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Search for double-beta decay of 136Xe to excited states of 136Ba with the KamLAND-Zen experiment

  • Author(s): Asakura, K;
  • Gando, A;
  • Gando, Y;
  • Hachiya, T;
  • Hayashida, S;
  • Ikeda, H;
  • Inoue, K;
  • Ishidoshiro, K;
  • Ishikawa, T;
  • Ishio, S;
  • Koga, M;
  • Matsuda, S;
  • Mitsui, T;
  • Motoki, D;
  • Nakamura, K;
  • Obara, S;
  • Otani, M;
  • Oura, T;
  • Shimizu, I;
  • Shirahata, Y;
  • Shirai, J;
  • Suzuki, A;
  • Tachibana, H;
  • Tamae, K;
  • Ueshima, K;
  • Watanabe, H;
  • Xu, BD;
  • Yoshida, 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
  • et al.
Abstract

A search for double-beta decays of 136Xe to excited states of 136Ba has been performed with the first phase data set of the KamLAND-Zen experiment. The 01+, 21+ and 22+ transitions of 0νββ decay were evaluated in an exposure of 89.5 kg⋅yr of 136Xe, while the same transitions of 2νββ decay were evaluated in an exposure of 61.8 kg⋅yr. No excess over background was found for all decay modes. The lower half-life limits of the 21+ state transitions of 0νββ and 2νββ decay were improved to T1/20ν(0+→21+)>2.6×1025 yr and T1/22ν(0+→21+)>4.6×1023 yr (90% C.L.), respectively. We report on the first experimental lower half-life limits for the transitions to the 01+ state of 136Xe for 0νββ and 2νββ decay. They are T1/20ν(0+→01+)>2.4×1025 yr and T1/22ν(0+→01+)>8.3×1023 yr (90% C.L.). The transitions to the 22+ states are also evaluated for the first time to be T1/20ν(0+→22+)>2.6×1025 yr and T1/22ν(0+→22+)>9.0×1023 yr (90% C.L.). These results are compared to recent theoretical predictions.

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