Associate Professor, Yoshinari Hayato
Yumiko Takenaga
(Kamioka Observatory, ICRR,
The University of Tokyo)



Two Nobel Prizes Born
from the Kamiokande Series

It was in 1983 that a detector called “Kamiokande” was constructed in Kamioka-town, Hida City, Gifu Prefecture, and in 1996, “Super-Kamiokande,” which is still in operation today, began its operation.
The Hyper-Kamiokande, which is scheduled to start in 2027, is the latest version of the Kamiokande series. In the history of research in Kamioka, what kind of future have researchers envisioned, and what kind of results have they achieved? Here, we will unravel the history of their thoughts and research.

Left: Professor Takaaki Kajita,
Right: Professor Masatoshi Koshiba

Kamiokade: Observed neutrinos from a supernova explotion

The original purpose was to search for proton decay

Kamiokande was planned by Dr. Masatoshi Koshiba, who later received the Nobel Prize in Physics, for the purpose of proton decay search. The proton is a particle that makes up the nucleus of the atoms that build our bodies and matter. It has been believed that protons are stable and do not spontaneously change into other particles, but a new theory proposes that protons will break down someday. If this is true, the protons in the universe will eventually reach the end of their lives, and in the distant future, they may no longer constitute new matter or life. Above all, if we could observe “proton decay,” we would be one step closer to the completion of the “Grand Unified Theory,” which would unify the forces in the natural world. We needed to observe this somehow.

The average lifetime of a proton is about 1030 years, which is 10 billion times longer than the current age of the universe (about 13.8 billion years). Although this is a tremendous length, if we can collect and observe a large number of protons, it is possible to observe their decay. Therefore, by installing 1,000 optical sensors (photomultiplier tubes) inside a huge tank holding 3,000 tons of water (about 1033 protons), Kamiokande aimed to observe the small amount of light produced when the protons in the water decay.

Inside of the Kamiokande detctor and overview

Four years later, shfted to neutrino detection

Kamiokande revealed that protons have an even longer lifetime than initially assumed. On the other hand, the high performance of Kamiokande was also shown, and it was decided to use it to observe neutrinos coming from the sun, which was attracting much attention at the time. They believed that they could detect neutrinos by observing a small amount of light when a neutrino rarely hits water particles in the tank. The detector was modified, and full-scale observations began in early 1987.

Shortly after that, on February 25, they received a fax from a colleague at University of Pennsylvania to inform them of a supernova explosion.
“Can you see it? (Can you detect neutrinos from the supernova explosion in Kamiokande?)”

At that time, observation data was recorded on magnetic tapes, and every ten tapes were sent from Kamioka to the Koshiba Laboratory of the University of Tokyo. Researchers immediately ordered the data tapes for analysis.
On February 28, researchers confirmed that Kamiokande certainly detected the supernova explosion neutrinos. Dr. Koshiba was awarded the Nobel Prize in Physics in 2002 for the world’s first observation of neutrinos emitted from a supernova explosion.

Before the explosion of supernova SN1987A (right) and after the explosion (left)
Anglo-Australian Observatory/David Malin

Was it just good luck to detect neutrinos “for 10 seconds once in a few decades?”

A supernova explosion in our galaxy occurs once every 30 to 50 years. It is only for ten seconds to emit the neutrinos from a supernova explosion. Three minutes before the explosion, the adjustment program was running and stopped data taking for a few minutes. The airtight construction of the tank, which was originally scheduled for that day, was postponed. Dr. Koshiba was going to retire the following March. Considering those things, we may say that it was just good luck to detect the supernova neutrinos. However, as Dr. Koshiba said, “Neutrinos rained down on everyone equally. It’s whether they were ready for it or not.”

Dr. Masatoshi Koshiba (2002 Nobel Prize in Physics)

Super-Kamiokande: Discovered that neutrinos have mass

Big discovery in studying noise events

In 1986, before the observation of supernova explosion neutrinos, Dr. Takaaki Kajita, then a graduate student, was investigating the number of atmospheric neutrinos, which are noise for proton decay search. Atmospheric neutrinos are produced when particles from space (cosmic rays) collide with protons and other particles in the atmosphere and fall to the ground. They pass through us humans or other materials without interaction and reach Kamiokande underground. There are two types of atmospheric neutrinos: muon neutrinos and electron neutrinos. Dr. Kajita noticed that the muon neutrino component was only about half the expected value. This was the first sign of neutrino oscillations, which later earned him the Nobel Prize in Physics.

