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Takaaki Kajita and Arthur B. McDonald

The University of Tokyo/Queens University

Proof that neutrinos change identity bags physics Nobel

A weird identity shifting among ghostly particles called neutrinos has won the 2015 Nobel Prize in Physics for the leaders of massive underground experiments in Japan and Canada. Takaaki Kajita of the University of Tokyo led researchers working with the Super-Kamiokande detector in a zinc mine 250 kilometers northwest of Japan's capital that made its key discovery in 1998. Arthur McDonald of Queen's University in Kingston, Canada, led the team working with the Sudbury Neutrino Observatory (SNO) in a mine in Canada that confirmed and expanded on the Super-Kamiokande result in 2001.

Hardly interacting with other matter, neutrinos come in the three different types—electron, muon, and tauand the winners of this year's prize showed that the three types can morph into one another as the particles zip along at near-light speed. That, in turn, proves that the elusive particle must have mass. Researchers working with Super-Kamiokande found that muon neutrinos, which are produced as high-energy particles from space strike the atmosphere, change identity as they travel. To do that, they compared the number of muon neutrinos raining down from above with those coming a much longer distance upward through Earth and showing that the numbers were different, proof that the neutrinos had changed type in transit.

Physicists with the SNO looked at neutrinos from the sun, all of which start out as electron neutrinos. They discovered that along the way some of those neutrinos change into muon and tau neutrinos as the particles make a 150-million-kilometer journey from the sun. To do that, they employed two methods within one detectorone that could detect just the electron neutrinos, and another that would count the total flux of all neutrinos.

Both experiments were large collaborations involving hundreds of physicists. "I have many colleagues who share this prize with me," McDonald said on the phone during the press conference to announce the prize.

The observations proved that neutrinos aren't completely massless but must have some weight. That's because, according to special relativity, massless particles would have to travel at light speed, in which case time for them would stand still and any change would be impossible. The results kicked off the study of such "neutrino oscillations," which is now one of the major thrusts of particle physics, involving huge experiments in which neutrinos are fired hundreds of kilometers through Earth to distant detectors. The study of such oscillations could eventually shed light matters as fundamental as how the universe generated so much more matter than antimatter.

"I'm quite thrilled, and still speechless," says particle physicist Alfons Weber of the University of Oxford in the United Kingdom. "It came as a surprise, but perhaps it is not so unexpected. This is one of the very big discoveries." The results obtained by the new Nobel laureates' experiments are somewhat counterintuitive, he says. "It's a strange thing in nature for one thing to change into another. It's like you throw up an apple and someone catches an orange."

With reporting by Dan Clery.