Limit on Neutrinoless ββ Decay of Xe136 from the First Phase of KamLAND-Zen and Comparison with the Positive Claim in Ge76

We present results from the first phase of the KamLAND-Zen double-beta decay experiment, corresponding to an exposure of 89.5 kg yr of Xe136. We obtain a lower limit for the neutrinoless double-beta decay half-life of T_1/2^0ν>1.9×10²⁵yr at 90% C.L. The combined results from KamLAND-Zen and EXO-200 give T_1/2^0ν>3.4×10²⁵yr at 90% C.L., which corresponds to a Majorana neutrino mass limit of ⟨m_ββ⟩<(120-250)meV based on a representative range of available matrix element calculations. Using those calculations, this result excludes the Majorana neutrino mass range expected from the neutrinoless double-beta decay detection claim in Ge76, reported by a part of the Heidelberg-Moscow Collaboration, at more than 97.5% C.L.