Observation of Two-Neutrino Double-Beta Decay inXe136with the EXO-200 Detector

We report the observation of two-neutrino double-beta decay in $^{136}\mathrm{Xe}$ with ${T}_{1/2}=2.11\ifmmode\pm\else\textpm\fi{}0.04(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.21(\mathrm{syst})\ifmmode\times\else\texttimes\fi{}{10}^{21}\text{ }\text{ }\mathrm{yr}$. 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.