25 Years Since the Announcement of Atmospheric Neutrino Oscillation


Twenty-five years ago, on June 5, 1998, Dr. Takaaki Kajita presented the results of the analysis of atmospheric neutrino oscillations at the International Neutrino Conference held in Takayama, Gifu Prefecture, Japan. The title was “Atmospheric neutrino results from Super-Kamiokande & Kamiokande  -Evidence for numu oscillations-.”


During his presentation, Dr. Kajita first showed the observed ratio of electron neutrinos to muon neutrinos recorded by the Kamiokande detector. He pointed out that the observed ratio deviated from the theoretical value, indicating that the number of muon neutrinos detected was smaller than expected. Further, he presented the analysis results of the Super-Kamiokande data, concluding that the statistically significant deficit in the number of muon neutrinos coming from the opposite side of the Earth provided robust evidence of neutrino oscillations.


Dr. Kajita’s presentation was followed by a round of applause from the researchers in attendance. Neutrino oscillation is a phenomenon that can only occur if neutrinos have mass. Until then, the theory of elementary particles had assumed that neutrinos had zero mass, and this discovery went beyond the conventional wisdom of the time, opening the door to new frontiers in physics. The discovery of neutrino oscillations led to a worldwide experimental campaign using accelerators and reactors as the neutrino source. Recognizing this achievement, Dr. Kajita was awarded the Nobel Prize in Physics in 2015.


In the 25 years since this discovery, Super-Kamiokande has continuously expanded its observations and enhanced data accuracy. This ongoing accumulation has led to a more precise understanding of neutrino oscillations. Researchers expect that the number of observations will increase dramatically when the Hyper-Kamiokande starts observations, revealing new physics that has not been visible until now.


Dr. Kajita presenting at the International Neutrino Conference



Website of International Neutrino Conference (Neutrino98)
Dr. Kajita’s presentation slides
Movie of Dr. Kajita’s presentation