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

A new search for two-neutrino double-beta (2νββ) decay of ¹³⁶Xe to the excited state of ¹³⁶Ba 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 2νββ decay to the state of ¹³⁶Ba is yr using a total ¹³⁶Xe exposure of 234.1 kg yr. No statistically significant evidence for 2νββ decay to the state is observed, leading to a lower limit of yr at 90% confidence level, improved by 70% relative to the current world’s best constraint.

Chinese Physics C, 47, 103001 (2023)