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

We present an improved search for neutrinoless double-beta (0 nu beta beta) decay of Xe-136 in the KamLAND-Zen experiment. Owing to purification of the xenon-loaded liquid scintillator, we achieved a significant reduction of the Ag-110m contaminant identified in previous searches. Combining the results from the first and second phase, we obtain a lower limit for the 0 nu beta beta decay half-life of T-1/2(0 nu) > 1.07 x 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.