A Hong Kong-born physicist reflects on the honeymoon phase of cooperation and the “fortunate twist” that initiated the Daya Bay Reactor Neutrino Experiment.

For the second installment of a series celebrating the tenth anniversary of the Future Science Prize, Victoria Bela examines Professor Kam-Biu Luk’s groundbreaking experimental finding of a novel form of neutrino oscillation. This achievement led to his receipt of the 2019 prize in physical sciences. You can find the initial segment of this series [here].
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In the quiet cosmic ballet, countless ethereal particles flow through us each second—unseen and unperceived, yet carrying clues to nature’s profound enigmas. Notable among these are neutrinos, the universe’s most evasive chameleons.

Once upon a time, these tiny subatomic particles became an unexpected link connecting global adversaries China and the United States. About twenty years back, during a peculiar convergence of scientific aspirations and improved international relations, both countries united to pursue a quantum mystery through the Daya Bay Reactor Neutrino Experiment.

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The initiative, located in South China’s Guangdong Province, was designed to enhance our comprehension of neutrinos and a phenomenon known as neutrino oscillation, where these particles transform from one type to another. Gaining insights into this could provide valuable knowledge regarding the origin of the universe, the development of matter, and, fundamentally, the emergence of human life.

The study, conducted by researchers based in China and the United States, uncovered a novel type of neutrino oscillation. This breakthrough not only revised the content of physics textbooks but also symbolized a prosperous period of global cooperation that is gradually being forgotten.

As political storms now cloud joint research efforts, Daya Bay stands as a poignant relic of what science can achieve when giants unite – and a sobering reminder of the cost when they drift apart.

In 2006, Hong Kong native Kam-Biu Luk, who was a professor at the University of California, Berkeley at the time, had his proposal for the project—which he co-directed with Wang Yifang from the Beijing-based Institute of High Energy Physics—approved.

In 2019, their research on neutrino oscillations earned them the Future Science Prize in the field of physical sciences. This private initiative honors significant scientific accomplishments in the realms of physical science, life science, and mathematics and computer science, specifically within the Chinese mainland, Hong Kong, Macau, and Taiwan.

The success at Daya Bay was truly fortunate,” Kam-Biu Luk stated during an earlier interview. “Maintaining a positive relationship between the U.S. and China was definitely essential.

If, for instance, such an experiment were suggested today, I doubt it would get approved.

After all, I conducted an experiment utilizing nuclear power plants within China despite being based in the US at the time.

The researchers suggested conducting an experiment that involved installing antineutrino detectors at multiple locations near nuclear reactors.

Antineutrinos serve as the antimatter counterparts to neutrinos. Similar to neutrinos, both particles undergo oscillations, making their study valuable for understanding this process. Unlike neutrinos, which originate from cosmic sources such as the Sun and Earth’s atmosphere, antineutrinos are generated during the operation of nuclear reactors, providing researchers with a more stable setting for conducting experiments.

By contrasting the antineutrinos observed close to a power plant with those seen at a greater distance from it, researchers were able to determine the number of antineutrinos that had vanished—or oscillated—into another variety of antineutrino during their journey.

The Earth continually receives cosmic rays that can generate signals resembling those of antineutrinos, potentially disrupting detector readings. To mitigate this issue, the experiment should be conducted underground.

“Besides locating the strongest power plants globally, we were also tasked with finding one situated close to a mountain range,” explained Luk. The mountains would provide additional protection for the detectors against cosmic rays.

Luk scoured the globe for a location to conduct his experiment. The Daya Bay Nuclear Power Plant and Ling Ao Nuclear Power Plant, situated close to the eastern fringe of Shenzhen, proved to be suitable options.

Following discussions with associates based in Hong Kong and Beijing, they opted to team up for this collaborative effort. This initiative also included contributions from scholars in Taiwan, Russia, and the Czech Republic.

“When we proposed the Daya Bay experiment, people didn’t really focus on this type of research in China at the time,” Luk stated.

At the time, neutrino research wasn’t well developed in China, and the initiative they were suggesting would be quite an extensive endeavor.

Luk mentioned that after reaching out to several European colleagues for the collaboration, they received no replies. Later, Luk found out this was due to the belief that “it wouldn’t be feasible in China.”

Although organizing a massive experiment involving partners from various nations wasn’t simple, both universities and funding organizations recognized the importance of the initiative back then, as per Luk’s statement.

Among the challenges was getting the power plants to agree. Luk said they eventually got on board because the central government had identified the experiment as a “very important project for China”.

Luk said that getting the approval from the power plant had been the most challenging and exciting moment in the project.

Construction on the antineutrino detectors and other facilities broke ground in 2007, a year after the project was approved.

The data collection started in 2011, initiating an experiment that significantly broadened our comprehension of neutrinos.

Material substances consist of minute components, with the most fundamental being referred to as elementary particles—a classification that encompasses neutrinos.

Neutrinos are the most prevalent massive particles in the cosmos, generated during processes where atomic nuclei fuse, like solar nuclear fusion, or split apart, akin to what happens in nuclear fission reactions. These elusive particles continuously rain down upon Earth, with roughly 100 trillion passing through each of us every single second.

These particles are extremely small, have negligible mass, and seldom engage with other matter in a strong way — which makes their investigation quite challenging.

