Search for $2\hat{I}½{\hat{I}}^2{\hat{I}}^2$ decay of ${}^{136}$Xe to the 0${}_1^{+}$ excited state of ${}^{136}$Ba with EXO-200

Abstract

EXO-200 is a single phase liquid xenon detector designed to search for neutrinoless double-beta decay of ${}^{136}$Xe to the ground state of ${}^{136}$Ba. We report here on a search for the two-neutrino double-beta decay of ${}^{136}$Xe to the first $0^{+}$ excited state, $0_1^{+}$, of ${}^{136}$Ba based on a 100 kg$\cdot$yr exposure of ${}^{136}$Xe. Using a specialized analysis employing a machine learning algorithm, we obtain a 90% CL half-life sensitivity of $1.7\times 10^{24}$ yr. We find no statistically significant evidence for the $2\hat{I}½{\hat{I}}^2{\hat{I}}^2$ decay to the excited state resulting in a lower limit of $T_{1/2}^{2\hat{I}½}$ ($0^{+}\to 0_1^{+}$) $>6.9\times 10^{23}$ yr at 90% CL. This observed limit is consistent with the estimated half-life of $2.5\times 10^{25}$ yr.

10 pages, 7 figures, 2 tables