Solar neutrinos are tiny particles born in the Sun's core by the trillions every second. They pass through Earth almost without noticing it, and for a long time they were considered absolutely stable. But perhaps they too can "age"—decaying imperceptibly, vanishing on their way to us. Physicists working with dark matter detectors took up the challenge to test this.
They had three giant facilities at their disposal: XENONnT, PandaX-4T, and LUX-ZEPLIN. Each is a multi-ton vat of liquid xenon deep underground, shielded from cosmic rays. Their primary mission is to catch hypothetical dark matter particles. But these ultra-sensitive instruments also register rare kicks from solar neutrinos that occasionally strike xenon nuclei. Analyzing such "nuclear recoils" allowed, for the first time, an estimate of how long the neutrino state with number two lives. The resulting limit is nearly as stringent as that from the famous Sudbury Neutrino Observatory.
The authors then looked to the future. They calculated what a next-generation detector, with an even larger xenon mass, could achieve. Here a new channel opens up: detecting electron recoils rather than nuclear ones. Low-energy neutrinos from the first proton-proton chain reaction could scatter off electrons, betraying themselves with tiny flashes. This method promises to improve lifetime measurements for two neutrino states at once — by a factor of 10, or even 100, compared to the best current results.
Thus, dark matter hunters unexpectedly become the best timekeepers for particles born in the solar core. The very instrument that peers into the darkness helps measure the lifetime of light itself.
Solar neutrinos travel from the Sun's core to Earth in 8 minutes, but inside the Sun itself they may wander for thousands of years before escaping.