Takaaki Kajita is a Japanese physicist, known for neutrino experiments at the Kamiokande and its successor, Super-Kamiokande. In 2015, he was awarded the Nobel Prize in Physics jointly with Canadian physicist Arthur McDonald.
Since 1988 Kajita has been at the Institute for Cosmic Radiation Research, University of Tokyo, where he became an assistant professor in 1992 and professor in 1999.
He became director of the Center for Cosmic Neutrinos at the Institute for Cosmic Ray Research (ICRR) in 1999. As of 2017, he is a Principal Investigator at the Institute for Physics and Mathematics of the Universe in Tokyo, and Director of ICRR.
In 1998, Kajita's team at the Super-Kamiokande found that when cosmic rays hit the Earth's atmosphere, the resulting neutrinos switched between two flavours before they reached the detector under Mt. Kamioka. This discovery helped prove the existence of neutrino oscillation and that neutrinos have mass. In 2015, Kajita shared the Nobel Prize in Physics with Canadian physicist Arthur McDonald, whose Sudbury Neutrino Obervatory discovered similar results. Kajita's and McDonald's work solved the longstanding Solar neutrino problem, which was a major discrepancy between the predicted and measured Solar neutrino fluxes, and indicated that the Standard Model, which required neutrinos to be massless, had weaknesses. In a news conference at the University of Tokyo, shortly after the Nobel announcement, Kajita said, "I want to thank the neutrinos, of course. And since neutrinos are created by cosmic rays, I want to thank them, too."
Kajita is currently the principal investigator of another ICRR project located at the Kamoka Observatory, the KAGRA4 gravitational wave detector.