The first loading of gadolinium (Gd) into Super-Kamiokande in 2020 was successful, and the neutron capture efficiency on Gd reached 50\%. To further increase the Gd neutron capture efficiency to ...75\%, 26.1 tons of \(\rm Gd_2(\rm SO_4)_3\cdot \rm 8H_2O\) was additionally loaded into Super-Kamiokande (SK) from May 31 to July 4, 2022. As the amount of loaded \(\rm Gd_2(\rm SO_4)_3\cdot \rm 8H_2O\) was doubled compared to the first loading, the capacity of the powder dissolving system was doubled. We also developed new batches of gadolinium sulfate with even further reduced radioactive impurities. In addition, a more efficient screening method was devised and implemented to evaluate these new batches of \(\rm Gd_2(\rm SO_4)_3\cdot \rm 8H_2O\). Following the second loading, the Gd concentration in SK was measured to be \(333.5\pm2.5\) ppm via an Atomic Absorption Spectrometer (AAS). From the mean neutron capture time constant of neutrons from an Am/Be calibration source, the Gd concentration was independently measured to be 332.7 \(\pm\) 6.8(sys.) \(\pm\) 1.1(stat.) ppm, consistent with the AAS result. Furthermore, during the loading the Gd concentration was monitored continually using the capture time constant of each spallation neutron produced by cosmic-ray muons,and the final neutron capture efficiency was shown to become 1.5 times higher than that of the first loaded phase, as expected.
Neutrinos from very nearby supernovae, such as Betelgeuse, are expected to generate more than ten million events over 10\,s in Super-Kamokande (SK). At such large event rates, the buffers of the SK ...analog-to-digital conversion board (QBEE) will overflow, causing random loss of data that is critical for understanding the dynamics of the supernova explosion mechanism. In order to solve this problem, two new DAQ modules were developed to aid in the observation of very nearby supernovae. The first of these, the SN module, is designed to save only the number of hit PMTs during a supernova burst and the second, the Veto module, prescales the high rate neutrino events to prevent the QBEE from overflowing based on information from the SN module. In the event of a very nearby supernova, these modules allow SK to reconstruct the time evolution of the neutrino event rate from beginning to end using both QBEE and SN module data. This paper presents the development and testing of these modules together with an analysis of supernova-like data generated with a flashing laser diode. We demonstrate that the Veto module successfully prevents DAQ overflows for Betelgeuse-like supernovae as well as the long-term stability of the new modules. During normal running the Veto module is found to issue DAQ vetos a few times per month resulting in a total dead time less than 1\,ms, and does not influence ordinary operations. Additionally, using simulation data we find that supernovae closer than 800~pc will trigger Veto module resulting in a prescaling of the observed neutrino data.
Among multi-messenger observations of the next galactic core-collapse supernova, Super-Kamiokande (SK) plays a critical role in detecting the emitted supernova neutrinos, determining the direction to ...the supernova (SN), and notifying the astronomical community of these observations in advance of the optical signal. On 2022, SK has increased the gadolinium dissolved in its water target (SK-Gd) and has achieved a Gd concentration of 0.033%, resulting in enhanced neutron detection capability, which in turn enables more accurate determination of the supernova direction. Accordingly, SK-Gd's real-time supernova monitoring system (Abe te al. 2016b) has been upgraded. SK_SN Notice, a warning system that works together with this monitoring system, was released on December 13, 2021, and is available through GCN Notices (Barthelmy et al. 2000). When the monitoring system detects an SN-like burst of events, SK_SN Notice will automatically distribute an alarm with the reconstructed direction to the supernova candidate within a few minutes. In this paper, we present a systematic study of SK-Gd's response to a simulated galactic SN. Assuming a supernova situated at 10 kpc, neutrino fluxes from six supernova models are used to characterize SK-Gd's pointing accuracy using the same tools as the online monitoring system. The pointing accuracy is found to vary from 3-7\(^\circ\) depending on the models. However, if the supernova is closer than 10 kpc, SK_SN Notice can issue an alarm with three-degree accuracy, which will benefit follow-up observations by optical telescopes with large fields of view.
We present the results of the charge ratio (\(R\)) and polarization (\(P^{\mu}_{0}\)) measurements using the decay electron events collected from 2008 September to 2022 June by the Super-Kamiokande ...detector. Because of its underground location and long operation, we performed 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 \(E_{\mu}\cos \theta_{\mathrm{Zenith}}=0.7^{+0.3}_{-0.2}\) \(\mathrm{TeV}\), where \(E_{\mu}\) is the muon energy and \(\theta_{\mathrm{Zenith}}\) is the zenith angle of incoming cosmic-ray muons. This result is consistent with the Honda flux model while this suggests a tension with the \(\pi K\) model of \(1.9\sigma\). We also measured the muon polarization at the production location to be \(P^{\mu}_{0}=0.52 \pm 0.02\) \((\mathrm{stat.}{+}\mathrm{syst.})\) at the muon momentum of \(0.9^{+0.6}_{-0.1}\) \(\mathrm{TeV}/c\) 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 the atmospheric neutrino simulations.
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 \(\Delta m^{2}_{21,\,\mathrm{global}} = (7.50^{+ 0.19}_{-0.18}) \times 10^{-5}~\rm{eV}^{2}\).
This paper reports the first measurement of muon neutrino charged-current interactions without pions in the final state using multiple detectors with correlated energy spectra at T2K. The data was ...collected on hydrocarbon targets using the off-axis T2K near detector (ND280) and the on-axis T2K near detector (INGRID) with neutrino energy spectra peaked at 0.6 GeV and 1.1 GeV respectively. The correlated neutrino flux presents an opportunity to reduce the impact of the flux uncertainty and to study the energy dependence of neutrino interactions. The extracted double-differential cross sections are compared to several Monte Carlo neutrino-nucleus interaction event generators showing the agreement between both detectors individually and with the correlated result.
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) data set, ...including events from an expanded fiducial volume. The data set corresponds to 6511.3 live days and an exposure of 484.2 kiloton-years. Measurements of the neutrino oscillation parameters \(\Delta m^2_{32}\), \(\sin^2\theta_{23}\), \(\sin^2 \theta_{13}\), \(\delta_{CP}\), 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.
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 data set from August 2020 to June 2022, we measure the NCQE cross section to be 0.74 \(\pm\) 0.22(stat.) \(^{+0.85}_{-0.15}\) (syst.) \(\times\) 10\(^{-38}\) cm\(^{2}\)/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 Liege 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.
Phys.Rev.Lett 132, 241803 (2024) We report a search for time variations of the solar $^8$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.
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.
