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New Results from Daya Bay: Tracking the Disappearance of Ghostlike Neutrinos

22 Aug 2013

 

Daya Bay neutrino experiment releases precision measurement of subatomic shape shifting and new result on differences among neutrino masses

The international Daya Bay Collaboration has announced new results about the transformations of elusive, ghostlike neutrinos, particles that carry invaluable clues about the makeup of the early universe.  The latest findings include their first data on how neutrino oscillation—in which neutrinos mix and change into other “flavors,” or types, as they travel — varies with neutrino energy, allowing scientists to measure a key difference in neutrino masses known as “mass splitting.”

The new results are based on four times the data, with twice the precision, of the first Daya Bay results released last year, which established the value of the third and final neutrino “mixing angle.”

The Daya Bay Experiment is located close to the Daya Bay and Ling Ao nuclear power plants in China, 55 kilometers northeast of Hong Kong.  The Daya Bay Collaboration includes more than 200 scientists from six regions and countries.

Daya Bay measures neutrino oscillation with electron neutrinos — actually antineutrinos, essentially the same as neutrinos for the purpose of these kinds of measurements. Millions of quadrillions of them are created every second by six powerful reactors. As they travel up to two kilometers to underground detectors, some seem to disappear. 

The missing neutrinos don’t vanish; instead they have transformed, changing flavors and becoming invisible to the detectors. The rate at which they transform is the basis for measuring the mixing angle, and the mass splitting is determined by studying how the rate of transformation depends on the neutrino energy.

“Mass splitting represents the frequency of neutrino oscillation. Mixing angles, another measure of oscillation, represent the amplitude.  Both are crucial for understanding the nature of neutrinos which may help answer some of the most mysterious questions about the universe” says Dr. John K.C. Leung, a collaborator from the Department of Physics, The University of Hong Kong.

Daya Bay’s first results were announced in March 2012 and established the unexpectedly large value of the mixing angle theta one-three (θ₁₃), the last of three long-sought neutrino mixing angles. The new results from Daya Bay put the precise number for that mixing angle at sin²2θ₁₃=0.090 plus or minus 0.009. The improvement in precision is a result of having more data to analyze and having the additional measurements of how the oscillation process varies with neutrino energy.

Daya Bay scientists have now measured the magnitude of the mass splitting |Δm²_ee| to be (2.59±0.20)×10^-3 eV² and the result establishes that the electron neutrino has all three mass states.  Precision measurement of the energy dependence should further the goal of establishing a “hierarchy,” or ranking, of the three mass states for each neutrino flavor.

These new results represent another milestone in our ongoing efforts to understand the nature of these mysterious particles. “Our results confirm and strengthen the current model of neutrino oscillations.  It contributes yet another piece of the puzzle to our pursuit of the fundamental building blocks of matter,” says Dr Jason C. S. Pun, another collaborator from the Department of Physics, The University of Hong Kong.

For additional background information on the Daya Bay Neutrino Experiment, visit the experiment’s website at http://dayabay.ihep.ac.cn/

For press enquiry, please contact Ms Cindy Chan, Senior Communication Manager of Faculty of Science, at 2241-5286/ 6703- 0212 or by email at cindycst@hku.hk; or members of the Collaboration from HKU Department of Physics Dr John K C Leung at 2859-2858 or Dr Jason C S Pun at 2859-1962.