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Measurement of the solar neutrino interaction rate below 3.49 MeV in Super-Kamiokande-IV

A. Yankelevich7, K. Abe1,49, Y. Asaoka1,49, M. Harada1, Y. Hayato1,49, K. Hiraide1,49, T. H. Hung1, K. Hosokawa1, K. Ieki1,49 et al. (Super-Kamiokande Collaboration)

K. Ieki1,49, M. Ikeda1,49, J. Kameda1,49, Y. Kanemura1, Y. Kataoka1,49, S. Miki1, S. Mine1,7, M. Miura1,49, S. Moriyama1,49, K. Nakagiri1, M. Nakahata1,49, S. Nakayama1,49, Y. Noguchi1, G. Pronost1, K. Sato1, H. Sekiya1,49, R. Shinoda1, M. Shiozawa1,49, Y. Suzuki1, A. Takeda1,49, Y. Takemoto1,49, H. Tanaka1,49, T. Yano1, S. Chen2, Y. Itow2,32,33, T. Kajita2,49,23, R. Nishijima2, K. Okumura2,49, T. Tashiro2, T. Tomiya2, X. Wang2, F. J. de Garay Arcones3, P. Fernandez3, L. Labarga3, D. Samudio3, B. Zaldivar3, C. Yanagisawa4,36, B. Jargowsky5, E. Kearns5,49, J. Mirabito5, L. Wan5, T. Wester5, B. W. Pointon6,53, J. Bian7, B. Cortez7, N. J. Griskevich7, Y. Jiang7, M. B. Smy7,49, H. W. Sobel7,49, V. Takhistov7,25, J. Hill8, M. C. Jang9, S. H. Lee9, D. H. Moon9, R. G. Park9, B. S. Yang9, B. Bodur10, K. Scholberg10,49, C. W. Walter10,49, A. Beauchêne11, O. Drapier11, A. Ershova11, M. Ferey11, E. Le Blévec11, Th. A. Mueller11, P. Paganini11, C. Quach11, R. Rogly11, T. Nakamura12, J. S. Jang13, R. P. Litchfield14, L. N. Machado14, F. J. P. Soler14, J. G. Learned15, K. Choi16, S. Cao17, L. H. V. Anthony18, N. W. Prouse18, M. Scott18, Y. Uchida18, V. Berardi19, N. F. Calabria19, M. G. Catanesi19, N. Ospina19, E. Radicioni19, A. Langella20, G. De Rosa20, G. Collazuol21, M. Feltre21, M. Mattiazzi21, L. Ludovici22, M. Gonin23, L. Périssé23, B. Quilain23, S. Horiuchi24, A. Kawabata24, M. Kobayashi24, Y. M. Liu24, Y. Maekawa24, Y. Nishimura24, R. Akutsu25, M. Friend25, T. Hasegawa25, Y. Hino25, T. Ishida25, T. Kobayashi25, M. Jakkapu25, T. Matsubara25, T. Nakadaira25, Y. Oyama25, A. Portocarrero Yrey25, K. Sakashita25, T. Sekiguchi25, T. Tsukamoto25, N. Bhuiyan26, G. T. Burton26, F. Di Lodovico26, J. Gao26, T. Katori26, R. Kralik26, N. Latham26, R. M. Ramsden26, V. Siccardi26, H. Ito27, T. Sone27, A. T. Suzuki27, Y. Takeuchi27,49, S. Wada27, H. Zhong27, J. Feng28, L. Feng28, S. Han28, J. Hikida28, J. R. Hu28, Z. Hu28, M. Kawaue28, T. Kikawa28, T. V. Ngoc28, T. Nakaya28,49, R. A. Wendell28,49, S. J. Jenkins29, N. McCauley29, A. Tarrant29, M. Fanì30, M. J. Wilking30, Z. Xie30, Y. Fukuda31, H. Menjo32,33, Y. Yoshioka32, J. Lagoda34, M. Mandal34, Y. S. Prabhu34, J. Zalipska34, M. Mori35, J. Jiang36, K. Hamaguchi37, H. Ishino37, Y. Koshio37,49, F. Nakanishi37, T. Tada37, T. Ishizuka38, G. Barr39, D. Barrow39, L. Cook39,49, S. Samani39, D. Wark39,45, A. Holin40, F. Nova40, S. Jung41, J. Yoo41, J. E. P. Fannon42, L. Kneale42, M. Malek42, J. M. McElwee42, T. Peacock42, P. Stowell42, M. D. Thiesse42, L. F. Thompson42, H. Okazawa43, S. M. Lakshmi44, E. Kwon46, M. W. Lee46, J. W. Seo46, I. Yu46, Y. Ashida47, A. K. Ichikawa47, K. D. Nakamura47, S. Abe48, S. Goto48, S. Kodama48, Y. Kong48, H. Hayasaki48, Y. Masaki48, Y. Mizuno48, T. Muro48, K. Nakagiri48, Y. Nakajima48,49, N. Taniuchi48, M. Yokoyama48,49, P. de Perio49, S. Fujita49, C. Jesús-Valls49, K. Martens49, Ll. Marti49, A. D. Santos49, K. M. Tsui49, M. R. Vagins49,7, J. Xia49, S. Izumiyama50, M. Kuze50, R. Matsumoto50, R. Asaka51, M. Ishitsuka51, M. Sugo51, M. Wako51, K. Yamauchi51, Y. Nakano52, F. Cormier53, R. Gaur53, M. Hartz53, A. Konaka53, X. Li53, B. R. Smithers53, S. Chen54, Y. Wu54, B. D. Xu54, A. Q. Zhang54, B. Zhang54, H. Adhikary55, M. Girgus55, P. Govindaraj55, M. Posiadala-Zezula55, S. B. Boyd56, R. Edwards56, D. Hadley56, M. Nicholson56, M. O’Flaherty56, B. Richards56, A. Ali57,53, B. Jamieson57, C. Bronner58, D. Horiguchi58, A. Minamino58, Y. Sasaki58, R. Shibayama58, and R. Shimamura58 (Super-Kamiokande Collaboration)

