Search for Two-neutrino Double-Beta Decay of 136 Xe to the 0+1 excited state of 136 Ba with the Complete EXO-200 Dataset
Abstract
A new search for two-neutrino double-beta (2ββ) decay of 136 Xe to the 0+1 excited state of 136 Ba 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 2ββ decay to the 0+1 state of 136 Ba is 2.9 × 1024~ yr using a total 136 Xe exposure of 234.1~ kg~yr. No statistically significant evidence for 2ββ decay to the 0+1 state is observed, leading to a lower limit of T21/2(0+ → 0+1) > 1.4×1024~ yr at 90\% confidence level, improved by 70\% relative to the current world's best constraint.
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