Following the Passing of Professor Masatoshi Koshiba

Professor Masatoshi Koshiba, the founder of neutrino research in Kamioka, passed away due to natural causes on November 12th, 2020.

In 1983 Professor Koshiba started the Kamiokande experiment in Kamioka with the aim of observing the spontaneous decay of protons. The experiment used a 3,000 ton water Cherenkov detector lined with 1000 large-diameter photomultiplier tubes on its inner surface. Though proton decay was not observed at Kamiokande, Professor Koshiba realized that the detector also had the potential to detect low energy neutrinos and pursued an upgrade for that purpose. Kamiokande subsequently made the first detection of neutrinos from a supernova explosion, observing neutrinos from SN1987A in the Large Magellanic Cloud in 1987, and then in 1989 observed neutrinos from the sun. These measurements marked the birth of neutrino astronomy, a discipline which focuses on observing the universe with neutrinos. In recognition of these achievements Professor Koshiba was awarded the Nobel Prize in Physics in 2002.

On the other hand, Kamiokande observed an anomaly in the neutrinos produced by cosmic ray interactions with the atmosphere: the ratio of the number of muon neutrinos to electron neutrinos differed from the expectation. Similarly, though Kamiokande had observed solar neutrinos, their number was only about half of the expectation.

In order to resolve these problems, the Super-Kamiokande (Super-K) experiment was proposed with a volume 25 times that of Kamiokande and its construction started in 1991. Indeed, Professor Koshiba had already expressed the need for such a large detector soon after Kamiokande started. Following the start of observations in 1996, Super-K discovered that the anomalies seen in its predecessor’s measurements were due to a phenomenon known as neutrino oscillation in which neutrinos change types. This discovery was recognized with the awarding of the 2015 Nobel Prize in Physics to Professor Takaaki Kajita, one of Professor Koshiba’s students on the Kamiokande experiment.

Research on neutrino oscillations has continued to progress since that time and is currently focused on whether the oscillations of neutrinos and their antiparticles differ. Such a difference could help explain why there is only matter in today’s universe and no antimatter. For that purpose, construction of the Hyper-Kamiokande experiment, which will be 10 times larger than Super-Kamiokande, started earlier this year.

In this way the seeds of neutrino astronomy sown by Professor Koshiba have grown significantly and are expected to bear even more fruit in future. We are grateful to Professor Koshiba for his contributions and leadership and will continue to advance neutrino physics with research in Kamioka.

Masayuki Nakahata
Director of Kamioka Observatory


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