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• Search for Diffuse Supernova Neutrino Background with 956.2 Days of Super-Kamiokande Gadolinium Dataset

  Abe K., Abe S., Asaoka Y., Harada M., Hayato Y., Hiraide K., Hosokawa K., Hung T.H., Ieki K., Ikeda M., Kameda J., Kanemura Y., Kataoka Y., Miki S., Mine S., Miura M., Moriyama S., Nakahata M., Nakayama S., Noguchi Y., Pronost G., Sato K., Sekiya H., Shinoda R., Shiozawa M., Suzuki Y., Takeda A., Takemoto Y., Tanaka H., Yano T., Itow Y., Kajita T., Nishijima R., Okumura K., Tashiro T., Tomiya T., Wang X., Fernandez P., Labarga L., Samudio D., Zaldivar B., Yanagisawa C., Kearns E., Mirabito J., Wan L., Wester T., Pointon B.W., Bian J., Cortez B., Griskevich N.J., Jiang Y., Smy M.B., Sobel H.W., Takhistov V., Yankelevich A., Hill J., Jang M.C., Lee S.H., Moon D.H., Park R.G., Yang B.S., Bodur B., Scholberg K., Walter C.W., Beauchêne A., Le Blévec E., Drapier O., Ershova A., Ferey M., Mueller T.A., Santos A.D., Paganini P., Quach C., Rogly R., Nakamura T., Jang J.S., Litchfield R.P., Machado L.N., Soler F.J.P., Learned J.G., Choi K., Cao S., Anthony L.H.V., Prouse N.W., Scott M., Uchida Y., Berardi V., Calabria N.F., Catanesi M.G., Ospina N., Radicioni E., Langella A., De Rosa G., Collazuol G., Feltre M., Mattiazzi M., Ludovici L., Gonin M., Périssé L., Quilain B.

  Astrophysical Journal 1005 （ 1 ）  2026年07月

  ISSN  0004637X

  　概要を見る

  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 m³ 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 ∼1.2σ disagreement with a null DSNB hypothesis, comparable to a previous result from 5823 days of all SK pure-water phases.

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• Evaluating position dependence of detection performance for 50-cm Hyper-Kamiokande photomultiplier tube

  Horiuchi S., Nishimura Y.

  Journal of Instrumentation 21 （ 6 ）  2026年06月

  　概要を見る

  The R12860, a new 50-cm photomultiplier tube (PMT) manufactured by Hamamatsu Photonics K.K., was developed for Hyper-Kamiokande to enable measurements of neutrino oscillation parameters with unprecedented precision. To minimize systematic uncertainties, the PMT responses — detection timing, charge, and efficiency — were evaluated in detail with an emphasis on their uniformity with respect to the light incident position on the photosensitive area. Timing measurements performed over the entire PMT aperture revealed a small variation of less than 1.4 ns within the region |θ| < 70°. Here, θ parameterizes the light incident position, with θ = 0° at the PMT center and θ = ±90° at the edge. In addition, the magnetic-field dependence of the detection timing was evaluated along two orthogonal axes of the PMT aperture. It was found that a 100 mG magnetic field could either delay or advance the detection timing depending on its direction; however, the shift was less than 1 ns within |θ| < 60°. Finally, all three responses — timing, charge, and efficiency — were investigated along the two axes over the expected range of high-voltage in operation and found to be quite stable.

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• Neutron-Multiplicity Measurement in Muon Capture on Oxygen Nuclei in the Gadolinium-Loaded Super-Kamiokande Detector

  Miki S., 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., Mine S., Miura M., Moriyama S., Nakahata M., Nakayama S., Noguchi Y., Okamoto K., Pronost G., 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., Yanagisawa C., Kearns E., Raaf J.L., Wan L., Wester T., Bian J., Cortez B., 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., Yang B.S., Bodur B., Scholberg K., Walter C.W., Beauchêne A., Drapier O., Ershova A., Giampaolo A., Mueller T.A., Santos A.D., Paganini P., Quach C., Quilain B., Rogly R., Nakamura T., Jang J.S., Litchfield R.P., Machado L.N., Soler F.J.P., 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., Radicioni E.

  Physical Review Letters 136 （ 13 ）  2026年04月

  ISSN  00319007

  　概要を見る

  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)=70-2+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.

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• Sensitivity of the Hyper-Kamiokande experiment to neutrino oscillation parameters using accelerator neutrinos

  Zsoldos S., Zito M., Zietara K., Ziembicki M., Zhu T., Zhong H., Zhao X., Zarnecki G., Zaremba K., Zalipska J., Zaldivar B., Yuriy O., Yu T., Yu I., Yousefnejad S., Yoshioka Y., Yoshimoto Y., Yoshida T., Yoo J., Yokoyama M., Yevarouskaya U., Yershov N., Yano T., Yang B.S., Yamauchi K., Yamashita M., Yamamoto K., Yamaguchi Y., Xie Z., Xia J., Wuethrich J., Wronka S., Wojciechowski M., Wilson S., Wilson J.R., Wendell R., Wark D., Wachala T., Vyalkov M., Vogelaar R., Vladisavljevic T., Vivolo D., Virginet U., Vinning B., Vasseur G., Vallmajó O., Valder S., Vagins M., Urquijo P., Urano S., Uchida Y., Tzanov M., Tsushima T., Tsukamoto T., Tsui K.M., Tsuchii R., Tseng J., Tran N., Tomiya T., Tomatani Sánchez A.K., Toledo Alarcón J.F., Tiwari D., Thrane E., Thiesse M., Terada K., Tashiro T., Tanigawa H., Tanaka M., Tanaka H.K.M., Tanaka H., Takhistov V., Takeuchi Y., Taketa A., Takemoto Y., Takeda A., Taghayor S., Tada M., Svirida D., Suzuki Y., Suzuki S.Y., Suzuki A., Suwa Y., Sunanda , Suchenek M., Suárez Gómez S.L., Studenikin A., Stowell P., Stawarz L., Stavropoulos D., Stankevich K., Spradlin P., Spisso B., Spina R., Sozzi M.S., Soler F.J.P., Sobel H.W., Sobczyk J., Smy M., Smithers B.R., Skwarczynski K.

