Elementary Particle Observation at Keio University Faculty of Science and Technology: Exploring Small Particles and the Laws of the Universe with Large Equipment

Contributing in one's own area of expertise—that is the appeal of particle physics experiments.

A boy who was good with his hands and loved making things became a physicist, drawn to the fundamental principles of the universe. Associate Professor Nishimura is looking forward to gradually unraveling the mysteries of the origin of all things alongside graduate students and young researchers at Hyper-Kamiokande, where observations will continue over the long term.

In this issue, we feature Associate Professor Yasuhiro Nishimura, who seeks to approach the grand theme of the "origin of the universe" through the latest neutrino observations.

To solve the mysteries of the beginning and origin of the universe, an understanding of elementary particles—the smallest components of matter—is necessary. Among them, neutrinos are crucial particles that hold the key to this understanding. In 2028, experiments will begin at Hyper-Kamiokande, a new neutrino observation facility following Kamiokande and Super-Kamiokande. Associate Professor Nishimura is in charge of developing the photodetectors, which can be called the heart of the observation. As one of the leaders of this massive project, he is making meticulous preparations.

"Elementary particles are the smallest elements in the world, but by understanding their behavior, we can verify the laws and origins of the universe on the largest scale," says Nishimura. This world is composed of 17 types of elementary particles with different characteristics (Figure 1), which give rise to matter, forces, and mass.

For example, our bodies, which are matter, are mainly made of three types of elementary particles: up and down quarks, and electrons (leptons). Additionally, gauge bosons are involved in forces, and the Higgs boson is the particle that generates mass. The type and magnitude of force experienced by elementary particles vary by type; in particular, neutrinos (leptons) have extremely small mass and exert almost no force on their surroundings. Hundreds of trillions of neutrinos from space pass through our bodies every second, but most pass right through the Earth as well.

Japanese researchers have taken on the challenge of this extremely difficult neutrino observation. In 1987, they were the first in the world to discover neutrinos emitted by a supernova explosion. Furthermore, in 1998, they observed atmospheric neutrinos generated from cosmic rays entering the atmosphere and discovered that muon neutrinos transform into other types of neutrinos (neutrino oscillation), thereby proving that neutrinos have mass. These discoveries were made using Japan's world-class neutrino detectors, Kamiokande and Super-Kamiokande, located in Kamioka-cho, Hida City, Gifu Prefecture. Dr. Masatoshi Koshiba in 2002 and Dr. Takaaki Kajita in 2015 were awarded the Nobel Prize in Physics for these achievements.

When Dr. Kajita won the Nobel Prize, Nishimura was serving as an assistant professor under Professor Kajita at the University of Tokyo. "I heard the news of the award in the room next to Dr. Kajita's," he says. Nishimura has been consistently involved in elementary particle research. As a graduate student, he researched the unknown phenomenon of muon decay into electrons and gamma rays. This relates to the verification of the Grand Unified Theory, which incorporates the "strong force" that binds quarks together (forming protons and neutrons in the nucleus) into the theory that unified the "electromagnetic force" and the "weak force" (electroweak theory). Later, he became involved in neutrino observation and participated in the T2K experiment, where an artificially produced neutrino beam from the J-PARC accelerator facility in Tokai-mura, Naka-gun, Ibaraki Prefecture, is fired at Super-Kamiokande 295 km away for observation. Furthermore, in 2013, he discovered that muon neutrinos change into electron neutrinos. This confirmed all three types of neutrino oscillations.

This success served as momentum, and the construction of Hyper-Kamiokande, which has higher performance than the previously planned Super-Kamiokande, began (Figure 2). "Neutrino research is led by Japan. In particular, it has very high sensitivity to proton decay. We also aim to discover CP violation using neutrinos," says Nishimura, expressing his expectations for Hyper-Kamiokande.

Proton decay is predicted by the Grand Unified Theory but has not yet been discovered. If discovered, it would significantly deepen our understanding of the origin of all things. It might also reveal the state of the universe after proton decay has progressed. Additionally, CP violation refers to the change in particle behavior under C-transformation (particles and antiparticles like electrons and positrons) and P-transformation (parity transformation, which inverts space like right and left hands). While CP violation has been discovered in quarks, it has not yet been found in leptons. If it can be observed, even the magnitude of the violation could be determined. This might provide a clue to explaining why antiparticles have disappeared from the particles and antiparticles born in pairs at the creation of the universe.

