After analyzing signals of tens of thousands of electron antineutrinos emitted by the nuclear reactors, the researchers discovered that electron antineutrinos disappeared at a rate of six percent over the two kilometers between the near and far halls, a very short distance for a neutrino.
"Our precise measurement will complete the understanding of the neutrino oscillation and pave the way for the future understanding of matter-antimatter asymmetry in the universe," says Yifang Wang of China's Institute of High Energy Physics, co-spokesperson and Chinese project manager of the Daya Bay experiment.
The value is unexpectedly large and helps explain why the experiment was able to make a precise measurement so quickly, with less than two months' worth of data from just six of the planned eight detectors.
"Although we're still two detectors shy of the complete experimental design, we've had extraordinary success in detecting the number of electron antineutrinos that disappear as they travel from the reactors to the detectors two kilometers away," says Kam-Biu Luk of the U.S. Department of Energy's Lawrence Berkeley National Laboratory and the University of California at Berkeley. Luk is co-spokesperson of the Daya Bay Experiment and heads U.S. participation.
The researchers confirmed the finding with very high confidence, Heeger says – in statistical terms, greater than five sigma, which translates to a less than a 1 in 3.5 million chance that the result arose by random chance.
The findings fill in a major gap in understanding neutrino oscillation and will provide important guidance for future neutrino experiments, including looking for nonstandard effects outside of current theories.
Under the guidance of U.S. chief project engineer Jeff Cherwinka, an engineer at the UW–Madison Physical Sciences Laboratory (PSL), the collaboration is now assembling the last two detectors and will install them this summer to increase data collection and improve precision. The UW–Madison PSL and Department of Physics have been involved in designing and building the detectors since 2006.
"What made this possible is that the detectors worked really well. We have a very strong technical engineering team with PSL, which led the onsite assembly and installation of the detectors. This allowed us to come online ahead of schedule and make these measurements so quickly," Heeger says.
Heeger will also present the findings locally in a seminar at 3:00 p.m. on Mar. 13 in 4272 Chamberlin Hall on the UW–Madison campus.
The Daya Bay collaboration is jointly led by China and the United States, with additional participants from Russia, the Czech Republic, Hong Kong, and Taiwan.