Missing: Electron antineutrinos; Reward: Understanding of matter-antimatter imbalance

The result from the Daya Bay Reactor Neutrino Experiment describes a critical and previously unmeasured quality of neutrinos -- and their antiparticles, antineutrinos -- that may underlie basic properties of matter and explain why matter predominates over antimatter in the universe.

Embedded under a mountain near the China Guangdong Nuclear Power Group power plant about 55 kilometers from Hong Kong, the Daya Bay experiment used neutrinos emitted by powerful reactors to precisely measure the probability of an electron antineutrino transforming into one of the other neutrino types.

The results, detailed in a paper submitted to the journal Physical Review Letters, reveal that electron neutrinos transform into other neutrino types over a short distance and at a surprisingly high rate.

"Six percent of the electron antineutrinos emitted from the reactor transform over about two kilometers into another flavor of neutrino. Essentially they change identity," explains University of Wisconsin-Madison physics professor Karsten Heeger. Heeger is the U.S. manager for the Daya Bay antineutrino detectors.

Coincident with presentations by other principal investigators in the Daya Bay collaboration, Heeger is describing the results March 8 in a talk at the Symposium on Electroweak Nuclear Physics, held at Duke University.

Neutrinos oscillate among three types or "flavors" -- electron, muon, and tau -- as they travel through space. Two of those oscillations were measured previously, but the transformation of electron neutrinos into other types over this distance (a so called "mixing angle" named theta one-three, written ?13) was unknown before the Daya Bay experiment.

"We expected that there would be such an oscillation, but we did not know what its probability would be," says Heeger.

The Daya Bay experiment counted the number of electron antineutrinos recorded by detectors in two experimental halls near the Daya Bay and Ling Ao reactors and calculated how many would reach the detectors in a more distant hall if there were no oscillation. The number that apparently vanished on the way -- due to oscillating into other flavors -- gave the value of theta one-three.

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.

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.