From Newton to the Hyper-Kamiokande Experiment
For Professor Professor Masashi Yokoyama, now working on the construction of the Hyper-Kamiokande, a massive experiment that will be a worthy successor to the Super-Kamiokande, Newton was the gateway for his career in particle physics research. Not the physicist Sir Isaac Newton, but the science magazine Newton. His parents had subscribed to the magazine for his brother, four years his senior, and Yokoyama started reading it when he was in elementary school and found it exciting and interesting, even though it was full of strange concepts like relativity and quantum mechanics. In junior high and senior high school, he became fascinated with the mysteries of our world through books such as George Gamow’s Mr. Tompkins in Wonderland and Richard Feynman’s Surely You’re Joking, Mr. Feynman!. His keen interest in the fundamental origins of the world and how it works then led him naturally to a career in particle physics.
Hands-on Early Research, Mixing Adhesives
When Professor Hiroaki Aihara returned from the United States and established his own laboratory at the University of Tokyo, he joined a large-scale experiment called the Belle collaboration that was about to begin at the High Energy Accelerator Research Organization (KEK) in Tsukuba. Yokoyama knocked on the door of Professor Aihara’s laboratory, thinking that this might be his only opportunity to participate in a major project from its launch. In the Belle experiment, electrons were collided with their antimatter, positrons, to produce large numbers of particles called B mesons, which contain heavy quarks known as b-quarks. To study the properties of these B mesons, the researchers had to design and build a device called a silicon vertex detector — placed in the center of a huge structure about eight meters square — to precisely measure the position and trajectory of the particles produced by the collisions. Professor Yokoyama, who was responsible for assembling this detector, notes that the first year of his master’s degree began with research on the mixing of adhesives. “I was trying to figure out how long to mix adhesives and how long they should be left to rest before use to get the best results.” He recalls this seemingly un-physicist-like hands-on process as a lot of fun. In the second year of his master’s too, he maintained his hands-on building approach, or perhaps it would be more accurate to say that it went up another level. He spent nearly every waking moment at Hamamatsu Photonics, assembling and testing detectors, not only eating at the company cafeteria but also receiving an employee attendance tag. He was so enthusiastic that a staff member invited him to join the company if he couldn’t make it as a researcher.
Hand-crafted Equipment to Capture Elementary Particles
The following year, in the spring of the first year of his doctoral studies, the Belle experiment began. When the detector Yokoyama had built was able to capture the particles needed to investigate B mesons, he was excited to learn that his handiwork was operating successfully. He leaned forward to say: “This was only just the beginning, but what makes such experiments so rewarding is when the detectors we’ve built ourselves are able to capture elementary particles. The excitement never dims!” He explained the attraction of his research thus: “Theoreticians make sense of the origin of the world via mathematical formulas. We do the same from the data obtained using the equipment we have built. It’s fascinating to be able to grasp the nature of these elementary particles that you can’t detect in everyday life.” Using this detector, he discovered that CP symmetry (a fundamental difference in the properties of particles and antiparticles) was violated in B mesons, thereby verifying the Kobayashi-Maskawa theory, for which the 2008 Nobel Prize in Physics was awarded. “Particle physics provided me with the opportunity to do research in such a fun field; I completely fell in love with it. And that’s how I ended up becoming a researcher.”
Each Member Plays a Key Role in a Massive Project
Experiments in particle physics are massive projects involving hundreds or thousands of researchers, and tpeople may think that each researcher is responsible for only a minor part of the project, and graduate students an even a smaller part. Yokoyama recalls: “Now that I think about it, I was in charge of a key component of the Belle collaboration. Without the detector I made, we wouldn’t have been able to obtain the data that were the main focus of the experiment. I was so lucky to be entrusted with several important tasks, and even though I was only a doctoral student, I believed that the experiment would not have been possible without me. Smiling, he continued: “And, assuming that my experience wasn’t unique, it’s important to remember that each of us has different strengths, including graduate students, and there are always areas where we can make the most of these strengths in experimental work. Therefore, it often happens that a particular part of an experiment wouldn’t be possible without that person. That’s why I try to encourage students who want to join a laboratory or graduate students who are doing research, to play a key role in their research.”
