Details of a Researcher - NISHIMURA Yasuhiro

Papers - NISHIMURA Yasuhiro

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Impact of external radiation on dark rate noise measurement for the Hyper-Kamiokande photomultiplier tube

Mandokoro Y., Bronner C., Ishitsuka M., Tashiro T., Nishimura Y.

Journal of Instrumentation 20 ( 5 ) 2025.05

The Hyper-Kamiokande (HK) is a next-generation water Cherenkov detector currently under construction in Japan as of 2025, and scheduled to begin operation in 2027. A new PMT has been developed for HK with improvements in detection efficiency, timing resolution, and pressure tolerance by a factor of two with respect to those used in the Super-Kamiokande detector. The HK detector will be instrumented with 20,000 photomultiplier tubes (PMT). Currently, the PMTs are produced, and the performance testing of the delivered PMTs is underway in parallel. In this test, it was observed that the dark rate values differed depending on the measurement location. The difference is considered due to environmental radiation, and we have conducted measurements of dark rate variations with radiation sources. We measured correlations between the irradiation of γ rays and dark noise rate and the evaluated intrinsic dark noise rate of HK PMT after subtracting the contribution due to environmental radiation.

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Optimization of geomagnetic shielding based on detection efficiency

Cabo S.R., Nishimura Y., Gómez S.L.S., Bonavera L., Sanchez M.L., Santos J.D., de Cos F.J.

European Physical Journal Plus 140 ( 4 ) 2025.04

Due to the progressive increase in size of the latest Cherenkov-type detectors, it is becoming increasingly important to design a suitable compensation system based on coils of the Earth’s magnetic field to ensure the correct operation of the photomultipliers (PMTs). Until now, most studies have assessed the correct functioning of such a system by the proportion of PMTs experiencing more than 100 mG of magnetic field perpendicular to their axis. In the present study, we discuss whether this evaluation parameter is the most appropriate and propose the average residual perpendicular magnetic field <Bperp> as an alternative that more closely reflects the loss of detection efficiency of PMTs. A compensation system design is also proposed that offers good results as well as being economical to optimize this parameter.

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First Joint Oscillation Analysis of Super-Kamiokande Atmospheric and T2K Accelerator Neutrino Data

Abe K., Abe S., Bronner C., Hayato Y., Hiraide K., Hosokawa K., Ieki K., Ikeda M., Kameda J., Kanemura Y., Kaneshima R., Kashiwagi Y., Kataoka Y., Miki S., Mine S., Miura M., Moriyama S., Nakahata M., Nakano Y., Nakayama S., Noguchi Y., Sato K., Sekiya H., Shiba H., Shimizu K., Shiozawa M., Sonoda Y., Suzuki Y., Takeda A., Takemoto Y., Tanaka H., Yano T., Han S., Kajita T., Okumura K., Tashiro T., Tomiya T., Wang X., Yoshida S., Fernandez P., Labarga L., Ospina N., Zaldivar B., Pointon B.W., Kearns E., Mirabito J., Raaf J.L., Wan L., Wester T., Bian J., Griskevich N.J., Smy M.B., Sobel H.W., Takhistov V., Yankelevich A., Hill J., Jang M.C., Lee S.H., Moon D.H., Park R.G., Bodur B., Scholberg K., Walter C.W., Beauchêne A., Drapier O., Giampaolo A., Mueller T.A., Santos A.D., Paganini P., Quilain B., Rogly R., Nakamura T., Jang J.S., MacHado L.N., Learned J.G., Choi K., Iovine N., Cao S., Anthony L.H.V., Martin D., Prouse N.W., Scott M., Uchida Y., Berardi V., Calabria N.F., Catanesi M.G., Radicioni E., Langella A., De Rosa G., Collazuol G., Feltre M., Iacob F., Mattiazzi M., Ludovici L., Gonin M., Périssé L., Pronost G., Fujisawa C., Horiuchi S., Kobayashi M.

Physical Review Letters 134 ( 1 ) 2025.01

ISSN 00319007

The Super-Kamiokande and T2K Collaborations present a joint measurement of neutrino oscillation parameters from their atmospheric and beam neutrino data. It uses a common interaction model for events overlapping in neutrino energy and correlated detector systematic uncertainties between the two datasets, which are found to be compatible. Using 3244.4 days of atmospheric data and a beam exposure of 19.7(16.3)×1020 protons on target in (anti)neutrino mode, the analysis finds a 1.9σ exclusion of CP conservation (defined as JCP=0) and a 1.2σ exclusion of the inverted mass ordering.

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Search for proton decay via p →e+η and p →μ+η with a 0.37 Mton-year exposure of Super-Kamiokande

Taniuchi N., Abe K., Abe S., Asaoka Y., Bronner C., Harada M., Hayato Y., Hiraide K., Hosokawa K., Ieki K., Ikeda M., Kameda J., Kanemura Y., Kaneshima R., Kashiwagi Y., Kataoka Y., Miki S., Mine S., Miura M., Moriyama S., Nakahata M., Nakayama S., Noguchi Y., Pronost G., Okamoto K., Sato K., Sekiya H., Shiba H., Shimizu K., Shiozawa M., Sonoda Y., Suzuki Y., Takeda A., Takemoto Y., Takenaka A., Tanaka H., Watanabe S., Yano T., Kajita T., Okumura K., Tashiro T., Tomiya T., Wang X., Yoshida S., Megias G.D., Fernandez P., Labarga L., Ospina N., Zaldivar B., Pointon B.W., Kearns E., Mirabito J., Raaf J.L., Wan L., Wester T., Bian J., Griskevich N.J., Kropp W.R., Locke S., Smy M.B., Sobel H.W., Takhistov V., Yankelevich A., Hill J., Jang M.C., Kim J.Y., Lee S.H., Lim I.T., Moon D.H., Park R.G., Yang B.S., Bodur B., Scholberg K., Walter C.W., Beauchêne A., Bernard L., Coffani A., Drapier O., El Hedri S., Giampaolo A., Mueller T.A., Santos A.D., Paganini P., Rogly R., Nakamura T., Jang J.S., Machado L.N., Learned J.G., Choi K., Iovine N., Cao S., Anthony L.H.V., Martin D., Prouse N.W., Scott M., Sztuc A.A., Uchida Y., Berardi V., Calabria N.F., Catanesi M.G.

