Takaaki Kajita

University of Tokyo

Takaaki Kajita is a Japanese experimental physicist, Professor at the University of Tokyo and Director of its Institute for Cosmic Ray Research (ICRR), where he has spent his career probing the most elusive particles in nature. He shared the 2015 Nobel Prize in Physics with Arthur B. McDonald for the discovery of neutrino oscillations, which proved that neutrinos have mass — a result that overturned a core assumption of the Standard Model of particle physics. Working at the Kamioka Observatory deep under Mount Ikeno, Kajita analyzed atmospheric neutrinos produced when cosmic rays strike the upper atmosphere. In 1998, using the giant Super-Kamiokande water-Cherenkov detector, he and his collaborators showed that muon neutrinos arriving from below — having traversed the entire Earth — were depleted relative to those arriving from above, the signature of neutrinos transforming from one flavor into another in flight. This discovery opened the field of neutrino mass physics and reshaped both particle physics and astrophysics. Kajita earlier earned the Bruno Rossi Prize and Asahi Prize for the same body of work, and he has been honored with Japan's Order of Culture. As ICRR Director he now leads KAGRA, Japan's underground cryogenic gravitational-wave interferometer, extending his mastery of ultra-low-background underground physics from kiloton water-Cherenkov tanks to sapphire-mirror cryogenic interferometry beneath Kamioka.