Results and Prospects of KamLAND-Zen

KamLAND-Zen is a double beta decay experiment that exploits the existing KamLAND neutrino detector which realizes an ultra-low background environment. The xenon-loaded liquid scintillator in a spherical nylon balloon was deployed at the center of the KamLAND detector. The first search with 381 kg of xenon (KamLAND-Zen 400) demonstrated excellent sensitivity. To enhance the sensitivity, the KamLAND-Zen detector was upgraded to larger volume containing 745 kg xenon (KamLAND-Zen 800), corresponding to a twofold increase. The data with 1 ton-year exposure are useful to investigate low-rate backgrounds, especially from cosmic muon spallation of xenon, and have required the use of novel background rejection techniques. Based on the improved analysis, KamLAND-Zen has provided the most stringent on the effective neutrino mass, and started probing the inverted mass ordering region for the first time. In the future, the KamLAND detector is planned to be upgraded to improve KamLAND2-Zen to improve the search sensitivity (KamLAND2-Zen), and R&D studies for optimizations are ongoing.