Physical Review D 110 ( 11 ) 2024.12

ISSN 24700010

A search for proton decay into e+/μ+ and a η meson has been performed using data from a 0.373 Mton·year exposure (6050.3 live days) of Super-Kamiokande. Compared to previous searches this work introduces an improved model of the intranuclear η interaction cross section, resulting in a factor of 2 reduction in uncertainties from this source and ∼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 1.4×1034 years for p→e+η and 7.3×1033 years for p→μ+η 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.

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

Kitagawa H., Tada T., Abe K., Bronner C., Hayato Y., Hiraide K., Hosokawa K., Ieki K., Ikeda M., Kameda J., Kanemura Y., Kaneshima R., Kashiwagi Y., Kataoka Y., Miki S., Mine S., Miura M., Moriyama S., Nakahata M., Nakano Y., Nakayama S., Noguchi Y., Okamoto K., Sato K., Sekiya H., Shiba H., Shimizu K., Shiozawa M., Sonoda Y., Suzuki Y., Takeda A., Takemoto Y., Takenaka A., Tanaka H., Watanabe S., Yano T., Han S., Kajita T., Okumura K., Tashiro T., Tomiya T., Wang X., Yoshida S., Megias G.D., Fernandez P., Labarga L., Ospina N., Zaldivar B., Kearns E., Raaf J.L., Wan L., Wester T., Pointon B.W., Bian J., Griskevich N.J., Locke S., Smy M.B., Sobel H.W., Takhistov V., Yankelevich A., Hill J., Jang M.C., Lee S.H., Moon D.H., Park R.G., Bodur B., Scholberg K., Walter C.W., Beauchêne A., Drapier O., Giampaolo A., Mueller T.A., Paganini P., Quilain B., Rogly R., Santos A.D., Nakamura T., Jang J.S., Machado L.N., Learned J.G., Choi K., Iovine N., Cao S., Anthony L.H.V., Marin D., Prouse N.W., Scott M., Sztuc A.A., Uchida Y., Berardi V., Calabria N.F., Catanesi M.G., Radicioni E., Langella A., De Rosa G., Collazuol G., Iacob F., Lamoureux M., Mattiazzi M., Ludovici L.

Physical Review D 110 ( 8 ) 2024.10

ISSN 24700010

We present the results of the charge ratio (R) and polarization (P0μ) 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 EμcosθZenith=0.7-0.2+0.3 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 indicating a tension with the πK model of 1.9σ. We also measured the muon polarization at the production location to be P0μ=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 1.5σ. 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 atmospheric neutrino simulations.

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Combined Pre-supernova Alert System with KamLAND and Super-Kamiokande

Abe S., Eizuka M., Futagi S., Gando A., Gando Y., Goto S., Hachiya T., Hata K., Ichimura K., Ieki S., Ikeda H., Inoue K., Ishidoshiro K., Kamei Y., Kawada N., Kishimoto Y., Koga M., Kurasawa M., Mitsui T., Miyake H., Morita D., Nakahata T., Nakajima R., Nakamura K., Nakamura R., Nakane J., Ozaki H., Saito K., Sakai T., Shimizu I., Shirai J., Shiraishi K., Shoji R., Suzuki A., Takeuchi A., Tamae K., Watanabe H., Watanabe K., Yoshida S., Umehara S., Fushimi K., Kotera K., Urano Y., Berger B.E., Fujikawa B.K., Learned J.G., Maricic J., Fu Z., Smolsky J., Winslow L.A., Efremenko Y., Karwowski H.J., Markoff D.M., Tornow W., Dell’Oro S., O’Donnell T., Detwiler J.A., Enomoto S., Decowski M.P., Weerman K.M., Grant C., Song H., Li A., Axani S.N., Garcia M., Suzuki S., Abe K., Abe S., Bronner C., Hayato Y., Hiraide K., Hosokawa K., Ieki K., Ikeda M., Kameda J., Kanemura Y., Kaneshima R., Kashiwagi Y., Kataoka Y., Miki S., Mine S., Miura M., Moriyama S., Nakahata M., Nakano Y., Nakayama S., Noguchi Y., Sato K., Sekiya H., Shiba H., Shimizu K., Shiozawa M., Sonoda Y., Suzuki Y., Takeda A., Takemoto Y., Tanaka H., Yano T., Han S.

Astrophysical Journal 973 ( 2 ) 2024.10

ISSN 0004637X

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 and 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 <inf>⊙</inf> 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.

