Elementary Particle Observation from Keio's Faculty of Science and Technology Using enormous equipment to explore tiny particles and the laws of the universe

Observing Neutrinos with Hyper-Kamiokande

Unraveling the Mystery of the Origin of the Universe and the Theory of Everything

“Subatomic particles are the smallest elements in existence,” says Nishimura. “By understanding their behavior, we can parse the laws behind the very universe itself and verify how it came to be.” Our world is made up of matter, force, and mass[1.1][YN1.2] created by 17 subatomic particles with different characteristics (Figure 1).
To give an example, the matter of our bodies is mainly made up out of three types of particles: up quarks, down quarks, and electrons (a type of lepton). Likewise, there are gauge bosons that act as force carriers and Higgs bosons that produce mass. Different particles are subject to different types and magnitudes of forces, especially neutrinos (another type of lepton), which have very low mass and exert minimal forces on their surroundings. While hundreds of trillions of neutrinos come into contact with us every single second from space, most of them pass through the earth.
This is where certain Japanese researchers have taken on the challenge of observing these elusive particles. In 1987, they were the first in the world to discover neutrinos that had been emitted from supernova explosions. They went on to observe atmospheric neutrinos produced by cosmic rays entering the earth’s atmosphere and even found that muon neutrinos could change into other flavors (types) of neutrinos, a process known as neutrino oscillation, thereby proving that neutrinos have mass. These discoveries were made possible by Japan’s world-class subatomic particle detectors, Kamiokande and Super-Kamiokande, in Gifu Prefecture and earned Dr. Masatoshi Koshiba and Dr. Takaaki Kajita the Nobel Prize in Physics in 2002 and 2015, respectively.

When Dr. Kajita was awarded the Nobel Prize, Nishimura was working under him as an assistant professor at the University of Tokyo. He heard the news of the award from the next room over. Nishimura has been a core researcher involved in elementary particle research over the years. As a graduate student, he focused on the little-understood phenomenon of muons decaying into electrons and gamma rays. His work tested the Grand Unified Theory, which seeks to incorporate the “strong” force that binds the quarks making up protons and neutrons in atomic nuclei into the theory that unifies the “electromagnetic” and “weak” forces, known as the electroweak theory. After this project, he got involved in neutrino observation research, participating in the T2K experiment which involved beaming artificially produced neutrinos from the J-PARC accelerator facility in Ibaraki Prefecture to Super-Kamiokande over 295 kilometers away. Then, in 2013, his team discovered that muon neutrinos can oscillate into electron neutrinos. This was the final step in ascertaining that all three types of neutrino oscillation were possible.
This breakthrough provided the impetus for beginning construction on the long-considered Hyper-Kamiokande as a more advanced successor to the Super-Kamiokande (Figure 2). In discussing his expectations for Hyper-Kamiokande, Nishimura explains, “Japan is leading the way in research on neutrinos. Our detectors are especially sensitive to proton decay. We are also hoping to discover CP violations through neutrino observations.”
Proton decay is predicted by the Grand Unified Theory but has not yet been observed. If researchers are able to confirm this phenomenon, it will greatly enhance our understanding of the origin of the universe and matter. It may also provide insight into what the universe will look like after proton decay progresses. In addition, CP violations refer to the phenomenon in which a particle’s behavior changes under a combination of C transformations (charge conjugation, swapping particles with their antiparticles, such as electrons and positrons) and P transformations (parity inversion, reflecting spatial coordinates like switching left and right). While scientists have discovered such CP violations in quarks, they have yet to observe them in leptons. If researchers are able to detect them, they could determine how large the violations are. It may also provide clues as to why the antiparticles that were created alongside particles at the birth of the universe have now disappeared.

Preparations are currently underway for Hyper-Kamiokande, with the facility becoming fully operational for experiments in 2028. The underground water tank will grow from Super-Kamiokande’s 50 kilotons to 260 kilotons, an 8.4-fold expansion in detection volume. Likewise, the performance of the photodetectors (photomultiplier tubes) will be doubled. Because neutrinos cannot be observed directly, photodetectors sense the Cherenkov light produced when neutrinos collide with water. The brightness and pattern of the resulting light rings allow researchers to infer the neutrinos’ energy, direction, and type (Figure 3). In a way, the photodetectors in this machinery are the heart and soul of the entire system. Nishimura has been tasked with leading this development process since 2012.

After countless rounds of experimentation and prototyping, they have settled on a larger photodetector design that has a diameter of 50 cm. While this model quickly achieved the doubled performance that the researchers were targeting, they had difficulty ensuring that the detectors would be pressure resistant, durable, and stable. The water tanks have increased in depth from 40 m to 70 m, meaning that the photodetectors must be able to withstand much higher levels of pressure than in the past. Different shapes and protective housings were devised to withstand the new requirements. Additionally, the new system requires the installation of over 20,000 photodetectors, meaning that it will be difficult to replace them should they get damaged. Speaking on some of the time effort that has gone into this process, Nishimura explained, “It takes months just to drain the tanks. And who knows? While we’re doing that, we might miss a long-awaited supernova explosion. We had to ensure that the equipment was reliable for at least ten years so that experiments won’t be interrupted.” The researchers also ran a number of studies to figure out how to reduce noise. For instance, the team has examined the raw materials that go into glass during the manufacturing process to reduce potential impurities and improve the final product. They then conducted extensive testing to confirm the stability of the final product.
In 2018, the team was able to demonstrate proof of concept by inserting approximately 100 of these photodetectors into Super-Kamiokande for observation. However, it wasn’t until 2020 that they were able to fully meet all of the final performance parameters. Currently, 20,000 units are being manufactured and are scheduled to be installed in 2027. “Particle experiments are extremely time-consuming and expensive to set up. 2028 is going to be a major milestone once everything finally gets underway,” said Nishimura.
Over 600 researchers have been involved in the development of Hyper-Kamiokande. With so many people working together to make this project a success, good teamwork has been an essential ingredient. “The actual hands-on work is mostly handled by junior colleagues and graduate students, so they are playing very active roles in this process. This is a long-term research endeavor, so I really want to leverage the longevity of this project to help educate and train the next generation of researchers.” As a student, Dr. Kajita participated in the original development of Kamiokande. Similarly, Nishimura was involved with Super-Kamiokande early in his career. Hyper-Kamiokande is certain to draw in bright minds and continue building on this distinguished legacy.

