Takaaki Kajita’s Nobel Prize

Professor Takaaki Kajita was awarded the Nobel Prize in Physics in 2015
for discovering “neutrino oscillation” in 1998. As follows, “neutrino oscillation” will be explained.

The discovery of neutrino oscillations,
which shows that neutrinos have mass

For a long time, neutrinos were believed to have “zero” mass.

Neutrinos are a kind of elementary particle. There are three types of neutrinos. All neutrinos are extremely light, so they have long been thought to have zero mass.

Neutrinos belong to the same group as electrons. There are three kinds of neutrinos. Although electrons, muons and tauons have electric charges, neutrinos have no electric charge.

Discovery that the number of neutrinos coming from the other side of the earth is small.

In 1998, Professor Takaaki Kajita discovered that the number of neutrinos produced on the other side of the earth and traveled a long distance was only about half the number coming from just above the detector, by the observation on atmospheric neutrinos.

Atmospheric neutrinos are produced when cosmic-rays coming from space collide with the atmosphere on the earth.
Since neutrinos pass through anything, atmospheric neutrinos produced on the other side of the earth come to us through the earth.

The reason why the number of neutrinos looked small is that some of neutrinos had changed into another type of neutrinos.

This was due to the phenomenon called “neutrino oscillation” where a neutrino changes into another kind of neutrino while flying. Specifically, the number of muon neutrinos looked small because some of the muon neutrinos produced on the other side of the earth changed into tauon neutrinos while traveling through the earth.

Neutrinos change into other types of neutrinos while flying. This phenomenon is called neutrino oscillation.

Such phenomenon is the evidence that neutrinos have mass.

Neutrino oscillation is a phenomenon that happens only when the neutrino has mass. Therefore, the discovery of neutrino oscillation became the definitive evidence that a neutrino has a non-zero mass.

The Nobel Prize in Physics was awarded for the discovery beyond the established theory in particle physics.

The discovery of atmospheric neutrino oscillations beyond the established theory in particle physics opened the door to new physics. This achievement was recognized, and led to the awarding of the Nobel Prize in 2015.

Born in Saitama Prefecture.
Joined the Kamiokande experiment as a student.
March, 1986
Ph.D. for proton decay search
Autumn, 1986
Noticed the signs of neutrino oscillation.
Published the first paper on atmospheric neutrinos.
April, 1996
Start of the Super-Kamiokande experiment.
Presented the discovery of neutrino oscillations at the International Conference on Neutrino Physics and Astrophysics.
Director of the Institute of Cosmic Ray Research, The University of Tokyo

Former Director, Institute for Cosmic Ray Research, The University of Tokyo

Professor Takaaki Kajita