Search for $2\nu\beta\beta$ decay of $^{136}$Xe to the 0$_1^+$ excited state of $^{136}$Ba with EXO-200

TL;DR

This paper reports a search for two-neutrino double-beta decay of $^{136}$Xe to an excited state of $^{136}$Ba using the EXO-200 detector, setting a new lower limit on the half-life with no significant detection.

Contribution

First search for $^{136}$Xe two-neutrino double-beta decay to the first excited state using machine learning analysis on EXO-200 data.

Findings

No significant evidence for decay was observed.

Established a lower limit on the half-life of $6.9 imes 10^{23}$ years.

Sensitivity reached $1.7 imes 10^{24}$ years at 90% CL.

Abstract

EXO-200 is a single phase liquid xenon detector designed to search for neutrinoless double-beta decay of $^{136}$Xe to the ground state of $^{136}$Ba. We report here on a search for the two-neutrino double-beta decay of $^{136}$Xe to the first $0^+$ excited state, $0^+_1$, of $^{136}$Ba based on a 100 kg$\cdot$yr exposure of $^{136}$Xe. Using a specialized analysis employing a machine learning algorithm, we obtain a 90% CL half-life sensitivity of $1.7 \times 10^{24}$ yr. We find no statistically significant evidence for the $2\nu\beta\beta$ decay to the excited state resulting in a lower limit of $T^{2\nu}_{1/2}$ ($0^+ \rightarrow 0^+_1$) $> 6.9 \times 10^{23}$ yr at 90% CL. This observed limit is consistent with the estimated half-life of $2.5\times10^{25}$ yr.