Search

We present the results of searches for nucleon decays via $p\to \nu {\pi }^{+}$ and $n\to \nu {\pi }^0$ using a $0.484\mathrm{Mt}·\mathrm{yr}$ exposure of Super-Kamiokande I-V data covering the entire pure water phase of the experiment. Various improvements on the previous 2014 nucleon decay search [Phys. Rev. Lett. 113, 121802 (2014)], which used an exposure of $0.173\mathrm{Mt}·\mathrm{yr}$, are incorporated. The physics models related to pion production and nuclear interaction are refined with external data, and a more comprehensive set of systematic uncertainties, now including those associated with the atmospheric neutrino flux and pion production channels, is considered. Also, the fiducial volume has been expanded by 21%. No significant indication of a nucleon decay signal is found beyond the expected background. Lower bounds on the nucleon partial lifetimes are determined to be $3.5\times {10}^{32}\mathrm{yr}$ for $p\to \nu {\pi }^{+}$ and $1.4\times {10}^{33}\mathrm{yr}$ for $n\to \nu {\pi }^0$ at 90% confidence level.

In recent neutrino detectors, neutrons produced in neutrino reactions play an important role. Muon capture on oxygen nuclei is one of the processes that produce neutrons in water Cherenkov detectors. We measured neutron multiplicity in the process using cosmic ray muons that stop in the gadolinium-loaded Super-Kamiokande detector. For this measurement, neutron detection efficiency is obtained with the muon capture events followed by gamma rays to be ${50.2}_{-2.1}^{+2.0}\%$. By fitting the observed multiplicity considering the detection efficiency, we measure neutron multiplicity in muon capture as $P(0)=24\pm 3\%$, $P(1)=70_{-2}^{+3}\%$, $P(2)=6.1\pm 0.5\%$, $P(3)=0.38\pm 0.09\%$. This is the first measurement of the multiplicity of neutrons associated with muon capture on oxygen without neutron energy threshold.

A search for proton decay into $e^{+}/{\mu }^{+}$ and a $\eta$ meson has been performed using data from a $0.373\mathrm{Mton}·\mathrm{year}$ exposure (6050.3 live days) of Super-Kamiokande. Compared to previous searches this work introduces an improved model of the intranuclear $\eta$ interaction cross section, resulting in a factor of 2 reduction in uncertainties from this source and $\sim 10\%$ increase in signal efficiency. No significant data excess was found above the expected number of atmospheric neutrino background events resulting in no indication of proton decay into either mode. Lower limits on the proton partial lifetime of $1.4\times {10}^{34}\mathrm{years}$ for $p\to e^{+}\eta$ and $7.3\times {10}^{33}\mathrm{years}$ for $p\to {\mu }^{+}\eta$ at the 90% CL were set. These limits are around 1.5 times longer than our previous study and are the most stringent to date.

We present a measurement of neutrino oscillation parameters with the Super-Kamiokande detector using atmospheric neutrinos from the complete pure-water SK I–V (April 1996–July 2020) dataset, including events from an expanded fiducial volume. The dataset corresponds to 6511.3 live days and an exposure of 484.2 kiloton-years. Measurements of the neutrino oscillation parameters $\mathrm{\Delta }{m}_{32}^2$, ${\sin }^2{\theta }_{23}$, ${\sin }^2{\theta }_{13}$, ${\delta }_{\mathrm{CP}}$, and the preference for the neutrino mass ordering are presented with atmospheric neutrino data alone, and with constraints on ${\sin }^2{\theta }_{13}$ from reactor neutrino experiments. Our analysis including constraints on ${\sin }^2{\theta }_{13}$ favors the normal mass ordering at the 92.3% level.

Radioactivity induced by cosmic muon spallation is a dominant source of backgrounds for $\mathcal{O}(10\mathrm{MeV})$ neutrino interactions in water Cherenkov detectors. In particular, it is crucial to reduce backgrounds to measure the solar neutrino spectrum and find neutrino interactions from distant supernovae. In this paper we introduce new techniques to locate muon-induced hadronic showers and efficiently reject spallation backgrounds. Applying these techniques to the solar neutrino analysis with an exposure of $2790\times 22.5\mathrm{kton}·\mathrm{day}$ increases the signal efficiency by 12.6%, approximately corresponding to an additional year of detector running. Furthermore, we present the first spallation simulation at Super-Kamiokande, where we model hadronic interactions using fluka. The agreement between the isotope yields and shower pattern in this simulation and in the data gives confidence in the accuracy of this simulation, and thus opens the door to use it to optimize muon spallation removal in new data with gadolinium-enhanced neutron capture detection.

