2015 physics laureate Takaaki Kajita is affiliated with Super-Kamiokande (pictured right) and received the Nobel Prize for his neutrino oscillations research which proves that neutrinos have mass.

Few discoveries in modern particle physics have reshaped our understanding of nature as profoundly as the discovery of neutrino oscillations. In 2015, Takaaki Kajita shared the Nobel Prize in Physics with Arthur B. McDonald for demonstrating that neutrinos change flavour as they propagate — revealing that these particles possess mass and pointing beyond the Standard Model.

Kajita’s career has been closely tied to the underground detectors of the Kamioka Observatory in Japan, where experiments such as Kamiokande and Super-Kamiokande transformed neutrino physics. Today, he continues to explore fundamental questions about the Universe through large-scale experiments, including the gravitational-wave observatory KAGRA. In this conversation with EP News, Kajita reflects on the path that led him into physics, the experimental journey that uncovered neutrino oscillations, and the questions that continue to drive research in neutrino physics and beyond.

Panos Charitos: Professor Kajita, when you look back, what first drew you to physics?

Takaaki Kajita: It is actually difficult to identify a single reason. My family had nothing to do with physics or science, so there was no particular influence from that side.

Like many children, I was curious about how things worked, but I cannot point to a specific moment when I decided that I wanted to become a physicist. In fact, I only began to seriously consider this possibility when I was already a graduate student.

As an undergraduate at Saitama University, I knew that I enjoyed physics, especially topics related to elementary particles and the Universe. But at that stage, I was not thinking very concretely about becoming a researcher. That idea developed gradually as I began to work more closely with experiments and realised how fascinating they could be.

Panos Charitos: What were the topics you worked on during your doctoral studies?

Takaaki Kajita: During my graduate studies at the University of Tokyo, I joined the Kamiokande experiment. At that time, the main goal of Kamiokande was to search for proton decay.

This was an extremely important question in particle physics because several Grand Unified Theories predicted that protons should eventually decay, even though the predicted lifetimes were far longer than the age of the Universe.

My PhD thesis focused on searching for particular proton decay channels, specifically processes in which a proton decays into an anti-neutrino accompanied by mesons. Detecting such extremely rare events required a very large detector and very low backgrounds. Kamiokande used a large tank of ultra-pure water instrumented with photomultiplier tubes, allowing us to detect Cherenkov light produced by charged particles in the detector.

Although proton decay itself was not observed, Kamiokande proved extraordinarily fruitful in other ways. Because the detector was also sensitive to neutrinos, it began to reveal phenomena that had not originally been anticipated.

Panos Charitos: Kamiokande produced several unexpected discoveries. Which do you see as the most important?

Takaaki Kajita: Kamiokande was originally conceived as a detector for proton-decay searches, but it quickly revealed itself to be much more than that. In fact, some of its most important contributions came from phenomena that were not part of its primary design goal.

One of the most dramatic moments came in 1987, when Kamiokande detected neutrinos from Supernova 1987A. This was an extraordinary milestone: for the first time, neutrinos from a supernova explosion were observed directly. The result provided powerful confirmation of theoretical models describing how massive stars collapse and explode, and it opened a new observational window on the universe.

Kamiokande also played a crucial role in confirming the deficit of solar neutrinos — the long-standing solar-neutrino problem — which had already emerged from earlier experiments. In addition, it began to register indications that the number of atmospheric muon neutrinos was lower than expected.

At the time, the origin of these anomalies was not yet understood. But it was already clear that they were pointing toward something important, something that did not fit comfortably within the existing picture.

That, perhaps, is one of the enduring lessons of experimental science. An instrument may be built with one goal in mind, yet its most significant discoveries can come from entirely unanticipated directions. Kamiokande is a particularly beautiful example of that principle: a detector designed for one great question ended up helping to transform several others.

Panos Charitos: The next step in this story was Super-Kamiokande. What moment stands out most clearly from that experiment?

Takaaki Kajita: For me, the most exciting moment came when we began examining the zenith-angle distribution of multi-GeV muon neutrino events. What we observed was quite striking. Muon neutrinos that arrived from above — produced in the atmosphere relatively close to the detector — appeared at the expected rate. However, muon neutrinos coming from below, which had travelled through the Earth before reaching the detector, were significantly fewer than expected.

