Search for Periodic Time Variations of the Solar 8^8B Neutrino Flux Between 1996 and 2018 in Super-Kamiokande

We report a search for time variations of the solar $^8$B neutrino flux using 5,804 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 data set comprised of five-day interval solar neutrino flux measurements with a maximum likelihood method. We also applied the Lomb-Scargle method to this data set 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$0.35)\,\%, and a perihelion shift is ($-$1.5$\pm$13.5)\,days.Comment: 8 pages, 5 figures, 2 tables, and data file: "sksolartimevariation5804d.txt

Measurements of the νμ\nu_{\mu} and νˉμ\bar{\nu}_{\mu}-induced Coherent Charged Pion Production Cross Sections on 12C^{12}C by the T2K experiment

We report an updated measurement of the $\nu_{\mu}$-induced, and the first measurement of the $\bar{\nu}_{\mu}$-induced coherent charged pion production cross section on $^{12}C$ nuclei in the T2K experiment. This is measured in a restricted region of the final-state phase space for which $p_{\mu,\pi} > 0.2$ GeV, $\cos(\theta_{\mu}) > 0.8$ and $\cos(\theta_{\pi}) > 0.6$, and at a mean (anti)neutrino energy of 0.85 GeV using the T2K near detector. The measured $\nu_{\mu}$ CC coherent pion production flux-averaged cross section on $^{12}C$ is $(2.98 \pm 0.37 (stat.) \pm 0.31 (syst.) \substack{ +0.49 \\ -0.00 } \mathrm{ (Q^2\,model)}) \times 10^{-40}~\mathrm{cm}^{2}$. The new measurement of the $\bar{\nu}_{\mu}$-induced cross section on $^{12}{C}$ is $(3.05 \pm 0.71 (stat.) \pm 0.39 (syst.) \substack{ +0.74 \\ -0.00 } \mathrm{(Q^2\,model)}) \times 10^{-40}~\mathrm{cm}^{2}$. The results are compatible with both the NEUT 5.4.0 Berger-Sehgal (2009) and GENIE 2.8.0 Rein-Sehgal (2007) model predictions

Measurements of the νμ and ν¯μ -induced coherent charged pion production cross sections on C12 by the T2K experiment

We report an updated measurement of the ν μ -induced, and the first measurement of the ¯ ν μ -induced coherent charged pion production cross section on 12 C nuclei in the Tokai-to-Kamioka experiment. This is measured in a restricted region of the final-state phase space for which p μ , π > 0.2     GeV , cos ( θ μ ) > 0.8 and cos ( θ π ) > 0.6 , and at a mean (anti)neutrino energy of 0.85 GeV using the T2K near detector. The measured ν μ charged current coherent pion production flux-averaged cross section on 12 C is ( 2.98 ± 0.37 ( stat ) ± 0.31 ( syst ) + 0.49 − 0.00 ( Q 2   model ) ) × 10 − 40     cm 2 . The new measurement of the ¯ ν μ -induced cross section on 12 C is ( 3.05 ± 0.71 ( stat ) ± 0.39 ( syst ) + 0.74 − 0.00 ( Q 2   model ) ) × 10 − 40     cm 2 . The results are compatible with both the NEUT 5.4.0 Berger-Sehgal (2009) and GENIE 2.8.0 Rein-Sehgal (2007) model predictions

Scintillator ageing of the T2K near detectors from 2010 to 2021

The T2K experiment widely uses plastic scintillator as a target for neutrino interactions and an active medium for the measurement of charged particles produced in neutrino interactions at its near detector complex. Over 10 years of operation the measured light yield recorded by the scintillator based subsystems has been observed to degrade by 0.9–2.2% per year. Extrapolation of the degradation rate through to 2040 indicates the recorded light yield should remain above the lower threshold used by the current reconstruction algorithms for all subsystems. This will allow the near detectors to continue contributing to important physics measurements during the T2K-II and Hyper-Kamiokande eras. Additionally, work to disentangle the degradation of the plastic scintillator and wavelength shifting fibres shows that the reduction in light yield can be attributed to the ageing of the plastic scintillator. The long component of the attenuation length of the wavelength shifting fibres was observed to degrade by 1.3–5.4% per year, while the short component of the attenuation length did not show any conclusive degradation

Performance of SK-Gd's upgraded real-time supernova monitoring system

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∘ 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

Measurements of the charge ratio and polarization of cosmic-ray muons with the Super-Kamiokande detector

We present the results of the charge ratio (R) and polarization (Pμ0) 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±0.02 (stat.+syst.) at EμcosθZenith=0.7+0.3−0.2 TeV, where Eμ is the muon energy and θZenith 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 πK model of 1.9σ. We also measured the muon polarization at the production location to be Pμ0=0.52±0.02 (stat.+syst.) at the muon momentum of 0.9+0.6−0.1 TeV/c at the surface of the mountain; this also suggests a tension with the Honda flux model of 1.5σ. 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 the atmospheric neutrino simulations