  European Physical Journal C 86 （ 2 ）  2026年02月

  ISSN  14346044

  　概要を見る

  This paper presents the expected sensitivity to the neutrino oscillation parameters of the Hyper-Kamiokande long-baseline program. The Hyper-Kamiokande experiment, currently under construction in Japan, will measure the oscillations of accelerator-produced neutrinos with thousands of selected events per sample: this corresponds to an increase of statistics of a factor 25–100 with respect to recent results from the currently-running long-baseline neutrino oscillation experiment in Japan, T2K. In the most favorable scenario we will achieve the discovery of Charge-Parity (CP) violation in neutrino oscillation at 5σ C.L. in less than 3 years. With 10 years of data-taking, and assuming a neutrino : antineutrino beam running ratio of 1:3, a CP violation discovery at 5σ C.L. is possible for more than 60% of the actual values of the CP-violating phase, δCP. Moreover, we will measure δCP with a precision ranging from 20∘, in the case of maximal CP violation, to 6∘, in the case of CP conservation. We aim to achieve a 0.5% resolution on the Δm322 parameter, and a resolution between 3% and 0.5% on the sin2θ23 parameter, depending on its true value. These results are obtained by extending the analysis methods of T2K with dedicated tuning to take into account the Hyper-Kamiokande design: the larger far detector, the more powerful beam, the upgraded near detector ND280, and the planned additional Intermediate Water Cherenkov Detector.

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• First Associated Neutrino Search for a Failed Supernova Candidate with Super-Kamiokande

  Nakanishi F., Abe K., Abe S., Asaoka Y., Harada M., Hayato Y., Hiraide K., Hosokawa K., Hung T.H., Ieki K., Ikeda M., Kameda J., Kanemura Y., Kataoka Y., Miki S., Mine S., Miura M., Moriyama S., Nakahata M., Nakayama S., Noguchi Y., Pronost G., Sato K., Sekiya H., Shiozawa M., Suzuki Y., Takeda A., Takemoto Y., Tanaka H., Yano T., Itow Y., Kajita T., Nishijima R., Okumura K., Tashiro T., Tomiya T., Wang X., Fernandez P., Labarga L., Zaldivar B., Pointon B.W., Yanagisawa C., Kearns E., Wan L., Wester T., Bian J., Cortez B., Griskevich N.J., Jiang Y., Smy M.B., Sobel H.W., Takhistov V., Yankelevich A., Hill J., Jang M.C., Lee S.H., Moon D.H., Park R.G., Yang B.S., Bodur B., Scholberg K., Walter C.W., Beauchêne A., Le Blévec , Drapier O., Ershova A., Ferey M., Mueller T.A., Santos A.D., Paganini P., Quach C., Rogly R., Nakamura T., Jang J.S., Litchfield R.P., Machado L.N., Soler F.J., Learned J.G., Choi K., Cao S., Anthony L.H.V., Prouse N.W., Scott M., Uchida Y., Berardi V., Calabria N.F., Catanesi M.G., Ospina N., Radicioni E., Langella A., De Rosa G., Collazuol G., Feltre M., Mattiazzi M., Ludovici L., Gonin M., Périssé L., Quilain B., Horiuchi S., Kawabata A.

  Astrophysical Journal Letters 997 （ 1 ）  2026年01月

  ISSN  20418205

  　概要を見る

  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<inf>⊙</inf> and relatively high mean electron-antineutrino energies of about 23.2 MeV, yielding a 90% confidence level upper limit of 1.76 × 10⁵³ erg on the time-integrated electron antineutrino luminosity, moderately above the expected value of 1.35 × 10⁵³ erg.

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• 新光検出システム確立で目指す大型ニュートリノ・核子崩壊検出装置の高性能化

  西村, 康宏

  科学研究費補助金研究成果報告書 2021年

• 次期スーパーカミオカンデ実験で始めるニュートリノ宇宙観測技術の基礎研究

  西村, 康宏

  学事振興資金研究成果実績報告書 （慶應義塾大学）  2019年

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• 陽子崩壊の低エネルギー事象高感度探索

  2023年04月

  -

  2026年03月

  西村 康宏, 基盤研究(C), 補助金,  研究代表者

• 新光検出システム確立で目指す大型ニュートリノ・核子崩壊検出装置の高性能化

  2018年04月

  -

  2020年03月

  文部科学省・日本学術振興会, 科学研究費助成事業, 西村　康宏, 若手研究, 補助金,  研究代表者

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• 先端数物科学修士研究１

  2026年度

• 基礎理工学特別研究第１

  2026年度

• 基礎理工学課題研究

  2026年度

• 卒業研究

  2026年度

• 自然科学実験

  2026年度

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