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Development of a Data Overflow Protection System for Super-Kamiokande to Maximize Data from Nearby Supernovae

Mori M., Abe K., Hayato Y., Hiraide K., Hosokawa K., Ieki K., Ikeda M., Kameda J., Kanemura Y., Kaneshima R., Kashiwagi Y., Kataoka Y., Miki S., Mine S., Miura M., Moriyama S., Nakano Y., Nakahata M., Nakayama S., Noguchi Y., Okamoto K., Sato K., Sekiya H., Shiba H., Shimizu K., Shiozawa M., Sonoda Y., Suzuki Y., Takeda A., Takemoto Y., Takenaka A., Tanaka H., Watanabe S., Yano T., Han S., Kajita T., Okumura K., Tashiro T., Tomiya T., Wang X., Yoshida S., Megias G.D., Fernandez P., Labarga L., Ospina N., Zaldivar B., Pointon B.W., Kearns E., Raaf J.L., Wan L., Wester T., Bian J., Griskevich N.J., Locke S., Smy M.B., Sobel H.W., Takhistov V., Yankelevich A., Hill J., Jang M.C., Lee S.H., Moon D.H., Park R.G., Bodur B., Scholberg K., Walter C.W., Beauchêne A., Drapier O., Giampaolo A., Mueller T.A., Santos A.D., Paganini P., Quilain B., Rogly R., Ishizuka T., Nakamura T., Jang J.S., Learned J.G., Choi K., Iovine N., Cao S., Anthony L.H.V., Martin D., Scott M., Sztuc A.A., Uchida Y., Berardi V., Catanesi M.G., Radicioni E., Calabria N.F., Langella A., Machado L.N., De Rosa G., Collazuol G., Iacob F., Lamoureux M., Mattiazzi M., Ludovici L., Gonin M., Perisse L.

Progress of Theoretical and Experimental Physics 2024 ( 10 ) 2024.10

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 are critical for understanding the dynamics of the supernova explosion mechanism. In order to solve this problem, two new data-acquisition (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 photomultiplier tubes 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 the Veto module, resulting in a prescaling of the observed neutrino data.

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Evaluation of neutron tagging efficiency on 0.03% Gd mass concentration in SK-Gd experiment

Nakanishi F., Abe K., Bronner C., Hayato Y., Hiraide K., Hosokawa K., Ieki K., Ikeda M., Kameda J., Kanemura Y., Kaneshima R., Kashiwagi Y., Kataoka Y., Miki S., Mine S., Miura M., Moriyama S., Nakano Y., Nakahata M., Nakayama S., Noguchi Y., Sato K., Sekiya H., Shiba H., Shimizu K., Shiozawa M., Sonoda Y., Suzuki Y., Takeda A., Takemoto Y., Tanaka H., Yano T., Han S., Kajita T., Okumura K., Tashiro T., Tomiya T., Wang X., Yoshida S., Fernandez P., Labarga L., Ospina N., Zaldivar B., Pointon B.W., Kearns E., Raaf J.L., Wan L., Wester T., Bian J., Griskevich N.J., Locke S., Smy M.B., Sobel H.W., Takhistov V., Yankelevich A., Hill J., Lee S.H., Moon D.H., Park R.G., Bodur B., Scholberg K., Walter C.W., Beauchêne A., Drapier O., Giampaolo A., Mueller T.A., Santos A.D., Paganini P., Quilain B., Nakamura T., Jang J.S., Machado L.N., Learned J.G., Choi K., Iovine N., Cao S., Anthony L.H.V., Martin D., Prouse N.W., Scott M., Sztuc A.A., Uchida Y., Berardi V., Catanesi M.G., Radicioni E., Calabria N.F., Langella A., De Rosa G., Collazuol G., Iacob F., Mattiazzi M., Ludovici L., Gonin M., Pronost G., Fujisawa C., Maekawa Y., Nishimura Y., Okazaki R., Akutsu R., Friend M.

Proceedings of Science 444 2024.09

Super-Kamiokande (SK) experiment started the "SK-Gd experiment" by dissolving gadolinium (Gd) sulfate octa-hydrate in ultra-pure water from the tank in July 2020. The neutron tagging efficiency has been improved compared to the pure-water phase by detecting a total energy of 8 MeV gamma rays emitted by thermal neutron capture on Gd. It enables us to improve, for example, the sensitivity of diffuse supernova neutrino background, the accuracy of the reconstruction for the supernova direction, and so on. For these physics targets, accurate evaluation of neutron identification efficiency is essential. In this presentation, I report the calibration results using an Am/Be neutron source to evaluate the efficiency at a Gd mass concentration of 0.03%, starting from June 2022, and the comparison of the estimated neutron detection efficiency with simulation.

Time-dependent and Time-independent Directional Search for High-Energy Astrophysical Neutrino Point Sources in Super-Kamiokande

Wang X., Abe K., Bronner C., Hayato Y., Hiraide K., Hosokawa K., Ieki K., Ikeda M., Kameda J., Kanemura Y., Kaneshima R., Kashiwagi Y., Kataoka Y., Miki S., Mine S., Miura M., Moriyama S., Nakano Y., Nakahata M., Nakayama S., Noguchi Y., Sato K., Sekiya H., Shiba H., Shimizu K., Shiozawa M., Sonoda Y., Suzuki Y., Takeda A., Takemoto Y., Tanaka H., Yano T., Han S., Kajita T., Okumura K., Tashiro T., Tomiya T., Wang X., Yoshida S., Fernandez P., Labarga L., Ospina N., Zaldivar B., Pointon B.W., Kearns E., Raaf J.L., Wan L., Wester T., Bian J., Griskevich N.J., Locke S., Smy M.B., Sobel H.W., Takhistov V., Yankelevich A., Hill J., Lee S.H., Moon D.H., Park R.G., Bodur B., Scholberg K., Walter C.W., Beauchêne A., Drapier O., Giampaolo A., Mueller T.A., Santos A.D., Paganini P., Quilain B., Nakamura T., Jang J.S., Machado L.N., Learned J.G., Choi K., Iovine N., Cao S., Anthony L.H.V., Martin D., Prouse N.W., Scott M., Sztuc A.A., Uchida Y., Berardi V., Catanesi M.G., Radicioni E., Calabria N.F., Langella A., De Rosa G., Collazuol G., Iacob F., Mattiazzi M., Ludovici L., Gonin M., Pronost G., Fujisawa C., Maekawa Y., Nishimura Y., Okazaki R., Akutsu R., Friend M.

