Observation of Two-Neutrino Double-Beta Decay in Xe-136 with EXO-200
  We report the observation of two-neutrino double-beta decay in Xe-136 with T_1/2 = 2.11 +- 0.04 (stat.) +- 0.21 (sys.) 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 neutrino-less double-beta decay, the most sensitive probe for the existence of Majorana particles and the measurement of the neutrino mass scale. 
N. Ackerman, B. Aharmim, M. Auger, D. J. Auty, P. S. Barbeau, K.   Barry, L. Bartoszek, E. Beauchamp, V. Belov, C. Benitez-M...
nucl-ex hep-ex physics.ins-det