Search for $2\nu\beta\beta$ decay of $^136$Xe to the 0$_1^+$ excited state of $^136$Ba with EXO-200

摘要: EXO-200 is a single phase liquid xenon detector designed to search for neutrinoless double-beta decay of 136Xe to the ground state of 136Ba. We report here on a search for the two-neutrino double-beta decay of 136Xe to the first 0+ excited state, 0+1, of 136Ba based on a 100 kg⋅yr exposure of 136Xe. 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 T2ν1/2 (0+→0+1) >6.9×1023 yr at 90% CL. This observed limit is consistent with the estimated half-life of 2.5×1025 yr.