New neutrino detection experiment in China up and running

Deep under a hillside near Hong Kong, a pair of new antineutrino detectors are warming up for some serious physics.

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

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 today matter prevails, explains Karsten Heeger, a University of Wisconsin-Madison physics professor and one of the leading scientists in the experiment. "Right now there is not a good understanding of what causes the matter-antimatter imbalance in the universe," he says. "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 "flavours" - 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 ?13 or theta one-three) has not been measured.

"This is a remarkable achievement after eight years of effort - four years of planning and four years of construction - by hundreds of physicists and engineers from around the globe," says Yifang Wang of the Institute of High Energy Physics (IHEP) of the Chinese Academy of Sciences, a co-spokesperson for the Daya Bay Collaboration.

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.