.jpg "During installation, a bird's-eye view of the underground Daya Bay far detector hall was captured. Four antineutrino detectors are submerged in a large pool of ultra-pure water. Berkeley Lab's Roy Kaltschmidt") |
| During installation, a bird's-eye view of the underground Daya Bay far detector hall was captured. Four antineutrino detectors are submerged in a large pool of ultra-pure water. Berkeley Lab's Roy Kaltschmidt |
Over the course of nearly nine years, the Daya Bay Reactor Neutrino Experiment recorded an unprecedented 5.5 million interactions from subatomic particles known as neutrinos. The Daya Bay collaboration's international team of physicists has now reported the first result from the experiment's full dataset—the most precise measurement of theta13, a key parameter for understanding how neutrinos change their "flavor."
The discovery, announced today at the Neutrino 2022 conference in Seoul, South Korea, will aid physicists in unraveling some of the most profound mysteries about the nature of matter and the universe.
Neutrinos are subatomic particles that are both notoriously elusive and abundant. They bombard the Earth's surface indefinitely at nearly the speed of light but rarely interact with matter. They can travel a light year through lead without disturbing a single atom.
At the time Daya Bay was designed in 2007, only one of the three mixing angles was unknown: theta13. As a result, Daya Bay was designed to measure theta13* with greater sensitivity than any other experiment.
The Daya Bay Reactor Neutrino Experiment, located in Guangdong, China, consists of large, cylindrical particle detectors immersed in pools of water in three underground caverns. The eight detectors detect light signals produced by antineutrinos emitted by nearby nuclear power plants. Antineutrinos are neutrino antiparticles that are abundantly produced by nuclear reactors.
Daya Bay was built as a result of international collaboration and a first-of-its-kind partnership between China and the United States for a major physics project. The Chinese Academy of Sciences' Institute of High Energy Physics (IHEP) in Beijing leads China's participation, while the US Department of Energy's (DOE) Lawrence Berkeley National Laboratory and Brookhaven National Laboratory co-lead US participation.
The Daya Bay Reactor Neutrino Experiment, located in Guangdong, China, consists of large, cylindrical particle detectors immersed in pools of water in three underground caverns. The eight detectors detect light signals produced by antineutrinos emitted by nearby nuclear power plants. Antineutrinos are neutrino antiparticles that are abundantly produced by nuclear reactors.
Daya Bay was built as a result of international collaboration and a first-of-its-kind partnership between China and the United States for a major physics project. The Chinese Academy of Sciences' Institute of High Energy Physics (IHEP) in Beijing leads China's participation, while the US Department of Energy's (DOE) Lawrence Berkeley National Laboratory and Brookhaven National Laboratory co-lead US participation.
Daya Bay physicists made the world's first conclusive measurement of theta13 in 2012 and then improved on the precision of the measurement as the experiment continued to collect data. Daya Bay has far exceeded expectations after nine years of operation and the end of data collection in December 2020, with excellent detector performance and dedicated data analysis.
Using the entire dataset, physicists have now measured theta13 with a precision that is two and a half times greater than the experiment's design goal. No other existing or planned experiment is expected to achieve such fine precision.
"We had multiple analysis teams that painstakingly scrutinized the entire dataset, carefully accounting for the evolution of detector performance over the nine years of operation," said IHEP Daya Bay co-spokesperson Jun Cao. "The large dataset was used not only to refine the selection of antineutrino events but also to improve background determination. This devoted effort enabled us to achieve unrivaled precision."
Theta13 precision measurement will allow physicists to more easily measure other parameters in neutrino physics and develop more accurate models of subatomic particles and their interactions.
Physicists may gain insight into the universe's matter-antimatter imbalance by studying the properties and interactions of antineutrinos. Physicists believe that at the Big Bang, matter and antimatter were created in equal amounts. But if that were the case, these two polar opposites should have annihilated each other, leaving only light behind. Some difference between the two must have tipped the balance to explain the universe's current predominance of matter (and lack of antimatter).
"We expect there to be some difference between neutrinos and antineutrinos," said Kam-Biu Luk, a Berkeley physicist, and Daya Bay co-spokesperson. "We've never found differences between particles and antiparticles for leptons, which include neutrinos.
For quarks, we've only found differences between particles and antiparticles. However, the differences between quarks are insufficient to explain why there is more matter than antimatter in the universe. Neutrinos could be the smoking gun in this case."
The most recent analysis of the final dataset from Daya Bay also provided physicists with a precise measurement of the mass splitting. The frequency of neutrino oscillations is determined by this property.
"Measuring mass splitting was not one of Daya Bay's original design goals, but it became possible due to the relatively large value of theta13," Luk explained. "With the final Daya Bay dataset, we measured mass splitting to 2.3 percent precision, an improvement over the 2.8 percent precision of the previous Daya Bay measurement."
The international Daya Bay collaboration plans to report additional findings from the final dataset, including updates to previous measurements, in the coming months.
The Daya Bay results will be used by next-generation neutrino experiments, such as the Deep Underground Neutrino Experiment (DUNE), to precisely measure and compare neutrino and antineutrino properties. DUNE, which is currently under construction, will provide physicists with the world's most intense neutrino beam, underground detectors separated by 800 miles, and unprecedented opportunities to study neutrino behavior.
"As one of many physics goals, DUNE expects to eventually measure theta13 almost as precisely as Daya Bay," said Elizabeth Worcester, a Brookhaven experimental physicist, and Daya Bay collaborator. "This is exciting because we will then have precise theta13 measurements from various oscillation channels, putting the three-neutrino model to the test. Until DUNE achieves that level of precision, we can use Daya Bay's precise theta13 measurement as a constraint to search for differences in neutrino and antineutrino properties."
The large theta13 value and reactor neutrinos will also be used by scientists to determine which of the three neutrinos is the lightest. "Daya Bay's precise theta13 measurement improves the mass-ordering sensitivity of the Jiangmen Underground Neutrino Observatory (JUNO), which will be completed in China next year," said Yifang Wang, JUNO spokesperson, and IHEP director. "Furthermore, JUNO will achieve sub-percent precision on mass splitting measured by Daya Bay in a few years."
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Story Source: Materials provided by DOE/Brookhaven National Laboratory. Note: Content may be edited for style and length.