  • 1Kamioka Observatory, Institute for Cosmic Ray Research, University of Tokyo, Kamioka, Gifu 506-1205, Japan
  • 2Research Center for Cosmic Neutrinos, Institute for Cosmic Ray Research, University of Tokyo, Kashiwa, Chiba 277-8582, Japan
  • 3Department of Theoretical Physics, University Autonoma Madrid, 28049 Madrid, Spain
  • 4Science Department, Borough of Manhattan Community College/City University of New York, New York, New York, 1007, USA
  • 5Department of Physics, Boston University, Boston, Massachusetts 02215, USA
  • 6Department of Physics, British Columbia Institute of Technology, Burnaby, British Columbia, V5G 3H2, Canada
  • 7Department of Physics and Astronomy, University of California, Irvine, Irvine, California 92697-4575, USA
  • 8Department of Physics, California State University, Dominguez Hills, Carson, California 90747, USA
  • 9Institute for Universe and Elementary Particles, Chonnam National University, Gwangju 61186, Korea
  • 10Department of Physics, Duke University, Durham North Carolina 27708, USA
  • 11Ecole Polytechnique, IN2P3-CNRS, Laboratoire Leprince-Ringuet, F-91120 Palaiseau, France
  • 12Department of Physics, Gifu University, Gifu 501-1193, Japan
  • 13GIST College, Gwangju Institute of Science and Technology, Gwangju 500-712, Korea
  • 14School of Physics and Astronomy, University of Glasgow, Glasgow, Scotland, G12 8QQ, United Kingdom
  • 15Department of Physics and Astronomy, University of Hawaii, Honolulu, Hawaii 96822, USA
  • 16Center for Underground Physics, Institute for Basic Science (IBS), Daejeon, 34126, Korea
  • 17Institute For Interdisciplinary Research in Science and Education, ICISE, Quy Nhon, 55121, Vietnam
  • 18Department of Physics, Imperial College London, London, SW7 2AZ, United Kingdom
  • 19Dipartimento Interuniversitario di Fisica, INFN Sezione di Bari and Università e Politecnico di Bari, I-70125, Bari, Italy
  • 20Dipartimento di Fisica, INFN Sezione di Napoli and Università di Napoli, I-80126, Napoli, Italy
  • 21Dipartimento di Fisica, INFN Sezione di Padova and Università di Padova, I-35131, Padova, Italy
  • 22INFN Sezione di Roma and Università di Roma “La Sapienza,” I-00185, Roma, Italy
  • 23ILANCE, CNRS—University of Tokyo International Research Laboratory, Kashiwa, Chiba 277-8582, Japan
  • 24Department of Physics, Keio University, Yokohama, Kanagawa, 223-8522, Japan
  • 25High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan
  • 26Department of Physics, King’s College London, London, WC2R 2LS, United Kingdom
  • 27Department of Physics, Kobe University, Kobe, Hyogo 657-8501, Japan
  • 28Department of Physics, Kyoto University, Kyoto, Kyoto 606-8502, Japan
  • 29Department of Physics, University of Liverpool, Liverpool, L69 7ZE, United Kingdom
  • 30School of Physics and Astronomy, University of Minnesota, Minneapolis, Minnesota 55455, USA
  • 31Department of Physics, Miyagi University of Education, Sendai, Miyagi 980-0845, Japan
  • 32Institute for Space-Earth Environmental Research, Nagoya University, Nagoya, Aichi 464-8602, Japan
  • 33Kobayashi-Maskawa Institute for the Origin of Particles and the Universe, Nagoya University, Nagoya, Aichi 464-8602, Japan
  • 34National Centre For Nuclear Research, 02-093 Warsaw, Poland
  • 35National Institute of Technology, Numazu College, Numazu, Shizuoka 410-8501, Japan
  • 36Department of Physics and Astronomy, State University of New York at Stony Brook, New York 11794-3800, USA
  • 37Department of Physics, Okayama University, Okayama, Okayama 700-8530, Japan
  • 38Media Communication Center, Osaka Electro-Communication University, Neyagawa, Osaka, 572-8530, Japan
  • 39Department of Physics, Oxford University, Oxford, OX1 3PU, United Kingdom
  • 40Rutherford Appleton Laboratory, Harwell, Oxford, OX11 0QX, United Kingdom
  • 41Department of Physics and Astronomy, Seoul National University, Seoul 151-742, Korea
  • 42School of Mathematical and Physical Sciences, University of Sheffield, S3 7RH, Sheffield, United Kingdom
  • 43Department of Informatics in Social Welfare, Shizuoka University of Welfare, Yaizu, Shizuoka, 425-8611, Japan
  • 44August Chełkowski Institute of Physics, University of Silesia in Katowice, 75 Pułku Piechoty 1, 41-500 Chorzów, Poland
  • 45STFC, Rutherford Appleton Laboratory, Harwell Oxford, and Daresbury Laboratory, Warrington, OX11 0QX, United Kingdom
  • 46Department of Physics, Sungkyunkwan University, Suwon 440-746, Korea
  • 47Department of Physics, Faculty of Science, Tohoku University, Sendai, Miyagi, 980-8578, Japan
  • 48Department of Physics, University of Tokyo, Bunkyo, Tokyo 113-0033, Japan
  • 49Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo Institutes for Advanced Study, University of Tokyo, Kashiwa, Chiba 277-8583, Japan
  • 50Department of Physics, Institute of Science Tokyo, Meguro, Tokyo 152-8551, Japan
  • 51Department of Physics and Astronomy, Faculty of Science and Technology, Tokyo University of Science, Noda, Chiba 278-8510, Japan
  • 52Faculty of Science, University of Toyama, Toyama City, Toyama 930-8555, Japan
  • 53TRIUMF, 4004 Wesbrook Mall, Vancouver, British Columbia, V6T2A3, Canada
  • 54Department of Engineering Physics, Tsinghua University, Beijing, 100084, China
  • 55Faculty of Physics, University of Warsaw, Warsaw, 02-093, Poland
  • 56Department of Physics, University of Warwick, Coventry, CV4 7AL, United Kingdom
  • 57Department of Physics, University of Winnipeg, Winnipeg, Manitoba R3J 3L8, Canada
  • 58Department of Physics, Yokohama National University, Yokohama, Kanagawa, 240-8501, Japan