Proceedings of Science 444 2024.09

This contribution reports the results of the searches for high-energy astrophysical neutrino point sources in the energy range above GeV using Super-Kamiokande data. The searches include time-integrated and time-dependent full sky searches for both ν<inf>μ</inf> and ν<inf>e</inf> sources and coincidence check with candidates including TXS 0506+056 and NGC 1068. The searches use unbinned maximum likelihood method, and test statistics is calculated to find signal excess over atmospheric neutrino background. The time-integrated search method is updated from a previous search by adding the neutrino energy distribution in the likelihood to consider different power-law emission spectra with varying spectral indices. This is the first time to perform the time-dependent search, which has a better performance in searching for neutrino emission in a short time period, in Super-Kamiokande. Upper limits on neutrino flux or fluence are set for all searches.

Study of the neutrino-oxygen neutral-current quasielastic cross section using atmospheric neutrinos in the SK-Gd experiment

Sakai S., Abe K., Bronner C., Hayato Y., Hiraide K., Hosokawa K., Ieki K., Ikeda M., Kameda J., Kanemura Y., Kaneshima R., Kashiwagi Y., Kataoka Y., Miki S., Mine S., Miura M., Moriyama S., Nakano Y., Nakahata M., Nakayama S., Noguchi Y., Sato K., Sekiya H., Shiba H., Shimizu K., Shiozawa M., Sonoda Y., Suzuki Y., Takeda A., Takemoto Y., Tanaka H., Yano T., Han S., Kajita T., Okumura K., Tashiro T., Tomiya T., Wang X., Yoshida S., Fernandez P., Labarga L., Ospina N., Zaldivar B., Pointon B.W., Kearns E., Raaf J.L., Wan L., Wester T., Bian J., Griskevich N.J., Locke S., Smy M.B., Sobel H.W., Takhistov V., Yankelevich A., Hill J., Lee S.H., Moon D.H., Park R.G., Bodur B., Scholberg K., Walter C.W., Beauchêne A., Drapier O., Giampaolo A., Mueller T.A., Santos A.D., Paganini P., Quilain B., Nakamura T., Jang J.S., Machado L.N., Learned J.G., Choi K., Iovine N., Cao S., Anthony L.H.V., Martin D., Prouse N.W., Scott M., Sztuc A.A., Uchida Y., Berardi V., Catanesi M.G., Radicioni E., Calabria N.F., Langella A., De Rosa G., Collazuol G., Iacob F., Mattiazzi M., Ludovici L., Gonin M., Pronost G., Fujisawa C., Maekawa Y., Nishimura Y., Okazaki R., Akutsu R., Friend M.

Proceedings of Science 444 2024.09

The atmospheric neutrino-oxygen neutral-current quasielasatic (NCQE) reactions are one of the main background in the search for supernova relic neutrinos. Here, we report the first measurement of the atmospheric neutrino-oxygen NCQE cross section in the Super-Kamiokande Gadolinium (SK-Gd) experiment using 552.2 days of full SK-VI data. The measured NCQE cross section is 0.74 ± 0.22(stat.) +<inf>−</inf>0.86<inf>0.16</inf>(syst.) × 10−38 cm2, which is consistent with the theoretical NCQE cross section and the previous study in SK within the range of the uncertainty. Furthermore, we established the verification method for nucleon-nucleus interaction models using atmospheric neutrino events. We performed the verification using the Bertini Cascade model (BERT), the Binary Cascade model (BIC) and the Liège Intranuclear Cascade model (INCL++). As a result, we suggested that BIC and INCL++ are closer to the observed data.

Status and Prospect of SK-Gd Project

Hino Y., Abe K., Bronner C., Hayato Y., Hiraide K., Hosokawa K., Ieki K., Ikeda M., Kameda J., Kanemura Y., Kaneshima R., Kashiwagi Y., Kataoka Y., Miki S., Mine S., Miura M., Moriyama S., Nakano Y., Nakahata M., Nakayama S., Noguchi Y., Sato K., Sekiya H., Shiba H., Shimizu K., Shiozawa M., Sonoda Y., Suzuki Y., Takeda A., Takemoto Y., Tanaka H., Yano T., Han S., Kajita T., Okumura K., Tashiro T., Tomiya T., Wang X., Yoshida S., Fernandez P., Labarga L., Ospina N., Zaldivar B., Pointon B.W., Kearns E., Raaf J.L., Wan L., Wester T., Bian J., Griskevich N.J., Locke S., Smy M.B., Sobel H.W., Takhistov V., Yankelevich A., Hill J., Lee S.H., Moon D.H., Park R.G., Bodur B., Scholberg K., Walter C.W., Beauchêne A., Drapier O., Giampaolo A., Mueller T.A., Santos A.D., Paganini P., Quilain B., Nakamura T., Jang J.S., Machado L.N., Learned J.G., Choi K., Iovine N., Cao S., Anthony L.H.V., Martin D., Prouse N.W., Scott M., Sztuc A.A., Uchida Y., Berardi V., Catanesi M.G., Radicioni E., Calabria N.F., Langella A., De Rosa G., Collazuol G., Iacob F., Mattiazzi M., Ludovici L., Gonin M., Pronost G., Fujisawa C., Maekawa Y., Nishimura Y., Okazaki R., Akutsu R., Friend M.

