Search for 2νββ decay of Xe 136 to the 01+ excited state of Ba 136 with the EXO-200 liquid xenon detector

EXO-200 is a single phase liquid xenon detector designed to search for neutrinoless ββ decay of Xe136 to the ground state of Ba136. We report here on a search for the two-neutrino ββ decay of Xe136 to the first 0+ excited state, 01+, of Ba136 based on a 100 kgyr exposure of Xe136. Using a specialized analysis employing a machine learning algorithm, we obtain a 90% CL half-life sensitivity of 1.7×1024 yr. We find no statistically significant evidence for the 2νββ decay to the excited state resulting in a lower limit of T1/22ν (0+→01+) > 6.9 ×1023 yr at 90% CL. This observed limit is consistent with the estimated half-life of 2.5×1025 yr.