Measurement of the double-β decay half-life of 136Xe with the KamLAND-Zen experiment

We present results from the KamLAND-Zen double-beta decay experiment based on an exposure of 77.6 days with 129 kg of Xe-136. The measured two-neutrino double-beta decay half-life of Xe-136 is T-1/2(2 nu) = 2.38 +/- 0.02(stat) +/- 0.14(syst) x 10(21) yr, consistent with a recent measurement by EXO-200. We also obtain a lower limit for the neutrinoless double-beta decay half-life, T-1/2(0 nu) > 5.7 x 10(24) yr at 90% confidence level (C. L.), which corresponds to almost a fivefold improvement over previous limits.