The universe contains three kinds of neutrinos: electron neutrinos, muon neutrinos, and tau neutrinos.

Towards the close of the previous century, physicists examining solar electron neutrinos traveling from the Sun to Earth noticed that the quantity of neutrinos was markedly different from predictions, as Luk pointed out.

While examining atmospheric neutrinos—produced as cosmic rays from deep space interact with particles in the atmosphere—scientists discovered that certain anticipated neutrinos seemed to “vanish,” failing to register in terrestrial detection equipment.

The explanation for why this occurred—a mystery that confounded physicists for many years—is a mechanism known as neutrino oscillation. This involves a neutrino initially generated as one type—either an electron, muon, or tau—transforming or oscillating into another type.

In 2015, this finding earned a Nobel Prize in Physics for both Japanese and Canadian physicists, supporting the idea that neutrinos have mass contrary to previous beliefs of them being without mass.

Prior to the revelations about neutrino oscillations, the Standard Model of particle physics—which outlines how particles of matter interact with basic forces—needed neutrinos to have zero mass.

Neutrino oscillations are characterized through “mixing angles,” which are values indicating the likelihood of a neutrino changing from one type to another.

Prior studies involving solar and atmospheric neutrinos contributed to identifying two out of the anticipated three mixing angles. However, the value of the third angle remained undetermined until the Daya Bay experiment provided insights.

Luk mentioned that once he began contemplating how the leftover mixing angle might be investigated, he became intrigued by the notion of employing a nuclear reactor along with a substantial detector to capture the interacting particles.

Just a few days into the data collection process at Daya Bay, they began noticing something unusual.

The team anticipated that the leftover mixing angle—referred to as theta-13—would be quite minuscule, necessitating an experimental duration of several years before they could reach any definitive conclusions.

However, at a meeting held in January 2012, they realised several groups conducting analysis had similar results.

“We started to understand… this is an issue we must treat with seriousness,” Luk stated.

During a collaborative gathering in Hong Kong during the same February, they discovered that they possessed sufficient data to validate their hypothesis.

“We had sufficient data to persuade ourselves, indeed, we discovered a novel type of neutrino oscillation associated with theta-13,” Luk stated.

They swiftly composed a report to inform the global community that they had discovered theta-13 to have a nonzero value, a key finding for comprehending discrepancies in the behavior of neutrinos and antineutrinos during oscillations.

During the big bang, matter and antimatter should have been created in equal amounts, according to the prevailing theory. But if the amounts of matter and antimatter were equal in the universe, they would have “annihilated” each other, leaving only energy, and not the building blocks to form stars, planets and human life.

The leading explanation for this “asymmetry” between matter and antimatter is a phenomenon called the CP violation, which shows that “the behaviour of matter is different from the behaviour of antimatter”, according to Luk.

“If we have this condition, then there’s a chance that we can explain what’s going on in the universe, which is solely made up of matter at present,” he said.

For the CP violation to occur in neutrinos, the theta-13 mixing angle had to be a non-zero value. Confirming this has opened up the possibility of further study, and the Daya Bay results set the stage for the next generation of neutrino oscillation experiments, according to Luk.

Data collection at Daya Bay concluded in 2020. While analysis has continued, it is winding down as collaborators have moved on to other projects.

Luk said he expected the last batch of final results – using nearly a decade’s worth of data – to be released soon, potentially this year.

The team has earned numerous accolades for their work at Daya Bay.

Luk and Wang were awarded the 2016 Breakthrough Prize in Fundamental Physics.

In 2023, the Daya Bay collaboration received the
High Energy and Particle Physics Prize
from the European Physical Society, one of the highest awards in particle physics.

Not only did the Daya Bay experiment result in a significant discovery for the whole realm of physics, but it also raised the profile of experimental particle physics in China.

Luk mentioned that the perception regarding this research shifted in China following the successes of the Daya Bay project and others.

In 2021, Luk came back to the city where he was born, taking up the position of director at the Center for Fundamental Physics within the Hong Kong University of Science and Technology.

Luk assisted in setting up an experimental particle physics program in Hong Kong through his involvement with the Daya Bay collaboration. He mentioned that one of the key factors motivating him to return was his desire to mentor students who were eager about this field.

After Daya Bay, he began working on the US-led Deep Underground Neutrino Experiment (Dune), which seeks to understand different questions in physics, including matter and antimatter asymmetry.

Luk is also working with colleagues from the University of Hong Kong and the Chinese University of Hong Kong to set up a laboratory experiment to study a type of exotic atom called positronium that could be used to understand dark matter.

Around a third of the universe is made up of this unknown material. Luk said that the experiment they were setting up in Hong Kong would not only help train students but also conduct “forefront science”.

Wang, his collaborator from the Institute of High Energy Physics, has gone on to direct the Jiangmen Underground Neutrino Observatory (Juno), which will finish construction this year.

The project intends to gauge the mass hierarchy and oscillation parameters of neutrinos with the aim of shedding light on the origins of the universe and understanding how supernova explosions occur.

Luk observed that the Juno project involved numerous collaborators from Europe, marking a significant difference compared to 25 years ago.

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This article originally appeared on the South China Morning Post (www.scmp.com), the leading news media reporting on China and Asia.

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