture and annihilation of millicharged dark matter in the Sun, deriving new constraints in the strong interaction regime where the millicharge is $q_\chi \sim 10^{-3}$-$10^{-2}$, which extend to small fractional abundances where all existing constraints lose sensitivity. In this work, I point out that the lower energy threshold of water Cherenkov detectors makes Super-Kamiokande and the future Hyper-Kamiokande sensitive to neutrinos from the annihilation of lighter millicharged dark matter, complementing the high-mass reach of IceCube. I find that Super-Kamiokande can constrain previously unexplored parameter space at $m_{\chi}=5$-28 GeV for dark matter fraction of $f_{\chi}=10^{-4.5}$, while Hyper-Kamiokande can improve these constraints and will be sensitive to fractional abundances as small as $f_{\chi}\simeq 5\times 10^{-6}$, nearly an order of magnitude below current IceCube limits.