I was born on March 9, 1959, in Higashi-Matsuyama, a small city located about an hour’s train ride north of Tokyo. My house was located in the countryside, surrounded by rice fields on the north, east, and south. I grew up in such a peaceful environment.
I went to Kawagoe High School, a rather typical small-town school. This school had a tradition of allowing students to do whatever they liked rather freely. Therefore, I spent a lot of time practicing Kyudo (Japanese archery). I was not particularly good at Kyudo, but I liked it. During one’s time as a high school student, you have to decide what you intend to study as an undergraduate once you are admitted to a university. Since I was interested in physics as a high school student, my choice was rather clear: I decided to learn physics in the undergraduate course at Saitama University, a local university near Tokyo.
During my four years of undergraduate education at Saitama University, I continued to play Kyudo, even more seriously than during the high school. I regret that I should have learned more physics during my undergraduate studies, because these studies in undergraduate courses form the basis of everyday research. In any case, I found that physics was indeed interesting. So I decided to continue to studying physics at the graduate level.
I was particularly interested in experimental particle physics. Very fortunately, Professor Masatoshi Koshiba accepted me as a graduate course student in his group at the University of Tokyo. My life as a graduate course student began in April of 1981. Katsushi Arisaka was also a student in Prof. Koshiba’s group. He had just finished his Master’s thesis based on a Monte Carlo study of a nucleon decay experiment. This was the Kamioka Nucleon Delay Experiment (Kamiokande). He was the only student working on Kamiokande in early 1981. Just when I started my studies, production of newly developed photomultiplier tubes (PMTs) with a diameter of 50 centimeters began. Katsushi Arisaka convinced me that Kamiokande would be a very interesting experiment and asked me to work on it, which I started to do.
At that time the other main members of Kamiokande were Teruhiro Suda from the Institute for Cosmic Ray Research (ICRR) of the University of Tokyo, and Atsuto Suzuki and Kasuke Takahashi from the High Energy Accelerator Research Center (KEK). Soon after I joined the Kamiokande experiment, Yoji Totsuka returned from Deutsches Electron Synchroton (DESY) and started to help us. Soon he too joined Kamiokande. Kazumasa Miyano from Niigata University and Tadashi Kifune from ICRR joined during the preparation stage of the experiment. Also, Masayuki Nakahata, who was an undergraduate course student, worked with us.
I enjoyed the preparation work for Kamiokande. In early spring of 1983, we started the construction work on the Kamiokande proton decay detector in Kamioka. It took almost four months to finish building the detector. I liked the construction work, watching the detector being assembled slowly but steadily. After it was filled with water, data taking with the Kamiokande experiment began in early July of 1983.
Soon after the beginning of the data gathering, we came back to Tokyo to search for proton decays in the Kamiokande data. I enjoyed the data analysis as well. I decided to be a professional physicist as a result of experiencing the complete process of preparation, construction, and data analysis of an experiment through my work with Kamiokande.
I received my Ph.D. in March of 1986; my thesis was titled “Search for nucleon decays into anti-neutrino plus mesons.” No evidence for proton decay was observed. In the Japanese system at that time, one way to get a postdoctoral position was to be selected as a postdoctoral researcher by the Japan Society for the Promotion of Science (JSPS). I was not selected. Fortunately, Professor Koshiba offered me a position as a research associate at the International Center for Elementary Particle Physics (ICEPP) of the University of Tokyo for a fixed term of one year. However, I was unable to find a new position in a year, and so I stayed in this position for two years with the very kind understanding of the ICEPP people. I really appreciate their kind decision, because this two year period was the moment that I found the deficit of atmospheric νμ and carried out the initial studies.
In April of 1988, I moved to ICRR as a research associate working on the successor experiment to Kamiokande, the much larger Super-Kamiokande. From that point on I have been a member of ICRR. I was able to work on the atmospheric neutrinos and the preparation of Super-Kamiokande without worrying about finding a job. The construction of Super-Kamiokande was approved by the Japanese government in 1991. People from the USA, most of them from the IMB experiment, joined Super-Kamiokande in 1992. (IMB was a large water Cherenkov detector operated in the USA in the 1980s and in the early 1990s.) Since then, Super- Kamiokande has been an international collaboration. The onsite construction of the Super-Kamiokande detector started in April of 1995. I moved to Kamioka in March of 1995 for the construction of the detector and worked underground for a year. The construction was completed at the end of March of 1996.
After years of planning and construction, the Super-Kamiokande experiment started taking data precisely on schedule, at the stroke of midnight that began April 1, 1996. Since then, I have worked as a convener of the atmospheric neutrino analysis. In the initial stage of the Super-Kamiokande data analysis, analyses were carried out by two independent groups. After confirming that the analyses gave quite similar results, it was decided to merge the two groups into one. Since then, Ed Kearns from Boston University and I have led the atmospheric neutrino analysis. I stepped down from this role when I was appointed the director of ICRR in April 2008.
When I joined Kamiokande, underground experiments were just a very small sub-field of particle physics experiments. At present, after more than 30 years, these underground experiments have become some of the most promising, powerful, versatile, and efficient ways to explore both particle physics and the Universe itself. This research underground continues to stimulate my interest. I look forward to what new discoveries the future will hold.
This autobiography/biography was written at the time of the award and later published in the book series Les Prix Nobel/ Nobel Lectures/The Nobel Prizes. The information is sometimes updated with an addendum submitted by the Laureate.