The Daya Bay experiment measures the antineutrinos produced by the reactors of the Daya Bay Nuclear Power Plant and the Ling Ao Nuclear Power Plant in mainland China. The photo shows a panoramic view of the Daya Bay reactor complex. Credit: Roy Kaltschmidt, Berkeley Lab
Members of the International Daya Bay Collaboration, who track the production and flavor-shifting behavior of 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 2 intriguing discrepancies with theoretical predictions and provide an important measurement that will shape future reactor neutrino experiments.
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, and 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. In fact, the study of particles emitted by reactors led to the first detection of neutrinos in the 1950s, a finding once considered impossible due to the extreme inert nature of these particles, which were then only predicted. Since that time reactor experiments, including Daya Bay, have played a crucial role in uncovering the secrets of neutrino oscillation—their tendency to switch among 3 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 (the flux), and how many are being produced at particular energies (the energy distribution, or spectrum). 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 in the 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 collected over the course of 217 days. Through dedicated calibration and analysis effort, Daya Bay was able to measure the neutrino energy to an unprecedented precision, better than 1%, over a broad energy range of the reactor antineutrinos.
The measured reactor antineutrino spectrum shows a surprising feature: an excess of antineutrinos at 5 million electron volts (MeV) compared with theoretical expectations. This represents a deviation of about 10% between the experimental measurement and calculations based on the theoretical models—well beyond the uncertainties—leading to a discrepancy of up to 4 standard deviations. “This unexpected disagreement between our observation and predictions strongly suggested that the current calculations would need some refinement,” commented Kam-Biu Luk. 2 other experiments have shown a similar excess at this energy, though with less precision than the new Daya Bay result.
Such deviation shows the importance of the direct measurement of the reactor antineutrino spectrums. “We expect that the spectrum measured by Daya Bay will improve with more data and better understanding of the detector response. These improved measurements will be essential for next-generation reactor neutrino experiments such as JUNO,” said Jun Cao of IHEP).
Daya Bay’s measurement of antineutrino flux—total number of antineutrinos emitted across the entire energy range—indicates that the reactors are producing 6% fewer antineutrinos overall when compared to some of the model-based predictions. This result is consistent with past measurements. The deficit is called “Reactor Antineutrino Anomaly.” This disagreement could arise from the imperfection of the 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.
http://phys.org/news/2016-02-precise-reactor-antineutrino-spectrum-reveals.htmljCp
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