Measurement of a New Half-Lifetime Lower Limit for Neutrinoless Double-Beta Decay in Xe-136 Using the Full EXO-200 Phase 1 Data Set and Improvements to Barium Tagging for nEXO

Several major experiments are currently underway attempting to observe neutrinoless double-beta decay (0νββ). This decay is of interest in the physics community because a measurement of it’s half-life would lead directly to a measurement of the effective Majorana-neutrino mass and therefore the absolute neutrino mass scale. Furthermore, the decay is interesting because it demonstrates non-conservation of lepton number and the Majorana nature of neutrinos, two features which are not forbidden by the Standard Model but have not been observed experimentally. This work focuses on improvements to two such experiments, EXO-200 and it’s successor, nEXO. An improved analysis technique for EXO-200 is developed using Boosted Decision Trees, leading to a 12% improvement to sensitivity and a 13% improvement to the 0νββ half-life lower limit. Furthermore, improvements are made to the nEXO barium tagging system producing a 50% improvement in signal-to-background ratio as well as some improvement to the continuous excitation technique.