AsianScientist (Mar. 17, 2016) – Members of the International Daya Bay Collaboration, who study electron antineutrinos generated at a nuclear power complex in China, have obtained the most precise measurement of these subatomic particles’ energy spectrum ever recorded.
The data generated from the world’s largest sample of reactor antineutrinos indicate two intriguing discrepancies with theoretical predictions, and also provide an important measurement that will shape future reactor neutrino experiments. The results have been published in Physical Review Letters.
Studying the behavior of elusive neutrinos holds the potential to unlock many secrets of physics, including details about the history, makeup, and fate of our universe. Neutrinos were among the most abundant particles at the time of the Big Bang. They are still generated abundantly today in the nuclear reactions that power stars and in collisions of cosmic rays with Earth’s atmosphere.
They are also emitted as a by-product of power generation in man-made nuclear reactors, giving scientists a powerful way to study them on Earth in a controlled manner. Reactor experiments such as the one at Daya Bay play a crucial role in uncovering the secrets of neutrino oscillation—their tendency to switch among three known ‘flavors’: electron, muon, and tau—and other important neutrino properties.
A crucial factor for many of these experiments is knowing how many antineutrinos are emitted in total in these nuclear reactions. In early studies, scientists relied on calculations or other indirect means, such as electron spectrum measurements made on reactor fuels, to estimate these numbers, based on their understanding of the complex fission processes in the reactor core. These methods have rather strong dependence on theoretical models.
The Daya Bay Collaboration now provides the most precise model-independent measurement of the energy spectrum of these elusive particles, and a new measurement of total antineutrino flux.
The data were gathered by analyzing more than 300,000 reactor antineutrinos. The measured reactor antineutrino spectrum shows a surprising feature: an excess of antineutrinos at an energy of around five million electron volts (MeV) compared with theoretical expectations.
This represents a deviation of about 10 percent between the experimental measurement and calculations based on theoretical models.
“This unexpected disagreement between our observation and predictions strongly suggested that the current calculations would need some refinement,” said Luk Kam-Biu, the co-spokesperson of the Daya Bay Collaboration.
Two other experiments have shown a similar excess, though with less precision than the new Daya Bay result.
Such deviation shows the importance of the direct measurement of the reactor antineutrino spectrum, particularly for experiments that use the spectrum to measure neutrino oscillations, and may indicate the need to revisit the models underlying the calculations.
Daya Bay’s measurement of antineutrino flux—the total number of antineutrinos emitted across the entire energy range—indicates that the reactors are producing six percent fewer antineutrinos overall when compared to some of the model-based predictions. This result is consistent with past measurements, and this observed deficit has been named the “Reactor Antineutrino Anomaly.”
This discrepancy could have risen from the imperfection of the theoretical models. Or, more intriguingly, it could be the result of an oscillation involving a new kind of neutrino, the so-called sterile neutrino, postulated by some theories but yet to be detected.
The article can be found at: An et al. (2016) Measurement of the Reactor Antineutrino Flux and Spectrum at Daya Bay.
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Source: Chinese Academy of Sciences; Photo: Shutterstock.
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