Limit on Neutrinoless $\beta \beta$ Decay of ${}^{136}\mathrm{Xe}$ from the First Phase of KamLAND-Zen and Comparison with the Positive Claim in ${}^{76}\mathrm{Ge}$

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 ${}^{136}\mathrm{Xe}$. We obtain a lower limit for the neutrinoless double-beta decay half-life of $T_{1/2}^{0\nu }>1.9\times {10}^{25}\mathrm{yr}$ at 90% C.L. The combined results from KamLAND-Zen and EXO-200 give $T_{1/2}^{0\nu }>3.4\times {10}^{25}\mathrm{yr}$ at 90% C.L., which corresponds to a Majorana neutrino mass limit of $⟨m_{\beta \beta }⟩<(120–250)\mathrm{meV}$ 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 ${}^{76}\mathrm{Ge}$, reported by a part of the Heidelberg-Moscow Collaboration, at more than 97.5% C.L.

Figure 1

(a) Energy spectrum of selected candidate events together with the best-fit backgrounds and $2\nu \beta \beta$ decays, and the 90% C.L. upper limit for $0\nu \beta \beta$ decays, for the combined data from DS-1 and DS-2; the fit range is $0.5<E<4.8\mathrm{MeV}$. (b) Closeup of (a) for $2.2<E<3.0\mathrm{MeV}$ after subtracting known background contributions.

Figure 2

Event rate variation in the energy region $2.2<E<3.0\mathrm{MeV}$ (${}^{136}\mathrm{Xe}$ $0\nu \beta \beta$ window) after subtracting known background contributions. The three fitted curves correspond to the hypotheses that all events in the $0\nu \beta \beta$ window are from ${}^{110m}\mathrm{Ag}$ (dashed line), ${}^{208}\mathrm{Bi}$ (dotted line), or ${}^{88}\mathrm{Y}$ (double-dot-dashed line). The gray band indicates the Xe-LS filtration period; no reduction in the fitted isotope is assumed for the ${\chi }^2$ calculation.

Figure 3

Experimental results on $0\nu \beta \beta$ decay half-life ($T_{1/2}^{0\nu }$) in ${}^{76}\mathrm{Ge}$ and ${}^{136}\mathrm{Xe}$. The 68% C.L. limit from the claim in Ref. [1] is indicated by the gray band. The limits for KamLAND-Zen (this work), EXO-200 [3], and their combination are shown at 90% C.L. The correlation between the ${}^{76}\mathrm{Ge}$ and ${}^{136}\mathrm{Xe}$ half-lives predicted by various NME calculations [8, 9, 10, 11] is drawn as diagonal lines together with the $⟨m_{\beta \beta }⟩(\mathrm{eV})$ scale. The band for QRPA and RQRPA represents the range of these NME calculations under the variation of model parameters.