Takaaki Kajita

Nobel Prize in Physics


Few scientists can say that they have changed the world’s understanding of the innermost workings of matter itself. Professor Takaaki Kajita, who is now the director of the University of Tokyo’s Institute for Cosmic Ray Research, is one of them. 

In 1986, just six months after he had received his doctoral degree, Prof Kajita noticed an odd phenomenon in one of his experiments. He had been trying to see if protons, the building blocks of ordinary matter, decay, but the data from the experiment showed a surprising deficit of a type of particle called muon neutrinos. 

When radioactive rays from outer space come into contact with particles in Earth’s atmosphere, particles called atmospheric neutrinos are formed. After photons, these neutrinos are the most numerous particles in the cosmos – their collective weight is about equal to that of the stars – and they come in three “flavours”: electron, tau and muon.

While Prof Kajita and his colleagues could not explain the deficit, they knew that it might be significant, concluding in their research paper on the experiment that “some as-yet-unaccounted-for physics such as neutrino oscillations might explain the data”.

“It was then that I made up my mind about pursuing this investigation as it felt like something that had never been imagined in the past. I felt that if I continued with my research, it might lead to some important findings,” recalled Prof Kajita. 

Indeed, in 1998, Prof Kajita discovered that the deficit was caused by neutrino oscillations, which is when neutrinos transform into other types of neutrinos while they are in flight. In his latest experiment, as in the earlier one, the data showed fewer muon neutrinos than expected because some of them had turned into tau ones. 

For the field of particle physics, this was a historic discovery, as it flew in the face of the Standard Model, a well-established theory of fundamental particles and how they interact. The model requires neutrinos to be massless, but neutrino oscillation can occur only if the neutrinos have mass.  

In 2015, Prof Kajita and another scientist who had observed similar changes with electron neutrinos were awarded the Nobel Prize in Physics for their discoveries. 

The Nobel Prize committee noted: “The new observations clearly showed that the Standard Model cannot be the complete theory of the fundamental constituents of the universe. The discoveries have yielded crucial insight into the all-but-hidden world of neutrinos, changed our understanding of the innermost workings of matter and can prove crucial to our view of the universe.”

Aside from the Nobel Prize, Prof Kajita has received several other accolades, including the Yoji Totsuka Memorial Prize, the Japan Academy Prize, the Breakthrough Prize in Fundamental Physics and the Order of Culture, which is awarded by the Emperor of Japan for significant contributions to Japan’s art, literature, science and culture in general. Prof Kajita is currently working on a project that aims to detect gravitational waves.