This asymmetry in the zenith-angle distribution was difficult to explain using conventional models of atmospheric neutrino production. The most natural interpretation was that some of the muon neutrinos had transformed into another type of neutrino during their journey through the Earth.

Super-Kamiokande began taking data in April 1996, and by the summer of 1997 we began to consider seriously that the observed pattern might be evidence for neutrino oscillations.

Panos Charitos: How did the collaboration gain confidence that this effect was real?

Takaaki Kajita: The collaboration approached the interpretation with great caution. In our earliest publications, we presented the atmospheric neutrino data but deliberately avoided making a strong claim about the underlying explanation. Only after additional analyses and consistency checks did we interpret the results in terms of neutrino oscillations. In 1998, the Super-Kamiokande collaboration published evidence supporting this interpretation.

Statistics played an essential role in reaching this conclusion. In the earlier Kamiokande experiment, atmospheric neutrino interactions were relatively rare — only about two events per week. In Super-Kamiokande, thanks to its much larger detector volume, the rate increased to roughly ten events per day.

This dramatic increase in statistics allowed us to study the angular distributions of neutrinos with much greater precision, making the oscillation pattern in the data much clearer.

Panos Charitos: Your discovery also helped resolve the long-standing solar neutrino problem.

Takaaki Kajita: From the outset, the Super-Kamiokande collaboration organised dedicated analysis groups for both atmospheric and solar neutrinos, recognising that these were two distinct but equally important lines of investigation.

At the same time, other experiments were tackling the solar-neutrino problem through complementary approaches. Among them, the Sudbury Neutrino Observatory played a particularly important role.

Taken together, these results showed that neutrinos produced in the Sun change flavour on their way to the Earth. This was a profound finding, because it implied that neutrinos must have mass — something not accounted for in the original formulation of the Standard Model of particle physics.

In that sense, the resolution of the solar-neutrino problem did far more than explain an experimental anomaly. It provided clear evidence that the Standard Model, though successful, is incomplete.

Panos Charitos: Looking back historically, what lessons do you draw from the transition from Kamiokande to Super-Kamiokande?

Takaaki Kajita: Kamiokande provided some of the earliest truly intriguing hints, but it had not yet delivered definitive answers. It pointed us in a promising direction, while also making clear the limitations of what could be achieved with the available statistics.

To pursue those hints properly, a much larger detector was needed. In neutrino physics, statistics are absolutely crucial, so increasing the detector volume was not simply an incremental improvement — it was essential to reaching robust conclusions.

In that sense, the transition from Kamiokande to Super-Kamiokande illustrates a broader pattern in particle physics. A first-generation experiment may reveal anomalies or suggest that something important is at work, but it often takes a more powerful and more ambitious successor to establish the picture clearly. That progression — from suggestive indications to convincing evidence — is one of the field's characteristic ways of advancing.

Panos Charitos: What are the most important questions in neutrino physics today?

Takaaki Kajita: Two questions stand out as particularly important for the future of neutrino physics.

The first concerns the possibility of CP violation in the neutrino sector. If neutrinos exhibit CP violation, it could provide an important clue to one of the most profound puzzles in cosmology: why the Universe today is dominated by matter rather than antimatter. Some theoretical scenarios suggest that neutrino processes in the early Universe may have contributed to generating this asymmetry, so experimental evidence of CP violation in neutrinos would be a major step toward understanding this question.

The second question concerns the fundamental nature of neutrinos themselves. It remains possible that neutrinos are Majorana particles, meaning that they are identical to their own antiparticles. If this is the case, it would have profound implications for particle physics, particularly for mechanisms that generate neutrino masses and for theories that extend beyond the Standard Model.

Addressing these questions will require very sensitive experiments and represents one of the central challenges in particle physics today.

Panos Charitos: Which experiments could address these questions?

Takaaki Kajita: Next-generation long-baseline neutrino experiments will play a central role in exploring these issues. Two major projects currently under construction are Hyper-Kamiokande in Japan and the Deep Underground Neutrino Experiment in the United States. Both experiments aim to measure neutrino oscillations with unprecedented precision and to search for signs of CP violation.

