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We report on a search for neutrinoless double-beta decay of $^{136}$Xe with EXO-200. No signal is observed for an exposure of 32.5 kg-yr, with a background of ~1.5 x 10^{-3} /(kg yr keV) in the $pm 1sigma$ region of interest. This sets a lower limit on the half-life of the neutrinoless double-beta decay $T_{1/2}^{0 ubetabeta}$($^{136}$Xe) > 1.6 x 10$^{25}$ yr (90% CL), corresponding to effective Majorana masses of less than 140-380 meV, depending on the matrix element calculation.
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EXO-200 is a single phase liquid xenon detector designed to search for neutrinoless double-beta decay of $^{136}$Xe. Here we report on a search for various Majoron-emitting modes based on 100 kg$cdot$yr exposure of $^{136}$Xe. A lower limit of $T^{^{136}Xe}_{1/2} >1.2 cdot 10^{24}$ yr at 90% C.L. on the half-life of the spectral index = 1 Majoron decay was obtained, corresponding to a constraint on the Majoron-neutrino coupling constant of $|< g^{M}_{ee} >|<$ (0.8-1.7)$cdot$10$^{-5}$.
A search for Majoron-emitting modes of the neutrinoless double-beta decay of $^{136}$Xe is performed with the full EXO-200 dataset. This dataset consists of a total $^{136}$Xe exposure of 234.1 kg$cdot$yr, and includes data with detector upgrades that have improved the energy threshold relative to previous searches. A lower limit of T$_{1/2}^{rm{^{136}Xe}}>$4.3$cdot$10$^{24}$ yr at 90% C.L. on the half-life of the spectral index $n=1$ Majoron decay was obtained, a factor of 3.6 more stringent than the previous limit from EXO-200, corresponding to a constraint on the Majoron-neutrino coupling constant of $|langle g_{ee}^{M}rangle|$$<(0.4$-$0.9)cdot10^{-5}$. The lower threshold and the additional data taken resulted in a factor 8.4 improvement for the $n=7$ mode compared to the previous EXO search. This search provides the most stringent limits to-date on the Majoron-emitting decays of $^{136}$Xe with spectral indices $n=1,2,3,$ and 7.
A search for neutrinoless double-beta decay ($0 ubetabeta$) in $^{136}$Xe is performed with the full EXO-200 dataset using a deep neural network to discriminate between $0 ubetabeta$ and background events. Relative to previous analyses, the signal detection efficiency has been raised from 80.8% to 96.4$pm$3.0% and the energy resolution of the detector at the Q-value of $^{136}$Xe $0 ubetabeta$ has been improved from $sigma/E=1.23%$ to $1.15pm0.02%$ with the upgraded detector. Accounting for the new data, the median 90% confidence level $0 ubetabeta$ half-life sensitivity for this analysis is $5.0 cdot 10^{25}$ yr with a total $^{136}$Xe exposure of 234.1 kg$cdot$yr. No statistically significant evidence for $0 ubetabeta$ is observed, leading to a lower limit on the $0 ubetabeta$ half-life of $3.5cdot10^{25}$ yr at the 90% confidence level.
Results from a search for neutrinoless double-beta decay $0 ubetabeta$ of $^{136}$Xe are presented using the first year of data taken with the upgraded EXO-200 detector. Relative to previous searches by EXO-200, the energy resolution of the detector has been improved to $sigma/E$=1.23%, the electric field in the drift region has been raised by 50%, and a system to suppress radon in the volume between the cryostat and lead shielding has been implemented. In addition, analysis techniques that improve topological discrimination between $0 ubetabeta$ and background events have been developed. Incorporating these hardware and analysis improvements, the median 90% confidence level $0 ubetabeta$ half-life sensitivity after combining with the full data set acquired before the upgrade has increased 2-fold to $3.7 times 10^{25}$ yr. No statistically significant evidence for $0 ubetabeta$ is observed, leading to a lower limit on the $0 ubetabeta$ half-life of $1.8times10^{25}$ yr at the 90% confidence level.
Searches for double beta decay of $^{134}$Xe were performed with EXO-200, a single-phase liquid xenon detector designed to search for neutrinoless double beta decay of $^{136}$Xe. Using an exposure of $29.6text{ kg}!cdot!text{yr}$, the lower limits of $text{T}_{1/2}^{2 ubeta!beta}>8.7cdot10^{20}text{ yr}$ and $text{T}_{1/2}^{0 ubeta!beta}>1.1cdot10^{23}text{ yr}$ at 90% confidence level were derived, with corresponding half-life sensitivities of $1.2cdot10^{21}text{ yr}$ and $1.9cdot10^{23}text{ yr}$. These limits exceed those in the literature for $^{134}$Xe, improving by factors of nearly $10^{5}$ and 2 for the two antineutrino and neutrinoless modes, respectively.
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