In our group, we conduct experiments and observations on elementary and astroparticle physics. In particular, we are promoting the study of the properties of neutrinos, the search for new phenomena such as proton decay, and observation of astrophysical objects using neutrinos, using Super-Kamiokande and its successor Hyper-Kamiokande.
The Super-Kamiokande is the world’s largest underground neutrino observation and proton decay experiment with more than 11,000 50-cm diameter photomultiplier tubes in a cylindrical tank with a diameter of about 40 m and a height of about 42 m. Since 1996, over the long time the Super-Kamiokande has continued to improve its equipment and analysis methods, leading neutrino research in the world.
In the summer of 2020, gadolinium, one of rare earth element, was added and a new observation was started as SK-Gd. Gadolinium has an extremely high neutron capture efficiency and emits relatively high energy gamma rays that can be easily detected in Super-Kamiokande after capture, dramatically improving the measurement sensitivity of neutrino interactions.
Supernova Relic Neutrinos
Among the many studies possible with SK-Gd, we aim to make the world’s first observation of supernova relic neutrinos. Supernova relic neutrinos are neutrinos that are generated by supernova explosions that occurred in the history of the past universe and are thought to accumulate and drift into the present universe. The expected number of events is extremely small at a few per year, and even Super-Kamiokande could not observe so far. SK-Gd aims to dramatically reduce background and achieve the world’s first observation of supernova background neutrinos by measuring neutrons emitted at the same time as neutrino reactions. We hope that the study of supernova relic neutrinos will provide clues to the history of star formation in the universe, the mechanism of supernova explosions, and the properties of neutrinos themselves.
Neutrino oscillations and proton decay
SK-Gd will also continue to measure neutrino oscillations using solar neutrinos, atmospheric neutrinos, and T2K beam neutrinos. In particular, in atmospheric neutrinos and T2K beam neutrinos, we believe that neutrons can be used to classify neutrino interactions more precisely, and research on neutrino oscillations will advance further. In addition, SK-Gd reduces background events caused by atmospheric neutrinos and enables further high-sensitivity searches for proton decay predicted in the grand unified theory of elementary particles.
The T2K (Tokai-to-Kamioka) experiment is an experiment in which a high-intensity muon neutrino beam artificially created by J-PARC, a high-intensity proton accelerator facility, is observed at Super-Kamiokande 295 km away. In T2K, the phenomenon called “neutrino oscillation” in which neutrinos change into another type of neutrino is studied with high accuracy. Firm evidence of neutrino oscillations was first reported in 1998 by a study using Super-Kamiokande. For this achievement, Professor Takaaki Kajita of the Institute of Cosmic Ray Research of The University of Tokyo was awarded the Nobel Prize in Physics in 2015.
Towards the discovery of CP violation
In the T2K experiment, in 2011, we discovered “change from muon type to electron type,” a type of neutrino oscillation that had not been observed before. By measuring the probability of neutrino oscillations from muon-type to electron-type in neutrinos and anti-neutrinos, respectively, and looking at the differences, we can observe the differences in the properties of matter and antimatter in neutrinos (CP violation).
All elementary particles have “antiparticles” that have the same mass and have the opposite sign of charge. When the universe began with the Big Bang, particles and antiparticles should have produced in the same number. However, there are almost no antiparticles in the current universe, and everything like stars and we are composed of only particles. The mystery of the disappearance of antiparticles from universe is one of the major challenges in particle physics. The CP violation in neutrinos, which has yet to be discovered, may hold the key to solving this mystery, and we are leading the world’s research in T2K experiments towards its discovery.
Near detector upgrade
In order to discover CP violation, we need to observe slight differences between neutrinos and anti-neutrinos. For that purpose, it is essential to correctly understand the interaction between neutrinos and nuclei and the properties of neutrino beams, and to reduce the systematic uncertainties. In the T2K experiment, neutrinos before oscillation are measured with near detectors installed in the J-PARC site to reduce these systematic errors. In order to measure neutrino interactions more precisely, our group proposed a project to upgrade this near detector to a better performance device. We are promoting the project as one of the central groups.
SuperFGD, one of new neutrino detectors we are mainly working on, can capture detailed information about neutrino interactions inside the detector with approximately 2 million plastic scintillator cubes of 1 cm3.
For the past 25 years, Super-Kamiokande continued to open up new research field by creating new experimental methods such as SK-Gd and T2K. However, higher statistics and high-precision experiments are required to make further progress based on the past achievement.
Therefore, we are proceeding with the construction of the Hyper-Kamiokande detector, which is an order of magnitude larger than the current Super-Kamiokande with higher-performance photosensors. Hyper-Kamiokande enables the world’s best research in a wide range of fields across the elementary particle physics and astrophysics, including the study of neutrino oscillations, the search for proton decay predicted by the grand unified theory of elementary particles, and neutrino detection from supernovae.
The Hyper-Kamiokande project was approved in Japan in early 2020, and construction began for the start of operation in 2027. Mass production of high-performance photomultiplier tubes, which improves performance such as light detection efficiency, charge resolution, and time resolution twice as much as those used in Super-Kamiokande, has also been started. We are conducting research to obtain the maximum performance of detectors, such as performance inspection of photomultiplier tubes and establishment of calibration methods.