After neutrino oscillations have been confirmed by several experiments, neutrino mass hierarchy is the key issue to understand neutrino nature. Search for neutrino-less double beta decay is one of the way to investigate the neutrino mass hierarchy.
KamLAND-Zen400 is an experiment to search for neutrino-less Double beta decay with 400 kg of e-136 and the mission has been completed after 4 years of data taking.
The final result does not see the signal of neutrino-less double beta decay, but constrains on mass hierarchy was improved and getting to "Inverted" hierarchy region. We are upgrading KamLAND-Zen400 to KamLAND-Zen800 to reach to the Inverted hierarchy region.
The main focus of this talk is the KamLAND-Zen experiment: final result and upgrade status. Since KamLAND-Zen is a part of KamLAND experiment that has several on-going physics programs, such as Neutrino Oscillation from Nuclear Reactors and Geo-Neutrinos, the KamLAND experiment is briefly introduced prior to KamLAND-Zen.