Search for Majorana neutrinos with the first two years of EXO-200 data

Many extensions of the Standard Model of particle physics suggest that neutrinos should be Majorana-type fermions, but this assumption is difficult to confirm. Observation of neutrinoless double-beta decay ($0 u beta beta$), a spontaneous transition that may occur in several candidate nuclei, would verify the Majorana nature of the neutrino and constrain the absolute scale of the neutrino mass spectrum. Recent searches carried out with $^{76}$Ge (GERDA experiment) and $^{136}$Xe (KamLAND-Zen and EXO-200 experiments) have established the lifetime of this decay to be longer than $10^{25}$ yr, corresponding to a limit on the neutrino mass of 0.2-0.4 eV. Here we report new results from EXO-200 based on 100 kg$cdot$yr of $^{136}$Xe exposure, representing an almost fourfold increase from our earlier published datasets. We have improved the detector resolution at the $^{136}$Xe double-beta-decay Q-value to $sigma$/E = 1.53% and revised the data analysis. The obtained half-life sensitivity is $1.9cdot10^{25}$ yr, an improvement by a factor of 2.7 compared to previous EXO-200 results. We find no statistically significant evidence for $0 u beta beta$ decay and set a half-life limit of $1.1cdot10^{25}$ yr at 90% CL. The high sensitivity holds promise for further running of the EXO-200 detector and future $0 u beta beta$ decay searches with nEXO.