We searched for proton decay via $p\to {\mu }^{+}{K}^0$ in $0.37\mathrm{Mton}·\mathrm{years}$ of data collected between 1996 and 2018 from the Super-Kamiokande water Cherenkov experiment. The selection criteria were defined separately for $K_S^0$ and $K_L^0$ channels. No significant event excess has been observed. As a result of this analysis, which extends the previous search by an additional $0.2\mathrm{Mton}·\mathrm{years}$ of exposure and uses an improved event reconstruction, we set a lower limit of $3.6\times {10}^{33}\mathrm{years}$ on the proton lifetime.

The Super-Kamiokande detector has measured solar neutrinos for more than 25 years. The sensitivity for solar neutrino measurement is limited by the uncertainties of energy scale and background modeling. Decays of unstable isotopes with relatively long half-lives through nuclear muon capture, such as ${}^{16}\mathrm{N}$, ${}^{15}\mathrm{C}$, ${}^{12}\mathrm{B}$, and ${}^{13}\mathrm{B}$, are detected as background events for solar neutrino observations. In this study, we developed a method to form a pair of stopping muon and decay candidate events and evaluated the production rates of such unstable isotopes. We then measured their branching ratios considering both their production rates and the estimated number of nuclear muon capture processes as $\mathrm{Br}\left({}^{16}\mathrm{N}\right)=(9.0\pm 0.1)\%$, $\mathrm{Br}\left({}^{15}\mathrm{C}\right)=(0.6\pm 0.1)\%$, $\mathrm{Br}\left({}^{12}\mathrm{B}\right)=(0.98\pm 0.18)\%$, $\mathrm{Br}\left({}^{13}\mathrm{B}\right)=(0.14\pm 0.12)\%$, respectively. The result for ${}^{16}\mathrm{N}$ has world-leading precision at present and the results for ${}^{15}\mathrm{C}$, ${}^{12}\mathrm{B}$, and ${}^{13}\mathrm{B}$ are the first branching ratio measurements for those isotopes.

The Calorimetric Electron Telescope (CALET), in operation on the International Space Station since 2015, collected a large sample of cosmic-ray (CR) iron and sub-iron events over a wide energy interval. In this Letter, we report an update of our previous measurement of the iron flux and we present—for the first time—a high statistics measurement of the spectra of two sub-iron elements Cr and Ti in the energy interval from 10 to $250\mathrm{GeV}/n$. The analyses are based on 8 years of data. Differently from older generations of cosmic-ray instruments which, in most cases, could not resolve individual sub-iron elements, CALET can identify each nuclear species from proton to nickel (and beyond) with a measurement of their electric charge. Thanks to the improvement in statistics and a more refined assessment of systematic uncertainties, the iron spectral shape is better resolved, at high energy, than in our previous paper, and we report its flux ratio to chromium and titanium. The measured fluxes of Cr and Ti show energy dependences compatible with a single power law with spectral indices $-2.74\pm 0.06$ and $-2.88\pm 0.06$, respectively.

We report a search for time variations of the solar ${}^8\mathrm{B}$ neutrino flux using 5804 live days of Super-Kamiokande data collected between May 31, 1996, and May 30, 2018. Super-Kamiokande measured the precise time of each solar neutrino interaction over 22 calendar years to search for solar neutrino flux modulations with unprecedented precision. Periodic modulations are searched for in a dataset comprising five-day interval solar neutrino flux measurements with a maximum likelihood method. We also applied the Lomb-Scargle method to this dataset to compare it with previous reports. The only significant modulation found is due to the elliptic orbit of the Earth around the Sun. The observed modulation is consistent with astronomical data: we measured an eccentricity of $(1.53\pm 0.35)\%$, and a perihelion shift of ($-1.5\pm 13.5$) days.

We present measurements of total neutron production from atmospheric neutrino interactions in water, analyzed as a function of electron-equivalent visible energy over a range of 30 MeV to 10 GeV. These results are based on 4,270 days of data collected by Super-Kamiokande, including 564 days with 0.011 wt% gadolinium added to enhance neutron detection. Neutron signal selection is based on a neural network trained on simulation, with its performance validated using an Am/Be neutron point source. The measurements are compared to predictions from neutrino event generators combined with various hadron-nucleus interaction models, which include an intranuclear cascade model and a nuclear deexcitation model. We observe significant variations in the predictions depending on the choice of hadron-nucleus interaction model. We discuss key factors that contribute to describing our data, such as in-medium effects in the intranuclear cascade and the accuracy of statistical evaporation modeling.

