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Search for astrophysical electron antineutrinos in Super-Kamiokande with 0.01wt% gadolinium-loaded water. 

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. 

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. 

A search for neutrinos produced in coincidence with gamma-ray bursts (GRBs) was conducted with the Super-Kamiokande (SK) detector. Between December 2008 and March 2017, the Gamma-ray Coordinates Network recorded 2208 GRBs that occurred during normal SK operation. Several time windows around each GRB were used to search for coincident neutrino events. No statistically significant signal in excess of the estimated backgrounds was detected. For all GRBs, upper bounds were obtained on the fluence as a function of neutrino energy. Additionally, for GRBs at known distances, upper limits were set for the neutrino energy emission at the GRB. 

We performed a n − ¯n oscillation search with SK-I-IV data using a multivariate analysis. Compared to previous results [15], the updated final state interaction model predicts fewer pions and less separation between signal and neutrino backgrounds. With the advanced MVA method and the inclusion of multiple new variables, the sensitivity of this analysis is still greatly enhanced. For the 0.37 megaton-year exposure at SK, we observed 11 events with an expected background of 9.3  2.7 events. There is no statistically significant excess of data events, so a lower limit on the neutron lifetime is set at 3.6 × 1032 years at 90% C.L., corresponding to a lower limit on the neutron-antineutron oscillation time in 16O of τn→¯n > 4.7 × 108 s. This is the world’s most stringent limit on neutron-antineutron oscillation so far, with 90% improvement from the previous best limit [15], and is reaching the predicted parameter space of some theoretical models. 

The Super-Kamiokande detector can be used to search for neutrinos in time coincidence with gravitational waves detected by the LIGO–Virgo Collaboration (LVC). Both low-energy (7–100 MeV) and high-energy (0.1–105 GeV) samples were analyzed in order to cover a very wide neutrino spectrum. Follow-ups of 36 (out of 39) gravitational waves reported in the GWTC-2 catalog were examined; no significant excess above the background was observed, with 10 (24) observed neutrinos compared with 4.8 (25.0) expected events in the high-energy (low-energy) samples. A statistical approach was used to compute the significance of potential coincidences. For each observation, p-values were estimated using neutrino direction and LVC sky map; the most significant event (GW190602_175927) is associated with a post-trial p-value of 7.8% (1.4σ). Additionally, flux limits were computed independently for each sample and by combining the samples. The energy emitted as neutrinos by the identified gravitational wave sources was constrained, both for given flavors and for all flavors assuming equipartition between the different flavors, independently for each trigger and by combining sources of the same nature. 

Search for proton decay via p → e+ π0 and p → μ+ π0 with an enlarged fiducial volume in Super-Kamiokande I-IV.