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The PandaX-4T detector listened to 148 kilograms of xenon for a decay that could reveal the nature of the neutrino, and heard only background.
Picture a room where hundreds of people are talking at once while you strain to hear a pin drop. That is essentially how physicists with the PandaX-4T collaboration listened for silence inside 3.7 tonnes of liquid xenon. Their "pin" is the neutrinoless quadruple beta decay of xenon-136, an event so rare that in a kilogram of material less than one atom would decay over a year. The detector sits deep underground in China's Jinping laboratory, where the rock overburden muffles cosmic noise.
Why quadruple? Ordinary neutrinoless double beta decay has long been sought because it would point to the Majorana nature of the neutrino, meaning the neutrino would be its own antiparticle. But decades of searching have come up empty. In 2013, physicists Heeck and Rodejohann proposed looking for the quadruple decay instead: it is possible even for Dirac neutrinos, which differ from their antiparticles. If the quadruple decay is found while the double one is not, that would be a strong argument in favor of the Dirac nature.
To hear such a faint signal, you need not only to eliminate outside noise but also to know exactly how the instrument itself sounds. The scientists collected data from two detector runs: commissioning and the first science run. The total exposure came to 148.4 kilogram-years of the xenon-136 isotope. They widened the search window to energies from 20 to 140 kiloelectronvolts in order to capture the expected peak at 79 kiloelectronvolts. Then they modeled every possible background source: krypton-85 and radon impurities, double beta decay of xenon itself, double electron capture of xenon-124, and even solar neutrinos.
What is a kilogram-year? It is a measure of accumulated sensitivity. One kilogram-year means the detector ran for a year with one kilogram of material. If you have 148 kilograms of xenon-136, then over a year of observation you accumulate 148 kilogram-years. The larger this number, the better your odds of catching a rare event.
The listening result turned out to be silence. The spectrum showed no statistically significant excess over background. But that silence allowed a new limit to be set: the half-life of neutrinoless quadruple beta decay of xenon-136 is greater than 6.01×10²⁴ years. That is 1.6 times tighter than the previous constraint set by the XMASS detector. Now one can say with confidence: if such a decay does occur, the wait for it would be longer than the age of the Universe, 400 trillion times longer.
For now the silence does not answer the big question about the nature of the neutrino, but it narrows the field for theorists. Models that predicted a faster quadruple decay are now ruled out. And the hunt goes on: ahead lie the analysis of PandaX-4T's second science run and the construction of the PandaX-xT detector with a mass of tens of tonnes. Perhaps in an even deeper silence the pin will finally drop.
A half-life of 6.01×10²⁴ years is 400 trillion times longer than the age of the Universe. If such a decay occurred, less than one atom per year would decay in a kilogram of xenon-136.
In Liu Cixin's novel The Three-Body Problem, neutrinos are used to communicate with an alien civilization. The search for exceedingly rare decays such as 0ν4β resembles attempts to catch nearly imperceptible signals from the depths of the microworld.