Search for Majorana Neutrinos Near the Inverted Mass Hierarchy Region with KamLAND Zen.
We present an improved search for neutrinoless double beta (0nbb) 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 0nbb decay half life of T_{1/2}{0n}>1.07x10^{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.
Physical review letters