Observation of Two-Neutrino Double-Beta Decay in ${}^{136}\mathrm{Xe}$ with the EXO-200 Detector

We report the observation of two-neutrino double-beta decay in ${}^{136}\mathrm{Xe}$ with $T_{1/2}=2.11\pm 0.04(\mathrm{stat})\pm 0.21(\mathrm{syst})\times {10}^{21}\mathrm{yr}$. This second-order process, predicted by the standard model, has been observed for several nuclei but not for ${}^{136}\mathrm{Xe}$. The observed decay rate provides new input to matrix element calculations and to the search for the more interesting neutrinoless double-beta decay, the most sensitive probe for the existence of Majorana particles and the measurement of the neutrino mass scale.

Figure 1

Drawing of the EXO-200 TPC. The chamber contains $\sim 175\mathrm{kg}$ of liquid xenon enriched to 80.6% in the isotope 136.

Figure 2

Energy spectrum for a ${}^{228}\mathrm{Th}$ calibration source at the midplane of the TPC, 3 cm outside the LXe volume. The intensity (vertical scale) is not fit: The agreement between the Monte Carlo calculations (solid line) and the data tests the accuracy of the simulation against the absolute, National Institute of Standards and Technology traceable, source activity. The energy scale (horizontal axis) has been corrected for ${\tau }_e$ at the time of collection and has had separate energy calibrations applied for single- (main plot) and multiple-cluster (inset) interactions.

Figure 3

Top: Fractional residuals between the energy calibration points and the linear model discussed in the text. The single- (solid line) and multicluster (dotted line) uncertainty bands are systematic, stemming from the finite accuracy of the position reconstruction and the ${\tau }_e$ correction. The thick dashed line represents the central value of the shift predicted by the simulation for pointlike energy depositions. Bottom: Measured energy resolution (points) along with a parameterization (line).

Figure 4

Energy distributions from 752.66 h of EXO-200 single-cluster events (main panel) and multicluster events (inset). The result of a likelihood fit to a model including the $2\nu \beta \beta$ decay and several backgrounds is shown (solid line) along with the $2\nu \beta \beta$ component (shaded region) and some prominent background components at the radius of the TPC vessel (${}^{232}\mathrm{Th}$, long dashed line; ${}^{40}\mathrm{K}$, dashed line; ${}^{60}\mathrm{Co}$, dash-dotted line; ${}^{54}\mathrm{Mn}$, thin dashed line; ${}^{65}\mathrm{Zn}$, thin solid line; ${}^{238}\mathrm{U}$ chain in equilibrium, dash-double-dotted line). Other background components fitting to negligible amounts are not shown, for clarity. The energy scale used for the main panel is consistent with that of single-cluster, $\beta$-like events, while the scale of the inset is consistent with the multicluster events it represents. The combined ${\chi }^2/\mathrm{degrees}$ of freedom between the model and the data for the two binned distributions shown here is $85/90$.

Figure 5

Top: Measured $2\nu \beta \beta$ decay rate of ${}^{136}\mathrm{Xe}$ (large points) and largest background contribution (${}^{40}\mathrm{K}$, small points) as a function of the standoff distance from detector components. Bottom: Event rates of $2\nu \beta \beta$ and ${}^{40}\mathrm{K}$ decays as a function of time.