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Takaaki Kajita

Nobel Prize in Physics

2015

That neutrinos have mass is important because it provides a deeper knowledge of the fundamental tenets of the universe. This discovery, contrary to conventional studies, has paved the way for new horizons in physics. 

In 2015, Professor Takaaki Kajita was awarded the Nobel Prize in Physics for proving that neutrinos have mass. 

Neutrinos are a type of elementary particle. They are ghost-like particles which can pass through anything. They can also travel out to the far reaches of outer space. Scientists have uncovered trillions of neutrinos, which come from Earth’s atmosphere, inside the Sun and stars and other places. They are also extremely light, and it has been long thought that a neutrino’s weight was zero. 

In fact, this theory was accepted as a fact of the Standard Model of particle physics until Prof Kajita’s discovery of neutrino oscillations - which means they have mass - re-wrote it. 

His work began in 1983 when he joined the Kamiokande experiment, which is a tank containing 3,000 tons of the purest water located about 1km underground in the Kamioka mine, because such a depth reduces cosmic rays. Most neutrinos pass right through the tank but on rare occasions, a neutrino would collide with a water molecule creating a charged particle. This is a good way to study and observe neutrinos. 

“During my research, in 1986, I found that the neutrino data was not up to expectations. I thought it was a mistake in the data analysis. Or could it be a hint of unexpected phenomena?” This was the motivation for Prof Kajita to continue his research. 

His breakthrough discovery was made at the Super-Kamiokande, a replacement tank for the earlier detector. This facility can hold 50,000 tons of ultrapure water. 

Prof Kajita is presently involved in developing the Hyper-Kamiokande. It will be able to hold 260,000 tons of ultrapure water, about 10 times more volume for the research than the Super-Kamiokande. The new facility comprises a cylindrical tank with a water depth of 71m and a diameter of 68 m. On the tank wall, 40,000 ultra-high sensitivity photosensors will be installed to detect neutrino oscillations, allowing more data to be collected. 

Experiments are expected to start at the new facility in 2027, which has been built at a cost of US$600 million. More than 400 scientists from 19 countries have shown interest in research collaborations at the new facility. 

Prof Kajita is the Special University Professor at the University of Tokyo and also the Director of the Institute of Cosmic Ray Research.