The discovery that subatomic particles called neutrinos have mass has won Takaaki Kajita of the University of Tokyo and Arthur McDonald of Queen’s University in Kingston, Canada, the 2015 Nobel Prize in physics. The scientists led two sophisticated experiments that found that the elusive particles can morph from one variety into another — a phenomenon that can occur only if neutrinos have mass. The discovery delivered a jolt to particle physics because prevailing theories had predicted that neutrinos were massless.
Physicists knew there are three types of neutrinos: muon, electron and tau. But in 1998, Kajita and his team at the Super-Kamiokande experiment found evidence that neutrinos produced in Earth’s atmosphere switched identities before striking the detector, located under a Japanese mountain. Three years later, McDonald’s Sudbury Neutrino Observatory collaboration discovered that some neutrinos emitted by the sun change flavors en route to Earth.
Today physicists around the world are working to identify the particles’ exact masses and understand neutrinos’ importance throughout the history of the universe.