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Phys. Rev. D 113, 112001 – Published 1 June, 2026

DOI: https://doi.org/10.1103/yz79-h758

Abstract

Super-Kamiokande (SK) has observed B8 solar neutrino elastic scattering at recoil electron kinetic energies (Ekin) as low as 3.49 MeV to study neutrino flavor conversion within the Sun. At SK-observable energies, these conversions are dominated by the Mikheyev-Smirnov-Wolfenstein effect. An upturn in the electron neutrino survival probability in which vacuum neutrino oscillations become dominant is predicted to occur at lower energies, but radioactive background increases exponentially with decreasing energy. New machine learning approaches provide substantial background reduction below 3.49 MeV such that statistical extraction of solar neutrino interactions becomes feasible. This article presents an analysis of the solar neutrino interaction rate at Ekin<3.49  MeV with the full SK-IV period, using data from a wideband intelligent trigger when available and with a boosted decision tree for event selection. A solar neutrino signal is observed between 2.99  MeV<Ekin<3.49  MeV with 2.76σ significance and a data to unoscillated Monte Carlo ratio of 0.307−0.111+0.112. These additional low-energy data have a negligible effect on the 1σ intervals of the fits to the solar neutrino energy spectrum but have a noticeable effect on the best fit when using the exponential parametrization.

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Phys. Rev. D 113, 112001– Published 1 June, 2026
  • Received 5 January 2026
  • Accepted 26 March 2026
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