Twin detectors recently installed in the first of three experimental halls in the Daya Bay Reactor Neutrino Experiment are now recording interactions of elementary antineutrinos produced by powerful reactors at the China Guangdong Nuclear Power Group power plant located about 55 kilometers from Hong Kong.

China and the U.S. lead the Daya Bay Reactor Neutrino Experiment, which also includes participants from Russia, the Czech Republic, Hong Kong, and Taiwan and the event marks the first step in the international effort to measure a puzzling property of neutrinos and antineutrinos that may underlie basic properties of matter and why matter predominates over antimatter in the universe.

Theories state that equal portions of matter and antimatter were created during the Big Bang  but we know today matter prevails.  "Right now there is not a good understanding of what causes the matter-antimatter imbalance in the universe," says Karsten Heeger, a University of Wisconsin-Madison physics professor and one of the scientists in the experiment. "We live in a world of matter and don't know where all the antimatter went."

Using antineutrinos as a probe, the Daya Bay experiment seeks to understand how the difference came about by measuring with unprecedented precision a crucial type of transformation called neutrino mixing. Neutrinos come in three types or "flavors" – electron, muon, and tau – that can morph or oscillate from one form to another as they travel through space and matter. Two of the oscillations have been studied but one transformation of electron neutrinos (called θ13 or theta one-three) has not been measured.

Because they are tiny and uncharged, neutrinos and antineutrinos can pass through even huge amounts of matter such as the planet Earth with no interactions, a property that makes them very difficult to detect and study. The large size and sensitivity of the detectors and power of the reactors at Daya Bay will provide the best opportunity to date to collect enough antineutrinos to precisely measure the last unknown neutrino mixing angle.

"The results will be a major contribution to understanding the role of neutrinos in the evolution of basic kinds of matter in the earliest moments after the Big Bang, and why there is more matter than antimatter in the universe today," says co-spokesperson Kam-Biu Luk of the U.S Department of Energy's Lawrence Berkeley National Laboratory and the University of California at Berkeley.

The massive antineutrino detectors are positioned underground and submerged in pools of ultrapure water to shield them from cosmic rays, natural sources of radiation, and other background signals. When completed, the experiment will consist of eight 125-ton antineutrino detectors, two each in two experimental halls near the Daya Bay and Ling Ao nuclear power reactors and four in a far hall about two kilometers away.

"By having these different locations, we can see the neutrinos at different distances from the reactors and how they change as they travel through space," says UW-Madison's Heeger, who is the U.S. manager for the antineutrino detectors.

Heeger has worked on the experiment for the past eight years and since 2006 his group in the physics department has been responsible for much of the design and development of the antineutrino detectors. Together with engineers from the UW-Madison Physical Sciences Laboratory (PSL), they developed the acrylic target vessels in the interior of each detector, developed precise calibration of the target liquids, modeled the integration of all detector components, and are providing oversight of the assembly and installation process currently underway in China.

The next two detectors are now being assembled in the second experimental hall and are expected to come on line early this fall. The remaining four detectors will be completed next year.