Search for 2 nu beta beta decay of Xe-136 to the 0(1)(+) 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 pp decay of Xe-136 to the ground state of Ba-136. We report here on a search for the two -neutrino pp decay of Xe-136 to the first 0+ excited state, 0(1)(+), of Ba-136 based on a 100 kg yr exposure of Xe-136. Using a specialized analysis employing a machine learning algorithm, we obtain a 90% CL half-life sensitivity of 1.7 x 10(24) yr. We find no statistically significant evidence for the 20P decay to the excited state resulting in a lower limit of Ti-1/2(7 nu) (0(+) -> 0(1)(+)) > 6.9 x10(23) yr at 90% CL. This observed limit is consistent with the estimated half-life of 2.5 x 10(25) yr.