The Daya Bay Reactor Neutrino Experiment, a multinational collaboration operating in the south of China, has reported its first results. The team has analysed tens of thousands of interactions of electron-antineutrinos caught by six massive detectors buried in the mountains adjacent to the powerful nuclear reactors of the China Guangdong Nuclear Power Group (CERN Courier October 2011 p5).

The copious data revealed for the first time a strong signal of the mixing angle θ13, related to the type of neutrino oscillation in which electron-neutrinos morph into the other two flavours. This is the last mixing angle to be measured precisely and could reveal clues leading to an understanding of why matter predominates over antimatter in the universe. Once thought to be near zero, the first results indicate that sin213 is equal to 0.092 ± 0.017.

The Daya Bay experiment counts the number of electron-antineutrinos detected in the halls nearest the Daya Bay and Ling Ao reactors and calculates how many would reach the detectors in the Far Hall if there were no oscillation. The number that apparently vanish on the way (by oscillating into other flavours) gives the value of θ13. Because of the near-hall/far-hall arrangement, it is unnecessary to have a precise estimate of the antineutrino flux from the reactors.

The initial results will in the coming months and years be honed by collecting more data and reducing statistical and systematic errors. Refined results will open the door to further investigations and influence the design of future neutrino experiments, including how to determine which neutrino flavours are the most massive and whether there is a difference between neutrino and antineutrino oscillations.