Search for Majorana Neutrinos Near the Inverted Mass Hierarchy Region with KamLAND-Zen

• Published in: Physical review letters Vol. 117; no. 8; p. 082503
• Main Authors: Gando, A, Gando, Y, Hachiya, T, Hayashi, A, Hayashida, S, Ikeda, H, Inoue, K, Ishidoshiro, K, Karino, Y, Koga, M, Matsuda, S, Mitsui, T, Nakamura, K, Obara, S, Oura, T, Ozaki, H, Shimizu, I, Shirahata, Y, Shirai, J, Suzuki, A, Takai, T, Tamae, K, Teraoka, Y, Ueshima, K, Watanabe, H, Kozlov, A, Takemoto, Y, Yoshida, S, Fushimi, K, Banks, T I, Berger, B E, Fujikawa, B K, O'Donnell, T, Winslow, L A, Efremenko, Y, Karwowski, H J, Markoff, D M, Tornow, W, Detwiler, J A, Enomoto, S, Decowski, M P
• Format: Journal Article
• Language: English
• Published: United States 19.08.2016
• ISSN: 1079-7114
• DOI: 10.1103/PhysRevLett.117.082503

We present an improved search for neutrinoless double-beta (0νββ) decay of ^{136}Xe in the KamLAND-Zen experiment. Owing to purification of the xenon-loaded liquid scintillator, we achieved a significant reduction of the ^{110m}Ag contaminant identified in previous searches. Combining the results from the first and second phase, we obtain a lower limit for the 0νββ decay half-life of T_{1/2}^{0ν}>1.07×10^{26}  yr at 90% C.L., an almost sixfold improvement over previous limits. Using commonly adopted nuclear matrix element calculations, the corresponding upper limits on the effective Majorana neutrino mass are in the range 61-165 meV. For the most optimistic nuclear matrix elements, this limit reaches the bottom of the quasidegenerate neutrino mass region.