An Improved Search for Two-neutrino Double-Beta Decay of ¹³⁶Xe to the 0₁⁺excited state of ¹³⁶Ba with the Complete EXO-200 Dataset

The EXO-200 detector was a single phase liquid xenon time projection chamber located at the Waste Isolation Pilot Plant to search for neutrinoless double beta decay (0νββ) of ¹³⁶Xe. EXO-200 stopped operation in Dec 2018 after successful Phase-I and II operation. The complete dataset offers valuable opportunities for many interesting physics searches in addition to 0νββ. Here we will present a search for the 2νββ decay of ¹³⁶Xe to the 0₁⁺ excited state of ¹³⁶Ba. Once discovered, it could help improve the understanding of nuclear matrix elements, potentially contributing to the determination of the effective Majorana neutrino mass from 0νββ half-life measurements. The previous search published by EXO-200 in 2016 based on 100 kg·yr exposure of ¹³⁶Xe in Phase-I obtained a half-life limit of 6.9×10²³ yr at 90% CL. With doubled exposure including Phase-II data, new machine learning based background discrimination approaches, and greatly increased detection efficiency due to optimized event selection criteria, an improved search utilizing the complete EXO-200 dataset is performed. In this talk, we present the analysis methods and results of searching for the 2νββ decay of ¹³⁶Xe to the 0₁⁺ excited state of ¹³⁶Ba with the complete EXO-200 dataset.