The Nobel Prize in Physics 2015 recognised Takaaki Kajita in Japan and Arthur McDonald in Canada for their contributions to experiments which demonstrated that neutrinos change identities.
“This metamorphosis requires that neutrinos have mass. The discovery has changed our understanding of the innermost workings of matter and can prove crucial to our view of the universe,” said the Royal Swedish Academy of Sciences.
Several years ago, Takaaki Kajita presented the discovery that neutrinos from the atmosphere switch between two identities on their way to the Super-Kamiokande detector in Japan.
The research group in Canada led by Arthur B. McDonald demonstrated that the neutrinos from the sun were not disappearing on their way to Earth.
Instead, they were captured with a different identity when arriving to the Sudbury Neutrino Observatory.
Compared to theoretical calculations of the number of neutrinos, up to two thirds of the neutrinos were missing in measurements performed on Earth.
The two experiments then discovered that the neutrinos had changed identities.
The discovery led to the conclusion that neutrinos, which for a long time were considered massless, must have some mass, however small.
For particle physics this was a historic discovery. Its Standard Model of the workings of matter had been successful, having resisted all experimental challenges for more than 20 years.
However, as it requires neutrinos to be massless, the new observations showed that the Standard Model cannot be the complete theory of the fundamental constituents of the universe.