Professor Chiaki Yanagisawa

On October 6, 2015 the Royal Swedish Academy of Sciences announced that the Nobel Prize for Physics is awarded to two physicists, Professors Takaaki Kajita of the University of Tokyo, Japan and Arthur McDonald of Queen’s University, Canada, for discovery of neutrino oscillation. See the link:

Recognizing the potentials of a new project in Japan, I and Professor C. K. Jung of Stony Brook University were the first physicists in the U.S. to have negotiated for our participation with Prof. Y. Totsuka of the University of Tokyo who started the project to build a 50,000 tons water Cherenkov detector 1,000 m below a mountain in Japan, now known as Super-Kamiokande.

Eventually we, together with a group of physicisits from the University of California/Irvine, Boston University, and Brookhaven National Lab, joined the Super-Kamiokande experiment in 1992. A water Chrekov detector is based on the fact that electrically charged particle radiates light when it moves faster than speed of light in a medium such as water.

The detector then captures light coming from Chrenkov radiation with photosensors on the water tank wall. In the case of Super-Kamiokande, we installed 11,000 50 cm and 2,000 20 cm photomultipleirs as photosensors.

Since 1992, I have been a member of the Super-Kamiokande (SK) Collaboration. I have also been a member of a research team lead by Prof. Kajita that studies neutrinos produced in atmosphere by cosmic rays. We noticed that the observed number of a type of neutrinos (muon neutrinos) was much less than what we expected.

As we accumulated more data on atmospheric neutrinos, we were finally convinced that missing muon neutrinos actually turn into another type of neutrinos (tau neutrinos) that we cannot detect. Prof. Kajita presented our discovery at an international conference in Japan in 1998. This announcement took the particle physics community by storm.

Neutrino oscillation is a macroscopic manifestation of quantum effect in which neutrino changes its identity called “flavor” while it is in flight.

This implies that, although for a long time we thought neutrino was massless, neutrino has actually tiny mass. This discovery is the first crack in the highly successful Standard Model of particle physics which assumes that neutrino is mass-less. Using neutrino oscillation as a tool, we now try to answer one of the greatest mysteries of the Universe: why we see, overwhelmingly, matter but not anti-matter in the Universe.

In addition to the study of neutrino properties, Super-Kamiokande is also capable of detecting proton decay, a sign of the Grand Unified Theory, detecting supernova neutrinos coming from the center of Milky Way hopefully to witness the birth of a blackhole some day, and looking for some kinds of dark matter candidates.

As a side note in addition to this prize, my adviser, Prof. M. Koshiba of the University of Tokyo, is a member of the Super-Kamokande Collaboration without whom this experiment would not have been funded. He was also awarded the Nobel Prize for Physics in 2002 for his successful detection of neutrinos from a supernova from Large Magellanic Cloud in 1987 using the predecessor of Super-Kamiokande called Kamiokande.

I would also like to mention that without the late Prof. Y. Totsuka, who was the founding father of Super-Kamiokande, we would not be where we are now in understanding of properties of neutrinos. I feel extremely lucky to have had the opportunity to work with the people I have mentioned.