Japan, Italy In Big Bang Particle Physics Collaboration

According to the US Department of Energy,“The neutrino is perhaps the best-named particle in the Standard Model of Particle Physics: it is tiny, neutral, and weighs so little that no one has been able to measure its mass.

Neutrinos are the most abundant particles that have mass in the universe. Every time atomic nuclei come together (like in the sun) or break apart (like in a nuclear reactor), they produce neutrinos. Even a banana emits neutrinos-they come from the natural radioactivity of the potassium in the fruit....”

The DOE goes on to say that scientists are now trying to determine the mass of a neutrino,“how it interacts with matter, and whether the neutrino is its own antiparticle (a particle with the same mass but opposite electric or magnetic properties) or not. Some scientists think neutrinos might be why all antimatter (the antiparticles of all matter) disappeared after the Big Bang, leaving us in a universe made of matter.”

Hyper-Kamiokande will mark the third stage of a series of observations and experiments that began with the commissioning of the original Kamiokande in 1983 and continued with the completion of Super-Kamiokande in 1996.

Situated in old zinc mines under a mountain in the town of Kamioka, Gifu Prefecture, in central Japan, the dome-shaped caverns that house these observatories were excavated by Mitsui Kinzoku (Mitsui Mining and Smelting Co), which owns the mines. KamiokaNDE stands for Kamioka Nucleon Decay Experiment.

Mitsui Kinzoku writes that“Neutrinos come from space and hardly interact with other matter at all. This means that they pass straight through the earth, making them extremely difficult to observe.

As a result, it is necessary to install a vast facility to increase the efficiency of observation, within an environment that blocks out other cosmic rays that get in the way of observation. The facility also has to be in a location with access to pure water, which is another essential component for observation.”

Professor Masatoshi Koshiba of the University of Tokyo identified Kamioka as an ideal site for the study of neutrinos in 1981. He designed the original detector, was also instrumental in the construction of Super-Kamiokande and remained actively involved in neutrino research throughout his career.

In 2002, he was awarded the Nobel Prize in Physics for his contribution to the detection of cosmic neutrinos, including the first observation of neutrinos emanating from a supernova in the Large Magellanic Cloud. Professor Koshiba died in November 2020.

Professor Takaaki Kajita, also of the University of Tokyo, won the Nobel Prize in Physics in 2015 for his work with Super-Kamiokande, which led to the discovery of neutrino oscillations and demonstrated that neutrinos have mass. Kajita said that he joined Professor Koshiba's research team because he thought that neutrinos might be interesting.

Like its predecessors, Hyper-Kamiokande will be operated by the Kamioka Observatory of the Institute for Cosmic Ray Research at the University of Tokyo. Super-Kamiokande, which is 1,000 meters underground, consists of a stainless-steel tank 39.3 meters across and 41.4 meters deep filled with 50,000 metric tons of ultra-pure water and about 13,000 photo-multiplier tubes, or PMT sensors to detect neutrinos.

PMTs are extremely sensitive vacuum tube devices that amplify incident light by up to 100 million times. They collect the pale blue light emitted by neutrinos passing through the water.

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Hyper-Kamiokande, 600 meters underground, is much bigger. Its water tank will be 68 meters across and 71 meters deep, containing 260,000 metric tons of water with a fiducial mass (comparable detector volume) eight times greater than its predecessor. Hyper-Kamiokande will have about 20,000 PMTs, most if not all of them made by Hamamatsu Photonics, the Japanese company that dominates the market for PMTs.

Super-Kamiokande is the world's largest water Cherenkov detector used for neutrino and nucleon decay experiments. Hyper-Kamiokande will most probably inherit that title. The University of Sheffield points out that using water as the neutrino detection medium provides a very large target mass at a reasonable cost, noting that all the world's largest neutrino detectors are water Cherenkov experiments. The concept was devised by Pavel Cherenkov, a Soviet physicist who was awarded the Nobel Prize in Physics in 1958.

Professor Masato Shiozawa, the Kamioka Observatory's director, says“The Super-Kamiokande is currently undergoing an upgrade to increase its sensitivity to neutrinos from supernova explosions.

This is expected to help elucidate the mechanism of supernova explosions and understand the history of star formation. In addition, development studies are underway for the next phase of the project for the discovery of neutrino-less double-beta decay and direct detection of dark matter.”

Super-Kamiokande is used by about 200 researchers from some 50 research institutes in Japan, South Korea, China, Vietnam, France, Italy, Spain, Poland, the UK, the US and Canada. Hyper-Kamiokande will probably have a similar list of participants in due time.

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