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Physical Chemistry
Arthur B. McDonald and Takaaki Kajita Win 2015 Nobel Prize in Physics
Awards: Discovery that neutrinos have mass upended subatomic physics
by Elizabeth Wilson
October 6, 2015
For their discoveries that the elusive but ubiquitous subatomic particles known as neutrinos have mass, Takaaki Kajita of Japan’s Super-Kamiokande and Arthur B. McDonald of Canada’s Sudbury Neutrino Observatory (SNO) will receive this year’s Nobel Prize in Physics.
Kajita and McDonald led teams that performed critical experiments near the turn of the 21st century that showed that neutrinos, once thought to be massless, oscillate between three different forms, a phenomenon only possible if the particles have mass.
The discovery has huge implications for particle physics and cosmology, Nobel committee members said at a conference in Sweden announcing the prize.
Neutrinos, which have no charge, are the second most abundant particle in the universe, behind only photons. They are produced, for example, in the core of the sun, or during the collision of cosmic rays. But they are so small that they can pass through the earth without interacting with anything, making their detection a monumental task.
The experiments at Super-Kamiokande and Sudbury involved enormous underground tanks filled with H2O and D2O, respectively. The water filtered out other, heavier particles, allowing the teams to detect neutrinos streaming in from the sun and from the atmosphere via the electronic bursts that occurred when they interacted with an atom in the water tanks.
Physicists had already found that neutrinos come in three varieties, or “flavors”—electron, muon, and tau neutrinos. The flavors were thought to be intrinsic to each neutrino. But then Kajita’s and McDonald’s teams discovered that some neutrinos actually changed their flavors in wavelike oscillations.
“There was a eureka moment when [the neutrinos] appeared to change from one type to the other,” said McDonald, who was reached by phone at the conference. Such oscillations could only be possible if the particles have mass.
“I think it’s a great prize—it’s richly deserved,” said Robert G. W. Brown, CEO of the American Institute of Physics. “It was a very complex, huge engineering challenge, and they produced an extraordinary result.”
Though physicists know the mass differences between the three neutrinos, they still don’t know the absolute mass of each one. The upper limit for each neutrino mass is believed to be 0.2 eV, which is more than a million times lighter than an electron.
Discovering the neutrinos’ absolute mass is “one of the outstanding challenges,” Nobel Committee member Olga Botner said at the conference. Various groups around the world are tweaking detection liquids to try to solve that problem.
The prizewinning discovery has made it clear that neutrinos are very different from other subatomic particles, said Mark A. Thomson, professor of experimental and particle physics at the University of Cambridge, and codirector of the university’s neutrino oscillation experiment.
“The real big mystery is that we know they have mass, but we don’t understand why the masses are so small,” Thomson said. That understanding, he told C&EN, “may provide us a clue as to why neutrinos are special.”
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