AsianScientist (Mar. 8, 2012) – The Daya Bay Reactor Neutrino Experiment, a multinational collaboration operating in the south of China, today reported the first results of its search for the last, most elusive piece of a long-standing puzzle: how is it that neutrinos can appear to vanish as they travel?
Neutrinos, the wispy particles that flooded the universe in the earliest moments after the big bang, are continually produced in the hearts of stars and other nuclear reactions.
They come in three types or “flavors” — electron, muon, and tau neutrinos — that morph, or oscillate, from one form to another, interacting hardly at all as they travel through space and matter, including people, buildings, and planets like Earth.
However, this activity is extremely difficult to detect. The challenge of capturing these elusive particles inspired the Daya Bay collaboration in the design and precise placement of six massive detectors which are buried in the mountains adjacent to the powerful nuclear reactors of the China Guangdong Nuclear Power Group.
These reactors, at Daya Bay and nearby Ling Ao, produce millions of quadrillions of elusive electron antineutrinos every second. From 24 December 2011 until 17 February 2012, scientists in the Daya Bay collaboration, which started collecting data in August 2011, observed tens of thousands of interactions of electron antineutrinos captured on the detectors.
This large amount of data provided the first evidence that antineutrinos actually oscillate (transform) into other flavors. The researchers were able to detect and precisely measure the strong signal of this effect, a so called “mixing angle” named theta one-three (written θ13).
This mixing angle expresses how electron neutrinos and their antineutrino counterparts mix and change into other flavours.
“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.
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 of antineutrinos that apparently vanish on the way (now revealed to have oscillated into other flavors) gives the value of theta one-three.
“The first Daya Bay results show that theta one-three, once feared to be near zero, instead is comparatively huge,” according to Kam-Biu Luk who is co-spokesperson of the Daya Bay Experiment and heads U.S. participation.
In coming months and years the initial results will be honed by collecting far 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 flavors are the most massive, and whether there is a difference between neutrino and antineutrino oscillations.
Eventually, scientists hope to answer the question of why there is more matter than antimatter in the universe – because these were presumably created in equal amounts in the big bang and should theoretically have completely annihilated one another.
Source: Lawrence Berkeley National Laboratory; Image: Roy Kaltschmidt/Lawrence Berkeley National Laboratory.
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