Physics Nobel Prize: What's the story with neutrinos?

The Nobel committee has awarded the physics prize to Takaaki Kajita and Arthur B. McDonald for their independent research on the oscillation of neutrinos, the elusive subatomic particles that have been proven to have mass.

Issei Kato
Takaaki Kajita, director of the Institute for Cosmic Ray Research and professor at University of Tokyo, smiles as he speaks with Japanese Prime Minister Shinzo Abe at the start of a news conference in Tokyo October 6, 2015. Kajita and Arthur McDonald, a Canadian scientist, won the 2015 Nobel Prize for Physics on Tuesday for discovering that elusive subatomic particles called neutrinos have mass, opening a new window onto the fundamental nature of the universe.

The Nobel Prize committee have chosen Takaaki Kajita of Japan and Arthur B. McDonald of Canada as the 2015 recipients of the prize in physics for their discovery of the oscillation of neutrinos.

Perhaps the least obtrusive entity known to science, a neutrino is a subatomic particle with no electric charge. Coming into existence soon after the birth of the universe, neutrinos also are produced by high-energy collisions, such as those found in the nuclear reactions and particle accelerators. They are ubiquitous; at any given moment, trillions of neutrinos are streaming through your body at nearly the speed of light. 

First theorized in the 1930s, the neutrino is now thought to be the most abundant particle in the cosmos, after the photon, the particle that conveys light and other electromagnetic radiation. Like the photon, the neutrino was long thought to be massless, until Dr. Kajita and Dr. McDonald proved otherwise.

Scientists first began to suspect neutrinos might be up to something special in the late 1960s, when measurements of neutrinos coming from the sun came up short of mathematical expectations: up to two thirds of the ghostly particles seemed to be missing.

Using the Super-Kamiokande, a neutrino observatory deep inside a zinc mine in Japan, in 1998 Kajita and his colleagues discovered neutrinos streaming through the atmosphere were shifting "flavors." 

Around the same time, McDonald was leading a team in observing neutrinos from the sun at the Sudbury Neutrino Observatory in Canada, and came to a similar conclusion. Rather than vanishing on their way to Earth, the neutrinos were changing to a new flavor that the solar neutrino detectors hadn't been looking out for.

It turns out that neutrinos can oscillate between three distinct flavors: electron, tau, and muon. And if neutrinos are able to change their flavors, they must have mass.

For particle physics this was a historic discovery,” read Tuesday's press release from the Nobel Committee. “As it requires neutrinos to be massless, the new observations had clearly showed that the Standard Model cannot be the complete theory of the fundamental constituents of the universe.”

Since acquiring its modern form in the 1970s, the Standard Model of particle physics has remained largely unchallenged, predicting, with astonishing accuracy, an array of subatomic phenomena. In 2013, François Englert and Peter Higgs won the Nobel Prize in physics for their confirmation of the existence of the Higgs boson, filling in a large gap in the Standard Model.

But this theoretical framework still cannot account for neutrino oscillation, suggesting that the Standard Model will at some point need to be revised.

“Now the experiments continue and intense activity is underway worldwide in order to capture neutrinos and examine their properties,” the Nobel committee concluded. “New discoveries about their deepest secrets are expected to change our current understanding of the history, structure, and future fate of the universe.”

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