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% $^{ 136}$Xe and 19.098% $^{ 134}$Xe. The detector operated at WIPP between 2010 and 2018 and was designed to search for double beta decay of $^{ 136}$Xe. Data was acquired in two phases separated by a period of detector upgrades. We report on the search for $0\nu\beta\beta$ and $2\nu\beta\beta$ decay of $^{ 134}$Xe with Phase II EXO-200 data, with median 90% C.L. exclusion sensitivity $T_{ 1/2}^{ 0\nu} \geq 3.7\times 10^{ 23}$ yr and $T_{ 1/2}^{ 2\nu} \geq 2.6 \times 10^{ 21}$ 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 $^{ 134}$Xe of $T_{ 1/2}^{ 0\nu} \geq 8.7\times10^{ 23}$ (90% C.L.) and the second strongest constraint on the two-neutrino decay of $T_{ 1/2}^{ 2\nu} \geq 2.9\times10^{ 21}$ (90% C.L.), a 3-fold improvement over the EXO-200 Phase I measurement. New constraints are also set for the $2\nu\beta\beta$ and $0\nu\beta\beta$ decays of $^{ 134}$Xe to the lowest excited state of $^{ 134}$Ba.
Categories: nucl-ex