Search for 2ββ decay of 136Xe to the 01+ excited state of 136Ba with EXO-200
Abstract
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 2ββ decay to the excited state resulting in a lower limit of T21/2 (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.
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