Search for Two-neutrino Double-Beta Decay of ${}^{136}\mathrm{Xe}$ to the $0_1^{+}$ excited state of ${}^{136}\mathrm{Ba}$ with the Complete EXO-200 Dataset

Abstract

A new search for two-neutrino double-beta ($2\hat{I}½{\hat{I}}^2{\hat{I}}^2$) decay of ${}^{136}\mathrm{Xe}$ to the $0_1^{+}$ excited state of ${}^{136}\mathrm{Ba}$ 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 $2\hat{I}½{\hat{I}}^2{\hat{I}}^2$ decay to the $0_1^{+}$ state of ${}^{136}\mathrm{Ba}$ is $2.9\times 10^{24}\mathrm{yr}$ using a total ${}^{136}\mathrm{Xe}$ exposure of $234.1\mathrm{kg}\mathrm{yr}$. No statistically significant evidence for $2\hat{I}½{\hat{I}}^2{\hat{I}}^2$ decay to the $0_1^{+}$ state is observed, leading to a lower limit of $T_{1/2}^{2\hat{I}½}(0^{+}\to 0_1^{+})>1.4\times 10^{24}\mathrm{yr}$ at 90$\%$ confidence level, improved by 70$\%$ relative to the current world's best constraint.

9 pages, 7 figures, 2 tables