The Royal Swedish Academy of Sciences has announced that T2K collaboration member Takaaki Kajita will be awarded the 2015 Nobel prize in physics. Prof Kajita, Director of The Institute for Cosmic Ray Research (ICRR), University of Tokyo, shares the award with Prof Arthur McDonald (Queen’s University, CA) “for the discovery of neutrino oscillations, which shows that neutrinos have mass.”
Prof Kajita’s Nobel-prize winning work was on the Super-Kamiokande (Super-K) experiment. Super-K, which serves as the far detector for T2K, is a gigantic underground water Cherenkov detector that discovered atmospheric neutrino oscillation. When cosmic rays interact in the earth’s atmosphere, they create both electron and muon neutrinos, which Super-K can distinguish with high accuracy. Although the rate of electron neutrinos coming from above is consistent with rate of electron neutrinos from below, Prof Kajita and his Super-K colleagues found that the rate of muon neutrinos coming from below, which is to say those that travel a long distance through the earth, is much smaller than the rate of muon neutrinos from above. We now know that the muon neutrinos are turning into tau neutrinos, and because the neutrino energy is too low to create the heavy tau particles, they do not interact. This phenomenon is called neutrino disappearance and its discovery by Prof Kajita and Super-K is considered the dawn of a new field of particle physics, which T2K is now continuing.
Congratulations to Kajita-san!