Missing: Electron antineutrinos and Understanding matter-antimatter imbalance

An international particle physics collaboration has announced its first results toward answering a longstanding question - how the elusive particles called neutrinos can appear to vanish as they travel through space.

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 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.