Proceedings of Science 444 2024.09

In 2020, a new phase of Super-Kamiokande experiment, SK-Gd, was started by loading gadolinium into the pure water. This led to a high neutron detection efficiency, which allows us to distinguish different neutrino reactions, enhance signals and remove backgrounds more efficiently. In the summer of 2022, an update of SK-Gd was done by adding additional Gd, and thus totally 39 tonnes of gadolinium sulfate octahydrate has been introduced in the water. The Gd concentration was estimated as 0.0332±0.0002% based on the dissolved weight of gadolinium sulfate octahydrate powder. It is consistent with the result of the direct sampling from the detector as 334 ± 4 ppm. The spallation neutron measurement and AmBe calibration confirms the shorter capture time constant and higher capture event rate due to the second loading. Therefore, we achieved 1.5 times higher than that of the first loaded phase as expected.

Search for Boosted Dark Matter in Super-Kamiokande with low-energy electrons

Iovine N., Abe K., Bronner C., Hayato Y., Hiraide K., Hosokawa K., Ieki K., Ikeda M., Kameda J., Kanemura Y., Kaneshima R., Kashiwagi Y., Kataoka Y., Miki S., Mine S., Miura M., Moriyama S., Nakano Y., Nakahata M., Nakayama S., Noguchi Y., Sato K., Sekiya H., Shiba H., Shimizu K., Shiozawa M., Sonoda Y., Suzuki Y., Takeda A., Takemoto Y., Tanaka H., Yano T., Han S., Kajita T., Okumura K., Tashiro T., Tomiya T., Wang X., Yoshida S., Fernandez P., Labarga L., Ospina N., Zaldivar B., Pointon B.W., Kearns E., Raaf J.L., Wan L., Wester T., Bian J., Griskevich N.J., Locke S., Smy M.B., Sobel H.W., Takhistov V., Yankelevich A., Hill J., Lee S.H., Moon D.H., Park R.G., Bodur B., Scholberg K., Walter C.W., Beauchene A., Drapier O., Giampaolo A., Mueller T.A., Santos A.D., Paganini P., Quilain B., Nakamura T., Jang J.S., Machado L.N., Learned J.G., Choi K., Cao S., Anthony L.H.V., Martin D., Prouse N.W., Scott M., Sztuc A.A., Uchida Y., Berardi V., Catanesi M.G., Radicioni E., Calabria N.F., Langella A., De Rosa G., Collazuol G., Iacob F., Mattiazzi M., Ludovici L., Gonin M., Pronost G., Fujisawa C., Maekawa Y., Nishimura Y., Okazaki R., Akutsu R., Friend M.

Proceedings of Science 444 2024.09

Despite efforts from numerous experiments to grasp the nature of dark matter, no convincing dark matter signal has been observed so far and the very properties of this invisible matter remain unknown. Nevertheless, strong constraints on these properties are set by current observations, particularly for GeV to TeV WIMP-like dark matter particles. In contrast, sub-GeV dark matter scenarios are largely unexplored and still offer promising prospects for dark matter detection. In this contribution, we conduct a search for low-energy electrons scattered by boosted dark matter particles in the Super-Kamiokande detector. In particular, a two-component dark matter scenario is considered, which results in a natural boosting of the non-secluded secondary dark matter particles produced by the decay or annihilation of the dominant cold dark matter component. This work focuses on boosted dark matter from the centre of our Galaxy, with scattered electron energies ranging from a few MeV to a few hundred of MeV. We present the first sensitivities of such low-energy boosted dark matter search using Super-Kamiokande.

Measurement of the kaon to pion production ratio with the Super-Kamiokande

Tada T., Nakano Y., Koshio Y., Abe K., Bronner C., Hayato Y., Hiraide K., Hosokawa K., Ieki K., Ikeda M., Kameda J., Kanemura Y., Kaneshima R., Kashiwagi Y., Kataoka Y., Miki S., Mine S., Miura M., Moriyama S., Nakano Y., Nakahata M., Nakayama S., Noguchi Y., Sato K., Sekiya H., Shiba H., Shimizu K., Shiozawa M., Sonoda Y., Suzuki Y., Takeda A., Takemoto Y., Tanaka H., Yano T., Han S., Kajita T., Okumura K., Tashiro T., Tomiya T., Wang X., Yoshida S., Fernandez P., Labarga L., Ospina N., Zaldivar B., Pointon B.W., Kearns E., Raaf J.L., Wan L., Wester T., Bian J., Griskevich N.J., Locke S., Smy M.B., Sobel H.W., Takhistov V., Yankelevich A., Hill J., Lee S.H., Moon D.H., Park R.G., Bodur B., Scholberg K., Walter C.W., Beauchêne A., Drapier O., Giampaolo A., Mueller T.A., Santos A.D., Paganini P., Quilain B., Nakamura T., Jang J.S., Machado L.N., Learned J.G., Choi K., Iovine N., Cao S., Anthony L.H.V., Martin D., Prouse N.W., Scott M., Sztuc A.A., Uchida Y., Berardi V., Catanesi M.G., Radicioni E., Calabria N.F., Langella A., De Rosa G., Collazuol G., Iacob F., Mattiazzi M., Ludovici L., Gonin M., Pronost G., Fujisawa C., Maekawa Y., Nishimura Y., Okazaki R.

