A yet unanswered question in contemporary science is why the Universe is dominated by matter over anti-matter, a fact supported by astronomical observations, when the equations governing physics processes at the smallest scales and those describing cosmic evolution would suggest that matter and anti-matter should have annihilated almost completely shortly after the Big Bang. This award is to support work at the University of Massachusetts, Amherst, to complete the analysis of the data collected by the EXO-200 experiment during its operation between 2010 and 2018. EXO-200 was the first of a novel generation of experiments seeking a possible microscopic origin of the matter/anti-matter imbalance through the study of whether the mass of the lightest fundamental particle having mass, the neutrino, is matter or anti-matter. Indeed, there exists the tantalizing possibility that neutrinos could simultaneously be both, enabling a nuclear process known as neutrino-less double-beta decay, in which a nucleus transforms into another by emitting two electrons and nothing else. If observed, this decay would determine that neutrinos and anti-neutrinos are the same particle. Particles having this property are known as Majorana particles. The UMass group proposes to continue to analyze the large EXO-200 data set to search for all possible mechanisms of neutrino-less double beta decay in the isotopes of xenon and to perform measurements that will inform the design and the technical solutions o