Search for Majorana Neutrinos Near the Inverted Mass Hierarchy Region with KamLAND-Zen

TLDR

The study aims to improve the search for neutrinoless double‑beta decay of ^{136}Xe in the KamLAND‑Zen experiment. By purifying the xenon‑loaded liquid scintillator, the experiment significantly reduced ^{110m}Ag background, enhancing sensitivity. The combined data set sets a 90 % C.L.

Abstract

We present an improved search for neutrinoless double-beta (0νββ) decay of ^{136}Xe in the KamLAND-Zen experiment. Owing to purification of the xenon-loaded liquid scintillator, we achieved a significant reduction of the ^{110m}Ag contaminant identified in previous searches. Combining the results from the first and second phase, we obtain a lower limit for the 0νββ decay half-life of T_{1/2}^{0ν}>1.07×10^{26} yr at 90% C.L., an almost sixfold improvement over previous limits. Using commonly adopted nuclear matrix element calculations, the corresponding upper limits on the effective Majorana neutrino mass are in the range 61-165 meV. For the most optimistic nuclear matrix elements, this limit reaches the bottom of the quasidegenerate neutrino mass region.