44 Search Results

We present a search for neutrinoless double-beta (0⁢𝜈⁢𝛽⁢𝛽) decay of ¹³⁶Xe using the full KamLAND-Zen 800 dataset with 745 kg of enriched xenon, corresponding to an exposure of 2.1 ton yr of ¹³⁶Xe. This updated search benefits from a more than twofold increase in exposure, recovery of photo-sensor gain, and reduced background from muon-induced spallation of xenon. Combining with the search in the previous KamLAND-Zen phase, we obtain a lower limit for the 0⁢𝜈⁢𝛽⁢𝛽 decay halflife of 𝑇$$^{0⁢𝜈}_{1/2}$$ >3.8 ×10²⁶ yr at 90% CL, a factor of 1.7 improvement over the previous limit. The corresponding upper limits on the effectivemore » Majorana neutrino mass are in the range 28–122 meV using phenomenological nuclear matrix element calculations.« less

The electron antineutrino flux limits are presented for the brightest gamma-ray burst (GRB) of all time, GRB221009A, over a range of 1.8–200 MeV using the Kamioka Liquid Scintillator Antineutrino Detector. Using multiple time windows ranging from minutes to days surrounding the event to search for electron antineutrinos coincident with the GRB, we set an upper limit on the flux under the assumption of several power-law neutrino source spectra, with power-law indices ranging from 1.5 to 3 in steps of 0.5. No excess was observed in any time windows ranging from seconds to days around the event trigger time T₀. Formore » a power-law index of 2 and a time window of T₀ ± 500 s, a flux upper limit of 2.34 × 10⁹ cm⁻² was calculated. The limits are compared to the results presented by IceCube.« less

Preceding a core-collapse supernova (CCSN), various processes produce an increasing amount of neutrinos of all flavors characterized by mounting energies from the interior of massive stars. Among them, the electron antineutrinos are potentially detectable by terrestrial neutrino experiments such as KamLAND and Super-Kamiokande (SK) via inverse beta decay interactions. Once these pre-supernova (pre-SN) neutrinos are observed, an early warning of the upcoming CCSN can be provided. In light of this, KamLAND and SK, both located in the Kamioka mine in Japan, have been monitoring pre-SN neutrinos since 2015 and 2021, respectively. Recently, we performed a joint study between KamLAND andmore » SK on pre-SN neutrino detection. A pre-SN alert system combining the KamLAND detector and the SK detector was developed and put into operation, which can provide a supernova alert to the astrophysics community. Fully leveraging the complementary properties of these two detectors, the combined alert is expected to resolve a pre-SN neutrino signal from a 15 M_⊙ star within 510 pc of the Earth at a significance level corresponding to a false alarm rate of no more than 1 per century. For a Betelgeuse-like model with optimistic parameters, it can provide early warnings up to 12 hr in advance.« less

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 inmore » 3.49–19.49 MeV electron kinetic energy region during SK-IV is $$65,443^{+390}_{–388}⁢$$(stat.) ± 925⁢(syst.) events. Corresponding ⁸B solar neutrino flux is (2.314 ± 0.014⁢(stat.) ± 0.040⁢(syst.)) × 10⁶ 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 ± 0.011⁢(stat.) ± 0.043⁢(syst.)) × 10⁶ cm^–2 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 sin²θ_12,solar = 0.306 ± 0.013 and $$Δm^2_{21,\text{solar}}$$ = ($$6.1⁢0^{+0.95}_{–0.81}$$) × 10^–5 eV², with a deviation of about 1.5⁢σ from the Δ⁢$$m^2_{21}$$ parameter obtained by KamLAND. The best-fit neutrino oscillation parameters obtained from all solar experiments and KamLAND are sin²θ_12,global = 0.307 ± 0.012 and $$Δm^2_{21,\text{global}}$$ = ($$7.5⁢0^{+0.19}_{–0.18}$$) × 10^–5 eV².« less

This paper reports the development and detailed properties of about 13 metric tons of gadolinium sulfate octahydrate, Gd₂(SO₄)₃ • 8H₂O⁠, which has been dissolved into Super-Kamiokande (SK) in the summer of 2020. We evaluate the impact of radioactive impurities in Gd₂(SO₄)₃ • 8H₂O on diffuse supernova neutrino background searches and solar neutrino observation and confirm the need to reduce radioactive and fluorescent impurities by about three orders of magnitude from commercially available high-purity Gd₂(SO₄)₃ • 8H₂O⁠. In order to produce ultra-high-purity Gd₂(SO₄)₃ • 8H₂O, we have developed a method to remove impurities from gadolinium oxide, Gd₂O₃, consisting of acid dissolution,more » solvent extraction, and pH control processes, followed by a high-purity sulfation process. All of the produced ultra-high-purity Gd₂(SO₄)₃ • 8H₂O is assayed by inductively coupled plasma mass spectrometry and high-purity germanium detectors to evaluate its quality. Because of the long measurement time of high-purity germanium detectors, we have employed several underground laboratories for making parallel measurements including the Laboratorio Subterráneo de Canfranc in Spain, Boulby in the UK, and Kamioka in Japan. In the first half of production, the measured batch purities were found to be consistent with the specifications. However, in the latter half, the Gd₂(SO₄)₃ • 8H₂O contained one order of magnitude more 228Ra than the budgeted mean contamination. This was correlated with the corresponding characteristics of the raw material Gd₂O₃, in which an intrinsically large contamination was present. Based on their modest impact on SK physics, they were nevertheless introduced into the detector. To reduce ²²⁸Ra for the next stage of gadolinium loading to SK, a new process has been successfully established.« less

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 μs.