Hyper-Kamiokande, the successor to Super-Kamiokande, is expected to begin data taking around 2028. One of the most demanding engineering challenges for the project is excavating an extremely large underground cavity capable of housing the detector. Such massive underground infrastructures are necessary to shield the experiment from cosmic radiation while providing the large target mass required for studying rare neutrino interactions.

The detector itself will also incorporate significant technological improvements. In particular, Hyper-Kamiokande will use a new generation of photomultiplier tubes with higher photon-detection efficiency and improved timing resolution, enabling the experiment to reconstruct neutrino events with greater accuracy than in Super-Kamiokande.

Together with other experiments searching for neutrinoless double beta decay, these projects will help clarify whether neutrinos violate CP symmetry and whether they are Majorana particles—two questions that lie at the heart of the next phase of neutrino physics.

Panos Charitos: What role might neutrinos play in understanding the early Universe?

Takaaki Kajita: Because neutrinos have very small masses and interact only weakly with other particles, they could have influenced processes in the early Universe in ways that are still not fully understood.

However, we now know that the known neutrinos cannot account for the main component of dark matter. Their masses are simply too small for that. This means that the nature of dark matter remains one of the major open questions in physics.

To address this problem, we need several complementary approaches. Collider experiments may produce new particles associated with dark matter, underground detectors can search directly for interactions of dark matter particles, and astrophysical observations may reveal signals produced by dark matter annihilation or decay. Each of these methods provides a different window on the same problem.

Panos Charitos: You are also involved in gravitational-wave astronomy through KAGRA.

Takaaki Kajita: I am currently working on the KAGRA experiment. KAGRA joined the global network of gravitational-wave detectors somewhat later than LIGO and Virgo, but it will play an important role as the network continues to improve its sensitivity.

One of the key strengths of a global network of detectors is that it allows us to determine the direction of gravitational-wave sources much more precisely. With only one detector, it is essentially impossible to localise the origin of a signal, but when several detectors observe the same event, we can triangulate its position in the sky. This is extremely important because it enables follow-up observations with electromagnetic telescopes and other instruments. In this way, gravitational-wave detectors become part of a broader effort in multi-messenger astronomy.

Panos Charitos: What advice would you give young researchers entering physics today?

Takaaki Kajita: Physics remains an extremely exciting field, and there are still many fundamental questions waiting to be answered.

My advice to young researchers would be to follow their curiosity and maintain a strong interest in basic science. Sometimes research progresses slowly, and important discoveries may take many years to emerge. But when they do, they can profoundly change our understanding of nature. For that reason, it is important to remain patient and continue exploring fundamental questions.

Professor Takaaki Kajita, Institute for Cosmic Ray Research of the University of Tokyo, received the 2015 Nobel Prize in Physics. Devotedly carrying on the research pioneered by Professors Masatoshi Koshiba and Yoji Totsuka, he achieved great success with his discovery of neutrino oscillations, which shows that neutrinos have mass. This groundbreaking research expands humanity's horizons, connecting the tiniest elementary particles to the vastness of the universe. 

Panos Charitos: Finally, what went through your mind when you received the phone call announcing the Nobel Prize?

Takaaki Kajita: To be honest, my first thought was simply that the phone call had come from an unexpected source. Everything happened very suddenly, and at that moment, it was difficult to fully understand what was happening.

Later, however, I realised that receiving the Nobel Prize also brings a certain responsibility. It provides an opportunity to speak more broadly about science and about the importance of supporting basic research. It is important to use that opportunity to encourage curiosity-driven science and to explain why fundamental research matters for society.

Editor’s note: We would also like to pay tribute to Yoji Totsuka, whose leadership was instrumental in the construction and recovery of Super-Kamiokande. Recalling the difficult period following the 2001 accident, Takaaki Kajita said after receiving the Nobel Prize: “The most difficult period of my research life was the accident. It was under very tough circumstances, and we were able to overcome the difficulties thanks to the leadership of Dr. Totsuka. I want everyone to remember this.”