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

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