Search for Majorana Neutrinos with the Complete KamLAND-Zen Dataset

Abstract

We present a search for neutrinoless double-beta ($0\hat{I}½{\hat{I}}^2{\hat{I}}^2$) 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 $0\hat{I}½{\hat{I}}^2{\hat{I}}^2$ decay half-life of $T_{1/2}^{0\hat{I}½}>3.8\times 10^{26}$\,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.

arXiv admin note: text overlap with arXiv:2203.02139