From Director

I would like to extend my greetings at the beginning of FY2022 in Japan.

In April 2022, I became the director of the Institute for Cosmic Ray Research (ICRR), succeeding Professor Takaaki Kajita. I am committed to doing my best for ICRR.

“Cosmic rays” are particles that travel from the universe with high energy. Since they were discovered in 1912, various research has been carried out with the aim of elucidating their origins and acceleration mechanisms. In the initial cosmic ray research, “cosmic rays” were charged particles such as protons and helium nuclei, but in recent years, uncharged elementary particles such as neutrinos, gamma rays and gravitational waves (gravitons) are also considered to be “cosmic rays,” broadening the scope of cosmic ray research. Since cosmic rays travel over long distances, larger than the size of the earth, and are accelerated to extremely high energies, they facilitate research on elementary particles that cannot be conducted in accelerator experiments on the ground. ICRR conducts fundamental research on the universe and elementary particles using cosmic rays.

The history of ICRR began with a cosmic ray observation hut called Asahi Hut on Mt. Norikura at an altitude of 2,770m. This small hut, built in 1950 with science project funding from the Asahi Shimbun, developed into the Cosmic Ray Observatory (commonly called Norikura Observatory) of the University of Tokyo in 1953. It was the first inter-university research facility in Japan. The Cosmic Ray Observatory was then reorganized to become the Institute for Cosmic Ray Research (ICRR) of the University of Tokyo in 1976. Since then, ICRR has carried out various research activities on cosmic rays as an inter-university research institute.

ICRR has three research divisions—the Neutrino and Astroparticle division, the High Energy Cosmic Ray division, and the Astrophysics and Gravity division—in order to conduct these scientific activities effectively. ICRR has four observatories in Japan; Kamioka Observatory (underground in Kamioka, Gifu Prefecture), KAGRA Observatory (underground in Kamioka, Gifu Prefecture), Norikura Observatory (2,770 meters alt., Mt. Norikura, Gifu Prefecture) and Akeno Observatory (Yamanashi Prefecture), and one research center (Research Center for Cosmic Neutrinos, Kashiwa, Chiba Prefecture). In addition, there are four observation facilities outside Japan, located in Utah, USA, Yangbajing in Tibet, China, High Energy Astrophysics Facility in Canarias (La Palma, Spain), and Chacaltaya, Bolivia. ICRR is an “International Joint Research/Usage Center,” which is a system of the Japanese government, and over 150 inter-university research programs have been carried out at ICRR by cosmic ray researchers in Japan and other countries. We note that most of the scientific outcomes from this institute are the results of the collaborative efforts by researchers from many institutions around the world. Great results have been achieved through this joint research. Please see the individual research group websites for more details.

In recent cosmic ray research at ICRR, observations using large-scale equipment have been carried out in order to make highly sensitive observations. At Kamioka Observatory, observations are being carried out using a 50,000-ton water Cherenkov detector (Super-Kamiokande) installed 1000 m underground, observing neutrinos produced by cosmic ray interactions in the atmosphere and neutrinos from the sun. In addition, an artificial neutrino experiment from Tokai to Kamioka (T2K) is also conducted. In the same mountain at Kamioka, a laser interferometer (Large-scale Cryogenic Gravitational Wave Telescope (KAGRA)) with an arm length of 3 km was constructed for gravitational wave observation. Large-sized Telescopes (LSTs) with a diameter of 23 m are being constructed at La Palma, one of Spain’s Canary Islands for the Cherenkov Telescope Array (CTA) project for the observation of very high energy gamma rays. The first telescope has already started observations. In Utah, USA, particle detectors are placed on about 700 square kilometers (expanding to about 3000 square kilometers) to search for the highest energy cosmic rays. In this way, observations of various cosmic rays have been conducted with surprisingly large-scale experimental equipment in ICRR.

Super-Kamiokande (SK) discovered atmospheric neutrino oscillations in 1998 and Professor Takaaki Kajita, who led the research, received the Nobel Prize in Physics in 2015. Since then, SK has achieved results such as “discovery of solar neutrino oscillation” in 2001 and “discovery of third oscillation by T2K” in 2011, and has elucidated the mass and mixing of neutrinos. If the “CP phase angle” is measured, the whole picture of the neutrino mass-mixing structure will be clarified (except for the absolute value of mass). This CP phase angle is more than just a property of elementary particles. If it has a finite value, it solves the “matter-dominant mystery” of why “matter” exists but “antimatter” does not exist in the universe. To measure the CP phase angle requires a next-generation experiment (Hyper-Kamiokande), which is about ten times larger than Super-Kamiokande. In order to realize Hyper-Kamiokande, ICRR and Kavli-IPMU, the school of Science and the Earthquake Research Institute of The University of Tokyo, have created a new organization, called “Next-Generation Neutrino Science Organization.” In early 2020, the Japanese government approved the construction of the Hyper-Kamiokande project, which is co-hosted by The University of Tokyo and KEK. Hyper-Kamiokande will begin the experiment in 2027.

In FY2020 and FY2021, the COVID-19 pandemic meant that ICRR was only able to accept a limited number of researchers at the institute. We hope that we can resume face-to-face collaborative research as soon as possible.

Finally, we sincerely appreciate the strong support of our colleagues in this research field, The University of Tokyo, and the Japanese Ministry of Education, Culture, Sports, Science and Technology. They are indispensable for the continuing development and exciting scientific outcomes at ICRR.

Director, Institute for Cosmic Ray Research
The University of Tokyo