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KamLAND-Zen Collaboration

Search for the Majorana Nature of Neutrinos in the
Inverted Mass Ordering Region with KamLAND-Zen

S. Abe Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan    S. Asami Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan    M. Eizuka Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan    S. Futagi Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan    A. Gando Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan    Y. Gando Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan    T. Gima Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan    A. Goto Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan    T. Hachiya Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan    K. Hata Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan    S. Hayashida Present address: Imperial College London, Department of Physics, Blackett Laboratory, London SW7 2AZ, UK Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan    K. Hosokawa Present address: Kamioka Observatory, Institute for Cosmic-Ray Research, The University of Tokyo, Hida, Gifu 506-1205, Japan Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan    K. Ichimura Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan    S. Ieki Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan    H. Ikeda Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan    K. Inoue Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo Institutes for Advanced Study, The University of Tokyo, Kashiwa, Chiba 277-8583, Japan    K. Ishidoshiro Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan    Y. Kamei Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan    N. Kawada Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan    Y. Kishimoto Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo Institutes for Advanced Study, The University of Tokyo, Kashiwa, Chiba 277-8583, Japan    M. Koga Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo Institutes for Advanced Study, The University of Tokyo, Kashiwa, Chiba 277-8583, Japan    M. Kurasawa Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan    N. Maemura Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan    T. Mitsui Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan    H. Miyake Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan    T. Nakahata Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan    K. Nakamura Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan    K. Nakamura Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan    R. Nakamura Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan    H. Ozaki Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan Graduate Program on Physics for the Universe, Tohoku University, Sendai 980-8578, Japan    T. Sakai Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan    H. Sambonsugi Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan    I. Shimizu Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan    J. Shirai Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan    K. Shiraishi Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan    A. Suzuki Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan    Y. Suzuki Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan    A. Takeuchi Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan    K. Tamae Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan    K. Ueshima Present address: National Institutes for Quantum and Radiological Science and Technology (QST), Sendai 980-8579, Japan Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan    H. Watanabe Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan    Y. Yoshida Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japan    S. Obara Present address: National Institutes for Quantum and Radiological Science and Technology (QST), Sendai 980-8579, Japan Frontier Research Institute for Interdisciplinary Sciences, Tohoku University, Sendai, 980-8578, Japan    A.K. Ichikawa Department of Physics, Tohoku University, Sendai, 980-8578, Japan    D. Chernyak Present address: Department of Physics and Astronomy, University of Alabama, Tuscaloosa, AL 35487, USA Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo Institutes for Advanced Study, The University of Tokyo, Kashiwa, Chiba 277-8583, Japan    A. Kozlov Present address: National Research Nuclear University “MEPhI” (Moscow Engineering Physics Institute), Moscow, 115409, Russia Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo Institutes for Advanced Study, The University of Tokyo, Kashiwa, Chiba 277-8583, Japan    K.Z. Nakamura Kyoto University, Department of Physics, Kyoto 606-8502, Japan    S. Yoshida Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan    Y. Takemoto Present address: Kamioka Observatory, Institute for Cosmic-Ray Research, The University of Tokyo, Hida, Gifu 506-1205, Japan Research Center for Nuclear Physics, Osaka University, Ibaraki, Osaka 567-0047, Japan    S. Umehara Research Center for Nuclear Physics, Osaka University, Ibaraki, Osaka 567-0047, Japan    K. Fushimi Department of Physics, Tokushima University, Tokushima 770-8506, Japan    K. Kotera Graduate School of Integrated Arts and Sciences, Tokushima University, Tokushima 770-8502, Japan    Y. Urano Graduate School of Integrated Arts and Sciences, Tokushima University, Tokushima 770-8502, Japan    B.E. Berger Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo Institutes for Advanced Study, The University of Tokyo, Kashiwa, Chiba 277-8583, Japan Nuclear Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA    B.K. Fujikawa Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo Institutes for Advanced Study, The University of Tokyo, Kashiwa, Chiba 277-8583, Japan Nuclear Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA    J.G. Learned Department of Physics and Astronomy, University of Hawaii at Manoa, Honolulu, Hawaii 96822, USA    J. Maricic Department of Physics and Astronomy, University of Hawaii at Manoa, Honolulu, Hawaii 96822, USA    S.N. Axani Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA    J. Smolsky Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA    Z. Fu Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA    L.A. Winslow Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA    Y. Efremenko Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo Institutes for Advanced Study, The University of Tokyo, Kashiwa, Chiba 277-8583, Japan Department of Physics and Astronomy, University of Tennessee, Knoxville, Tennessee 37996, USA    H.J. Karwowski Triangle Universities Nuclear Laboratory, Durham, North Carolina 27708, USA;
Physics Departments at Duke University, Durham, North Carolina 27708, USA;
North Carolina Central University, Durham, North Carolina 27707, USA;
and The University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, USA    D.M. Markoff Triangle Universities Nuclear Laboratory, Durham, North Carolina 27708, USA;
Physics Departments at Duke University, Durham, North Carolina 27708, USA;
North Carolina Central University, Durham, North Carolina 27707, USA;
and The University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, USA    W. Tornow Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo Institutes for Advanced Study, The University of Tokyo, Kashiwa, Chiba 277-8583, Japan Triangle Universities Nuclear Laboratory, Durham, North Carolina 27708, USA;
Physics Departments at Duke University, Durham, North Carolina 27708, USA;
North Carolina Central University, Durham, North Carolina 27707, USA;
and The University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, USA    S. Dell’Oro Center for Neutrino Physics, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, USA    T. O’Donnell Center for Neutrino Physics, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, USA    J.A. Detwiler Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo Institutes for Advanced Study, The University of Tokyo, Kashiwa, Chiba 277-8583, Japan Center for Experimental Nuclear Physics and Astrophysics, University of Washington, Seattle, Washington 98195, USA    S. Enomoto Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo Institutes for Advanced Study, The University of Tokyo, Kashiwa, Chiba 277-8583, Japan Center for Experimental Nuclear Physics and Astrophysics, University of Washington, Seattle, Washington 98195, USA    M.P. Decowski Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo Institutes for Advanced Study, The University of Tokyo, Kashiwa, Chiba 277-8583, Japan Nikhef and the University of Amsterdam, Science Park, Amsterdam, the Netherlands    C. Grant Boston University, Boston, Massachusetts 02215, USA    A. Li Boston University, Boston, Massachusetts 02215, USA Triangle Universities Nuclear Laboratory, Durham, North Carolina 27708, USA;
Physics Departments at Duke University, Durham, North Carolina 27708, USA;
North Carolina Central University, Durham, North Carolina 27707, USA;
and The University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, USA    H. Song Boston University, Boston, Massachusetts 02215, USA
Abstract