Proceedings of Science 444 2024.09

Cosmic-ray muons are generated from the decay of mesons which are secondary particles produced via hadronic interaction between primary cosmic-ray particles and atmospheric nuclei at the upper the atmosphere. The mesons such pions and kaons mostly decay into muons immediately, reflecting the details of the hadronic interactions depending on their energy and lifetime. The modulation of the cosmic-ray muon rate as well as the flux and the atmospheric kaon-to-pion production ratio are important to determine the flux and energy of the atmospheric neutrino. In particular, the kaon-to-pion production ratio r<inf>K/π</inf> is measured with the correlation coefficient α<inf>T</inf> between the cosmic-ray muon rate in the underground detector and the atmospheric temperature. As a result, we obtained the correlation coefficient r<inf>K/π</inf> = 0.85 ± 0.01 (statistical uncertainty only) and r<inf>K/π</inf> = 0.12 ± 0.03 (statistical uncertainty only) using the data taken in 10 years at the Super-Kamiokande detector. In this article, we report the current status of measurement of the modulation of the comic-ray muon and the atmospheric kaon-to-pion production ratio.

Measurement of the charge ratio and polarization of cosmic-ray muon using the tagged decay-e events in Super-Kamiokande

Nakano Y., Tada T., Kitagawa H., Koshio Y., Abe K., Bronner C., Hayato Y., Hiraide K., Hosokawa K., Ieki K., Ikeda M., Kameda J., Kanemura Y., Kaneshima R., Kashiwagi Y., Kataoka Y., Miki S., Mine S., Miura M., Moriyama S., Nakano Y., Nakahata M., Nakayama S., Noguchi Y., Sato K., Sekiya H., Shiba H., Shimizu K., Shiozawa M., Sonoda Y., Suzuki Y., Takeda A., Takemoto Y., Tanaka H., Yano T., Han S., Kajita T., Okumura K., Tashiro T., Tomiya T., Wang X., Yoshida S., Fernandez P., Labarga L., Ospina N., Zaldivar B., Pointon B.W., Kearns E., Raaf J.L., Wan L., Wester T., Bian J., Griskevich N.J., Locke S., Smy M.B., Sobel H.W., Takhistov V., Yankelevich A., Hill J., Lee S.H., Moon D.H., Park R.G., Bodur B., Scholberg K., Walter C.W., Beauchêne A., Drapier O., Giampaolo A., Mueller T.A., Santos A.D., Paganini P., Quilain B., Nakamura T., Jang J.S., Machado L.N., Learned J.G., Choi K., Iovine N., Cao S., Anthony L.H.V., Martin D., Prouse N.W., Scott M., Sztuc A.A., Uchida Y., Berardi V., Catanesi M.G., Radicioni E., Calabria N.F., Langella A., De Rosa G., Collazuol G., Iacob F., Mattiazzi M., Ludovici L., Gonin M., Pronost G., Fujisawa C., Maekawa Y., Nishimura Y.

Proceedings of Science 444 2024.09

Neutrinos and muons from cosmic-ray interactions in the atmosphere originate from mesons, such as pions and kaons, in air-showers. An accuracy of the absolute atmospheric neutrino flux and its flavor ratio are limited due to the uncertainty of meson productions in hadronic interaction models. In order to reduce such uncertainty, measurements of the charge ratio and the polarization of cosmic-ray muons are important as new inputs to theoretical models which simulate the atmospheric neutrino flux. In this proceedings, we present the results of the charge ratio (R) and polarization (Pμ<inf>0</inf>) of cosmic-ray muon measurements using the decay electrons collected from 2008 September to 2022 June by the Super-Kamiokande (SK) detector. We measured muon charge ratio to be R = 1.32±0.02 (stat.+syst.) and the muon polarization at the production location to be Pμ<inf>0</inf> = 0.52±0.01 (stat.+syst.). The SK detector uniquely measured the polarization of cosmic-ray muons with energies more than 1 TeV at sea-level because of its underground location.

Measurement of cosmogenic 9Li in Super-Kamiokande with gadolinium loaded water

Shinoki M., Abe K., Bronner C., Hayato Y., Hiraide K., Hosokawa K., Ieki K., Ikeda M., Kameda J., Kanemura Y., Kaneshima R., Kashiwagi Y., Kataoka Y., Miki S., Mine S., Miura M., Moriyama S., Nakano Y., Nakahata M., Nakayama S., Noguchi Y., Sato K., Sekiya H., Shiba H., Shimizu K., Shiozawa M., Sonoda Y., Suzuki Y., Takeda A., Takemoto Y., Tanaka H., Yano T., Han S., Kajita T., Okumura K., Tashiro T., Tomiya T., Wang X., Yoshida S., Fernandez P., Labarga L., Ospina N., Zaldivar B., Pointon B.W., Kearns E., Raaf J.L., Wan L., Wester T., Bian J., Griskevich N.J., Locke S., Smy M.B., Sobel H.W., Takhistov V., Yankelevich A., Hill J., Lee S.H., Moon D.H., Park R.G., Bodur B., Scholberg K., Walter C.W., Beauchêne A., Drapier O., Giampaolo A., Mueller T.A., Santos A.D., Paganini P., Quilain B., Nakamura T., Jang J.S., Machado L.N., Learned J.G., Choi K., Iovine N., Cao S., Anthony L.H.V., Martin D., Prouse N.W., Scott M., Sztuc A.A., Uchida Y., Berardi V., Catanesi M.G., Radicioni E., Calabria N.F., Langella A., De Rosa G., Collazuol G., Iacob F., Mattiazzi M., Ludovici L., Gonin M., Pronost G., Fujisawa C., Maekawa Y., Nishimura Y., Okazaki R., Akutsu R., Friend M.

