A new neutrino experiment in China has put the world on notice that it’s poised to make a breakthrough. The Jiangmen Underground Neutrino Observatory (JUNO), a plastic sphere 10 stories high filled with a liquid that flashes when certain particles pass through it, detects neutrinos streaming from nuclear power plants 53 kilometers away. Neutrinos come in three types that “oscillate,” or morph into one another, as they zip along at near–light-speed, a phenomenon physicists have yet to fully puzzle out. Now, JUNO has measured with unequaled precision two of the six parameters that describe the oscillations, as reported today in Nature. The result, based on just 2 months of data, suggests JUNO is on track to reach its main goal: sorting the neutrinos by mass.
Knowing that mass hierarchy would guide other neutrino experiments and shed light on cosmic phenomena from supernovas to the evolution of the galaxies. “The thing they are ultimately after is incredibly ambitious, but it looks like there’s nothing that’s obviously going to prevent them from reaching their goal,” says Kate Scholberg, a neutrino physicist at Duke University. “It’s very impressive.” Yifang Wang, a physicist at the Institute of High Energy Physics of the Chinese Academy of Sciences and spokesperson for the 750-member JUNO team, says, “We started to have a high-quality physics data almost at the first shot.”
Nearly massless, neutrinos interact with other matter so rarely that every second trillions pass through each of us. JUNO directly detects just one type, or “flavor”: electron neutrinos, which emanate from the Sun and nuclear reactors. Muon neutrinos rain down from the atmosphere, generated as cosmic rays strike the air, and can be created with particle accelerators. The third flavor, tau neutrinos, comes from decay of rare and massive tau particles made with accelerators.
Bizarrely, each flavor has no definite mass but is a quantum mechanical combination of three different mass states. The mass states evolve at different rates, a bit like intermeshed gears of different sizes turning at different speeds, which causes a neutrino’s flavor to change. So, for example, an electron neutrino born in the Sun can evolve to a muon or tau neutrino and back again. As a neutrino zings along, it oscillates at different frequencies among the three flavors.
Theorists can encapsulate all this in a model with just a handful of parameters, describing the mixtures of flavors in different mass states and the differences between masses. Experimenters have already determined that two mass states are close and one is quite different. But they don’t know whether there are two light mass states and one heavy one, or vice versa. (Neutrinos’ masses are too small to measure directly, at least for now.)
That mass hierarchy could determine the feasibility of experiments to probe whether the neutrino is its own antiparticle, which could be key to how neutrinos get their tiny masses, says Patricia Vahle, a neutrino physicist at the College of William and Mary. It could also help explain how supernova explosions work and the universe evolved. Past neutrino experiments have typically focused on one particular oscillation and parameter. For example, JUNO’s predecessor, the Daya Bay Reactor Neutrino Experiment, wowed the world by beating competitors in measuring a crucial missing parameter. But JUNO, which cost roughly $300 million, aims to measure multiple oscillations at once.
To do that, JUNO researchers seek to measure the energy spectrum of electron neutrinos gushing from the eight power plant reactors with a precision of 3%, twice the precision of any previous similar detector. JUNO’s size helps: The giant sphere holds 20,000 tons of a liquid that flashes when a charged particle zips through it. Neutrinos themselves are uncharged, but when an electron neutrino from the reactors—actually an antineutrino—strikes a proton in the liquid, it turns into a positron that travels about 10 centimeters, radiating light whose total energy reveals the neutrino’s energy.
To collect the light, 43,183 phototubes large and small line the sphere. The phototubes had to collect light more efficiently than any the team could buy, Wang explains. So researchers built a factory to make their own. “We invented a new kind of phototube,” Wang says.
The shape of the resulting energy spectrum holds clues to all three oscillations, even though two of the flavors are invisible to JUNO. As the neutrinos come out of the reactors, their spectrum peaks broadly at about 3 megaelectron volts. By the time the neutrinos reach JUNO, the oscillation between the two close mass states flattens the spectrum of the remaining electron neutrinos, turning the peak into a mesa. On top of that, the oscillation driven by the more different mass state should create small, tightly spaced ripples. Those ripples point to the mass ordering, as they will shift one way or the other depending on the mass hierarchy, Wang explains.
With just 59 days of data, JUNO researchers have already reduced the uncertainty on two oscillation parameters by one-third, they report. And their neutrino spectrum already shows tantalizing ripples. “They’re pretty pronounced,” Vahle says. JUNO physicists seem sure to meet their once aspirational goal, says Patrick Huber, a theorist at Virginia Tech. “It’s now just a matter of time and collecting the data.”
But there may still be a race for that prize, some physicists predict. Groups in Japan and the United States are building huge accelerator-driven neutrino experiments known respectively as Hyper-Kamiokande (Hyper-K) and the Deep Underground Neutrino Experiment. DUNE in particular should be able to determine the mass hierarchy very quickly, says Zoya Vallari, a physicist at Ohio State University. “Within a year of data taking, DUNE will make it clear as night and day which ordering it is.” However, Hyper-K won’t start to take data until 2028, and DUNE won’t receive a neutrino beam until 2031.
For now, JUNO and China are clearly in the lead.
Correction, 12 June, 4:50 p.m.: This story has been changed to reflect that Yifang Wang is no longer director of the Institute of High Energy Physics.