Illustration of the Japnese astrophysicist Takaaki Kajita (born 1959). Kajita is best known for his work on the neutrino, an elementary particle. Billions of neutrinos generated from nuclear reactions in the Sun pass through Earth every second. Neutrinos have no charge, but it was not known if they had mass. It was thought that neutrinos came in three different types, or flavours and that they oscillated between these. If so, they would have to have mass. Kajita joined the Kamiokande-II neutrino experiment, a heavy water neutrino detector, at the Institute for Cosmic Radiation Research, University of Tokyo in 1986 and became the director of its Centre for Cosmic Neutrinos in 1999. In 1998 Kajita and colleagues published evidence of solar neutrino oscillation, showing that neutrinos have mass.Kajita was awarded the 2015 Nobel Prize in Physics along with Arthur McDonald, whos team at the Sudbury Neutrino Observatory in the USA made the same discovery.