Proceedings of Science 444 2024.09

Super-Kamiokande is a large water Cherenkov detector located approximately 1,000 m underground in Kamioka, Japan. The detector is a cylindrical tank 39.3 m in diameter and 41.4 m high, filled with about 50 kton of gadolinium loaded water. We measured 9Li isotopic nuclei produced by muon spallation using the data taken from 2020 to 2022 by the Super-Kamiokande detector with 0.011% gadolinium concentration in water. Cosmic-ray muons that penetrate the detector form hadron showers in water and secondary hadrons produce unstable radioisotopes through the spallation. Among those spallation products, 9Li is a long-lived radioactive isotope with a lifetime of about 0.26 seconds. It emits an electron and a neutron at a branching ratio of 50.8%, which is difficult to distinguish from the inverse beta decay caused by electron antineutrinos. Therefore, 9Li is one of the main background sources in the observation of diffuse supernova neutrino background. In this study, the energy spectrum of the electrons was measured with a threshold at 4.5 MeV which is lowered from the previous result with pure water. We will report the analysis method and results.

First result of a search for Diffuse Supernova Neutrino Background in SK-Gd experiment

Harada M., Abe K., Bronner C., Hayato Y., Hiraide K., Hosokawa K., Ieki K., Ikeda M., Kameda J., Kanemura Y., Kaneshima R., Kashiwagi Y., Kataoka Y., Miki S., Mine S., Miura M., Moriyama S., Nakano Y., Nakahata M., Nakayama S., Noguchi Y., Sato K., Sekiya H., Shiba H., Shimizu K., Shiozawa M., Sonoda Y., Suzuki Y., Takeda A., Takemoto Y., Tanaka H., Yano T., Han S., Kajita T., Okumura K., Tashiro T., Tomiya T., Wang X., Yoshida S., Fernandez P., Labarga L., Ospina N., Zaldivar B., Pointon B.W., Kearns E., Raaf J.L., Wan L., Wester T., Bian J., Griskevich N.J., Locke S., Smy M.B., Sobel H.W., Takhistov V., Yankelevich A., Hill J., Lee S.H., Moon D.H., Park R.G., Bodur B., Scholberg K., Walter C.W., Beauchêne A., Drapier O., Giampaolo A., Mueller T.A., Santos A.D., Paganini P., Quilain B., Nakamura T., Jang J.S., Machado L.N., Learned J.G., Choi K., Iovine N., Cao S., Anthony L.H.V., Martin D., Prouse N.W., Scott M., Sztuc A.A., Uchida Y., Berardi V., Catanesi M.G., Radicioni E., Calabria N.F., Langella A., De Rosa G., Collazuol G., Iacob F., Mattiazzi M., Ludovici L., Gonin M., Pronost G., Fujisawa C., Maekawa Y., Nishimura Y., Okazaki R., Akutsu R., Friend M.

Proceedings of Science 444 2024.09

Since 2020, Super-Kamiokande (SK) detector has been updated by loading gadolinium (Gd) as a new experimental phase, ‘SK-Gd.’ In the SK-Gd experiment, we can search low-energy electron antineutrinos via inverse-beta decay with efficient neutron identification thanks to high cross-section and high-energy gamma-ray emission of thermal neutron capture on Gd. Until July 2022, the observation is operated with the 0.01% Gd mass concentration. The neutron capture fraction on Gd is about 50% in this situation. In this paper, the first search result for the flux of astrophysical electron antineutrinos for the energy range of O(10) MeV in SK-Gd with a 22.5 × 552 kton·day exposure at 0.01% Gd mass concentration of the initial stage of SK-Gd is reported.

New methods and simulations for cosmogenic induced spallation removal in Super-Kamiokande-IV

Locke S., Coffani A., Abe K., Bronner C., Hayato Y., Ikeda M., Imaizumi S., Ito H., Kameda J., Kataoka Y., Miura M., Moriyama S., Nagao Y., Nakahata M., Nakajima Y., Nakayama S., Okada T., Okamoto K., Orii A., Pronost G., Sekiya H., Shiozawa M., Sonoda Y., Suzuki Y., Takeda A., Takemoto Y., Takenaka A., Tanaka H., Yano T., Hirade K., Kanemura Y., Miki S., Watabe S., Han S., Kajita T., Okumura K., Tashiro T., Xia J., Wang X., Megias G.D., Bravo-Berguño D., Labarga L., Marti L., Zaldivar B., Pointon B.W., Blaszczyk F.D.M., Kearns E., Raaf J.L., Stone J.L., Wan L., Wester T., Bian J., Griskevich N.J., Kropp W.R., Mine S., Smy M.B., Sobel H.W., Takhistov V., Weatherly P., Yankelevich A., Hill J., Kim J.Y., Lim I.T., Park R.G., Bodur B., Scholberg K., Walter C.W., Bernard L., Drapier O., Hedri S.E., Giampaolo A., Gonin M., Mueller T.A., Paganini P., Quilain B., Santos A.D., Ishizuka T., Nakamura T., Jang J.S., Learned J.G., Anthony L.H.V., Sztuc A.A., Uchida Y., Martin D., Scott M., Berardi V., Catanesi M.G., Radicioni E., Calabria N.F., Machado L.N., De Rosa G., Collazuol G., Iacob F., Lamoureux M., Ospina N., Mattiazzi M., Ludovici L., Nishimura Y., Maewaka Y., Cao S.

Physical Review D ( 3 ) 2024.08

ISSN 24700010

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 2790×22.5 kton·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.

