Giants United: Kam-Biu Luk's Neutrino Discovery Shows What's Possible When the U.S. and China Collaborate

Hong Kong-born physicist recalls honeymoon era of collaboration and 'stroke of luck' that launched Daya Bay Reactor Neutrino Experiment

In the second instalment of a series to mark the 10th anniversary of the Future Science Prize, Victoria Bela looks at Professor Kam-Biu Luk's experimental discovery of a new type of neutrino oscillation, which earned him the 2019 award for physical science. The first part of the series can be found here .

In the quiet cosmic ballet, countless ethereal particles flow through us each second—unseen and untouched, yet carrying clues to nature’s profoundest enigmas. Chief among these are neutrinos, the universe’s most evasive chameleons.

Once upon a time, these tiny subatomic particles became an unexpected link connecting rival countries—China and the United States. About twenty years back, driven by a shared scientific objective and a momentary state of international cooperation, both nations collaborated on a groundbreaking quest known as the Daya Bay Reactor Neutrino Experiment, aiming to unravel a mystery within the realm of quantum physics.

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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 insight into this could provide knowledge about the formation of the universe, the development of matter, and, fundamentally, the emergence of human life.

Led by researchers based in China and the United States, this experiment uncovered a novel type of neutrino oscillation. This breakthrough didn’t just rewrite physics textbooks; it also symbolized a flourishing period of global scientific cooperation that has since slipped from public recollection.

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.

Hong Kong-born Kam-Biu Luk was a professor at the University of California, Berkeley in 2006 when his proposal for the project, which he co-led with Wang Yifang of the Beijing-based Institute of Higher Energy Physics, was approved.

In 2019, their research on neutrino oscillations earned them the Future Science Prize in the field of physical sciences. This private award honors significant scientific accomplishments in the areas of physical science, life science, and mathematics and computer science, covering achievements from the Chinese mainland as well as Hong Kong, Macau, and Taiwan.

The success at Daya Bay was truly fortunate," Kam-Biu Luk stated during an earlier interview. "Maintaining good relations between the U.S. and China played a vital role.

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

After all, I conducted an experiment involving 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 are the antimatter counterpart of neutrinos. Both neutrinos and antineutrinos oscillate, so studying either can yield insight into how oscillation works. Antineutrinos are produced when power plants run, allowing for a more controlled environment in experiments compared to neutrinos, which come from the sun and atmosphere.

By contrasting the antineutrinos observed close to a reactor with those further away, 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 produced by antineutrinos, potentially disrupting detector readings. To mitigate this issue, the experiment should be conducted beneath the surface.

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

Luk scoured the globe for an appropriate 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 suitable for his needs.

Following discussions with associates based in Hong Kong and Beijing, they opted to unite their efforts for the collaboration. This initiative also included contributions from scholars hailing from 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 was not firmly rooted in China, and the initiative they were suggesting would be an enormous 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 people believing "it wouldn’t be feasible in China."

Although organizing this extensive experiment with partners from various nations was "challenging," both universities and funding organizations recognized the importance of the project back then, as per Luk.

One of the difficulties was persuading the power plants to cooperate. Luk mentioned that they ultimately agreed when the central government designated the experiment as a "highly significant project for China."

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

The construction of the antineutrino detectors and other facilities began in 2007, one year following the approval of the project.

If, say, the experiment were proposed now, I don't think it would go anywhere

Kam-Biu Luk

Data collection finally began in 2011, marking the beginning of an experiment that would greatly expand our understanding of neutrinos.

Material substances consist of minute components, with the most fundamental being referred to as elementary particles—an inclusive term for entities such as neutrinos.

Neutrinos are the most prevalent massive particles in the cosmos, generated during processes where atomic nuclei merge, like nuclear fusion within stars, or split apart, akin to what happens in nuclear fissions. Trillions of these elusive entities continually pelt our planet, with roughly 100 trillion passing through each person’s body every single second.

