Exclusive | Neutrinos Have Mass—How the Kamiokande Experiments Built a Nobel Legacy

As Taiwan pours resources into artificial intelligence infrastructure and talent development, Nobel Prize-winning physicist Takaaki Kajita (梶田隆章) is raising a quieter concern: who is paying attention to basic science — the kind of research that demands decades of commitment with no guaranteed return?

"Basic science expands the boundaries of human knowledge," Kajita told Storm Media in an exclusive interview. "Even when there is no immediate application, I hope people understand how important it is."

Kajita, a distinguished professor at the Institute for Cosmic Ray Research at the University of Tokyo and the 2015 Nobel laureate in physics, was visiting Taiwan as part of the Taiwan Bridge Program — a collaborative initiative led by Academia Sinica, National Taiwan University, and other academic institutions together with the International Peace Foundation to foster deep exchanges between Taiwan and leading global scholars. On April 23, he delivered a lecture at Academia Sinica titled "International Cooperation in Basic Science: Sharing Practical Experience."

Neutrinos are the second most abundant particles in the universe and among the most elusive. The Standard Model of particle physics long assumed they were massless. Kajita's research team provided the first experimental confirmation of neutrino oscillation — the phenomenon by which neutrinos change between types as they travel — thereby proving that neutrinos do carry mass. The discovery overturned a foundational assumption of the Standard Model and earned Kajita the Nobel Prize in Physics jointly with Arthur McDonald in 2015.

The scientific lineage behind Kajita's discovery runs through a disused zinc mine in Hida, Gifu Prefecture, Japan, where shafts descend more than 1,000 meters underground. That location houses both the Kamiokande and Super-Kamiokande neutrino detectors — facilities whose data have supported two Nobel prizes in physics: Masatoshi Koshiba's in 2002 and Kajita's in 2015.

The Kamioka experiment is Japan's flagship basic science program. Building on the success of the first-generation Kamiokande, the Japanese government funded the Super-Kamiokande at a cost of approximately $100 million; it began operation in 1996. Placed deep within the bedrock to shield it from external interference, the highly sensitive detector became a uniquely powerful instrument for probing the fundamental particles of the universe.

To further investigate the relationship between neutrinos and the origins of cosmic matter, Japan is now constructing the next-generation Hyper-Kamiokande detector, at an estimated cost exceeding 65 billion yen. Scheduled to begin operation in 2028, it will be the largest neutrino detector ever built.

Kajita's doctoral supervisor, Masatoshi Koshiba (小柴昌俊), won the Nobel Prize in Physics in 2002. Koshiba's own supervisor, Sin-Itiro Tomonaga (朝永振一郎), won the same prize in 1965. Kajita's award in 2015 completed an unbroken chain of Nobel laureates across three academic generations — an unprecedented distinction in Japanese scientific history.

Now 67 and visibly energetic, Kajita received Storm Media for an exclusive interview following his lecture at Academia Sinica, discussing his intellectual journey, the people who shaped him, the neutrino breakthrough, and the imperatives of international scientific cooperation.

Kajita traces his commitment to physics to a specific moment early in his graduate career. In 1981, as a new master's student, he joined the Kamiokande experiment — then a small operation of roughly ten people, where everyone had to contribute across the board. In his first year of doctoral study, he was required to work on-site, deep underground, helping to build the detector itself.

"I really enjoyed that work," he said. "I thought the data Kamiokande would collect could make a major contribution to physics. I was very excited about the experiment and had high expectations. That was when I decided to become a physicist."

The question of how the Kamioka experiment originally secured government support came up during his Academia Sinica lecture. Kajita explained that in the beginning, much of the Japanese scientific community believed observing proton decay was a critical problem. Universities including the University of Tokyo and multiple other institutions participated enthusiastically and actively sought private funding. The Japanese government was not initially persuaded.

Because the experiment required access to the Kamioka mine as well as government funding, a formal proposal was submitted — though the initial budget was very limited. "The scientific community believed the project was very important and tried to persuade the government to support it and get it started as soon as possible," he said.

From his entry into the Kamioka experiment in 1981 until the publication of his breakthrough neutrino findings in 1998, Kajita spent seventeen years working toward a discovery he could not have anticipated. When he presented the results at an international conference that year, they caused a sensation.

The research drew on neutrinos produced when cosmic rays interact with Earth's atmosphere, tracking those particles as they traveled through the planet toward the Super-Kamiokande detector in Japan. The data showed that neutrinos changed their type in transit — the first confirmation of neutrino oscillation, and therefore proof that neutrinos have mass, directly contradicting the Standard Model's long-held assumption.

Kajita described neutrinos as particles similar to electrons but without electric charge — among the most fundamental constituents of the universe. They pass effortlessly through the sun and the Earth, carrying information about the basic processes at stellar cores. They come in three types: electron neutrinos, muon neutrinos, and tau neutrinos. And for a long time, they were assumed to have no mass at all.

The path to the Nobel-winning discovery began not with a dramatic hypothesis but with a stubborn discrepancy. "We were very lucky to be part of the Kamioka experiment," Kajita said. "The original purpose was to search for proton decay. To detect proton decay, you have to understand the background — which meant neutrinos. We had to separate the proton decay signal from the neutrino background, and that required improving our data analysis. Eventually, we found that the neutrino data we were collecting didn't match our simulations."