Atmospheric neutrinos are produced then cosmic rays collide with the earth’s atmosphere.

Big enough news that even the U.S. President mentioned it in his speech.

In 1996, Kamiokande’s successor, Super-Kamiokande, was completed. Sufficient data on “neutrino oscillations” could not be obtained with Kamiokande, but with Super-Kamiokande, one year of observations is equivalent to 20 years of data at Kamiokande. This has revealed that the deficit in muon neutrinos varies depending on the flight length. The number of downward muon neutrinos coming from above Kamioka was the same as expected, but the number of upward muon neutrinos coming from the other side of the earth was found to be only about half the expected value.

In 1998, Dr. Takaaki Kajita presented the neutrino oscillation study results at the International Neutrino Conference in Takayama, Japan, and was applauded. The observation of neutrino oscillations means that neutrinos have non-zero mass. This was a major breakthrough that surpassed the standard theory of elementary particle theory, known as the Standard Model. It made the front page of The New York Times the next day, and was mentioned by then-President Clinton in his commencement address at the Massachusetts Institute of Technology, creating quite a stir.

Dr. Takaaki Kajita (2015 Nobel Prize in Physics)

Vacant “Third Nobel Laureate.”

For this achievement, Dr. Kajita was awarded the Nobel Prize in Physics in 2015. Usually, up to three Nobel Prizes can be awarded at the same time, but on this occasion, only two laureates were awarded. It is said that the remaining one seat was left open for Dr. Yoji Totsuka, who passed away from colon cancer in 2008.

Dr. Yoji Totsuka (Leader in Super-Kamiokande construction)

Dr. Totsuka led the construction of the Super-Kamiokande to demonstrate neutrino oscillation. When a major accident occurred in 2001, in which more than half of the photomultiplier tubes were broken, he made an announcement to the disappointed researchers, “We will rebuild in one year!” He led the restoration work with his strong leadership, even though he was already ill. After receiving the Nobel Prize, Dr. Kajita told the press, “The most difficult period of my research life was the accident. It was under very tough circumstances, and we were able to overcome the difficulties thanks to the leadership of Dr. Totsuka. I want everyone to remember this.”

Hyper-Kamiokande: Confront of Further Mysteries of Elementary Particles and the Universe

Pursuing new research topics

The discovery of neutrino oscillations has opened up many more issues to be studied regarding elementary particles and the universe. For example, the current universe is composed almost entirely of matter, but it is believed that there was an equal amount of antimatter, which annihilates matter when it encounters it, at the beginning of the universe. Why did only matter remain even though it would be possible that nothing existed in the universe? It is thought that this may be due to an asymmetry between matter and antimatter, called “CP violation.” The key to solving this mystery may lie in “neutrino oscillations.” A successor detector, the Hyper-Kamiokande, is currently under construction to investigate this in more detail.

In order to investigate neutrino oscillation in detail, the T2K experiment has been conducted with Super-Kamiokande, where neutrinos emitted from the J-PARC accelerator in Tokai-mura, Naka-gun, Ibaraki Prefecture, are captured at Super-Kamiokande in Kamioka, Hida City, Gifu Prefecture, 295 km away. The beam intensity from J-PARC is planned to be increased at the Hyper-Kamiokande project for further investigation of neutrinos.

The Hyper-Kamiokande, currently under construction, is scheduled for completion in 2027.

Realization of the Large-Scale Detector Envisioned by Dr. Koshiba

On the other hand, Hyper-Kamiokande also explores proton decay, which was the initial goal of Kamiokande. Although similar experiments are planned in the U.S. and China, we aim to take the lead by our scale and long experience in neutrino experiments.

In fact, a paper published by Dr. Koshiba in 1992 already proposed a 1-million-ton scale detector that would surpass Super-Kamiokande. Even then, researchers believed that a large-scale detector was necessary to verify “proton decay” and other phenomena. Various other types of detectors were also proposed over the years, including a horizontal type, a dual detector, and others. Through various innovations, such as increasing the sensitivity of the photomultiplier tubes, the Hyper-Kamiokande will achieve the same level of sensitivity as Dr. Koshiba’s conception on a 260,000-ton scale. We expect that neutrinos will solve the mystery of the universe’s evolution and that proton decay will reveal the secrets of elementary particles in the near future. Challenges to the unknown move into the next generation. Please keep an eye on research at the Hyper-Kamiokande to be started in 2027.