Yoji Totsuka, an honorary professor emeritus at the University of Tokyo, died of cancer on 10 July 2008 in Kashiwa, Japan. He will be most remembered for his leadership on the Super-Kamiokande experiment, which discovered that neutrinos have tiny, nonzero masses.

Born in Fuji City, near Japan’s Mount Fuji, on 6 March 1942, Totsuka spent most of his boyhood in the area. After he graduated with a degree in physics from the University of Tokyo in 1965, he entered the university’s graduate course in physics under the supervision of Masatoshi Koshiba. Totsuka’s thesis work, for which he received his PhD in 1972, was on the underground measurement of the flux of cosmic-ray muon bundles at the Kamioka mine. Thus began his 30-year research career in Kamioka.

From 1972 to 1981, Totsuka worked on the DASP and JADE e+e experiments at DESY, the German Electron Synchrotron in Hamburg. He returned to Tokyo in 1981 and joined Koshiba’s Kamiokande experiment, which used a 3-kiloton water Cherenkov detector to search for proton decay; it began operating in July 1983. Because the detector worked so well, two new ideas were soon proposed: the detection of solar neutrinos in Kamiokande and the construction of a 50-kiloton water Cherenkov detector, Super-Kamiokande. From 1984 to early 1987, the detector underwent a major improvement so it could detect solar neutrinos. Luckily, that work was finished one month before the arrival of supernova neutrinos in February 1987.

After Koshiba retired from the University of Tokyo, Totsuka was appointed leader of the Kamiokande and Super-Kamiokande experiments in April 1987. To realize the Super-Kamiokande project, he moved to the university’s Institute for Cosmic Ray Research (ICRR) the following year.

Thanks to Totsuka’s effort, in 1991 Super-Kamiokande was approved by the Japanese government. A large number of US collaborators joined the experiment; of the roughly 100 researchers, about 60% were from Japan and 40% from the US. The construction was scheduled for completion on 31 March 1996, the end of Japan’s 1995 fiscal year; the Super-Kamiokande experiment came on line as scheduled at midnight on 1 April.

Totsuka’s strong leadership kept the construction on track, and his engaging character helped the collaboration work cohesively toward its goal. His warmth and friendliness made him very easy to work with.

From the beginning of Super-Kamiokande, its primary goals were clear: to resolve why experiments detected fewer solar neutrinos than predicted by theory and to explain anomalous observed ratios of atmospheric neutrinos. Through the detailed studies of atmospheric neutrinos, Totsuka and his colleagues in 1998 discovered neutrino oscillations between muon neutrinos and tau neutrinos and thus solved the atmospheric neutrino anomaly. In 2001 they determined, from solar neutrino data gathered by Super-Kamiokande and the Sudbury Neutrino Observatory (SNO) in Canada, that the solar neutrino deficit was due to neutrino oscillations between electron neutrinos and the other types. Those discoveries implied that neutrinos have tiny, nonzero masses, and they were the first experimental evidence for physics beyond the standard model of particle physics. Due to those fundamental contributions to the field, Totsuka was honored with numerous prizes and awards, including the Franklin Institute’s Franklin Medal, which he received in 2007 with Art McDonald, leader of the SNO experiment.

Totsuka’s excellent leadership skills were especially critical to the collaboration when an accident on 12 November 2001 destroyed more than half of the photomultiplier tubes in Super-Kamiokande. The following day he declared that the detector would be reconstructed. In a letter posted on the home page of the ICRR’s Kamioka Observatory, he wrote to his colleagues, “We will rebuild the detector. There is no question.” Without Totsuka’s guidance, the present, rebuilt Super-Kamiokande could not have been imagined.

As a result of his science and management skills, Totsuka was asked to serve in numerous leadership positions: as director of the ICRR from 1997 to 2001; as director general of Japan’s high-energy physics organization, KEK, from 2003 to 2006; and as director of the Research Center for Science Systems of the Japan Society for the Promotion of Science from 2006 to 2008. He kept working on proposals and making important decisions until right before his death.

Totsuka pushed the next-generation neutrino oscillation experiment, the T2K (Tokai to Kamioka), which is scheduled to start in late 2009. Although he will not see the project through, his younger colleagues continue his dedication by exploring neutrinos to better understand the physics of elementary particles and the universe.