Scientists routinely encounter anomalies, he noted; finding the reason behind them is often where breakthroughs lie. "At the time, we didn't know what was causing it. But I thought it was an important problem, and I decided to investigate further and find the answer."

Kajita completed his doctorate at the University of Tokyo in 1986, the same year he began working as a research associate under Koshiba. It was during this period that he first identified the discrepancy in the neutrino data and resolved to pursue it — a turning point he regards as decisive.

When the Kamiokande data refused to align with simulations, Kajita's first instinct was not excitement — it was suspicion of his own results. He spent a full year methodically checking for mistakes. "When I saw the data didn't match expectations, I thought perhaps we had made an error," he said. "I kept checking, trying to find what had gone wrong. After spending a whole year carefully going through everything, I couldn't find any."

Even then, he was reluctant to declare the data sound. "I thought, maybe it isn't our mistake after all — but I still couldn't be one hundred percent certain our data was correct," he said with a laugh. "At that point I was almost sure it wasn't our error, but I remained very cautious, because humans always make mistakes, and those mistakes are not always easy to find."

Neutrinos travel at close to the speed of light, pass through virtually all matter, and remain among the most difficult particles to detect. The discovery that they carry mass — however small — opened new questions that the Standard Model cannot yet answer.

"We found that neutrinos have mass, but compared to other particles, that mass is extremely tiny," Kajita said. "We need to understand this further. The current Standard Model cannot explain it. I believe the extremely small mass of neutrinos is a key to understanding the fundamental laws of the universe."

In 2010, Kajita shifted part of his research focus to gravitational waves — another frontier of fundamental inquiry, where observations can shed light on the nature of gravity and illuminate extreme cosmic events such as black hole mergers, neutron star collisions, and supernova explosions. Asked how one sustains commitment to basic science that may take decades to yield results — if any — his answer was characteristically understated: "At the beginning, I simply wanted to understand why the neutrino data didn't match what we expected."

On the broader value of basic science, Kajita was direct. "We were very fortunate to discover neutrino oscillation and prove that neutrinos have mass," he said. "The purpose of this research is to expand what humanity knows. I hope people can understand that basic science is very important." Asked about potential future applications of neutrino research, he shook his head: "I don't know."

When asked which person had most influenced his life, his answer was immediate: his doctoral supervisor, Masatoshi Koshiba. "His influence on me was the greatest — his thinking, his approach to science, his attitude. I was very lucky to have such a good mentor, and I benefited enormously."

The Kamioka experiment began taking data in July 1983, initially aimed at detecting proton decay. When proton decay proved far rarer than theoretical models had predicted and the detector failed to register it, Koshiba redirected the program toward solar neutrino detection. The team spent three years upgrading Kamiokande's sensitivity. Japan lacked the necessary detection technology at the time; critical contributions came from the University of Pennsylvania. By January 1987, the upgraded detector was ready.

One month later, in February 1987, a supernova exploded in the Large Magellanic Cloud — the first visible to the naked eye in roughly four centuries. "The detector immediately proved its worth," Kajita said. "It detected ten neutrino events from that explosion." The data Koshiba gathered — humanity's first direct detection of cosmic neutrinos — represented a foundational contribution to astrophysics and earned him the Nobel Prize in 2002.

Kajita recalled how Koshiba responded to those who called him lucky: "Many people said I was lucky. But when the cosmic neutrinos arrived at Earth, only those who were prepared could capture that data."

That the Kamioka experiment — originally designed to observe proton decay — ultimately detected supernova neutrinos and later enabled the discovery of neutrino oscillation was not accidental, in Kajita's telling. He emphasized one of Koshiba's guiding principles: "Keep many research ideas ready. Some of them may hatch successfully in the future."

Asked what he considers the world's greatest challenge, Kajita pointed to climate change. "One of the biggest challenges facing the world today is climate change, and the most urgent priority is reducing carbon dioxide emissions," he said. "But there are still many people unwilling to accept it. We need more and stronger scientific evidence to demonstrate global warming. We cannot persuade everyone, so we must work harder to produce stronger evidence and more precise predictions about the future. We need to encourage everyone to engage with this problem."

On the scientific side, Kajita noted that as detectors have grown in scale and complexity, the need for international talent has grown with them. The Hyper-Kamiokande project, currently under construction — a detector 72 meters tall and 68 meters in diameter — will involve approximately 650 experts from 23 countries and regions.

"This project requires international collaboration," Kajita said. "We need the brightest minds from every country. We need the best talent working together to explore the great questions of the universe."

To young people considering a career in science, his advice was straightforward: "Based on my own experience, basic science is exciting. I hope young people who are interested in science will have the courage to pursue a scientific career and become scientists."

Reflecting on more than 45 years of research — from neutrinos to gravitational waves through the KAGRA detector project — Kajita said the excitement has not diminished. "Throughout the research process, there have been many thrilling moments. Basic science is genuinely exciting. And some scientific results and discoveries are very important for the future of humanity."

For Kajita, a farmer's son from Saitama who grew up to overturn a pillar of modern physics, the absence of obvious practical application has never been the point. The opportunity to expand what humanity knows about the universe, he said, is a contribution in itself. （Related： Exclusive | Nobel Laureate Urges Taiwan to Ease Organ Laws, Eye Cross-Strait Exchange ｜ Latest ）

​Original Article in Chinese​