2015 Nobel Prize in Physics

Picture of a Japanese man in his mid-fifties, in a suite with a blue tie, smiling while talking on a cellphone.
Takaaki Kajita speaks on the phone to Prime Minister of Japan Shinzo Abe during a press conference on October 6, 2015 in Tokyo, Japan. The Nobel committee announced on October 6, 2015 that Takaaki Kajita of the University of Tokyo and Arthur B. McDonald of Queen's University in Canada won the Nobel Prize in Physics for discovering that the elusive subatomic particles known as neutrinos have mass. Chris McGrath/Getty Images

On October 6, 2015, the Nobel Prize committee announced that the 2015 Nobel Prize in Physics was awarded jointly to Takaaki Kajita and Arthur B. McDonald "for the discovery of neutrino oscillations, which shows that neutrinos have mass."

The Science: Neutrino Oscillation

In 1914, James Chadwick first demonstrated differences in the spectrum of energy released in various forms of radioactive decay. At the time, there was suspicion that this might suggest a violation of conservation of energy, but in 1930 Wolfgang Pauli instead suggested that the missing energy could be accounted for by incorporating a new, undetected particle into the model of the beta decay.

Pauli originally suggested the name "neutron" for this particle but in 1932, Chadwick discovered the nucleon called the neutron, so Enrico Fermi suggested the name neutrino to describe the weakly interacting, electrically neutral, half-spin particle predicted by Pauli. Fermi also expanded the basic concept put forth by Pauli into a more comprehensive theoretical model.

The neutrino was experimentally detected in 1956, in research that was ultimately recognized with a portion of the 1995 Nobel Prize in Physics. Physicists used these results to incorporate neutrinos into the Standard Model of physics through the 1970s, though the assumption was that these weakly-interacting particles contained no mass.

There were three types of neutrinos in this model: the electron-neutrino, muon-neutrino, and tau-neutrino. These types of neutrino are called the three "flavours" of neutrinos.

Cosmic rays interacting in the upper atmosphere produce muon-neutrinos.

Research presented in 1998 by Takaaki Kajita, working at the Super-Kamiokande neutrino detector in Japan, showed that many fewer of these atmospheric muon-neutrinos reach the detector.  

In 2001 and 2002, Arthur B. McDonald presented research from the Sudbury Neutrino Observatory (NO) Collaboration which demonstrated evidence that neutrinos could convert between the different flavours.

 This neutrino oscillation explained what happened to the missing atmospheric neutrinos in the Super-Kamiokande results, and together provided evidence that the previous understanding of neutrinos was incomplete. 

As mentioned above, the Standard Model assumes that they are massless particles, and also that they cannot oscillate between different flavours. Applying the mathematics of quantum mechanics to incorporate the neutrino oscillation into the model resulted in predictions that the neutrino actually contained mass.

The study of neutrino properties remains a strong area of particle physics research, as the Standard Model continues to be modified to incorporate the new understanding of these particles. 

The Scientists: Takaaki Kajita

Takaaki Kajita was born in 1959 in Higashimatsuyama, Japan. He received an undergraduate degree from Saitama University in 1981, followed by a 1986 doctorate from the University of Tokyo in Kashiwa, Japan. He has been worked at the University of Tokyo's Institute for Cosmic Radiation Research since 1988, including conducting research at the Super-Kamiokande neutrino detector.

The Scientists: Arthur B. McDonald

Arthur B. McDonald was born in 1943 in Sydney, Nova Scotia, Canada.

He received a bachelor's degree in physics from Dalhousie University, in Nova Scotia, followed by a 1965 master's degree. He went to the California Institute of Technology for his physics PhD.

He worked at the Chalk River Nuclear Laboratories near Ottawa, Ontario, from 1970 to 1982. He was a physics professor at Princeton University from 1982 to 1989. After leaving Princeton, he joined the faculty of Queen's University in Kingston, Ontario, Canada, as University Research Chair. He is a board member of the Perimeter Institute for Theoretical Physics in Waterloo, Ontario.