KamLAND-Zen 800: neutrinoless double-beta decay recent results and future prospects*

The detection of neutrinoless double beta decay (0νββ) would confirm that neutrinos are Majorana particles, providing insights into the neutrino mass and the predominance of matter in the universe. The KamLAND-Zen experiment has set the most stringent limit on the 0νββ decay half-life in ¹³⁶Xe, using a xenon-loaded liquid scintillator. Here, we report the final results from the KamLAND-Zen 800 experiment, which utilized the upgraded detector, an ultra-low-radioactivity environment, and nearly 800 kg of enriched xenon, achieving a total exposure of 2097 kg·yr of ¹³⁶Xe. No significant evidence of 0νββ was observed. Consequently, we set a lower limit on the half-life of this process at T_1/2 > 4.3×10²⁶ years at 90% confidence level, translating to an upper limit on the effective Majorana neutrino mass of 〈m_ββ〉 < (28−122) meV. With improved photon sensing technology, brighter scintillator, and reduced background, we are now entering the KamLAND2-Zen experiment, further enhancing our sensitivity to 0νββ.