A Nobel for Neutrinos: Super-Kamiokande

Discovery of neutrino oscillations, which shows that neutrinos have mass, garners the 2015 Nobel Prize in Physics.

Image courtesy of University of Tokyo
Inside the Super-Kamiokande detector, scientists clean light-detecting photomultiplier tubes from a raft as the large underground tank is slowly filled with 50,000 metric tons of ultra-pure water.

The Science

The 2015 Nobel Prize in Physics was shared by Arthur B. McDonald, from the Sudbury Neutrino Observatory (SNO), and Takaaki Kajita, from the Super-Kamiokande collaboration, “for the discovery of neutrino oscillations, which shows that neutrinos have mass.”

The Impact

The discovery of neutrino oscillations and mass has profoundly affected our understanding of these elusive particles, their role in the theoretical underpinning of physics, and the evolution of the universe. The discovery opens new doors to an understanding of basic questions about solar energy generation, about why the universe contains more matter than antimatter, and what properties a new standard model of particle physics must have.


Neutrinos are neutral, weakly interacting particles produced abundantly in the big bang, by the sun, and by cosmic rays striking the earth’s atmosphere. They have long been thought to be massless, a prediction of the standard model of particles and fields. Beginning in the 1960s, Raymond Davis, Jr., a scientist at Brookhaven National Laboratory, began to measure the flux of neutrinos from the sun. His experiment and subsequent ones at Kamiokande in Japan, SAGE in Russia, and GALLEX in Italy all found that the solar neutrino flux was much smaller than expected. In 1985, Herbert Chen a professor at University of California-Irvine, observed that if neutrinos oscillated, they would still arrive at earth but in “flavors” undetectable in the Davis experiment, which was designed for electron neutrinos. In the mid-1980’s, experiments designed to look for proton decay, Kamiokande and the Irvine-Brookhaven-Michigan (IMB) experiment in the U.S., found an anomaly in the flux of neutrinos from cosmic rays in the earth’s atmosphere. Resolving these two puzzles were prime motivating factors for the second generation experiments, SNO and Super-Kamiokande.

There are 3 neutrino flavors, called electron, muon, and tau, but the sun can only make electron neutrinos. Chen proposed the SNO detector, which could use heavy water to detect all flavors equally, and in 2001 SNO showed that two-thirds of the electron neutrinos produced by the sun had converted to non-electron flavors. In 1992, U.S. physicists joined the Kamiokande group in Japan to collaborate on the next-generation Super-Kamiokande detector. The U.S. group brought significant expertise in large water Cherenkov detectors and the atmospheric neutrino flux garnered from their work on the IMB experiment. The Super-Kamiokande detector began data taking in 1996 and in 1998 discovered that muon neutrinos produced in the atmosphere were converting to a non-muon flavor. Super-Kamiokande also made measurements of the solar neutrino flux that supported the interpretation of solar electron neutrinos converting to another flavor.

The flavor conversions discovered by Super-Kamiokande and SNO can only occur if neutrino masses are non-zero, a discovery that revised our basic model of particles and fields and transforms our understanding of the universe. Although neutrinos have a very low mass, their high abundance affects the form and evolution of the large-scale structure of galaxies in the universe. Future experiments seek to understand whether neutrinos played a role in causing the imbalance of matter and antimatter that has led to the existence of the universe as it is today.


Hank Sobel
University of California Irvine

Jim Stone
Boston University


The research was supported by the following: U.S.:

Department of Energy, Office of Science, Offices of High Energy Physics and Nuclear Physics, and the National Science Foundation

Japan: Japanese Ministry of Education, Science, Sports and Culture


Study of the atmospheric neutrino flux in the multi-GeV energy range By Super-Kamiokande Collaboration (Y. Fukuda et al.). Phys.Lett. B436 (1998) 33-41.

Evidence for oscillation of atmospheric neutrinos By Super-Kamiokande Collaboration (Y. Fukuda et al.). Phys.Rev.Lett. 81 (1998) 1562-1567.

Evidence for an oscillatory signature in atmospheric neutrino oscillation By Super-Kamiokande Collaboration (Y. Ashie et al.). Phys.Rev.Lett. 93 (2004) 101801.

Related Links

A Nobel for Neutrinos: Sudbury Neutrino Observatory

Nobel Prize Press Release

Nobel Prize Scientific Background

Highlight Categories

Program: HEP , NP

Additional: Collaborations , Non-DOE Interagency Collaboration