We report the first measurement of the atmospheric neutrino-oxygen neutral-current quasielastic (NCQE) cross section in the gadolinium-loaded Super-Kamiokande (SK) water Cherenkov detector. In June 2020, SK began a new experimental phase, named SK-Gd, by loading 0.011% by mass of gadolinium into the ultrapure water of the SK detector. The introduction of gadolinium to ultrapure water has the effect of improving the neutron-tagging efficiency. Using a 552.2 day dataset from August 2020 to June 2022, we measure the NCQE cross section to be $0.74\pm 0.22(\text{stat}{)}_{-0.15}^{+0.85}(\text{syst})\times {10}^{-38}{\mathrm{cm}}^2/\text{oxygen}$ in the energy range from 160 MeV to 10 GeV, which is consistent with the atmospheric neutrino-flux-averaged theoretical NCQE cross section and the measurement in the SK pure-water phase within the uncertainties. Furthermore, we compare the models of the nucleon-nucleus interactions in water and find that the binary cascade model and the Liège intranuclear cascade model provide a somewhat better fit to the observed data than the Bertini cascade model. Since the atmospheric neutrino-oxygen NCQE reactions are one of the main backgrounds in the search for diffuse supernova neutrino background (DSNB), these new results will contribute to future studies—and the potential discovery—of the DSNB in SK.

The Super-Kamiokande and T2K Collaborations present a joint measurement of neutrino oscillation parameters from their atmospheric and beam neutrino data. It uses a common interaction model for events overlapping in neutrino energy and correlated detector systematic uncertainties between the two datasets, which are found to be compatible. Using 3244.4 days of atmospheric data and a beam exposure of $19.7(16.3)\times {10}^{20}$ protons on target in (anti)neutrino mode, the analysis finds a $1.9\sigma$ exclusion of $CP$ conservation (defined as $J_{CP}=0$) and a $1.2\sigma$ exclusion of the inverted mass ordering.

An analysis of solar neutrino data from the fourth phase of Super-Kamiokande (SK-IV) from October 2008 to May 2018 is performed and the results are presented. The observation time of the dataset of SK-IV corresponds to 2970 days and the total live time for all four phases is 5805 days. For more precise solar neutrino measurements, several improvements are applied in this analysis: lowering the data acquisition threshold in May 2015, further reduction of the spallation background using neutron clustering events, precise energy reconstruction considering the time variation of the PMT gain. The observed number of solar neutrino events in 3.49–19.49 MeV electron kinetic energy region during SK-IV is ${\mathrm{65,443}}_{-388}^{+390}(\mathrm{stat}.)\pm 925(\mathrm{syst}.)$ events. Corresponding ${}^8\mathrm{B}$ solar neutrino flux is $(2.314\pm 0.014(\mathrm{stat}.)\pm 0.040(\mathrm{syst}.))\times {10}^6{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}$, assuming a pure electron-neutrino flavor component without neutrino oscillations. The flux combined with all SK phases up to SK-IV is $(2.336\pm 0.011(\mathrm{stat}.)\pm 0.043(\mathrm{syst}.))\times {10}^6{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}$. Based on the neutrino oscillation analysis from all solar experiments, including the SK 5805 days dataset, the best-fit neutrino oscillation parameters are ${\sin }^2{\theta }_{12,\mathrm{solar}}=0.306\pm 0.013$ and $\mathrm{\Delta }{m}_{21,\text{solar}}^2=(6.10_{-0.81}^{+0.95})\times {10}^{-5}{\mathrm{eV}}^2$, with a deviation of about $1.5\sigma$ from the $\mathrm{\Delta }{m}_{21}^2$ parameter obtained by KamLAND. The best-fit neutrino oscillation parameters obtained from all solar experiments and KamLAND are ${\sin }^2{\theta }_{12,\text{global}}=0.307\pm 0.012$ and $\mathrm{\Delta }{m}_{21,\text{global}}^2=(7.50_{-0.18}^{+0.19})\times {10}^{-5}{\mathrm{eV}}^2$.

