Observation of Two-Neutrino Double-Beta Decay in ^(136)Xe with the EXO-200 Detector

We report the observation of two-neutrino double-beta decay in ^(136)Xe with T_(1/2)=2.11±0.04(stat)±0.21(syst)×10^(21)  yr. This second-order process, predicted by the standard model, has been observed for several nuclei but not for ^(136)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.