Trillions of neutrinos whiz through our bodies every day, pulsing from the sun, outer space and deep beneath Earth. Yet these elusive subatomic particles have proven difficult to study. That could soon change, however. Buried 700 meters beneath the rolling hills of southern China, an enormous neutrino observatory called JUNO has released its first results after a mere 59 days of operation. And so far, they are very promising, physicists say.
“The physics result is already world-leading in the areas that it touches,” says particle physicist Juan Pedro Ochoa-Ricoux of the University of California, Irvine, who co-leads a team on JUNO.
“In particular, we measured two neutrino oscillation parameters, and that measurement is already for both parameters the best in the world,” he says. The results were published in two separate preprints on arXiv.org.
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JUNO—short for Jiangmen Underground Neutrino Observatory—has been tasked with a tall order: determine the ordering of masses of the three types of neutrino. In other words, do they follow a “normal mass ordering,” where the first flavor of neutrino is the lightest and the third the heaviest, or an inverted one, in which the third neutrino mass state is the lightest?
The answer to this question holds myriad implications, from informing other experiments to uncovering new physics to explaining certain cosmological mysteries. That’s because despite being such lightweights, neutrinos are so incredibly numerous that they may play an outsized role in the distribution of matter in the universe.
JUNO’s spherical detector, which is akin to a 13-story-tall fishbowl, primarily measures so-called electron antineutrinos spewing from the nearby Yangjian and Taishan nuclear plants. When the particles strike a proton inside the detector, a reaction triggers two light flashes that ping photomultiplier tubes and get converted into electrical signals.
The new measurements from these neutrino-proton collisions are now considered the most precise for two oscillation parameters, which act as proxies for differences in their mass, according to Ochoa-Ricoux.
“It is the first time we’ve turned on a scientific instrument like JUNO that we’ve been working on for over a decade. It’s just tremendously exciting,” Ochoa-Ricoux says. “And then to see that we’re able to already do world-leading measurements with it, even with such a small amount of data, that’s also really exciting.”
Still, the physicists will need years’ worth of neutrino detections to answer the mass-ordering conundrum.