A search for proton decay into e(+)/mu(+) and a. meson has been performed using data from a 0.373 Mton center dot year exposure (6050.3 live days) of Super-Kamiokande. Compared to previous searches this work introduces an improved model of the intranuclear. interaction cross section, resulting in a factor of 2 reduction in uncertainties from this source and similar to 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 x 10(34) years for p -> e(+)eta and 7.3 x 10(33) years for p -> 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.

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 data set 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 $65,443^{+390}_{-388}\,(\mathrm{stat.})\pm 925\,(\mathrm{syst.})$ events. Corresponding $\mathrm{^{8}B}$ solar neutrino flux is $(2.314 \pm 0.014\, \rm{(stat.)} \pm 0.040 \, \rm{(syst.)}) \times 10^{6}~\mathrm{cm^{-2}\,s^{-1}}$, assuming a pure electron-neutrino flavor component without neutrino oscillations. The flux combined with all SK phases up to SK-IV is $(2.336 \pm 0.011\, \rm{(stat.)} \pm 0.043 \, \rm{(syst.)}) \times 10^{6}~\mathrm{cm^{-2}\,s^{-1}}$. Based on the neutrino oscillation analysis from all solar experiments, including the SK $5805$~days data set, the best-fit neutrino oscillation parameters are $\rm{sin^{2} \theta_{12,\,solar}} = 0.306 \pm 0.013 $ and $\Delta m^{2}_{21,\,\mathrm{solar}} = (6.10^{+ 0.95}_{-0.81}) \times 10^{-5}~\rm{eV}^{2}$, with a deviation of about 1.5$\sigma$ from the $\Delta m^{2}_{21}$ parameter obtained by KamLAND. The best-fit neutrino oscillation parameters obtained from all solar experiments and KamLAND are $\sin^{2} \theta_{12,\,\mathrm{global}} = 0.307 \pm 0.012 $ and $\Delta m^{2}_{21,\,\mathrm{global}} = (7.50^{+ 0.19}_{-0.18}) \times 10^{-5}~\rm{eV}^{2}$.

Radioactivity induced by cosmic muon spallation is a dominant source of backgrounds for O(10 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 x 22.5 kton center dot 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.

Cosmic-ray muons that enter the Super-Kamiokande detector cause hadronic showers due to spallation in water, producing neutrons and radioactive isotopes. Those are a major background source for studies of MeV-scale neutrinos and searches for rare events. Since 2020, gadolinium was introduced in 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 $(2.76 \pm 0.02\,\mathrm{(stat.) \pm 0.19\,\mathrm{(syst.)}}) \times 10^{-4}\,\mu^{-1} \mathrm{g^{-1} cm^{2}}$ at 259 GeV of average muon energy at the Super-Kamiokande detector.

This corrects the article DOI: 10.1103/PhysRevLett.130.031802.

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}cm^{2} and 10^{-27}cm^{2} for dark matter mass from 1  MeV/c^{2} to 300  MeV/c^{2}.

In order to improve Super-Kamiokande’s neutron detection efficiency and to thereby increase its sensitivity to the diffuse supernova neutrino background flux, 13 tons of Gd2(SO4)3 · 8H2O (gadolinium sulfate octahydrate) was dissolved into the detector’s otherwise ultrapure water from July 14 to August 17, 2020, marking the start of the SK-Gd phase of operations. During the loading, water was continuously recirculated at a rate of 60 m/h, extracting water from the top of the detector and mixing it with concentrated Gd2(SO4)3 · 8H2O solution to create a 0.02% solution of the Gd compound before injecting it into the bottom of the detector. A clear boundary between the Gd-loaded and pure water was maintained through the loading, enabling monitoring of the loading itself and the spatial uniformity of the Gd concentration over the 35 days it took to reach the top of the detector. During the subsequent commissioning the recirculation rate was increased to 120 m/h, resulting in a constant and uniform distribution of Gd throughout the detector and water transparency equivalent to that of previous purewater operation periods. Using an Am-Be neutron calibration source the mean neutron capture time was measured to be 115.6± 0.6 μs, which corresponds to a Gd concentration of 110.9±1.4 (stat. only) ppm, as expected for this level of doping. This paper describes changes made to the water circulation system for this detector upgrade, the Gd loading procedure, detector commissioning, and the first neutron calibration measurements in SK-Gd.

