Whether neutrinos are Majorana particles—that is, whether neutrinos are their own antiparticles—remains one of the most fundamental open questions in particle physics. Scientists worldwide are searching deep underground for the extremely rare process known as neutrinoless double-beta decay (0νββ), which would provide a crucial test of the Majorana nature of neutrinos. To reliably identify 0νββ, however, it is first essential to precisely measure the spectral shape and half-life of the Standard Model-allowed two-neutrino double-beta decay process (2νββ).
Recently, the PandaX Collaboration, using the ton-scale natural xenon PandaX-4T detector at the China Jinping Underground Laboratory, completed the most systematic and precise experimental study to date of the 2νββ spectrum of xenon-136, which also yields the world’s most precise half-life result for this isotope. The results were published on April 21, 2026, in the leading international physics journal Physical Review Letters.
https://journals.aps.org/prl/abstract/10.1103/v4c6-h6l6
Group photo of PandaX students and researchers in front of the detector
Xenon-136 is one of the most important candidate isotopes for 0νββ searches. Its natural abundance in xenon is about 8.9%. Experiments such as KamLAND-Zen and EXO-200/nEXO typically rely on costly isotopically enriched Xe-136. Once a detector reaches the ton scale, however, a natural xenon target with significant amount of Xe-136, can also enable high-precision measurements, offering a cost-effective and scalable alternative path. Leveraging its large target mass, low background level, and broad energy coverage, PandaX-4T measured the half-life of Xe-136 2νββ to be (2.14±0.05)×1021 years, with a total relative uncertainty of 2.3%. This precision surpasses that of leading enriched-xenon experiments such as KamLAND-Zen and EXO-200, making it the most precise measurement worldwide to date. Beyond the half-life itself, this study also measures the relative size of higher-order contributions to the 2νββ nuclear matrix element and to search for double-beta decay modes accompanied by Majoron emission, setting the world’s most stringent constraint on the spectral-index n=7 model.