Speaker
Description
EXO-200 is a single phase time projection chamber filled with liquid xenon enriched in $^{136}$Xe to 80.6$\%$ to search for its neutrinoless double beta decay ($0\nu\beta\beta$). EXO-200 stopped operation in Dec 2018 after two multi-year phases of successful operation. The complete dataset offers valuable opportunities for many interesting physics searches in addition to $0\nu\beta\beta$. Here we will present a search for the $2\nu\beta\beta$ decay of $^{136}$Xe to the $0^+_1$ excited state of $^{136}$Ba. Once discovered, it could help improve the understanding of nuclear matrix elements, potentially contributing to the precise determination of the effective Majorana neutrino mass from $0\nu\beta\beta$ half-life measurements. The last search published by EXO-200 in 2016 based on 100 kg$\cdot$yr exposure of $^{136}$Xe in Phase-I obtained a half-life limit of 6.9$\times 10^{23}$ yr at 90$\%$ CL. With doubled exposure including additional Phase-II data, new machine learning based background discrimination approaches, and greatly increased detection efficiency due to optimized event selection conditions, an improved search utilizing the complete EXO-200 dataset is performed. In this poster, the details of the analysis will be presented.
| Collaboration | EXO-200 |
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