Ling Xin looks at the impact of the Daya Bay Reactor Neutrino Experiment, which ended recently, on neutrino physics, US–China partnerships, and future neutrino facilities.

On December 12, 2020, authorities gathered in an underground laboratory near Shenzhen, southern China, to bid farewell to a decade-old experiment that not only revealed neutrino secrets but also encouraged China–US scientific partnership. Yifang Wang of the Chinese Academy of Sciences’ Institute of High Energy Physics (IHEP) pulled a red button that halted the Daya Bay Reactor Neutrino Experiment from collecting data shortly after 10.30 a.m. Covers were removed a few minutes later, and four large, cylindrical tanks showed in a pool of highly cleansed water.

During the ceremony, Jun Cao, a co-spokesperson for the partnership, said, “Today we are here to celebrate the completion of the Daya Bay Reactor Neutrino Experiment, which has accomplished all of its tasks.” Due of coronavirus restrictions, only a tiny crowd was present, but 1.7 million people tuned in online to watch the trial conclude. Kam-Biu Luk, a particle physicist at the University of California at Berkeley and the Lawrence Berkeley National Laboratory, and the experiment’s US spokesperson, was one of them, watching the webcast from his California home. He told Physics World, “I’ve worked on a number of studies throughout my life, but Daya Bay has accomplished so much that it is incredibly fulfilling.” For all of us, this is unquestionably a happy ending.”

The beginning

Neutrinos are exceedingly light and difficult to catch, although they are found all around us as a result of nuclear interactions. They are divided into three types: electrons, muons, and taus, which morph into one another as they approach the speed of light. By the early 2000s, scientists had a decent idea of how electron neutrinos turn into muon and tau neutrinos (as in solar neutrino oscillation) and how muon neutrinos transform into tau neutrinos thanks to large-scale neutrino detectors in Japan, the United States, Canada, and other nations (as in atmospheric neutrino oscillation). However, the case of electron–muon oscillation, which is specified by the parameter “theta-13” and is the final missing piece in the puzzle of neutrino oscillations, remained unsolved.

Because reactors are well-understood neutrino sources, several scientists recommended utilizing nuclear reactors to examine this neutrino oscillation, and Luk concluded that this could be the ideal option to address the theta-13 problem. He began looking at suitable locations in Japan, South Korea, and the United States. Luk, a Hong Kong native, was also aware of the Daya Bay nuclear power project and included it to his list.

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The Daya Bay and Ling Ao reactors are powerful enough to produce a huge quantity of antineutrinos, which made Daya Bay stand out in many ways. The location is also adjacent to a mountain range, making construction and cosmic ray shielding much easier. Given that comparing antineutrino events at near and far sites was the most efficient way to deduce theta-13, the team planned eight detectors, four of which would be located within 300 and 500 meters from the reactors (named “near detectors”) and four of which would be placed 2 kilometers away.

Luk chose Daya Bay and contacted IHEP in late 2003 about collaborating. Despite a scarcity of neutrino researchers in China, Wang, who was in charge of the institute’s experimental section, recognized that this was a once-in-a-lifetime chance and began looking for funds and personnel. The US Department of Energy rapidly backed the plan, contributing nearly a third of the entire cost, with Cao being one of the first to join. He immediately got down to basic design challenges including the form of the detectors and the construction of the liquid scintillator, which he did with collaborators at Brookhaven National Laboratory after finishing his postdoctoral study in the United States at Fermilab.

The project drew the participation of students as well. Liangjian Wen, a nuclear physics student at the Chinese University of Science and Technology in Hefei, traveled to IHEP Beijing to work on his undergraduate project. He was instructed to design the reflecting panels located at the top and bottom of the detector, a technique never utilized in such tests before, despite his lack of experience with creating detectors. Wen explains, “Because the panels can reflect photons to the side, we were able to employ less photomultiplier tubes and save roughly 20 million yuan.” Wen learnt what materials to use for the supporting framework, how to put the reflecting film between the panels, and how to assemble the panels with great precision by starting from scratch. “In the end, we made it,” he continues. “The detectors had a simpler shape and improved performance thanks to the reflecting panels.”

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Surprising results

On December 24, 2011, Daya Bay began collecting data with only six of the eight detectors in place. From data acquired during the first few days, researchers were able to quickly filter out background signals and uncover something significant. Cao recalls how they worked late into the night, held numerous meetings, and utilized a variety of cross-checking techniques to ensure that the results were accurate. The group then held a press conference in Beijing on March 8, 2012, to unveil its pioneering results on theta-13.

Approximately 6% of the reactor’s antineutrinos converted into other forms of neutrinos on their route from the reactors to the remote location, according to tens of thousands of antineutrino events detected. Wang was able to proclaim the finding of a novel type of neutrino oscillation because the transformation rate was unexpectedly high. It was a pleasant surprise for Cao, given that he only had to wait 55 days for a final answer to the crucial theta-13 problem, the value of which turned out to be significantly higher than predicted. As the team collected and analyzed more data over the next eight years, theta-13’s measurement precision improved sixfold to 3.4 percent, a milestone that no other experiment is projected to surpass in the next 20 years.

Aside from theta-13, the experiment yielded additional significant results. It, for example, cast doubt on the existence of a fourth sort of neutrino, the sterile neutrino. The reactors emitted significantly less antineutrinos than projected, possibly because some had transitioned into sterile neutrinos, according to observations at the close detector. To be sure, the scientists measured uranium and plutonium, two key reactor fuel components and antineutrino sources, separately. They discovered that whereas modeling and observation matched well for plutonium, uranium had a significant divergence. “This effectively ruled out the notion of sterile neutrinos as a cause of the deficit,” Wang explains. “If sterile neutrinos existed, they should have operated in the same way on plutonium and uranium.”

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The legacy of Daya Bay

Daya Bay is still the largest basic scientific relationship between China and the United States, and it has benefited experts on both sides. The neutrino research team in China has grown from a few persons in the early 2000s to around 100 presently. The Daya Bay project proved to be significantly cheaper and faster for the United States than if it had been done alone.

The eight detectors have been dismantled since the ceremony, with some components, such as the electronics, being repurposed in China’s next big neutrino experiment, the Jiangmen Underground Neutrino Observatory (JUNO). Other components, such as 32 tonnes of gadolinium-doped liquid scintillator and 50 tonnes of undoped liquid scintillator, were contributed to a Japanese experiment called JSNS2.

The remainder of the study will be distributed to schools for educational or outreach purposes. Meanwhile, the main laboratory hall will be converted into a display space for the experiment. The team will also continue to analyze the entire dataset, which is expected to take another year or two.

JUNO is expected to be operational by the end of 2022, according to IHEP experts. It will attempt to determine the mass ordering of various types of neutrinos, which will aid future neutrino facilities such as the Deep Underground Neutrino Experiment in the United States and the Hyper-Kamiokande neutrino observatory in Japan in determining their absolute values and possibly revealing why the universe is made up of matter rather than antimatter. “These topics will keep particle physicists busy for the next few decades,” Luk predicts.

Researchers are also creating critical technologies for JUNO’s second phase, which will conduct a neutrino-less double-beta decay experiment to see if neutrinos are their own antiparticles and quantify neutrinos’ absolute masses. Cao, on the other hand, is not sorry to see Daya Bay come to an end. “On the contrary, we look forward to tomorrow,” he says, “when the new generation of neutrino experiments will unveil additional unknowns in neutrino physics.”

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