Measurement of the double-$\mathit{\beta }$ decay half-life of ${}^{136}$Xe 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 ${}^{136}$Xe. The measured two-neutrino double-beta decay half-life of ${}^{136}$Xe is $T_{1/2}^{2\nu }=2.38\pm 0.02\left(\mathrm{stat}\right)\pm 0.14\left(\mathrm{syst}\right)\times {10}^{21}$ yr, consistent with a recent measurement by EXO-200. We also obtain a lower limit for the neutrinoless double-beta decay half-life, $T_{1/2}^{0\nu }>5.7\times {10}^{24}$ yr at 90$\%$ confidence level (C. L.), which corresponds to almost a fivefold improvement over previous limits.

Figure 1

Schematic diagram of the KamLAND-Zen detector.

Figure 2

Visible energy distributions of (a) $\gamma$'s from the ${}^{208}$Tl calibration source and (b) ${}^{214}$Bi ($\beta +\gamma$'s) decays in the Xe-LS. The lines indicate the best fits to the analytical spectral models with the resolution and energy scale parameters floating. The fit to (a) has a ${\chi }^2$/d.o.f. = 5.0/8 and (b) has a ${\chi }^2$/d.o.f. = 27.0/29.

Figure 3

$R^3$ vertex distribution of candidate events for (a) $1.2<E<2.0\mathrm{MeV}$ and (b) $2.2<E<3.0\mathrm{MeV}$. The curves show the best-fit model components: (a) $2\nu \beta \beta$ (dashed) and ${}^{134}$Cs (dotted). (b) 2.6 MeV $\gamma$'s (dashed) and ${}^{214}$Bi (dotted); the long-dashed curve is for $0\nu \beta \beta$ instead of $\gamma$'s. (c) ${}^{214}$Bi events from Xe-LS (dashed) and IB (dotted). The vertical lines show the fiducial radius of 1.2 m (dashed) and the IB radius (dotted).

Figure 4

(a) Energy spectrum of selected $\beta \beta$ decay candidates 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; the fit range is $0.5<E<4.8\mathrm{MeV}$. The coincident $\beta /\gamma$ events from ${}^{208}$Tl give the main contribution from 3 to 5 MeV. (b) Closeup of (a) for $2.2<E<3.0\mathrm{MeV}$. (c)–(e) Closeups for $2.2<E<3.0\mathrm{MeV}$ with different background assumptions around the $0\nu \beta \beta$ peak: (c) only 0$\nu \beta \beta$, (d) ${}^{110}$Ag${}^m$ + 0$\nu \beta \beta$, (e) ${}^{208}$Bi + 0$\nu \beta \beta$, for comparison. ${\chi }^2$ are from the data in this energy range.