Physics Nobel goes to neutrino work by Kajita and McDonald.

The 2015 Nobel Prize in Physics was awarded Tuesday to two researchers ”for the discovery of neutrino oscillations, which shows that neutrinos have mass.” To understand that prize, you have to look deep underground to complex experiments performed in enormous tanks of water, all with an eye toward evaluating some of the smallest particles in the universe.

This year’s prize was split between Takaaki Kajita, director of the wonderfully named Super-Kamiokande Collaboration at the University of Tokyo, and Arthur B. McDonald, of the Sudbury Neutrino Observatory Collaboration at Queen’s University in Kingston, Canada. Though they worked separately, their efforts jointly demonstrated that neutrinos change their form as they travel.

First proposed theoretically by the Austrian scientist Wolfgang Pauli in 1930, neutrinos are a species of subatomic particles produced by radioactive decay. Neutrinos continued to puzzle physicists until the 1950s, when they were at last detected. Though countless billions of these particles—produced in stars and nuclear reactors alike—pass through our bodies every second, they remain extraordinarily difficult to record and describe. Measuring them—and, it turns out, weighing them—has required astonishing feats of experimental ingenuity.

Consider the Super-Kamiokande project, which unfolded more than half a mile underground in a repurposed zinc mine near Tokyo. (Neutrinos pass through the Earth easily, but the surrounding rock shields the underground detector from other types of cosmic radiation.) Almost alien in its beauty, Super-Kamiokande allowed researchers to conclusively demonstrate in the late 1990s that neutrinos have mass. By way of evidence, they pointed to the discovery that neutrinos changed form as they traveled. As Adrian Cho explains in Science, if neutrinos were truly massless, they “would have to travel at light speed, in which case time for them would stand still and any change would be impossible.”

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In a phone interview with Adam Smith of the Nobel Prize organization, Kajita describes the award as “kind of unbelievable,” later adding that he had no idea what the hours ahead held for him. Cautious about the scope of his work, Kajita nevertheless acknowledged that it helped upset the Standard Model in physics. Previously, the Standard Model had failed to account for the mass of neutrinos.

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While some scientists remained skeptical when Kajita first announced Super-Kamiokande’s results, McDonald’s work at the Sudbury Neutrino Observatory confirmed and advanced them in 2001. Also located in an unused mine (nickel rather than zinc), the observatory is principally composed of a ball of heavy water (in which the hydrogen in H₂O has an extra particle) deep underground. McDonald’s experiments accounted for the ways that neutrinos change as they travel from the sun to the Earth, shifting their “flavor” as they go. 

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Elaborate, costly projects like Super-Kamiokande and the Sudbury Neutrino Observatory are necessary in part because neutrinos are extraordinarily elusive. Even as it has solved long-standing enigmas, Kajita and McDonald’s work in these facilities has opened up a host of other important questions, not least of which involves the actual mass of neutrinos, which has only been measured in terms of the differentials between the particles’ three distinct flavors. As a paper on the background of the prize from the Royal Swedish Academy of Sciences notes, the true consequences of this research are still hard to predict. A more comprehensive picture of neutrinos should enrich our knowledge of the very underpinnings of the universe.

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• Physics

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