Precision Analysis of the ^{136}Xe Two-Neutrino ββ Spectrum in KamLAND-Zen and Its Impact on the Quenching of Nuclear Matrix Elements

• Published in: Physical review letters 2019-05, Vol.122 (19), p.192501-192501
• Main Authors: Gando, A, Gando, Y, Hachiya, T, Ha Minh, M, Hayashida, S, Honda, Y, Hosokawa, K, Ikeda, H, Inoue, K, Ishidoshiro, K, Kamei, Y, Kamizawa, K, Kinoshita, T, Koga, M, Matsuda, S, Mitsui, T, Nakamura, K, Ono, A, Ota, N, Otsuka, S, Ozaki, H, Shibukawa, Y, Shimizu, I, Shirahata, Y, Shirai, J, Sato, T, Soma, K, Suzuki, A, Takeuchi, A, Tamae, K, Ueshima, K, Watanabe, H, Chernyak, D, Kozlov, A, Obara, S, Yoshida, S, Takemoto, Y, Umehara, S, Fushimi, K, Hirata, S, Berger, B E, Fujikawa, B K, Learned, J G, Maricic, J, Winslow, L A, Efremenko, Y, Karwowski, H J, Markoff, D M, Tornow, W, O'Donnell, T, Detwiler, J A, Enomoto, S, Decowski, M P, Menéndez, J, Dvornický, R, Šimkovic, F
• Format: Article
• Language: English
• ISSN: 1079-7114
• DOI: 10.1103/PhysRevLett.122.192501

We present a precision analysis of the ^{136}Xe two-neutrino ββ electron spectrum above 0.8 MeV, based on high-statistics data obtained with the KamLAND-Zen experiment. An improved formalism for the two-neutrino ββ rate allows us to measure the ratio of the leading and subleading 2νββ nuclear matrix elements (NMEs), ξ_{31}^{2ν}=-0.26_{-0.25}^{+0.31}. Theoretical predictions from the nuclear shell model and the majority of the quasiparticle random-phase approximation (QRPA) calculations are consistent with the experimental limit. However, part of the ξ_{31}^{2ν} range allowed by the QRPA is excluded by the present measurement at the 90% confidence level. Our analysis reveals that predicted ξ_{31}^{2ν} values are sensitive to the quenching of NMEs and the competing contributions from low- and high-energy states in the intermediate nucleus. Because these aspects are also at play in neutrinoless ββ decay, ξ_{31}^{2ν} provides new insights toward reliable neutrinoless ββ NMEs.