In an underground laboratory near Shenzhen, southern China, officials gathered on 12 December 2020 to say goodbye to a decade-old experiment that not only unveiled secrets of the neutrino but also fostered China–US scientific collaboration. A little after 10.30 a.m., Yifang Wang from the Institute of High Energy Physics (IHEP), Chinese Academy of Sciences, pressed a red button that stopped the Daya Bay Reactor Neutrino Experiment from taking data. A few minutes later, covers were removed and four massive, cylindrical tanks appeared in a pool filled with highly purified water.

“Today we are here to celebrate the completion of the Daya Bay Reactor Neutrino Experiment, which has fulfilled all its missions,” noted Jun Cao, co-spokesperson of the collaboration, during the ceremony. Only a small audience was present due to coronavirus restrictions, but 1.7 million people joined online to see the experiment come to an end. Among them was Kam-Biu Luk, a particle physicist from the University of California at Berkeley and Lawrence Berkeley National Laboratory and the experiment’s US spokesperson, who watched the livestream from his home in California. “I’ve worked on a number of experiments in my life,” he told Physics World, “but Daya Bay has achieved so much that it is extremely rewarding. This is certainly a happy ending for all of us.”

Early days

Born in nuclear interactions, neutrinos are extremely light and hard to catch, yet they are everywhere around us. They come in three types – electron, muon and tau – that morph into each other as they travel near the speed of light. Thanks to large-scale neutrino detectors in Japan, the US, Canada and other countries, by the early 2000s physicists had a good idea about how electron neutrinos transform into muon and tau neutrinos (as in solar neutrino oscillation) and how muon neutrinos transform into tau neutrinos (as in atmospheric neutrino oscillation). However, the case of electron–muon oscillation – the last missing piece in the puzzle of neutrino oscillations and dictated by the parameter “theta-13” – remained unclear.

Some scientists proposed using nuclear reactors to study this neutrino oscillation, since reactors are well-understood neutrino sources, and Luk realized it could be the best way to solve the theta-13 problem. He started searching for potential sites in Japan, South Korea and the US. Originally from Hong Kong, Luk also knew about the Daya Bay nuclear power plant and added it to his list.

Daya Bay stood out in many ways, not least because the Daya Bay and Ling Ao reactors are powerful enough to produce a large number of antineutrinos. The site is also next to a mountain range, making the construction and shielding of cosmic rays much easier. Given that the most efficient scheme to infer theta-13 was to compare antineutrino events at the near and far sites, the team planned eight detectors, four placed between 300 and 500 m from the reactors – dubbed “near detectors” – and four positioned 2 km away.

Daya Bay remains the largest collaboration between China and the US in basic research and has benefited scientists from both sides

Luk opted for Daya Bay and in late 2003 contacted IHEP for potential collaboration. Despite a lack of neutrino researchers in China, Wang, who was leading the institute’s experimental department, knew that it was an opportunity not to be missed and began searching for funding and people. The idea also quickly won support from the US’s Department of Energy, which later contributed about one third of the total cost, with Cao among the first to join. Wrapping up his postdoctoral research in the US at Fermilab, he immediately got down to basic design issues such as the shape of the detectors and the development of the liquid scintillator, which was done together with collaborators at Brookhaven National Laboratory.

Students also got involved in the project. They included Liangjian Wen, who was studying nuclear physics at the University of Science and Technology of China in Hefei, and came to IHEP Beijing to work on his undergraduate project. Inexperienced with building detectors, he was asked to develop the reflecting panels placed at the top and bottom inside the detector, a technique never used in similar experiments before. “The panels can reflect photons to the side, so we got to use fewer photomultiplier tubes and save about 20 million yuan,” says Wen. Doing everything from scratch, Wen learned what materials to use for the supporting structure, how to apply the reflecting film between the panels, and how to assemble the panels with high precision. “We made it in the end,” he adds. “The reflecting panels gave the detectors a simpler structure and better performance.”

Surprising findings

Daya Bay began taking data on 24 December 2011, when only six of the eight detectors were in place. Researchers were quick to remove noise signals and identify something indicative from data collected within the first few days. Cao remembers vividly how they worked late into the night, had lots of meetings and used a variety of cross-checking methods to make sure the results were correct. Then on 8 March 2012 the collaboration announced its groundbreaking findings on theta-13 at a press conference in Beijing.

Based on tens of thousands of antineutrino events observed, about 6% of the reactor’s antineutrinos transformed into other types of neutrinos on their way from the reactors to the far site. The transformation rate was surprisingly large, allowing Wang to announce the discovery of a new type of neutrino oscillation. For Cao, it was a wonderful surprise given it only took 55 days to get a definitive answer to the critically important theta-13 problem, the value of which turned out to be much larger than expected. In the eight years that followed, as the team collected and analysed more data, the measurement precision of theta-13 improved by sixfold to 3.4%, a milestone no other experiment is expected to surpass in the next 20 years.

Besides theta-13, the experiment also made other important findings. For example, it strongly challenged the assumption that a fourth type of neutrino, the sterile neutrino, exists. Observations at the near detector clearly showed that the reactors gave off far fewer antineutrinos than predicted – potentially because some had morphed into sterile neutrinos. To clarify the case, the team made separate measurements on uranium and plutonium, two major reactor fuel components and antineutrino sources. They found that the modelling and observation matched well for plutonium but there was a major discrepancy with uranium. “This largely ruled out the theory of explaining the deficit with sterile neutrinos,” says Wang. “If sterile neutrinos did exist, they should have acted on plutonium and uranium the same way.”

The Daya Bay legacy

Daya Bay remains the largest collaboration between China and the US in basic research and has benefited scientists from both sides. For China, the neutrino research team has grown from a small number of people in the early 2000s to about 100 today. For the US, the Daya Bay experiment turned out to be much cheaper and quicker than if the US had done the experiment alone.

We yearn for tomorrow, to reveal more unknowns in neutrino physics with the new generation of experiments

Jun Cao

Since the shutdown ceremony, the eight detectors have been taken apart, with some components such as the electronics being reused in the Jiangmen Underground Neutrino Observatory (JUNO) – China’s next major neutrino experiment. Other parts have been donated to overseas experiments, including 32 tonnes of gadolinium-doped liquid scintillator and 50 tonnes of undoped liquid scintillator to a Japanese experiment called JSNS2.

The rest of the experiment will be given to schools for educational or outreach use. The main laboratory hall, meanwhile, will be repurposed into an exhibition facility about the experiment. The team will also continue to analyse the complete dataset, which will take another year or two to complete.

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Neutrinos: ghosts of matter

IHEP researchers are working hard to make sure that JUNO will be up and running by the end of 2022. It will seek to work out the mass ordering of different types of neutrinos, which will help other upcoming neutrino facilities such as the Deep Underground Neutrino Experiment in the US and the Hyper-Kamiokande neutrino observatory in Japan to examine their absolute values as well as possibly reveal why the universe is made up of matter instead of antimatter. “These questions will keep particle physicists fully occupied for a few decades from now,” says Luk.

Researchers are also developing crucial technologies for a second phase of JUNO, which will conduct a neutrino-less double-beta decay experiment to study whether neutrinos are their own antiparticles and seek to measure the absolute masses of neutrinos. Yet Cao does not feel sorry to witness the end of Daya Bay. “On the contrary, we yearn for tomorrow,” he says, “to reveal more unknowns in neutrino physics with the new generation of experiments.”