Search for Double Beta Decays of ${}^{134}$Xe with EXO-200 Phase II

Abstract

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 $0\hat{I}½{\hat{I}}^2{\hat{I}}^2$ and $2\hat{I}½{\hat{I}}^2{\hat{I}}^2$ decay of ${}^{134}$Xe with Phase II EXO-200 data, with median 90% C.L. exclusion sensitivity $T_{1/2}^{0\hat{I}½}\ge 3.7\times 10^{23}$ yr and $T_{1/2}^{2\hat{I}½}\ge 2.6\times 10^{21}$ 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 $T_{1/2}^{0\hat{I}½}\ge 8.7\times 10^{23}$ (90% C.L.) and the second strongest constraint on the two-neutrino decay of $T_{1/2}^{2\hat{I}½}\ge 2.9\times 10^{21}$ (90% C.L.), a 3-fold improvement over the EXO-200 Phase I measurement. New constraints are also set for the $2\hat{I}½{\hat{I}}^2{\hat{I}}^2$ and $0\hat{I}½{\hat{I}}^2{\hat{I}}^2$ decays of ${}^{134}$Xe to the lowest excited state of ${}^{134}$Ba.

7 pages, 2 figures