The Three Main Objectives of Hyper-Kamiokande
Professor Yokoyama is currently working on the construction of the Hyper-Kamiokande neutrino detector, a huge experimental facility with a diameter of 68 meters and a height of 71 meters, about the size of a 20-story apartment building. He is still in the process of developing the detector with his fellow researchers and graduate students, thinking of ways to improve the performance of the photomultiplier tubes and coordinating the design of the entire device that is being built in an international collaborative process. The Hyper Kamiokande experiment has three main objectives.
The first objective is to study the phenomenon known as neutrino oscillation, in which a neutrino changes its type in flight. Thorough investigation of this phenomenon is expected to demonstrate that CP symmetry is violated in neutrinos. If neutrinos are found to violate CP symmetry, we will be one step closer to solving the physics mystery of why there were equal amounts of matter and antimatter at the beginning of the universe, but now there is only matter left. The Super-Kamiokande experiment is also exploring neutrino oscillations, but even this huge facility is capable of only detecting a very few neutrinos. It is anticipated that Hyper-Kamiokande will definitely show the differences between neutrinos and antineutrinos.
The second objective is to discover proton decay. The Kamiokande experiment, started by Professor Koshiba, who was awarded the Nobel Prize in 2002, was originally intended to observe the decay of protons, electrically charged particles found in the nucleus of an atom. Protons are stable and do not decay according to the current theories of elementary particles, but the Grand Unified Theories assert that they will eventually decay, albeit with an extremely long lifetime of at least 1034 years, much longer than the age of the universe. Since it would be impossible to watch and wait for a single proton to decay, the strategy is to collect and observe a large number of protons. Super-Kamiokande, the successor to the Kamiokande experiment, has been observing protons for 25 years, but has yet to detect any proton decay. The Hyper-Kamiokande is the third attempt to observe proton decay, and success would constitute a major discovery in particle physics.
The third objective is to use neutrinos to study the universe, with the plan to capture neutrinos originating from supernova explosions. Such explosions that produce detectable neutrinos are believed to occur two to three times every 100 years, but none have occurred since the 1987 event that led to Professor Koshiba’s Nobel Prize. At that time, 11 neutrinos could be observed. Now, if we could capture a similar supernova explosion in Super-Kamiokande, we could observe thousands of neutrinos, and in Hyper-Kamiokande, tens of thousands. This would yield detailed chronological information about the star at the moment of the supernova explosion. Neutrinos can provide new insights not only into elementary particles, but also into the history and evolution of the universe!
Hyper-Kamiokande to Yield Unexpected Findings
“Kamiokande and Super-Kamiokande are both superb pieces of equipment. I was reminded of this when I was planning the design of the Hyper-Kamiokande.” One of the reasons that Super-Kamiokande is still able to continue its world-leading research after 25 years is not only its wide range of applications, from the observation of proton decay to the study of neutrino oscillations and the early universe, but also the fact that it is possible to expand its potential by making improvements to the equipment. For example, in 2008, the data acquisition system was redesigned to make it possible to study phenomena that previously couldn’t be monitored, and in 2020, the rare earth element gadolinium was added to the ultrapure water to improve the sensitivity of our observations. During the course of research with the detector, new ideas and dormant possibilities continued to emerge. There is no doubt that the Hyper-Kamiokande will also find application in a variety of research fields for decades to come. Professor Yokoyama has high hopes for the next generation of research. “I hope that in 10 or 20 years, Hyper-Kamiokande will be able to do research that we, its designers, would never have dreamed of. We will also need new ideas to figure out what kind of research we should do in experiments after Hyper-Kamiokande. I would like to encourage future researchers to try experiments that go beyond our imagination.”
※Year of interview:2021
Interview and text: Naoto Horibe
Photography: Junichi Kaizuka
Illustration and video provided Kamioka Observatory, ICRR(Institute for Cosmic Ray Research), The University of Tokyo