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Second gadolinium loading to Super-Kamiokande

Abe K., Bronner C., Hayato Y., Hiraide K., Hosokawa K., Ieki K., Ikeda M., Kameda J., Kanemura Y., Kaneshima R., Kashiwagi Y., Kataoka Y., Miki S., Mine S., Miura M., Moriyama S., Nakano Y., Nakahata M., Nakayama S., Noguchi Y., Sato K., Sekiya H., Shiba H., Shimizu K., Shiozawa M., Sonoda Y., Suzuki Y., Takeda A., Takemoto Y., Tanaka H., Yano T., Han S., Kajita T., Okumura K., Tashiro T., Tomiya T., Wang X., Yoshida S., Fernandez P., Labarga L., Ospina N., Zaldivar B., Pointon B.W., Kearns E., Raaf J.L., Wan L., Wester T., Bian J., Griskevich N.J., Smy M.B., Sobel H.W., Takhistov V., Yankelevich A., Hill J., Jang M.C., Lee S.H., Moon D.H., Park R.G., Bodur B., Scholberg K., Walter C.W., Beauchêne A., Drapier O., Giampaolo A., Mueller T.A., Santos A.D., Paganini P., Quilain B., Rogly R., Nakamura T., Jang J.S., Machado L.N., Learned J.G., Choi K., Iovine N., Cao S., Anthony L.H.V., Martin D., Prouse N.W., Scott M., Uchida Y., Berardi V., Calabria N.F., Catanesi M.G., Radicioni E., Langella A., De Rosa G., Collazuol G., Iacob F., Mattiazzi M., Ludovici L., Gonin M., Périssé L., Pronost G., Fujisawa C., Maekawa Y., Nishimura Y., Okazaki R., Akutsu R., Friend M.

Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 1065 2024.08

ISSN 01689002

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 Gd<inf>2</inf>(SO<inf>4</inf>)<inf>3</inf>⋅8H<inf>2</inf>O was additionally loaded into Super-Kamiokande (SK) from May 31 to July 4, 2022. As the amount of loaded Gd<inf>2</inf>(SO<inf>4</inf>)<inf>3</inf>⋅8H<inf>2</inf>O 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 Gd<inf>2</inf>(SO<inf>4</inf>)<inf>3</inf>⋅8H<inf>2</inf>O. Following the second loading, the Gd concentration in SK was measured to be 333.5±2.5 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 ± 6.8(sys.) ± 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.

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Performance of SK-Gd’s Upgraded Real-time Supernova Monitoring System

Kashiwagi Y., Abe K., Bronner C., Hayato Y., Hiraide K., Hosokawa K., Ieki K., Ikeda M., Kameda J., Kanemura Y., Kaneshima R., Kataoka Y., Miki S., Mine S., Miura M., Moriyama S., Nakano Y., Nakahata M., Nakayama S., Noguchi Y., Sato K., Sekiya H., Shiba H., Shimizu K., Shiozawa M., Sonoda Y., Suzuki Y., Takeda A., Takemoto Y., Tanaka H., Yano T., Han S., Kajita T., Okumura K., Tashiro T., Tomiya T., Wang X., Yoshida S., Fernandez P., Labarga L., Ospina N., Zaldivar B., Pointon B.W., Kearns E., Raaf J.L., Wan L., Wester T., Bian J., Griskevich N.J., Locke S., Smy M.B., Sobel H.W., Takhistov V., Yankelevich A., Hill J., Jang M.C., Lee S.H., Moon D.H., Park R.G., Bodur B., Scholberg K., Walter C.W., Beauchêne A., Drapier O., Giampaolo A., Mueller T.A., Santos A.D., Paganini P., Quilain B., Rogly R., Nakamura T., Jang J.S., Machado L.N., Learned J.G., Choi K., Iovine N., Cao S., Anthony L.H.V., Martin D., Prouse N.W., Scott M., Sztuc A.A., Uchida Y., Berardi V., Catanesi M.G., Radicioni E., Calabria N.F., Langella A., De Rosa G., Collazuol G., Iacob F., Mattiazzi M., Ludovici L., Gonin M., Périssé L., Pronost G., Fujisawa C., Maekawa Y., Nishimura Y., Okazaki R.

Astrophysical Journal 970 ( 1 ) 2024.07

ISSN 0004637X

Among multimessenger 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. In 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 has been upgraded. SK_SN Notice, a warning system that works together with this monitoring system, was released on 2021 December 13, and is available through GCN Notices. 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° to 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.

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

Abe K., Bronner C., Hayato Y., Hiraide K., Hosokawa K., Ieki K., Ikeda M., Kameda J., Kanemura Y., Kaneshima R., Kashiwagi Y., Kataoka Y., Miki S., Mine S., Miura M., Moriyama S., Nakano Y., Nakahata M., Nakayama S., Noguchi Y., Sato K., Sekiya H., Shiba H., Shimizu K., Shiozawa M., Sonoda Y., Suzuki Y., Takeda A., Takemoto Y., Tanaka H., Yano T., Han S., Kajita T., Okumura K., Tashiro T., Tomiya T., Wang X., Yoshida S., Fernandez P., Labarga L., Ospina N., Zaldivar B., Pointon B.W., Kearns E., Raaf J.L., Wan L., Wester T., Bian J., Griskevich N.J., Locke S., Smy M.B., Sobel H.W., Takhistov V., Yankelevich A., Hill J., Lee S.H., Moon D.H., Park R.G., Jang M.C., Bodur B., Scholberg K., Walter C.W., Beauchêne A., Drapier O., Giampaolo A., Mueller T.A., Santos A.D., Paganini P., Quilain B., Nakamura T., Jang J.S., MacHado L.N., Learned J.G., Choi K., Iovine N., Cao S., Anthony L.H.V., Martin D., Prouse N.W., Scott M., Sztuc A.A., Uchida Y., Berardi V., Catanesi M.G., Radicioni E., Calabria N.F., Langella A., De Rosa G., Collazuol G., Iacob F., Mattiazzi M., Ludovici L., Gonin M., Pronost G., Fujisawa C., Maekawa Y., Nishimura Y., Okazaki R., Akutsu R., Friend M.

Physical Review Letters 132 ( 24 ) 2024.06

ISSN 00319007

We report a search for time variations of the solar B8 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±0.35)%, and a perihelion shift of (-1.5±13.5) days.

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