New Search for Dark Matter with Neutrons at Neutrino Detectors

Large-volume neutrino experiments are ideal for testing boosted dark matter (BDM) scenarios. We propose, for the first time, an approach to utilize knockout neutrons by detecting de-excitation $\gamma$ rays and coincident captured neutrons from dark-matter interactions with bound neutrons in oxygen, while previous studies have focused on knockout-protons and electrons. This method is especially crucial for water \v{C}erenkov detectors, where high proton \v{C}erenkov threshold ($\sim$1 GeV) suppresses signal acceptance. Recently, Super-Kamiokande (SK) was doped with 0.03\% gadolinium (SK-Gd) to enhance neutron tagging efficiency. Using SK-Gd as a target experiment, we demonstrate that this method increases sensitivity to BDM models by an order of magnitude compared to 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.