JUNO Neutrino Observatory Releases First Results

China’s giant underground neutrino observatory has just published its first results, and they are promising

By Jeanna Bryner edited by Claire Cameron

Billions of neutrinos pass through our bodies every day, originating from the sun, space and deep underground. Yet these elusive subatomic particles have proven difficult to study. But that could soon change. Buried 700 meters beneath the hills of southern China, a massive neutrino observatory called JUNO has published its first results after just 59 days of operation. And so far, they’re very promising, physicists say.

“The physics results are already among the best in the world in the areas they touch,” says Juan Pedro Ochoa-Ricoux, a particle physicist at the University of California, Irvine, who co-leads a team on JUNO.

“In particular, we measured two neutrino oscillation parameters, and this measurement is already for these two parameters the best in the world,” he says. The results have been published in two separate preprints on arXiv.org.

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JUNO, short for Jiangmen Underground Neutrino Observatory, was tasked with a daunting task: determining the order of the masses of the three types of neutrinos. In other words, do they follow a “normal mass order”, in which the first flavor of the neutrino is the lightest and the third the heaviest, or a reverse order, in which the third mass state of the neutrino is the lightest?

The answer to this question has countless implications, from informing other experiments to discovering new physics to explaining certain cosmological mysteries. Indeed, although they are so light, neutrinos are so incredibly numerous that they could play an outsized role in the distribution of matter in the universe.

JUNO’s spherical detector, which resembles a 13-story fishbowl, primarily measures electronic antineutrinos emitted by the nearby Yangjian and Taishan nuclear power plants. When the particles hit a proton inside the detector, a reaction triggers two flashes of light that ping the photomultiplier tubes and are converted into electrical signals.

The new measurements of these neutrino-proton collisions are now considered the most precise for two oscillation parameters, which serve as proxies for differences in their masses, according to Ochoa-Ricoux.

“This is the first time we’re operating a scientific instrument like JUNO that we’ve been working on for over a decade. It’s just extremely exciting,” says Ochoa-Ricoux. “And then to see that we are already able to do cutting-edge measurements with it, even with such a small amount of data, is also very exciting.”

Still, physicists will need several years of neutrino detection to answer the mass order puzzle.

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