By Ashley Yeager
Two anti-neutrino detectors at Daya Bay, shown here prior to the pool being filled with ultrapure water. Courtesy of Roy Kaltschmidt, Lawrence Berkeley National Laboratory.
Elusive particles called neutrinos can change their flavors, just like the Wrigley Company trying out a new taste of Starburst candy.
Now, physicists say they have gotten the best glimpse yet of the most elusive change in neutrino flavors. The result is the “missing piece in the puzzle to understand the phenomenon of how the particles transform,” said University of Wisconsin-Madison physicist Karsten Heeger, a collaborator at the Daya Bay experiment.
He announced the new result at a symposium at Duke on March 8. The team has also submitted a paper on the result to Physical Review Letters.
Neutrinos are elementary particles that come in three flavors — muon, electron and tau. In past experiments, physicists have measured two of the ways that neutrinos can change flavors.
But no one had seen the third transformation yet. “It revealed itself in the disappearance of electron-flavored antineutrinos over a distance of only two kilometers at the Daya Bay experiment,” Heeger said. An anti-neutrino is the anti-matter counterpart of a neutrino.
By observing the change over short distances, the physicists have measured the “mixing angle,” called theta one-three. Measuring the angle will help them design new experiments to better understand why matter predominates over antimatter in the universe.
Last year, physicists at the T2K neutrino experiment in Japan said they had seen hints of neutrinos flipping flavors in a way to give them theta one-three. But the experiment was interrupted when Japan was hit by an earthquake and tsunami on March 11, 2011. Their results at that point did not have enough significance to constitute a discovery in particle physics.
In the new neutrino experiment, Heeger and his collaborators looked for anti-neutrinos coming from the six nuclear reactors at the Daya Bay Nuclear Power Plant in southern China. The team built and installed six anti-neutrino detectors in the mountains near the plant. Three of the detectors sit only about 500 meters from the plant, while the other three sit 1700 meters from it.
The nuclear reactors produced tens of thousands of electron antineutrinos. Recording the particles’ signals, the scientists found that the far detector registered six percent fewer electron-flavored particles. The deficit, according to Heeger and his collaborators, is the signal for the elusive neutrino flavor changes in neutrinos. He thinks there is less than a 1 in 3.5 million chance that the result happened by random chance.