EXO-200 is a single phase liquid xenon detector designed to search for neutrinoless $\beta \beta$ decay of ${}^{136}\mathrm{Xe}$ to the ground state of ${}^{136}\mathrm{Ba}$. We report here on a search for the two-neutrino $\beta \beta$ decay of ${}^{136}\mathrm{Xe}$ to the first $0^{+}$ excited state, $0_1^{+}$, of ${}^{136}\mathrm{Ba}$ based on a $100 \mathrm{kg}\mathrm{yr}$ exposure of ${}^{136}\mathrm{Xe}$. 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 $2\nu \beta \beta$ decay to the excited state resulting in a lower limit of $T_{1/2}^{2\nu } (0^{+}\to 0_1^{+}) > 6.9 \times {10}^{23}$ yr at 90% CL. This observed limit is consistent with the estimated half-life of $2.5\times {10}^{25}$ yr.

• Received 16 November 2015

DOI:https://doi.org/10.1103/PhysRevC.93.035501