We searched for proton decay via p -> mu(+) K-0 Mton center dot 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 Mton center dot years of exposure and uses an improved event reconstruction, we set a lower limit of 3.6 x 10(33) years on the proton lifetime.

In 2020, the Super-Kamiokande (SK) experiment moved to a new stage (SK-Gd) in which gadolinium (Gd) sulfate octahydrate was added to the water in the detector, enhancing the efficiency to detect thermal neutrons and consequently improving the sensitivity to low energy electron anti-neutrinos from inverse beta decay (IBD) interactions. SK-Gd has the potential to provide early alerts of incipient core-collapse supernovae through detection of electron anti-neutrinos from thermal and nuclear processes responsible for the cooling of massive stars before the gravitational collapse of their cores. These pre-supernova neutrinos emitted during the silicon burning phase can exceed the energy threshold for IBD reactions. We present the sensitivity of SK-Gd to pre-supernova stars and the techniques used for the development of a pre-supernova alarm based on the detection of these neutrinos in SK, as well as prospects for future SK-Gd phases with higher concentrations of Gd. For the current SK-Gd phase, high-confidence alerts for Betelgeuse could be issued up to 9 hr in advance of the core collapse itself.

Non-Standard Interactions (NSI) between neutrinos and matter affect the neutrino flavor oscillations. Due to the high matter density in the core of the Sun, solar neutrinos are suited to probe these interactions. Using the $277$ kton-yr exposure of Super-Kamiokande to $^{8}$B solar neutrinos, we search for the presence of NSI. Our data favors the presence of NSI with down quarks at 1.8$\sigma$, and with up quarks at 1.6$\sigma$, with the best fit NSI parameters being ($\epsilon_{11}^{d},\epsilon_{12}^{d}$) = (-3.3, -3.1) for $d$-quarks and ($\epsilon_{11}^{u},\epsilon_{12}^{u}$) = (-2.5, -3.1) for $u$-quarks. After combining with data from the Sudbury Neutrino Observatory and Borexino, the significance increases by 0.1$\sigma$.

Due to a very low production rate of electron anti-neutrinos ((v) over bar (epsilon)) via nuclear fusion in the Sun, a flux of solar (v) over bar (epsilon). is unexpected. An appearance of (v) over bar (epsilon) in solar neutrino flux opens a new window for the new physics beyond the standard model. In particular, a spin-flavor precession process is expected to convert an electron neutrino into an electron anti-neutrino (v(epsilon) ->(v) over bar (epsilon)) when neutrino has a finite magnetic moment. In this work, we have searched for solar (v) over bar (epsilon). in the Super-Kamiokande experiment, using neutron tagging to identify their inverse beta decay signature. We identified 78 v(epsilon). candidates for neutrino energies of 9.3 to 17.3 MeV in 2970.1 live days with a fiducial volume of 22.5 kiloton water (183.0 kton.year exposure). The energy spectrum has been consistent with background predictions and we thus derived a 90% confidence level upper limit of 4.7 x 10(-4) on the v(epsilon) -> (v) over bar (epsilon) conversion probability in the Sun. We used this result to evaluate the sensitivity of future experiments, notably the Super-Kamiokande Gadolinium (SK-Gd) upgrade.

We present the development of neutron-tagging techniques in Super-Kamiokande IV using a neural network analysis. The detection efficiency of neutron capture on hydrogen is estimated to be 26%, with a mis-tag rate of 0.016 per neutrino event. The uncertainty of the tagging efficiency is estimated to be 9.0%. Measurement of the tagging efficiency with data from an Americium-Beryllium calibration agrees with this value within 10%. The tagging procedure was performed on 3,244.4 days of SK-IV atmospheric neutrino data, identifying 18,091 neutrons in 26,473 neutrino events. The fitted neutron capture lifetime was measured as 218 +/- 9 mu s.

