Capturing Neutrinos
The smallest elements that make us up are called elementary particles. While photons (the particles of light) and electrons (the source of electric current) are elementary particles, the protons and neutrons that make up atomic nuclei are not. Instead, they are composed of three elementary particles of two types, known as quarks. In addition to these stable elementary particles, other types have been discovered, including massive, unstable ones and light ones whose properties are not well understood.
Elementary particles experience different types and magnitudes of forces depending on their type, and we cannot notice particles that interact weakly. Neutrinos, a type of elementary particle, have extremely small mass and exert almost no force on their surroundings. Every second, hundreds of trillions of neutrinos from space pass through our bodies, and most of them pass right through the Earth as well. However, if we can capture the rare moments when they do interact, we can study their properties. For this purpose, the Super-Kamiokande detector uses 50,000 tons of water in a large cylindrical tank approximately 40 meters in both diameter and height. It captures the light from the occasional interacting neutrino, allowing for the observation of about 30 neutrinos per day.
Super-Kamiokande has been observing neutrinos for over 20 years, and in 2020, a new experiment began after an upgrade to the detector, which involved adding gadolinium to the water. Neutrinos are produced in vast quantities from stellar explosions (supernovae) that have occurred throughout history and drift through space, holding information about the end of stars and the evolution of the universe. The detector upgrade has made it possible to distinguish antineutrinos—antiparticles with reversed properties—and to filter out neutrinos from the sun, which interfere with observations. Consequently, observations are ongoing with the aim of discovering these ancient, drifting neutrinos.
In addition to observing the universe with neutrinos, we also aim to elucidate the laws governing elementary particles by observing neutrino and proton decay. It is not yet known whether neutrinos behave identically to their antiparticles as if in a mirror world. We are investigating whether differences with their antiparticles appear through a phenomenon called neutrino oscillation, where neutrino types change. For this purpose, we observe neutrinos and antineutrinos that are beamed from the high-intensity proton accelerator in Ibaraki Prefecture, 295 kilometers away. If this difference can be discovered, it is thought to be a clue to explaining why matter came to exist in the early universe. Furthermore, "proton decay," the decay of protons in water over a vast timescale of more than 10^34 years, is predicted by new theories that could unify our description of elementary particles. Its discovery is anticipated through the long-term observation of a massive volume of water.
To make these discoveries, construction is underway on Hyper-Kamiokande, a detector even larger than Super-Kamiokande, with a height and diameter of about 70 meters and approximately 10 times the sensitivity. To count the faint light in the water with higher precision, new large-scale 50-centimeter-diameter photosensors have been developed, achieving twice the performance of conventional ones. Precise calibration is being carried out to ensure high accuracy over the large light-receiving surface, and research is being conducted to improve not only the size but also the detection precision. The photosensors are scheduled to be installed in 2026, with the goal of starting observations in 2027. The next generation of neutrino and proton decay research is about to begin, aiming to unravel the mysteries of the universe's creation and the ultimate theory of everything.