  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 ($0\nu \beta \beta$), 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 $1.9\cdot10^{25}$ yr, an improvement by a factor of
2.7 compared to previous EXO-200 results. We find no statistically significant
evidence for $0\nu \beta \beta$ decay and set a half-life limit of
$1.1\cdot10^{25}$ yr at 90% CL. The high sensitivity holds promise for further
running of the EXO-200 detector and future $0\nu \beta \beta$ decay searches
with nEXO.
