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

• Published in: Physical review letters 2019-05, Vol.122 (19), Article 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
• Subjects: NUCLEAR PHYSICS AND RADIATION PHYSICS; PHYSICS OF ELEMENTARY PARTICLES AND FIELDS
• ISSN: 0031-9007; 1079-7114
• DOI: 10.1103/PhysRevLett.122.192501

In this paper, we present a precision measurement of the $^{136}$Xe two-neutrino $\beta\beta$ electron spectrum above 0.8 MeV, based on high-statistics data obtained with the KamLAND-Zen experiment. An improved formalism for the two-neutrino $\beta\beta$ rate allows us to measure the ratio of the leading and subleading $2\nu\beta\beta$ nuclear matrix elements (NMEs), $\xi^{2\nu}_{31} = -0.26^{+0.31}_{-0.25}$. 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 $\xi^{2\nu}_{31}$ range allowed by the QRPA is excluded by the present measurement at the 90%25 C.L. Our analysis reveals that predicted $\xi^{2\nu}_{31}$ values are sensitive to the quenching of NMEs and the competing contributions from low- and high-energy states in the intermediate nucleus. Since these aspects are also at play in neutrinoless $\beta\beta$ decay, $\xi^{2\nu}_{31}$ provides new insights towards reliable neutrinoless $\beta\beta$ NMEs.