Yoji Totsuka (March 6, 1942 – July 10, 2008) was a Japanese physicist and Special University Professor, Emeritus, University of Tokyo.

Totsuka became a Research Associate at the University of Tokyo in 1972, followed by seven years at Deutsches Electron Synchrotron (DESY) in Germany, where he investigated electron–positron collisions. Subsequently, he became an Associate Professor of the University of Tokyo from 1979 to 1987. In 1987, he was promoted to full Professor at the University of Tokyo. He later became the Director of the Kamioka Observatory, part of the Institute for Cosmic Ray Research (ICRR) at the University of Tokyo in 1995 and then Director of the Institute for Cosmic Ray Research in 1997. In 2003, Totsuka became the Director General of the High Energy Accelerator Research Organization (KEK).

Totsuka's career in neutrino physics began following his time at DESY, when he began working as part of the Kamiokande experiment with Nobel prize winner Masatoshi Koshiba. The experiment, though designed to detect proton decay, actually ended up successfully measuring the first and so far only neutrinos from a cosmogenic source on Earth, from SN 1987A, along with the Irvine–Michigan–Brookhaven (IMB) detector in the US.

The success of Kamiokande, through Totsuka's leadership, led to the funding of a substantially larger water Cherenkov detector in 1991, the storied Super-Kamiokande (Super-K) detector, which is still an active international collaboration. It was at Super-K that the first definitive evidence for neutrino oscillations was measured, via a high-statistics, high-precision measurement of the atmospheric neutrino flux. Super-K also confirmed, along with the Sudbury Neutrino Observatory(SNO), the solution to the solar neutrino problem.

The measurement of neutrino oscillations at such a high level of precision was a critical chapter in the history of particle physics. Neutrino oscillations, and thus the existence of neutrino mass, are not a prediction made by the Standard Model of particle physics. Indeed, the Standard Model requires that neutrinos are massless. Totsuka's experiment provided incontrovertible evidence that there is still much about particle physics yet to be understood.

For the remainder of his time as a physicist, as the Director General at KEK, he oversaw successful the K2K experiment and the BelleB-meson "factory".

Information and photo: wikipedia.org