Preparations for Hyper-Kamiokande are underway, aiming for the start of experiments in 2028. The underground water tank will increase from Super-Kamiokande's 50,000 tons to 260,000 tons, and the observation volume will be 8.4 times larger. Furthermore, the performance of the photodetectors (photomultiplier tubes) will double. Since neutrinos cannot be observed directly, photodetectors capture the light (Cherenkov light) emitted by charged particles kicked out when neutrinos collide with water. The amount of light and the shape of the ring are used to determine the neutrino's energy, direction, and type (Figure 3). The photodetectors can be called the heart of the detection device. Nishimura has been entrusted as the leader of this development since 2012.

After various considerations regarding its type and design, it finally became a large photodetector with a diameter of 50 cm. The target of doubling the performance was achieved relatively early, but ensuring pressure resistance, durability, and stability was difficult. Since the water tank depth increased from approximately 40m to 70m, the photodetectors must withstand higher water pressure than before. This was achieved through various innovations in high-strength shapes and covers. Furthermore, more than 20,000 of these photodetectors will be installed. They cannot be easily replaced if they fail. "Just draining the water from the tank takes several months. During that time, a long-awaited supernova explosion might occur. To avoid interrupting the experiment, we were required to ensure they wouldn't fail for decades," he says of the challenges. Additionally, repeated studies were conducted to reduce noise. For example, various improvements were made, such as reviewing the raw materials and manufacturing processes of the glass to reduce impurities. Extensive test measurements were then performed to confirm stability.

In 2018, about 100 units were installed in Super-Kamiokande for observation, demonstrating high performance (Figure 4). It was in 2020 that all detailed performance requirements were finally cleared. Currently, 20,000 units are being manufactured, with installation scheduled to begin in 2027. "Particle physics experiments take a lot of time and cost to set up. 2028 will be a major milestone when it finally starts moving," says Nishimura.

More than 600 researchers are involved in Hyper-Kamiokande. Since it is a massive project that relies on the cooperation of many people, teamwork is crucial. "The ones actually doing the work are the young researchers and graduate students, and they are very active. Since this is long-term research, I want to nurture the next generation." Dr. Kajita participated in research at Kamiokande since his student days. Nishimura has also been involved in Super-Kamiokande since he was a young researcher. Hyper-Kamiokande will undoubtedly leave a brilliant legacy by involving new talent.

(Interview/Composition: Yuko Hiratsuka)

An Interview with Associate Professor Yasuhiro Nishimura

I have loved making things since I was a child. From my kindergarten days, I would make various things out of clay and blocks. I was also very curious and was the type of child who questioned everything, wondering "why?" I was influenced by a biography of Edison that my father bought for me when I was in the first grade, and for a while, I wanted to be an inventor. I liked things that lit up, so I tried connecting miniature light bulbs to batteries, attempted to build motors with craft kits, and borrowed craft books from the library to make switches.

Around the third grade, I was allowed to take a correspondence course in electronic construction and learned how to solder from videos. I loved building everything from scratch by looking at circuit diagrams and made various electronic circuits. Gradually, I started combining electronic circuits or taking things apart and putting them into new containers I had made.

Along with electronic construction, I also started writing programs. We didn't have a home game console, so instead, I programmed games on a PC (NEC's PC-9800 series) to play.

Meanwhile, on TV, I watched the NHK Special "Einstein Roman" with great interest. It was the first time I learned about the theory of relativity, and I was amazed that if you assume the speed of light is constant, the length of an object and the flow of time change when it moves. From there, my interest expanded to elementary particles and quantum mechanics.

In the fifth and sixth grades, I came across manga titled "Dr. Atom's Scientific Exploration" and "Dr. Atom's Theory of Relativity." They were written so that advanced physics was understandable even for elementary school students, and I learned about relativity calculations and covalent bonding of molecules from these books. What I did as a hobby back then has unexpectedly connected to my current research.

