Two scientists from Canada and Japan have won the 2015 Nobel Prize in Physics 2015 for opening “a new realm in particle physics,” the Nobel Prize committee says. Working far apart, both Takaaki Kajita and Arthur B. McDonald showed how neutrinos shift identities like chameleons in space.
The revelation that neutrinos — the subatomic particles that are more numerous than any other in the universe except for particles of light — undergo a metamorphosis led to a second and shocking conclusion: that they have mass.
With that discovery, Kajita and McDonald changed the field of particle physics, which had for years held that neutrinos, which rarely interact with matter, have no mass.
The discoveries about a fundamental and yet mysterious particle were made in two huge physics labs that were built deep underground — one in an old nickel mine in Canada and the other in an abandoned mineral mine in Japan. At depths of a kilometer or more, thousands of sensors at each facility are shielded from cosmic rays, allowing them to focus on detecting neutrinos’ behavior.
The Nobel committee explains the puzzle Kajita and McDonald solved:
“Since the 1960s, scientists had theoretically calculated the number of neutrinos that were created in the nuclear reactions that make the Sun shine, but when carrying out measurements on Earth, up to two thirds of the calculated amount was missing. Where did the neutrinos go?
“There was no lack of suggestions. Maybe there was something wrong with the theoretical calculations of how the neutrinos are produced in the Sun? One of the other suggestions to solve the solar neutrino puzzle was that the neutrinos change identities. According to the Standard Model of particle physics there are three types of neutrinos — electron-neutrinos, muon-neutrinos and tau-neutrinos. Each has its respective charged partner, the electron, and its two much heavier and short-lived relatives, the muon and the tau. The Sun only produces electron-neutrinos. But if they would be transformed to muon-neutrinos or tau-neutrinos on their way to Earth, that would make the deficit of the captured electron-neutrinos understandable.”
And here’s the committee on how the answer arrived:
“Around the turn of the millennium, Takaaki Kajita presented the discovery that neutrinos from the atmosphere switch between two identities on their way to the Super-Kamiokande detector in Japan.
“Meanwhile, the research group in Canada led by Arthur B. McDonald could demonstrate 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.”
Kajita, born in 1959, and McDonald, born in 1943, will share a prize of 8 million Swedish Krona, or nearly $1 million.