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Open Access Publications from the University of California

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

  • Author(s): 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, TI;
  • Berger, BE;
  • Fujikawa, BK;
  • O'Donnell, T;
  • Winslow, LA;
  • Efremenko, Y;
  • Karwowski, HJ;
  • Markoff, DM;
  • Tornow, W;
  • Detwiler, JA;
  • Enomoto, S;
  • Decowski, MP;
  • KamLAND-Zen Collaboration
  • et al.

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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.

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