Search for Double Beta Decays of Xe 134 with EXO-200 Phase II

EXO-200 was a leading double beta decay experiment consisting of a single-phase, enriched liquid xenon time projection chamber filled with an admixture of 80.672% Xe136 and 19.098% Xe134. The detector operated at the Waste Isolation Pilot Plant between 2010 and 2018 and was designed to search for double beta decay of Xe136. Data were acquired in two phases separated by a period of detector upgrades. We report on the search for 0νββ and 2νββ decay of Xe134 with Phase II EXO-200 data, with median 90% CL exclusion sensitivity to half-life T1/20ν≥3.7×1023 yr and T1/22ν≥2.6×1021 yr, respectively. No statistically significant signal is observed for either decay mode. We set a world-leading lower limit on the half-life of the neutrinoless decay mode of Xe134 of T1/20ν≥8.7×1023 (90% CL) and the second strongest constraint on the two-neutrino decay of T1/22ν≥2.9×1021 (90% CL), a threefold improvement over the EXO-200 Phase I measurement. New constraints are also set for the 2νββ and 0νββ decays of Xe134 to the lowest excited state of Ba134.