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For a whole year, scientists listened to 148 kilograms of xenon, hoping to catch a decay that happens less than once in a trillion years. They heard nothing.
In a noisy room, it's hard to hear a pin drop. For a whole year, physicists with the PandaX-4T collaboration listened to liquid xenon, hoping to catch something even quieter: the decay of a xenon-136 nucleus that spits out four electrons at once and produces no neutrino. This decay is so rare that in a kilogram of xenon, less than one atom would decay over a year.
Why does this matter? Ordinary neutrinoless double beta decay would show that a neutrino is its own antiparticle. But it hasn't been found. Four-electron decay is also possible in the other scenario, where a neutrino differs from its antiparticle. If four-electron decay turns up and double decay doesn't, that would be a clue.
The detector sat deep underground, where rock blocks out cosmic noise. The scientists collected data over 148.4 kilogram-years, as if they had listened to one kilogram of xenon-136 for 148 years. They compared the signal they got with what they expected from the decay and with noise from impurities, other decays, and even solar neutrinos. It turned out there was no signal, only noise. But a missing signal is still a result. Now we know: if this decay happens at all, its half-life is longer than 6.01×10²⁴ years. That's like waiting for a pin to drop 400 trillion times longer than the Universe has existed.
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.