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Operating in Guangdong, China, the Daya Bay Reactor Neutrino Experiment consists of large, cylindrical particle detectors immersed in pools of water in three underground caverns. The eight detectors pick up light signals generated by antineutrinos streaming from nearby nuclear power plants. Antineutrinos are the antiparticles of neutrinos, and produced in abundance by nuclear reactors.
Neutrinos are subatomic particles that are both famously elusive and omnipresent. They endlessly bombard every inch of Earth's surface at nearly the speed of light, but rarely interact with matter.
One of their defining characteristics is their ability to oscillate between three distinct "flavours": muon neutrino, tau neutrino, and electron neutrino. The experiment was designed to investigate the properties that dictate the probability of those oscillations, or what are known as mixing angles and 'mass splittings.'
Only one of the three mixing angles remained unknown at the time Daya Bay was designed in 2007 called theta13. So, Daya Bay was built to measure theta13* with higher sensitivity than any other experiment.
Daya Bay physicists made the world's first conclusive measurement of theta13 in 2012 and subsequently improved upon the measurement's precision as the experiment continued taking data. Now, after nine years of operation and the end of data collection in December 2020, scientists say Daya Bay has far exceeded expectations. Working with the complete dataset, physicists have now measured the value of theta13 with a precision two and a half times greater than the experiment's design goal. No other existing or planned experiment is expected to reach such an "exquisite" level of precision, they note.
The precision measurement of theta13 will enable physicists to more easily measure other parameters in neutrino physics, as well as develop more accurate models of subatomic particles and how they interact.
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