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The Nobel Prize in Physics 2015

On October 6, 2015, the Nobel Prize in Physics 2015 was awarded jointly to Takaaki Kajita and Arthur B. McDonald "for the discovery of neutrino oscillations, which shows that neutrinos have mass".

A Breakthrough Discovery Toward Understanding the Origin of the Universe

After completing a 4-year undergraduate program, Dr. Kajita moved on to a graduate program at the University of Tokyo, driven by his desire to understand the principles of matter and nature. He became interested in accelerator experiments and studied under Masatoshi Koshiba, who would win the Nobel Prize in 2002. In 1986, Dr. Kajita started his research on atmospheric neutrinos, which are generated when cosmic rays collide with atomic nuclei in the atmosphere. He discovered in 1988 that the observed amount of muon neutrinos in the atmosphere is only about 60% of what is predicted by theory, and that the remaining 40% has changed into tau neutrinos. Furthermore, in 1998, the research group at Super-Kamiokande in which he worked observed that the amount of muon neutrinos decreases by about half as they travel long distances. In addition to muon and tau neutrinos, there are also electron neutrinos for a total of three neutrino “flavors.” The phenomenon in which neutrinos change flavors is called neutrino oscillation, and it can only occur if neutrinos have mass.

Additionally, in 2001 at Canada’s Sudbury Neutrino Observatory, Dr. McDonald, the co-recipient of the Nobel Prize, found that electron neutrinos traveling from the sun to the earth changed into muon and tau neutrinos. In this manner, Dr. Kajita, Dr. McDonald, and the research groups with which they are affiliated proved for the first time that neutrino oscillations occur. Although the original Standard Model of particle physics assumes that neutrinos are massless, their discovery proved that neutrinos have mass.

Many scientists around the world are currently working together to capture neutrinos and to understand their properties. The discovery by Dr. Kajita and Dr. McDonald brings us closer to revealing the mysteries of matter and could rewrite our understanding of the origin and the future of the universe.

(October 8, 2015/ Revised)

Takaaki Kajita

2012    Awarded the Japan Academy Prize
2008    Became director of the Institute of Cosmic Radiation Research, University of Tokyo
2002    Awarded the Panofsky Prize as part of the Super-Kamiokande research group
1999    Awarded the 45th Nishina Memorial Prize
1999    Became a professor of the Institute of Cosmic Radiation Research, University of Tokyo
1998    Awarded the Asahi Prize as part of the Super-Kamiokande research group
1992    Became an assistant professor of the Institute of Cosmic Radiation Research, University of Tokyo
1988    Joined the Institute of Cosmic Radiation Research, University of Tokyo as an assistant
1986    Became an assistant at the Faculty of Science, University of Tokyo
1986    Obtained the degree of Doctor of Science from the Graduate School of Science, University of Tokyo
1981    Graduated from the Department of Physics, Faculty of Science, Saitama University
1977    Graduated from Saitama Prefectural Kawagoe High School

See also
Nobel Prize
Hamamatsu Photonics K.K.: Photomultiplier Tubes
(a manufacturer and supplier of photomultiplier tubes for Kamiokande and Super-Kamiokande)