Search for Majorana Neutrinos with the Complete KamLAND-Zen Dataset

We present a search for neutrinoless double-beta ($0\nu \beta \beta$) decay of ${}^{136}$Xe using the full \mbox{KamLAND-Zen 800} dataset with 745,kg of enriched xenon, corresponding to an exposure of $2.1$,ton,yr of ${}^{136}$Xe. This updated search benefits from a more than twofold increase in exposure, recovery of photo-sensor gain, and reduced background from muon-induced spallation of xenon. Combining with the search in the previous KamLAND-Zen phase, we obtain a lower limit for the $0\nu \beta \beta$ decay half-life of $T_{1\mathrm{/}2}^{0\nu }>3.8\times {10}^{26}$,yr at 90% C.L., a factor of 1.7 improvement over the previous limit. The corresponding upper limits on the effective Majorana neutrino mass are in the range 28–122,meV using phenomenological nuclear matrix element calculations.