Neutrinoless Quadruple Beta Decay

While detection of neutrinoless double beta decay is a sign that neutrinos are Majorana particles, neutrinoless quadruple beta decay in the absence of neutrinoless double beta decay is a sign that neutrinos have Dirac mass. So far, neither form of decay has been observed.

The observation of neutrinoless quadruple beta decay (0ν4β) in the absence of neutrinoless double beta decay (0ν2β) has been argued to provide a strong indication that neutrinos are Dirac particles. We report a search for 0ν4β decay of 136Xe using a total 136Xe exposure of 148.4 kg⋅yr, collected during the commissioning and the first science runs of the PandaX-4T experiment. No significant excess of events over the background is observed. A lower limit on the 0ν4β decay half-life of 136Xe is set at 6.01 x 10^24 yr at the 90% confidence level. This result establishes the most stringent constraint on this process in xenon, demonstrating the unique capability of the PandaX-4T detector in probing lepton number violation and shedding light on the fundamental nature of neutrinos.

PandaX Collaboration, "Search for neutrinoless quadruple beta decay of 136Xe in PandaX-4T detector" arXiv:2609.10048 (September 9, 2026).