These particles are minuscule, have negligible mass, and seldom engage with other matter in a strong way — which makes their investigation particularly 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 discovered that the quantity of neutrinos was significantly different from predictions, as stated by Luk.

While examining atmospheric neutrinos—created as cosmic rays from outer space interact with particles in the atmosphere—scientists discovered that certain anticipated neutrinos seemed to vanish and were undetectable by terrestrial instruments.

The solution to why this occurred—a mystery that confounded physicists for many years—is a mechanism known as neutrino oscillation. This occurs when a neutrino initially generated as one variety—an electron, muon, or tau—transforms or shifts into a different type.

This finding earned Japanese and Canadian physicists a Nobel Prize in Physics in 2015 and supported the idea that neutrinos have mass, contrary to previous beliefs that they were without mass.

Prior to the observations of 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 by "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 the discovery of 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 reacting particles.

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

The team anticipated that the leftover mixing angle—referred to as theta-13—would be extremely minor, necessitating many years of experimentation before they could reach any significant findings.

At a gathering in January 2012, however, they discovered that multiple teams carrying out analyses had obtained comparable outcomes.

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

At another collaboration meeting held in Hong Kong in February of the same year, they realised they had the data they needed to confirm their hunch.

"We had enough data to convince ourselves, yes, we [did] discover a new kind of neutrino oscillation related to this theta-13," Luk said.

They quickly drafted a report to tell the world they had found theta-13 to be a non-zero value, which was crucial to help understand a difference in behaviour between neutrino and antineutrino oscillation.

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 primary explanation for this "imbalance" between matter and antimatter is a process known as CP violation, indicating that "matter behaves differently than antimatter," as stated by Luk.

He stated that if we possess this condition, there’s a possibility to elucidate the phenomena occurring within our current universe, which is entirely composed of matter.

To observe CP violation in neutrinos, the theta-13 mixing angle needed to have a non-zero value. Establishing this finding paves the way for additional research opportunities. According to Luk, the outcomes from the Daya Bay experiment are setting the foundation for future generations of neutrino oscillation studies.

Securing the Daya Bay project was truly fortunate. Maintaining strong ties between the United States and China played a key role for sure.

Kam-Biu Luk

The data collection phase at Daya Bay ended in 2020. Although analysis continues, it is gradually coming to an end with many collaborators having transitioned to different initiatives.

Luk mentioned that he anticipates the release of the final set of results shortly, which will incorporate almost ten years' worth of data, possibly within this year.

The team has received many awards for their efforts at Daya Bay.

Luk and Wang received the 2016 Breakthrough Prize in Fundamental Physics.

In 2023, the Daya Bay collaboration was awarded the High Energy and Particle Physics Award From the European Physical Society, one of the most prestigious accolades in particle physics.

The Daya Bay experiment was significant not just for advancing the broader realm of physics but also for boosting the prominence of experimental particle physics within China.

Luk mentioned that attitudes toward this research in China shifted following the successes of the Daya Bay experiment and other initiatives.

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

Via the Daya Bay collaboration, Luk played a key role in setting up an experimental particle physics research program in Hong Kong. He mentioned that one of his motivations for returning was to mentor students who were eager to enter this field.

Following his work at Daya Bay, he started contributing to the U.S.-led Deep Underground Neutrino Experiment (DUNE). This experiment aims to explore various unanswered questions in physics, such as the imbalance between matter and antimatter.

Luk is collaborating with fellow researchers at the University of Hong Kong and the Chinese University of Hong Kong to establish a lab experiment focusing on an unusual atom known as positronium. This research aims to utilize positronium for gaining insights into dark matter.

Approximately one-third of the universe consists of this mysterious substance. Luk mentioned that the experiment being established in Hong Kong would serve dual purposes: training students as well as carrying out cutting-edge scientific research.

Wang, who collaborated with researchers at the Institute of High Energy Physics, is now leading the Jiangmen Underground Neutrino Observatory (JUNO). The facility is expected to be completed later this year.

The project seeks to determine 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, highlighting this as a significant difference compared to 25 years prior.

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