A Chinese lab starts to tackle a giant mystery in particle physics

At the foot of the thickly forested Dashi Hill, in southern China’s Guangdong province, pre-approved visitors can take a ride aboard a unique yellow train. Rather than winding through the serene landscape, however, the train descends along a steeply-sloping track that disappears into the darkness under the mountainside. After ten minutes on the train and a few more on foot, visitors reach a vast chamber that has been gouged out of the Earth. Here, more than half a kilometre underground, is a 12-storey-high sphere made from steel and plexiglass—the Jiangmen Underground Neutrino Observatory (JUNO).

This week, this enormous scientific facility, ten years in the making, will begin its hunt for the most elusive particles in the universe. In doing so, its scientists hope to crack open a decades-long mystery in fundamental physics.

Neutrinos—which come in three “flavours” known as electron, muon and tau—are elementary particles, shrapnel born out of the nuclear reactions that fuel stars and atomic-power plants. They are extremely light, have no electric charge and rarely interact with anything else, meaning they mostly stream through the universe unimpeded and invisible. (Hundreds of trillions of neutrinos have passed through your body in the few seconds it’s taken you to read this sentence.)

They also present a problem for the Standard Model of particle physics. This description of the known particles and forces, one the most successful scientific ideas of all time, predicts that neutrinos should have no mass at all. That is at odds, however, with what physicists actually observe.

Around 30 years ago, scientists working at Super-Kamiokande, a neutrino observatory in Japan, noticed something odd. While the number of muon neutrinos arriving at its detectors from above (formed by the collision of high-energy cosmic rays with atoms in Earth’s upper atmosphere) was in line with predictions, the number of neutrinos coming from below (formed by the same processes in the atmosphere on the other side of planet and then travelling through Earth’s core) was too low. Shortly afterwards the Sudbury Neutrino Observatory in Canada reported a similar anomaly concerning neutrinos from the Sun: of the mix of particles it detected, too few were electron-flavoured. These observations led scientists to conclude that the neutrinos must be transforming from one flavour to another as they flew through space. They also knew that such “oscillation” would only be possible if the neutrinos had mass.

“Neutrino physics is physics beyond the Standard Model,” says Juan Pedro Ochoa-Ricoux, a physicist at the University of California, Irvine, who is part of the international team that works on JUNO. A deeper understanding of the masses of neutrinos is key to an improved Standard Model. One of JUNO’s goals, therefore, will be to work out which neutrino is heaviest and which is the lightest. Wang Yifang, the observatory’s lead scientist and the director of the Institute of High-Energy Physics at the Chinese Academy of Sciences, reckons the task will take about six years.

Standing inside JUNO’s underground experiment hall feels like being in a cathedral—people’s voices echo inside the enormous space, which is significantly colder than the forests and fields above ground. The tank at the core of the observatory holds a mix of around 20,000 tonnes of hydrogen-rich fluids, known as the liquid scintillator. The vast majority of neutrinos that enter this tank will pass through unnoticed. A few, however, will hit protons in the fluid, resulting in bursts of blue light. Around 40,000 photomultiplier tubes line the inside of the tank, ready to detect those rare flashes.

JUNO’s task will be to count the number of neutrinos that arrive from a pair of nuclear power plants, each situated 53km from the observatory. With around 700 metres of granite mountain above, the detector is well insulated against other sources of neutrinos (resulting from cosmic rays, for example) that might otherwise interfere with its primary measurements. Scientists know how many neutrinos of a specific type are produced at the power plants so those that make it to JUNO, therefore, represent the fraction that did not switch flavour en route. That will provide a measure of the rate at which oscillation occurs.

That oscillation rate is, in turn, linked to the neutrinos’ mass. Unlike other elementary particles, neutrinos do not have a fixed mass but, instead, each neutrino flavour is a mix of three underlying states, each of a different mass, known as v1, v2 and v3. As a neutrino flies through space, the exact composition of this mixture changes, pushing the particles to switch from one flavour to another.

The precise values of these three mass states are what physicists ideally would want to measure, but such direct observations have proven difficult. Results from other neutrino labs, however, have provided clues to how the mass states might be related. Current evidence leans towards “normal ordering” in which v1 is lighter than v2, both of which are much lighter than v3. The other option, known as “inverted ordering”, dictates that v3 is the lightest, with v1 and v2 at the heavier end.

JUNO’s data will look subtly different depending on the true ordering of the mass states, allowing scientists to pin down whether normal or inverted is more likely to be correct. When the observatory is fully operational, around 50 neutrino detections are expected every day. Around 100,000 detections will be required to get statistically significant results, hence Dr Wang’s confidence of an answer within six years.

Theoretical physicists will have a hard time waiting. Ever since neutrino oscillation was experimentally confirmed, says Dr Ochoa-Ricoux, he and his peers have been busy coming up with possible extensions to the Standard Model that could account for neutrino mass. Inverted ordering is the more exciting option, says Kaladi Babu, a theorist from Oklahoma State University. He says it would allow scientists to test whether neutrinos are, in fact, their own antiparticles.

The Standard Model says that all particles have antimatter equivalents, which have identical mass but (among other things) opposite electric charge. Some particles, such as the photon, are their own antiparticles. A group of proposals to extend the model suggests this could also be the case for neutrinos. They rely on the “seesaw mechanism”, in which neutrinos could have tiny masses if they were also connected to other, as-yet-undetected, neutrinos with much larger masses. It is an elegant mechanism preferred by theorists like Silvia Pascoli at the University of Bologna. These heavier neutrinos could even be candidates for dark matter, another mysterious physical phenomenon that can only be inferred thus far by its effect on its surroundings.

To test if neutrinos and antineutrinos are indeed the same, physicists need to study radioactive isotopes of elements such as calcium and germanium. Sometimes these elements will emit two electrons and two antineutrinos when they undergo radioactive decay. If neutrinos are indeed their own antiparticles then scientists should, very rarely, observe a version of this process in which no antineutrinos are emitted at all.

How long scientists would have to wait to spot such an event, if the hypothesis is correct, depends on the neutrino mass states. If the ordering is inverted, it should happen often enough that sensitive experiments, such as the LEGEND experiment in Italy or the NEXT experiment in Spain or their successors, should pick them up in the next ten to 15 years. “That would be new physics just around the corner,” says Dr Pascoli. But if the ordering is normal, the process would probably be too rare to show up in any detector that scientists know how to make.

Helping to resolve such debates will be JUNO’s most important legacy, but the observatory will also allow physicists to eventually use neutrinos as probes. JUNO will, for example, look for neutrinos from deep within Earth, which will shed light on the distribution of radioactive elements within the mantle and crust.

It will also look for neutrinos from exploding stars known as supernovae. Because neutrinos flow through matter in a way that light cannot, they can leave those stars and reach Earth before the actual explosion becomes visible. Detecting them will give astronomers time to properly orient their telescopes so that they can then watch the epic blasts in action.

It’s when they are being used like this—as a way to peer into places that are currently unknown—that the neutrino era will truly have begun. ■