Neutrinos associated with solar flares (solar-flare neutrinos) provide information on particle acceleration mechanisms during the impulsive phase of solar flares. We searched using the Super-Kamiokande detector for neutrinos from solar flares that occurred during solar cycles $23$ and $24$, including the largest solar flare (X28.0) on November 4th, 2003. In order to minimize the background rate we searched for neutrino interactions within narrow time windows coincident with $\gamma$-rays and soft X-rays recorded by satellites. In addition, we performed the first attempt to search for solar-flare neutrinos from solar flares on the invisible side of the Sun by using the emission time of coronal mass ejections (CMEs). By selecting twenty powerful solar flares above X5.0 on the visible side and eight CMEs whose emission speed exceeds $2000$ $\mathrm{km \, s^{-1}}$ on the invisible side from 1996 to 2018, we found two (six) neutrino events coincident with solar flares occurring on the visible (invisible) side of the Sun, with a typical background rate of $0.10$ ($0.62$) events per flare in the MeV-GeV energy range. No significant solar-flare neutrino signal above the estimated background rate was observed. As a result we set the following upper limit on neutrino fluence at the Earth $\mathit{\Phi}<1.1\times10^{6}$ $\mathrm{cm^{-2}}$ at the $90\%$ confidence level for the largest solar flare. The resulting fluence limits allow us to constrain some of the theoretical models for solar-flare neutrino emission.

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.

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. The ν̄e fluence in the range from 8 MeV to 100 MeV in positron total energy for ν̄e + p → e+ + n was found to be less than 5.07 × 105 cm−2 per GRB at a 90% confidence level. 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.

K. Abe, 32 Y. Haga, Y. Hayato, 32 M. Ikeda, K. Iyogi, J. Kameda, 32 Y. Kishimoto, 32 Ll. Marti, M. Miura, 32 S. Moriyama, 32 M. Nakahata, 32 T. Nakajima, S. Nakayama, A. Orii, H. Sekiya, 32 M. Shiozawa, 32 Y. Sonoda, A. Takeda, 32 H. Tanaka, Y. Takenaga, S. Tasaka, T. Tomura, K. Ueno, T. Yokozawa, R. Akutsu, T. Irvine, H. Kaji, T. Kajita, 32 I. Kametani, K. Kaneyuki, 32, ∗ K. P. Lee, Y. Nishimura, T. McLachlan, K. Okumura, 32 E. Richard, L. Labarga, P. Fernandez, F. d. M. Blaszczyk, J. Gustafson, C. Kachulis, E. Kearns, 32 J. L. Raaf, J. L. Stone, 32 L. R. Sulak, S. Berkman, S. Tobayama, M. Goldhaber, ∗ K. Bays, G. Carminati, N. J. Griskevich, W. R. Kropp, S. Mine, A. Renshaw, M. B. Smy, 32 H. W. Sobel, 32 V. Takhistov, P. Weatherly, K. S. Ganezer, B. L. Hartfiel, J. Hill, W. E. Keig, N. Hong, J. Y. Kim, I. T. Lim, R. G. Park, T. Akiri, J. B. Albert, A. Himmel, Z. Li, E. O’Sullivan, K. Scholberg, 32 C. W. Walter, 32 T. Wongjirad, T. Ishizuka, T. Nakamura, J. S. Jang, K. Choi, J. G. Learned, S. Matsuno, S. N. Smith, M. Friend, T. Hasegawa, T. Ishida, T. Ishii, T. Kobayashi, T. Nakadaira, K. Nakamura, 32 K. Nishikawa, Y. Oyama, K. Sakashita, T. Sekiguchi, T. Tsukamoto, Y. Nakano, A. T. Suzuki, Y. Takeuchi, 32 T. Yano, S. V. Cao, T. Hayashino, T. Hiraki, S. Hirota, K. Huang, K. Ieki, M. Jiang, T. Kikawa, A. Minamino, A. Murakami, T. Nakaya, 32 N. D. Patel, K. Suzuki, S. Takahashi, R. A. Wendell, 32 Y. Fukuda, Y. Itow, 20 G. Mitsuka, F. Muto, T. Suzuki, P. Mijakowski, K. Frankiewicz, J. Hignight, J. Imber, C. K. Jung, X. Li, J. L. Palomino, G. Santucci, I. Taylor, C. Vilela, M. J. Wilking, C. Yanagisawa, † D. Fukuda, H. Ishino, T. Kayano, A. Kibayashi, Y. Koshio, 32 T. Mori, M. Sakuda, J. Takeuchi, R. Yamaguchi, Y. Kuno, R. Tacik, 34 S. B. Kim, H. Okazawa, Y. Choi, K. Ito, K. Nishijima, M. Koshiba, Y. Totsuka, ∗ Y. Suda, M. Yokoyama, C. Bronner, R. G. Calland, M. Hartz, K. Martens, Y. Obayashi, Y. Suzuki, M. R. Vagins, 7 C. M. Nantais, J. F. Martin, P. de Perio, H. A. Tanaka, A. Konaka, S. Chen, H. Sui, L. Wan, Z. Yang, H. Zhang, Y. Zhang, K. Connolly, M. Dziomba, and R. J. Wilkes