I loved making and building things from a young age. Even in kindergarten, I remember making structures out of clay and blocks. I was the curious kid who wanted to know the “why” behind everything, always asking questions. In first grade, my father bought me a biography of Thomas Edison, so there was a long period of time when I wanted to become an inventor. I liked things that light up, so I would connect miniature bulbs to a battery, tried to make a motor using a kit, borrowed books on building and crafting from the library, and learned how to make switches.
Around third grade, I had my parents enroll me in an electronics correspondence course where I learned how to solder by watching instructional videos. I liked to look at electrical circuits and build them from scratch, so I tried my hand at various types. As time went on, I began to put different circuits together, take electronics apart and try to build my own by putting them in new casings that I had made.
In addition to my work with electronics, I also started to learn how to program. My family didn’t have a gaming console back then, so instead, I used to write my own games on our computer (an NEC PC-9800 series).
Another thing I remember was my obsession watching the NHK Special, Romantic Einstein. When I first learned about the theory of relativity, I was astonished by the idea that if the speed of light remains constant, an object’s length and the passage of time itself change as it moves. From there, my interest broadened to subatomic particles and quantum mechanics.
In fifth or sixth grade, I came across some manga called Dr. Atom’s Scientific Explorations and Dr. Atom’s Theory of Relativity. These volumes presented advanced physics in a way that even elementary school students could understand, and it introduced me to things like relativity calculations and molecular covalent bonds. Looking back, the things I once did for fun as a kid have, unexpectedly, have come full circle with the research I do today.

That’s right. I came to Keio in 2019. Hyper-Kamiokande involves a large number of researchers, and there is still a great deal of work to be done. Even before this, I had been conducting important research on photodetectors and detection techniques together with graduate students from various universities. At Keio, I have continued this work and finally completed it. Now, the photodetectors are at the stage where we are mass-producing them. In the future, we will work on installing them and making sure that things function on site. This is a very intensive process, so I am looking forward to doing it alongside the students at Keio.

At first glance, everyone is a serious student and gives off a similar impression, but on closer look they each have their own quirks and strengths. There’s endless potential here. Even if they don’t know what to do at first, they ask sharp questions, grow in unexpected ways, and have fun beyond their wildest expectations.
　Particle experiments require a variety of roles. Not only do you need people who can analyze data and run simulations, you need people who can write programs for the equipment and actually put together the machines. You also need people who are extremely detail oriented when doing the delicate work of adjusting the equipment and testing their sensitivity and output. The range of work is incredibly broad, and I love how this type of research project lets each individual and interest shine.

Most students have no idea what they are capable of. I think it’s best for them to start by trying out a variety of things, build up their basic skills, and then broaden their horizons from there. Eventually they will all need to choose their specialties, but my goal is to guide them to try different outlets and take a more scenic route to their destinations.

Actually, back in college I was very interested in modern art and even joined an art club. I made a variety of pieces using different media, including paper-cuttings and sand paintings. Even now, when I have some spare time on business trips, I will stop by local art or science museums. I also like the Echigo-Tsumari Art Triennale in Niigata and other regional art festivals. The entire area becomes part of the exhibition, and I enjoy seeing how creative the art pieces are, showcasing ideas I never would have imagined on my own.
Also, for some reason I tend to enjoy dark places, so I will often visit caves and other underground sites. Cappadocia and Naples have huge underground cities. I’ve always wanted to visit the underground tunnels beneath Keio’s Hiyoshi Campus, where the Imperial Japanese Navy’s Combined Fleet Headquarters and other command posts were located during World War II. I have also been to see the Metropolitan Area Outer Underground Discharge Channel that helps prevent flooding in Tokyo. I didn’t choose to get involved with Hyper-Kamiokande because I like to be underground, but I have to admit I find the atmosphere calming.

Observations of the universe began with light. However, light cannot reach the earth if it is blocked by matter. In recent years, gravitational waves have also begun to be observed, but they too are deflected by gravitational fields. Neutrinos, on the other hand, are able pass through most forms of matter unobstructed. To understand something that has traveled from far away is to understand the past. We can observe things on earth that are from when the universe was formed.
There were large numbers of supernova explosions in the early days of the universe, and the neutrinos from these phenomena have drifted to various places. At Super-Kamiokande, we are currently conducting research aimed at detecting ancient neutrinos drifting through the universe, while avoiding interference from neutrinos originating from the Sun and other sources.
With the Hyper-Kamiokande’s vast performance improvements, we should see breakthroughs in a variety of studies in the next ten to twenty years. In other words, it’s an incredibly exciting time for high school, college, and graduate students to get involved in this research.

● I came to Keio from Taiwan to study proton decay signals. Professor Nishimura has given me a ton of freedom to pursue this topic. (3rd year Ph.D. student)
● Professor Nishimura’s active role with Super-Kamiokande and Hyper-Kamiokande puts him at the cutting edge of research on subatomic particles. I’m currently involved with the photodetectors, and Professor Nishimura is great about giving me clear feedback and advice about the trajectory of my research during the daily meetings. He always takes the time to answer my questions and is very kind, which allows me to be comfortable while conducting my research (1st year master’s student).