In what’s arguable the most important physics discovery ever to come out of China, and a perfect example of “by the book” international collaborative effort, researchers report they’ve successfully identified the last piece of missing information needed to describe the mysterious neutrino oscillation. For a long time, scientists have been trying to discover how neutrinos apparently simply vanish as they travel. Armed with this new discovery, scientists are just a step closer to understanding the neutrino/anti-neutrino dance. Ultimately, it might spring the long sought answer to the riddle which has been puzzling physicists – why is there far more matter than antimatter in the Universe? Maybe more importantly, why is there any matter at all?

As they travel at razor close light speed, neutrinos morph into different types. So far, three “flavors” have been identified:  electron neutrinos, born in nuclear reactions; muon neutrinos, sprung from the decay of particles called pions; and tau neutrinos, only generated in particle collisions at accelerator labs. When traveling, neutrinos mix together and transform – a very difficult activity to detect. For instance, electron neutrinos, naturally ejected by the sun, morph into a different flavor on their route towards Earth, as much fewer of them eventually arrive than otherwise expect. Muon neutrinos, which are brought in by cosmic rays, similarly transform when they reach Earth’s atmosphere.

Panorama of the Daya Bay Nuclear Power Plant Complex

To describe the oscillation or transformation, researchers  working with the Daya Bay Reactor Neutrino Experiment at the Daya Bay Nuclear Power Plant and two neighboring plants in Da Peng, China, measured all but one parameter in a theoretical scheme that describes this peculiar activity. This last parameter, a “mixing angle” named theta one-three, was finally measured with unmatched precision.  The team presented its results today at a seminar at the Institute of High Energy Physics of the Chinese Academy of Sciences in Beijing.

The final piece of the puzzle

Scientists in the Daya Bay collaboration observed 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. What they looked for was the rate which these electron antineutrinos signaled the oscillation into a different flavor, and eventually found that  θ₁₃

equals 8.8 degrees.  The Daya Bay team started taking data on 24 December and needed only 55 days of running the detectors to make a definite measurement.

“This is a new type of neutrino oscillation, and it is surprisingly large,” says Yifang Wang of China’s Institute of High Energy Physics (IHEP), co-spokesperson and Chinese project manager of the Daya Bay experiment. “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.”

Curiously enough, θ₁₃ was once consider to be zero. Far from it, the new found, accurately measured value can only be considered colossal. Eventual systematic and statistical errors will be reduced from the initial result in the coming months.

“It has been very gratifying to be able to work with such an outstanding international collaboration at the world’s most sensitive reactor neutrino experiment,” says Steve Kettell of Brookhaven National Laboratory, the chief scientist for the U.S. effort. “This moment is exciting because we have finally observed all three mixing angles, and now the way is cleared to explore the remaining parameters of neutrino oscillation.”

“This is really remarkable,” says Wenlong Zhan, vice president of the Chinese Academy of Sciences and president of the Chinese Physical Society. “We hoped for a positive result when we decided to fund the project, but we never imagined it could come so quickly!”

“Exemplary teamwork among the partners has led to this outstanding performance,” says James Siegrist, DOE Associate Director of Science for High Energy Physics. “These notable first results are just the beginning for the world’s foremost reactor neutrino experiment.”

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