What are cosmic rays?
Cosmic rays (CRs) are high-energy subatomic particles, such as protons or nuclei, that travel from outer space to the Earth. They were discovered by Victor Hess in 1912, who won the Nobel prize in physics in 1936. As a natural experimental facility for particle physics, CRs have played a leading role in the discovery of various new particles and new phenomena, and have been a driving force in the early days of particle physics. CR astronomy or astroparticle physics with the use of the observation of CRs, such as neutrinos and gamma rays, provides information on high-energy phenomena in the universe, where CRs were born. Furthermore, CRs, which have a charge and are influenced by magnetic fields, provide means of studying the magnetic environment around the Sun and the Earth. The study of CRs is a broad interdisciplinary field that spans particle physics, astrophysics, and geophysics. Thus the students and the researchers with diverse interests are working on a wide variety of topics in CR research at the CR Research Division.
Research at the CR Research Division
CRs are a natural experimental field for elementary particles that can give us hints about ultrahigh energy phenomena and unknown elementary particles that can never be directly studied by ground-based experiments. We are tackling the mysteries that span the universe and elementary particles from the underground, such as neutrino research using the Super-Kamiokande/Hyper-Kamiokande, and the direct dark-matter searches using liquid xenon, XENONnT/DARWIN experiments.
We also conduct observations of cosmic gamma rays using the Fermi gamma-ray satellite and Cherenkov Telescope Array (CTA) to study the origin and the acceleration mechanism of CRs and search for gamma rays from dark-matter annihilation or decay.
The CR Research Division also conducts the LHCf and RHICf experiments using the LHC and RHIC colliders, respectively, to study the nuclear reactions that occur when ultrahigh energy CRs collide with atmospheric nuclei, and to better understand the air-shower phenomenon of CRs.
When CRs enter the Earth’s atmosphere, they cause ionization, and then produce nuclei such as radiocarbon-14 through nuclear reactions, bringing their energy to the Earth’s surface. By studying the cosmic-ray-produced nuclei left behind in annual rings and ice cores, we are trying to reveal the history of the sudden increase in CRs in the past, which may indicate the evidence of superflares of the Sun in the past.
History of the Cosmic-ray Research Division
The history of the cosmic-ray research group at Nagoya University, which is the root of the CR Research Division, goes back to the former “H-Lab”, which was opened by Prof. Yataro Sekido, who was a Research Assistant of Dr. Yoshio Nishina in RIKEN, moved to the Department of Physics of Nagoya University in 1946. Prof. Sekido built a CR telescope to observe the arrival direction of CR muons to find the source of CRs and established the Cosmic Ray Telescope Laboratory in the Faculty of Science in 1959. A large dome-roofed building where the Cosmic Ray Telescope No. 3 was installed is still standing on a hill on the Higashiyama Campus (see this website banner). In 1990, the Cosmic Ray Telescope Laboratory was merged with the Static Electricity Laboratory, which was located on the Toyokawa Campus, to form the Solar-Terrestrial Environment Laboratory (STE Lab). In 2015, The STE Lab merged with Hydrospheric Atmospheric Research Center (HyARC) and Center for Chronological Research, and Institute for Space–Earth Environmental Research (ISEE) was established.
The CR group is also responsible for research on astroparticle physics at the Kobayashi–Maskawa Institute for the Origin of Particles and the Universe (KMI) in collaboration with astrophysics and particle physics researchers in the Department of Physics.
With a history spanning more than 60 years, the CR group has produced researchers with a wide range of perspectives as a laboratory with a free atmosphere where diverse research from elementary particles to the universe and the earth can interact. With a policy of producing students who can build and operate their own detectors and who are strong in both hardware and software, our graduates are active in research and development at a variety of industries, universities, and research facilities all over the world. We have various research bases inside and outside of Japan such as the Kamioka Observatory (Japan), CERN (Switzerland), Max Planck Institute (Germany), Gran Sasso National Laboratory (Italy), Stanford University (USA), Brookhaven National Laboratory (USA), Mount John Observatory (New Zealand), and National Autonomous University of Mexico (Mexico), where students research with a global perspective.