I went to Kyoto University with the intention of studying physics. During my undergraduate graduation research, I first encountered particle experiments, such as building particle detectors and scattering electron beams. I became interested in muon decay experiments (the MEG experiment) to explore new physical phenomena, decided to go to graduate school at the University of Tokyo, and conducted research at the Paul Scherrer Research Centers and Institutes (PSI) in Switzerland. I wasn't very good at dealing with large numbers of people, so I avoided the 3,000-person scale experiments at CERN (European Organization for Nuclear Research), but collaborative experiments were unavoidable. Even now, when I immerse myself in electronic construction or programming alone, I feel like I can clear my mind and return to my true self. Since it was an environment where I could do anything, I was able to try various things myself. We observed gamma rays decaying from a large number of muons, and I used photomultiplier tubes that I had soldered myself, lining them up across the entire surface like in Kamiokande. After finishing my doctoral thesis, I was supposed to conduct research at Super-Kamiokande using a neutrino beam from Tokai Village, Naka District, Ibaraki Prefecture. However, in March 2011, just before graduation, the Great East Japan Earthquake occurred, and the beam stopped. As soon as flights resumed and I returned from Switzerland, I worked on analyzing the data collected up to that point in order to publish the first results within a few months, ahead of several other experiments that were progressing globally at the time. Later, I moved to the University of Tokyo Cosmic Ray Research Centers and Institutes in Kashiwa, Chiba Prefecture, and in 2013, I was able to achieve and announce the discovery that "muon neutrinos change into electron neutrinos." In 2012, design research for Hyper-Kamiokande started, and I also began working on "improving the performance of photodetectors," which leads to my current research. It was going to be a long-term study, and since I was already busy with the T2K experiment, I declined the invitation a few times, but I made up my mind on the third time, which led to my current research.

Yes. I moved to Keio in 2019. Hyper-Kamiokande has a large number of researchers and many things to be done. Even before then, I had been conducting research on important photodetectors and detection methods together with graduate students from various universities. I continued this at Keio and finally completed it, and mass production of the photodetectors has just begun. From now on, we must install them and make them actually work. This is also a very difficult task, so I want to do it together with the students at Keio.

At first glance, everyone seems serious and gives a similar impression, but each person has their own particularities and different strengths. They have high potential, so even if they are confused at first, there are sharp questions or unexpected growth, and there is a joy that exceeds my imagination. Various roles are required in particle experiments. In addition to analyzing detection data and simulations, we write programs for equipment design and actually assemble machinery. Furthermore, fine adjustment of the equipment and verification of sensitivity and results are necessary. Since the range of work is very wide, I hope students can conduct research according to their individual personalities and interests.

Students themselves sometimes don't know their own potential, so I think it's good for them to tackle various things at first and then expand their horizons once they've reached a stage where they can do basic things. Ultimately, expertise is required, but I am conscious of guiding them so they can take various detours until then.

Actually, I became interested in contemporary art in university and joined the art club. I enjoyed creating works using various methods, such as paper cutting and sand painting. Even now, when I have time during business trips, I visit art galleries and museums. I also like art festivals held in various regions, such as the "Echigo-Tsumari Art Triennale" in Niigata. It's fun to see works created with ideas I could never have thought of myself, with the entire region serving as an exhibition space.

Also, for some reason, I like dark places and often visit caves and underground facilities. There are vast underground cities in Cappadocia and Naples. I've always wanted to visit the underground bunkers at Keio's Hiyoshi Campus, which housed the Imperial Japanese Navy's Combined Fleet Headquarters during the war. I also went to see the Metropolitan Area Outer Underground Discharge Channel for flood control. It's not that I chose Kamiokande because I like underground spaces, but it is a place where I feel at ease.

In current theoretical physics, various physical models and theoretical systems, including the Standard Model, have been proposed. It is impossible to determine which theory is correct using mathematical methods alone; experimental verification is necessary. Observations of "CP violation" and "proton decay" planned for Hyper-Kamiokande are among these. I hope we can find a guidepost for how to describe this world.

Observation of the universe began with light. However, light cannot reach Earth if it is blocked by matter. Recently, observations of gravitational waves have also begun, but these also refract in places where there is gravity. On the other hand, neutrinos pass through almost everything and are not blocked by anything. Being able to understand distant things means being able to understand things from the past. We can observe things from the time the universe was born right here on Earth.

Supernova explosions have occurred frequently since the early days of the universe, and neutrinos from those times are drifting everywhere. At Super-Kamiokande, we are currently conducting research to see these ancient neutrinos drifting in space without letting them get buried by neutrinos from the sun and other sources.

With Hyper-Kamiokande's significantly improved performance, various things should become clear over the next 10 to 20 years. High school, undergraduate, and graduate students today will be able to conduct research during a very exciting time.

(Interview/Composition: Yuko Hiratsuka)