As neutrino astronomy comes of age, the Nobel Foundation has decided to award Takaaki Kajita and Arthur B. McDonald with the Physics Prize for 2015 for their discovery of neutrino oscillations – a property that indicates that the fundamental particle has mass.
Takaaki Kajita is affiliated with the Super-Kamiokande neutrino detector in Japan. Kajita used the detector to report in 1998 that neutrinos that were produced when cosmic rays struck Earth’s atmosphere were ‘disappearing’ as they travelled to the detector.
Then, in 2002, McDonald of the Sudbury Neutrino Observatory in Canada reported that electron neutrinos incoming from the Sun were metamorphosing into muon- or tau-neutrinos. Electron, muon and tau are three kinds of neutrinos. What McDonald and Kajita had together found was that neutrinos were changing from one kind to another as they travelled – a property called neutrino oscillations – that is definite proof that the particles have mass.
This was an important discovery for astroparticle physics because the Standard Model group of equations that define the behaviour of fundamental particles hadn’t anticipated it. Neutrino oscillations were first predicted by Bruno Pontecorvo in 1957.