Neutrinos are subatomic particles that are both elusive and extremely abundant. They endlessly bombard every inch of the Earth’s surface at nearly the speed of light, but rarely interact with matter. They can travel a light year of lead without ever disturbing a single atom.

One of the defining characteristics of these ghost particles is their ability to oscillate between three distinct “flavors”: the muon neutrino, the tau neutrino and the electron neutrino. The Daya Bay reactor neutrino experiment was designed to study the properties that dictate the probability of these oscillations, or what are known as mixing angles and mass divisions.

Only one of the three mixing angles remained unknown when Daya Bay was designed in 2007: theta13. Thus, Daya Bay was designed to measure theta13* with higher sensitivity than any other experiment.

Operating in Guangdong, China, the Daya Bay Reactor Neutrino Experiment consists of large cylindrical particle detectors submerged in pools of water in three underground caverns. The eight detectors pick up light signals generated by antineutrinos from nearby nuclear power plants. Antineutrinos are the antiparticles of neutrinos, and they are produced in abundance by nuclear reactors. Daya Bay was built through an international effort and a one-of-a-kind partnership for a major physics project between China and the United States. The Chinese Academy of Sciences’ Institute of High Energy Physics (IHEP), based in Beijing, is leading China’s role in the collaboration, while the US Department of Energy’s (DOE) Lawrence Berkeley National Laboratory and Brookhaven National Laboratory are co-leading the US participation.

To determine the value of theta13, Daya Bay scientists detected neutrinos of a specific flavor – in this case, electron antineutrinos – in each of the underground caverns. Two caverns are close to the nuclear reactors, and the third cavern is farther away, providing enough distance for the antineutrinos to oscillate. By comparing the number of electron antineutrinos picked up by the near and far detectors, physicists calculated how many changed flavors and, therefore, the value of theta13.

Daya Bay physicists made the world’s first conclusive measurement of theta13 in 2012 and later improved the accuracy of the measurement as the experiment continued to take data. Now, after nine years of operation and the end of data collection in December 2020, excellent detector performance and dedicated data analysis, Daya Bay has far exceeded expectations. Working with the full dataset, physicists have now measured the value of theta13 with two and a half times the accuracy of the experiment’s design goal. No other existing or planned experience is expected to achieve such an exquisite level of precision.

“We had multiple analysis teams who thoroughly reviewed the data set, carefully considering how the detector’s performance has changed over the nine years of operation,” the Daya Bay co-spokesperson said. , Jun Cao from IHEP. “Teams took advantage of the large dataset not only to refine the selection of antineutrino events but also to improve the determination of backgrounds. This dedicated effort allowed us to achieve an unprecedented level of precision.”

The precision measurement of theta13 will make it easier for physicists to measure other parameters of neutrino physics, as well as to develop more accurate models of subatomic particles and their interaction.

By studying the properties and interactions of antineutrinos, physicists could better understand the imbalance of matter and antimatter in the universe. Physicists believe that matter and antimatter were created in equal amounts at the time of the Big Bang. But if that were the case, these two opposites should have annihilated, leaving only light behind. Some difference between the two must have tipped the scales to explain the preponderance of matter (and lack of antimatter) in the universe today.

“We expect there to be a difference between neutrinos and antineutrinos,” said Berkeley physicist and Daya Bay co-spokesperson Kam-Biu Luk. “We have never detected differences between particles and antiparticles for leptons, the type of particles that includes neutrinos. We have only detected differences between particles and antiparticles for quarks. we see with quarks are not enough to explain why there is more matter than antimatter in the universe. It is possible that neutrinos are the irrefutable proof.”

The latest analysis of the final Daya Bay dataset has also provided physicists with an accurate measure of mass division. This property dictates the frequency of neutrino oscillations.

“Mass splitting measurement was not one of Daya Bay’s original design goals, but it became accessible due to the relatively high value of theta13,” Luk said. “We measured the mass split at 2.3% with the final Daya Bay dataset, an improvement over the 2.8% accuracy of the previous Daya Bay measurement.”

Going forward, the Daya Bay International Collaboration plans to report additional results from the final dataset, including updates to previous measurements.

Next-generation neutrino experiments, such as the Deep Underground Neutrino Experiment (DUNE), will exploit Daya Bay results to accurately measure and compare the properties of neutrinos and antineutrinos. Currently under construction, DUNE will provide physicists with the most intense neutrino beam in the world, underground detectors 800 miles away, and the ability to study neutrino behavior like never before.

“As one of many physics goals, DUNE expects to eventually measure theta13 almost as accurately as Daya Bay,” said Brookhaven experimental physicist and Daya Bay collaborator Elizabeth Worcester. “This is exciting because then we will have precise theta13 measurements from different oscillation channels, which will rigorously test the three-neutrino model. Until DUNE achieves this high precision, we can use the precise theta13 measurement of Daya Bay as a constraint to allow the search for differences between the properties of neutrinos and antineutrinos.”

Scientists will also exploit the large theta13 value and reactor neutrinos to determine which of the three neutrinos is the lightest. “The precise measurement of theta13 from Daya Bay improves the mass order sensitivity of the Jiangmen Underground Neutrino Observatory (JUNO), which will complete construction in China next year,” said Yifang Wang, gatekeeper. word of JUNO and director of IHEP. “Furthermore, JUNO will achieve sub-percentage accuracy on the mass division measured by Daya Bay several years from now.”

The Daya Bay Reactor Neutrino Experiment is supported by the Ministry of Science and Technology of China, DOE Office of Science High Energy Physics Program, Chinese Academy of Sciences, National Natural Science Foundation of China and other funding agencies. The Daya Bay collaboration has 237 participants at 42 institutions in Asia, Europe and North America.

*Physicists measure theta13 based on its amplitude of oscillation, or what is mathematically written as sin²2θ₁₃.