The KamLAND-Zen experiment has provided stringent constraints on the neutrinoless double-beta (0νββ0\nu\beta\beta) decay half-life in 136{}^{136}Xe using a xenon-loaded liquid scintillator. We report an improved search using an upgraded detector with almost double the amount of xenon and an ultralow radioactivity container, corresponding to an exposure of 970970 kg yr of 136{}^{136}Xe. These new data provide valuable insight into backgrounds, especially from cosmic muon spallation of xenon, and have required the use of novel background rejection techniques. We obtain a lower limit for the 0νββ0\nu\beta\beta decay half-life of T1/20ν>2.3×1026T_{1/2}^{0\nu}>2.3\times 10^{26} yr at 90% C.L., corresponding to upper limits on the effective Majorana neutrino mass of 36–156 meV using commonly adopted nuclear matrix element calculations.

The search for neutrinoless double-beta (0νββ0\nu\beta\beta) decay is the most practical way to probe the Majorana nature of neutrinos. In the context of light Majorana neutrino exchange between two nucleons, the decay rate is proportional to the square of the effective Majorana neutrino mass ⟨mββ⟩≡|ΣiUei2mνi|\left<m_{\beta\beta}\right>\equiv\left|\Sigma_{i}U_{ei}^{2}m_{\nu_{i}}\right|, providing information on the absolute neutrino mass scale and mass eigenstate ordering. To date, KamLAND-Zen has provided the most stringent constraint on ⟨mββ⟩\left<m_{\beta\beta}\right> of ≳\gtrsim 100 meV in the quasidegenerate neutrino mass region Gando2016. An improved search probing ⟨mββ⟩\left<m_{\beta\beta}\right> below 50 meV would provide a first test of the Majorana nature of neutrinos in the inverted mass ordering (IO) region beyond the quasidegenerate mass region. Such searches also test theoretical models predicting ⟨mββ⟩\left<m_{\beta\beta}\right> in this range Harigaya2012; Asaka2020; Asai2020.

KamLAND-Zen Gando2012a; Gando2012b; Gando2013a; Asakura2016; Gando2016; Gando2019 is a double-beta decay experiment that exploits the existing KamLAND neutrino detector. The ββ\beta\beta decay source is a Xe-loaded liquid scintillator (Xe-LS) contained in a spherical inner balloon (IB) at the center of the detector. The IB is surrounded by 1 kton of LS (Outer LS) contained in a 13-m-diameter spherical outer balloon made of 135-μ\mum-thick nylon/EVOH composite film. To detect scintillation light, 1,325 17-inch and 554 20-inch photomultiplier tubes (PMTs) are mounted on the inner surface of the stainless-steel containment tank (SST), providing 34% solid-angle coverage. The SST is surrounded by a 3.2 kton water-Cherenkov outer detector.

The previous search in KamLAND-Zen used 381 kg of enriched xenon (referred to as KamLAND-Zen 400) and probed 0νββ0\nu\beta\beta just above the IO region Gando2016. To further improve this limit, the KamLAND-Zen collaboration upgraded the experiment to 745 kg of enriched xenon (referred to as KamLAND-Zen 800), nearly twice the target mass of the previous experiment. To hold the additional xenon, a larger and cleaner 3.80-m-diameter IB was constructed with better mitigation measures to avoid dust attachment to the balloon surface Gando2021. The Xe-LS consists of 82% decane and 18% pseudocumene (1,2,4-trimethylbenzene) by volume, 2.4 g/liter of the fluor PPO (2,5-diphenyloxazole), and (3.13±0.01)(3.13\pm 0.01)% by weight of enriched xenon gas. The isotopic abundances in the enriched xenon were measured by a residual gas analyzer to be