Search for two-neutrino double-beta decay of ¹³⁶Xe to the 0⁺₁excited state of ¹³⁶Ba with the complete EXO-200 dataset*

Abstract

`A new search for two-neutrino double-beta (2 nu ss ss) decay of Xe-136 to the 0(1)(+)excited state of Ba-136 is performed with the full EXO-200 dataset. A deep learning-based convolutional neural network is used to discriminate signal from background events. Signal detection efficiency is increased relative to previous searches by EXO-200 by more than a factor of two. With the addition of the Phase II dataset taken with an upgraded detector, the median 90% confidence level half-life sensitivity of 2 nu ss ss decay to the state of Ba-136 is yr using a total Xe-136 exposure of 234.1 kg yr. No statistically significant evidence for 2 nu ss ss decay to the 0(1)(+) state is observed, leading to a lower limit of T-1/2(2 nu)(0+ -> 0(1)(+)) > 1.4 x 10(24) yr at 90% confidence level, improved by 70% relative to the current world's best constraint.