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Masatoshi Koshiba, Ph.D.

Senior Counselor, International Centre for Elementary Particle Physics

Emeritus Professor of Physics, University of Tokyo

Masatoshi Koshiba

Year: 2003

Subject: Physics

Award: Benjamin Franklin

Citation: Drs. Koshiba, Davis and Bahcall are awarded the 2003 Benjamin Franklin Medal in Physics for their work that led to an understanding of neutrino emission from the sun.

Biographical Information

The groundbreaking work of John Bahcall, Raymond Davis, and Masatoshi Koshiba led to the discovery of solar neutrinos, furthering our understanding of the internal processes of the Sun and stars in general. Their research demonstrated that the nuclear reactions that produce neutrinos also cause the Sun to shine. Specifically, nuclear fusion among light elements is responsible for energy generation in the Sun.

Koshiba led the design and construction of a giant Kamiokande detector in Japan, which precisely recorded the time of arrival, energy, and direction of incoming neutrinos. This experiment confirmed Davis's measurements, detected the type of neutrinos Davis's experiment was unable to detect, and actually showed that the neutrinos came from the Sun.

This research prepared the way for the discovery of a new property of neutrinos called "neutrino oscillations," which showed that neutrinos have mass. It had previously been believed that their mass, like that of a proton, was zero. On the way from the Sun, some neutrinos change identity and become invisible in the detectors like Davis designed, whereas, Koshiba designed his experiment to detect neutrino oscillations. The discoveries of Koshiba, along with Bahcall and Davis promise to open up new vistas in the fundamental physics of elementary particles.

Koshiba earned his B.S. in 1951 from the University of Tokyo, and his Ph.D. in physics from the University of Rochester, NY in 1955. Koshiba is a member of the American Physical Society, the Physical Society of Japan, and the Japanese Astronomical Society. He has been awarded numerous honors and prizes including the Academy Award from the Academy of Japan, the Order of Cultural Merit, which was conferred by the Emperor of Japan in person, the Wolf Prize, and in 2002 the Nobel Prize in Physics along with Dr. Davis.

Nuclear reactions at the cores of stars (including the sun, shown here) emit neutrinos.