Searching for neutrino-less double beta decay of 136Xe with PandaX-II liquid xenon detector * * Supported by grants from the Ministry of Science and Technology of China (2016YFA0400301, 2016YFA0400302), a Double Top-class grant from Shanghai Jiao Tong University, grants from National Science Foundation of China (11435008, 11505112, 11525522, 11775142, 11755001), grants from the Office of Science and Technology, Shanghai Municipal

Abstract

We report the Neutrino-less Double Beta Decay (NLDBD) search results from PandaX-II dual-phase liquid xenon time projection chamber. The total live time used in this analysis is 403.1 days from June 2016 to August 2018. With NLDBD-optimized event selection criteria, we obtain a fiducial mass of 219 kg of natural xenon. The accumulated xenon exposure is 242 kg·yr, or equivalently 22.2 kg·yr of 136Xe exposure. At the region around 136Xe decay Q-value of 2458 keV, the energy resolution of PandaX-II is 4.2%. We find no evidence of NLDBD in PandaX-II and establish a lower limit for decay half-life of 2.1 yr at the 90% confidence level, which corresponds to an effective Majorana neutrino mass eV. This is the first NLDBD result reported from a dual-phase xenon experiment.