We have searched for proton decay via $p\ensuremath{\rightarrow}{e}^{+}{\ensuremath{\pi}}^{0}$ and $p\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\pi}}^{0}$ modes with the enlarged fiducial volume data of Super-Kamiokande from April 1996 to May 2018, which corresponds to $450\text{ }\text{ }\mathrm{kton}\ifmmode\cdot\else\textperiodcentered\fi{}\mathrm{years}$ exposure. We have accumulated about 25% more livetime and enlarged the fiducial volume of the Super-Kamiokande detector from 22.5 kton to 27.2 kton for this analysis, so that $144\text{ }\text{ }\mathrm{kton}\ifmmode\cdot\else\textperiodcentered\fi{}\mathrm{years}$ of data, including $78\text{ }\text{ }\mathrm{kton}\ifmmode\cdot\else\textperiodcentered\fi{}\mathrm{years}$ of additional fiducial volume data, has been newly analyzed. No candidates have been found for $p\ensuremath{\rightarrow}{e}^{+}{\ensuremath{\pi}}^{0}$ and one candidate remains for $p\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\pi}}^{0}$ in the conventional 22.5 kton fiducial volume and it is consistent with the atmospheric neutrino background prediction. We set lower limits on the partial lifetime for each of these modes: $\ensuremath{\tau}/B(p\ensuremath{\rightarrow}{e}^{+}{\ensuremath{\pi}}^{0})>2.4\ifmmode\times\else\texttimes\fi{}{10}^{34}\text{ }\text{ }\mathrm{years}$ and $\ensuremath{\tau}/B(p\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\pi}}^{0})>1.6\ifmmode\times\else\texttimes\fi{}{10}^{34}\text{ }\text{ }\mathrm{years}$ at 90% confidence level.

Used for both proton decay searches and neutrino physics, large water Cherenkov (WC) detectors have been very successful tools in particle physics. They are notable for their large masses and charged particle detection capabilities. While current WC detectors reconstruct charged particle tracks over a wide energy range, they cannot efficiently detect neutrons. Gadolinium (Gd) has the largest thermal neutron capture cross section of all stable nuclei and produces an 8 MeV gamma cascade that can be detected with high efficiency. Because of the many new physics opportunities that neutron tagging with a Gd salt dissolved in water would open up, a large-scale R&D program called EGADS was established to demonstrate this technique's feasibility. EGADS features all the components of a WC detector, chiefly a 200-ton stainless steel water tank furnished with 240 photo-detectors, DAQ, and a water system that removes all impurities from water while keeping Gd in solution. In this paper we discuss the milestones towards demonstrating the feasibility of this novel technique, and the features of EGADS in detail.