NEWS & VIEWS NATURE|Vol 454|21 August 2008

OBITUARY Yoji Totsuka (1942–2008) Leader in the discovery of neutrino oscillations.

Yoji Totsuka, a major figure in the world of of the supernova and of the neutrinos particle physics, died on 10 July after a long themselves. battle with cancer. He was a leading light Shortly after the supernova, Koshiba in the thrilling advances in the physics of retired and Totsuka became leader of neutrinos over the past 30 years, and was the Kamiokande project. Two seminal a notably resolute figure in seeing through results from the group were subsequently the rebuilding of the Super-Kamiokande published. The first observed a deficit in neutrino detector after a disastrous accident atmospheric neutrinos that could not be in 2001. explained by systematic experimental errors Born on 6 March 1942, in Fuji, Japan, or uncertainties in the background neutrino Totsuka received his bachelor’s, master’s and flux. Instead, some “as-yet-unaccounted-for PhD degrees from the University of Tokyo. physics”, as the paper put it, might explain His thesis project studied ultra-high-energy the data. The second crucially confirmed the particle interactions, so kick-starting his solar neutrino deficit recorded by Davis. lifelong passion for particle physics. As a In 1991, Totsuka obtained funding for a Totsuka also supervised the successful research associate with the University of successor to Kamiokande. This was Super- Belle B-factory, where particles known as Tokyo, he then travelled to the Deutsches Kamiokande, an underground detector B-mesons were generated to study differences Electron Synchrotron Laboratory in that contained 50,000 tons of water. Super- between matter and antimatter. Germany to investigate electron–positron Kamiokande provided the first definitive Totsuka proved his mettle as a leader in the collisions. He went on to accept an associate evidence for neutrino oscillations and aftermath of the Super-Kamiokande accident professorship at Tokyo in 1979. neutrino mass. Neutrino oscillation is a in 2001, when thousands of photomultipliers Perhaps the turning point in Totsuka’s process in which neutrinos convert between in the detector imploded. The day after the career came in 1981, when he started work on one of three types: electron neutrinos, muon accident, he announced that the detector the Kamiokande experiment with Masatoshi neutrinos and tau neutrinos. Atmospheric would be reconstructed within a year. In Koshiba — who was later jointly awarded neutrinos are predominantly muon and just two months he established a road-map the Nobel Prize in Physics with Ray Davis Jr electron neutrinos. Because these particles to achieve this, and the detector was indeed for the detection of cosmic neutrinos. The can pass through the Earth without being ready for action 13 months after the accident. Kamiokande project was originally designed absorbed, it was predicted that as many During the last year or so of his illness, to detect proton decays. A tank of ultra- atmospheric neutrinos should come up from Totsuka wrote a blog in which he aired his pure water acted as a source of protons; the the ground as travel down from the sky. But views on science and science policy. He also products of proton decay would interact with the observations with Super-Kamiokande used this forum to describe his own illness, the water to produce distinctive Cherenkov found that, although there were equal plotting the extent of his cancer as a function radiation, which could be observed by numbers of electron neutrinos travelling in of time, and evaluating the effectiveness hundreds of photomultiplier tubes arranged these directions, fewer muon neutrinos were of the chemical treatment. And, revealing around the inner surface of the tank. A huge detected coming up than were going down. entirely different interests, he discussed the volume of water was required (3,000 tons), We now know that, because the upward- flowers in his garden and in the village where which was buried deep underground to block going muon neutrinos had travelled much Super-Kamiokande is located. out background noise from cosmic rays. farther — through the planet — than Totsuka received international recognition The experiment set stringent limits on the those that came directly down through for his work, winning many academic proton-decay process and was also able to the atmosphere, they had more time to prizes — perhaps most notably the Order study both low-energy neutrinos from the Sun convert into tau neutrinos (which were of Culture, the most prestigious prize in and neutrinos produced in Earth’s atmosphere essentially invisible to the detector). This Japan. It is a testament to his achievements by cosmic rays. Solar neutrinos had previously evidence of neutrino oscillation suggested that neutrino physics continues to generate been detected by Ray Davis Jr in the late 1960s an explanation for the solar neutrino deficit: enormous interest. The efforts of the with the use of a chlorine-based detector, but the ‘missing’ electron neutrinos from the Sun Super-Kamiokande collaboration laid only one-third of the expected neutrino flux might simply have been converted into tau the foundations for the current, detailed was observed. This discrepancy with theory neutrinos and muon neutrinos, which could understanding of the atmospheric neutrino became known as the solar neutrino deficit not be spotted by the earlier detectors. This anomaly and the solar neutrino problem. Yoji and was an early indication of a problem in was later confirmed through work at Super- Totsuka was central to those efforts, and his understanding neutrinos. Kamiokande and at the Sudbury Neutrino vision and leadership will be sorely missed. In 1987, the Kamiokande collaboration had Observatory in Canada. Henry W. Sobel & Yoichiro Suzuki a stroke of luck when a supernova exploded in In 2003, Totsuka became the Director Henry W. Sobel is at the School of Physical the Large Magellanic Cloud. Along with the General of Japan’s high-energy physics Sciences, University of California, Irvine, Irvine–Michigan–Brookhaven consortium in organization, KEK. Over the next three years, Irvine, California 92697-4575, USA. Yoichiro the United States, the Kamiokande detector he oversaw the K2K neutrino-oscillation Suzuki is at the Kamioka Observatory, Institute observed the resulting burst of neutrinos. experiment, in which a beam of neutrinos for Cosmic Ray Research, University of Tokyo, This event was hailed as the birth of neutrino was sent 250 km from the KEK accelerator Higashi-Mozumi, Kamioka, Hida-City, astronomy. More than 800 scientific papers to Super-Kamiokande. This experiment Gifu 506-1205, Japan. were subsequently written that used the data confirmed the earlier findings of atmospheric e-mails: [email protected]; to extract information about the properties neutrino oscillation. During this period, [email protected]

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