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Cosmic-ray muons that enter the Super-Kamiokande detector cause hadronic showers due to spallation in water, producing neutrons and radioactive isotopes. These are a major background source for studies of MeV-scale neutrinos and searches for rare events. In 2020, gadolinium was introduced into the ultra-pure water in the Super-Kamiokande detector to improve the detection efficiency of neutrons. In this study, the cosmogenic neutron yield was measured using data acquired during the period after the gadolinium loading. The yield was found to be ð2.76 0.02ðstatÞ 0.19ðsystÞÞ × 10−4 μ−1 g−1 cm2 at an average muon energy 259 GeV at the Super-Kamiokande detector.more » « less
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We report a search for cosmic-ray boosted dark matter with protons using the 0.37 megaton years data collected at Super-Kamiokande experiment during the 1996–2018 period (SKI-IV phase). We searched for an excess of proton recoils above the atmospheric neutrino background from the vicinity of the Galactic Center. No such excess is observed, and limits are calculated for two reference models of dark matter with either a constant interaction cross section or through a scalar mediator. This is the first experimental search for boosted dark matter with hadrons using directional information. The results present the most stringent limits on cosmic-ray boosted dark matter and exclude the dark matter–nucleon elastic scattering cross section between 10−33 cm2 and 10−27cm2 for dark matter mass from 1 MeV=c2 to 300 MeV=c2.more » « less
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