Probing Dark Matter with Knockout Neutrons in Neutrino Detectors

Large-volume neutrino experiments are ideal for testing boosted-dark-matter scenarios. We propose, for the first time, an approach to utilize knockout neutron from dark-matter interaction with a nucleus, while previous studies have focused on recoiling free protons. This method is especially crucial for water Čerenkov detectors, where high proton Čerenkov threshold (~1 GeV) suppresses signal acceptance. The knockout neutrons produce γ rays when captured by medium, which are detectable in coincidence with oxygen de-excitation γ rays. Recently, Super-Kamiokande (SK) was doped with gadolinium (SK-Gd) to enhance neutron detection efficiency. Using SK-Gd as a target experiment, we demonstrate that this method increases sensitivity to boosted-dark-matter models by an order of magnitude compared to free proton-based analysis, and it allows exploration of a wider range of light dark-matter models previously inaccessible with proton-based analysis. We also present the projected sensitivity for the upcoming Hyper-Kamiokande detector.