Super-Kamiokande (SK) has observed ${}^8\mathrm{B}$ solar neutrino elastic scattering at recoil electron kinetic energies ($E_{\mathrm{kin}}$) as low as 3.49 MeV to study neutrino flavor conversion within the Sun. At SK-observable energies, these conversions are dominated by the Mikheyev-Smirnov-Wolfenstein effect. An upturn in the electron neutrino survival probability in which vacuum neutrino oscillations become dominant is predicted to occur at lower energies, but radioactive background increases exponentially with decreasing energy. New machine learning approaches provide substantial background reduction below 3.49 MeV such that statistical extraction of solar neutrino interactions becomes feasible. This article presents an analysis of the solar neutrino interaction rate at $E_{\mathrm{kin}}<3.49\mathrm{MeV}$ with the full SK-IV period, using data from a wideband intelligent trigger when available and with a boosted decision tree for event selection. A solar neutrino signal is observed between $2.99\mathrm{MeV}<E_{\mathrm{kin}}<3.49\mathrm{MeV}$ with $2.76\sigma$ significance and a data to unoscillated Monte Carlo ratio of ${0.307}_{-0.111}^{+0.112}$. These additional low-energy data have a negligible effect on the $1\sigma$ intervals of the fits to the solar neutrino energy spectrum but have a noticeable effect on the best fit when using the exponential parametrization.

We report a search for cosmic-ray boosted dark matter with protons using the $0.37\text{megaton}\times \mathrm{years}$ 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 $10^{-33}{\mathrm{cm}}^2$ and $10^{-27}{\mathrm{cm}}^2$ for dark matter mass from $1\mathrm{MeV}/c^2$ to $300\mathrm{MeV}/c^2$.

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 $(2.76\pm 0.02(\mathrm{stat})\pm 0.19(\mathrm{syst}))\times {10}^{-4}{\mathrm{\mu }}^{-1}{\mathrm{g}}^{-1}{\mathrm{cm}}^2$ at an average muon energy 259 GeV at the Super-Kamiokande detector.

We have conducted a new search for the diffuse supernova neutrino background (DSNB) flux at Super-Kamiokande (SK), with a $22.5\times 2970\text{−}\mathrm{kton}·\mathrm{day}$ exposure from its fourth operational phase IV. With the new analysis we improve on the existing background reduction techniques and systematic uncertainties and take advantage of an improved neutron tagging algorithm to lower the energy threshold compared to the previous phases of SK. This allows for setting the world’s most stringent upper limit on the extraterrestrial ${\overline{\nu }}_e$ flux, for neutrino energies below 31.3 MeV. The SK-IV results are combined with the ones from the first three phases of SK to perform a joint analysis using $22.5\times 5823\mathrm{kton}·\mathrm{days}$ of data. This analysis has the world’s best sensitivity to the DSNB ${\overline{\nu }}_e$ flux, comparable to the predictions from various models. For neutrino energies larger than 17.3 MeV, the new combined 90% CL upper limits on the DSNB ${\overline{\nu }}_e$ flux lie around $2.7{\mathrm{cm}}^{-2}·{\sec }^{-1}$, strongly disfavoring the most optimistic predictions. Finally, potentialities of the gadolinium phase of SK and the future Hyper-Kamiokande experiment are discussed.

We searched for bound neutron decay via $n\to \overline{\nu }+K^0$ predicted by the grand unified theories in $0.401\mathrm{Mton}·\mathrm{years}$ exposure of all pure water phases in the Super-Kamiokande detector. About 4.4 times more data than in the previous search have been analyzed by a new method including a spectrum fit to kaon invariant mass distributions. No significant data excess has been observed in the signal regions. As a result of this analysis, we set a lower limit of $7.8\times {10}^{32}\mathrm{years}$ on the neutron lifetime at a 90% confidence level.

We present the results of the charge ratio ($R$) and polarization ($P_0^{\mu }$) measurements using decay electron events collected between September 2008 and June 2022 with the Super-Kamiokande detector. Because of its underground location and long operation, we are able to perform high-precision measurements by accumulating cosmic-ray muons. We measured the muon charge ratio to be $R=1.32\pm 0.02(\mathrm{stat}+\mathrm{syst})$ at $E_{\mu }\cos {\theta }_{\text{Zenith}}={0.7}_{-0.2}^{+0.3}\mathrm{TeV}$, where $E_{\mu }$ is the muon energy and ${\theta }_{\text{Zenith}}$ is the zenith angle of incoming cosmic-ray muons. This result is consistent with the Honda flux model while indicating a tension with the $\pi K$ model of $1.9\sigma$. We also measured the muon polarization at the production location to be $P_0^{\mu }=0.52\pm 0.02(\mathrm{stat}+\mathrm{syst})$ at the muon momentum of ${0.9}_{-0.1}^{+0.6}\mathrm{TeV}/c$ at the surface of the mountain; this also suggests a tension with the Honda flux model of $1.5\sigma$. This is the most precise measurement ever to experimentally determine the cosmic-ray muon polarization near $1\mathrm{TeV}/c$. These measurement results are useful to improve atmospheric neutrino simulations.