F. Nova

We report the search result for the Diffuse Supernova Neutrino Background (DSNB) in neutrino energies beyond 9.3 MeV in the gadolinium-loaded Super-Kamiokande (SK) detector with a 22,500 × 956.2 m ³ day exposure. Starting in the summer of 2020, SK introduced 0.01% gadolinium (Gd) by mass into its ultrapure water to enhance the neutron capture signal, termed the SK-VI phase. This was followed by a 0.03% Gd-loading in 2022, a phase referred to as SK-VII. We then conducted a DSNB search using 552.2 days of SK-VI data and 404.0 days of SK-VII data through 2023 September. This analysis includes several new features, such as two new machine learning neutron detection algorithms with Gd, an improved atmospheric background-reduction technique, and two parallel statistical approaches. No significant excess over background predictions was found in a DSNB spectrum-independent analysis, and 90% confidence level (C.L.) upper limits on the astrophysical electron antineutrino flux were set. Additionally, a spectral fitting result exhibited a ∼1.2 σ disagreement with a null DSNB hypothesis, comparable to a previous result from 5823 days of all SK pure-water phases.

Super-Kamiokande (SK) has observed B 8 solar neutrino elastic scattering at recoil electron kinetic energies ( E 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 kin < 3.49 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 MeV < E kin < 3.49 MeV with 2.76 σ 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 σ intervals of the fits to the solar neutrino energy spectrum but have a noticeable effect on the best fit when using the exponential parametrization.

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 ± 3 % , P ( 1 ) = 7 0 − 2 + 3 % , P ( 2 ) = 6.1 ± 0.5 % , P ( 3 ) = 0.38 ± 0.09 % . This is the first measurement of the multiplicity of neutrons associated with muon capture on oxygen without neutron energy threshold.

In 2024, a failed supernova (SN) candidate, M31-2014-DS1, was reported in the Andromeda galaxy (M31), located at a distance of approximately 770 kpc. In this Letter, we search for neutrinos from this failed SN using data from Super-Kamiokande (SK). Based on the estimated time of black hole formation inferred from optical and infrared observations, we define a search window for neutrino events in the SK data. Using this window, we develop a dedicated analysis method for failed SNe and apply it to M31-2014-DS1, by conducting a cluster search using the timing and energy information of candidate events. No significant neutrino excess is observed within the search region. Consequently, we place an upper limit on the time-integrated electron antineutrino luminosity from M31-2014-DS1 and discuss its implications for various failed SN models and their neutrino emission characteristics. Despite the 18 MeV threshold adopted to suppress backgrounds, the search remains sufficiently sensitive to constrain the Shen-TM1 equation of state, in a more optimistic emission scenario with progenitor stars of 40 M _⊙ and relatively high mean electron-antineutrino energies of about 23.2 MeV, yielding a 90% confidence level upper limit of 1.76 × 10 ⁵³ erg on the time-integrated electron antineutrino luminosity, moderately above the expected value of 1.35 × 10 ⁵³ erg.

In 2024, a failed supernova candidate, M31-2014-DS1, was reported in the Andromeda galaxy (M31), located at a distance of approximately 770 kpc. In this paper, we search for neutrinos from this failed supernova using data from Super-Kamiokande (SK). Based on the estimated time of black hole formation inferred from optical and infrared observations, we define a search window for neutrino events in the SK data. Using this window, we develop a dedicated analysis method for failed supernovae and apply it to M31-2014-DS1, by conducting a cluster search using the timing and energy information of candidate events. No significant neutrino excess is observed within the search region. Consequently, we place an upper limit on the electron antineutrino luminosity from M31-2014-DS1 and discuss its implications for various failed SN models and their neutrino emission characteristics. Despite the 18 MeV threshold adopted to suppress backgrounds, the search remains sufficiently sensitive to constrain the Shen-TM1 EOS, yielding a 90% confidence level upper limit of 1.76 \times 10^{53} erg on the electron antineutrino luminosity, slightly above the expected value of 1.35 \times 10^{53} erg.

We present the results of the charge ratio ( R ) and polarization ( P 0 μ ) 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 ± 0.02 ( stat + syst ) at E μ cos θ Zenith = 0.7 − 0.2 + 0.3 TeV , where E μ is the muon energy and θ 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 π K model of 1.9 σ . We also measured the muon polarization at the production location to be P 0 μ = 0.52 ± 0.02 ( stat + syst ) at the muon momentum of 0.9 − 0.1 + 0.6 TeV / c at the surface of the mountain; this also suggests a tension with the Honda flux model of 1.5 σ . This is the most precise measurement ever to experimentally determine the cosmic-ray muon polarization near 1 TeV / c . These measurement results are useful to improve atmospheric neutrino simulations. Published by the American Physical Society 2024