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By Calla Cofield
In early March, the Daya Bay neutrino experiment announced a measurement of the highly-sought- after mixing angle θ13. The experiment made headlines by reaching its goal after only 55 days of data- taking, ahead of two competing experiments.
“It’s a shining moment for us,” said U.S. Project Manager Bill Edwards of Lawrence Berkeley National Laboratory (LBNL). By us, Edwards means himself, Kam-Biu Luk, the American spokesperson for Daya Bay, and Yifang Wang, the Chinese spokesperson and project manager. Daya Bay is the largest US-Chinese physics collaboration in history, but the accomplishment wasn’t without struggle. In addition to the difficulties that face any large experiment, there was also the challenge of combining cultures. It was, as Edwards describes it, “An experiment within an experiment.”
In 2003, Luk, a member of the LBNL physics division and a professor of physics at UC Berkeley, conducted an international search for potential sites for a multi-detector, reactor-based neutrino experiment. The fission processes inside nuclear power plants naturally produce a flux of electron antineutrinos, which over a short distance become muon neutrinos; θ13 measures the rate of this transformation. Two other sites presented themselves in the US, but the site that won out was the one Luk found 35 miles east of Hong Kong, on the coast of the Daya Bay.
Building the experiment in China would mean shared cost, combined expertise, and new collaborations among scientists; but for the American members, many aspects of the project, such as communicating with the power plant and the Chinese government, would be left in the hands of their Chinese colleagues. In fact, Luk’s first attempt to contact the Daya Bay power plant to discuss the possibility of building the experiment there was met with silence; it wasn’t until his Chinese collaborators got involved that the wheels started to turn. And there was simply the challenge of having to work together largely over the phone, across time zones and language barriers.
“Certainly at the beginning, there were concerns even by myself,” said Luk. “Because even though I had worked with Chinese collaborators on another experiment before, it’s not the same as doing it in China.”
Luk says in some instances things seemed to go much faster in China than they do in the US. In other cases, they went very slowly, such as waiting for a year to secure approval of the radioactive material used to test the detector.
Wang says many challenges the project faced weren’t always made apparent to his American colleagues because, “We didn’t know how to explain it to them…and even if they knew they couldn’t help.”
With many American members joining the experiment in Daya Bay, the collaboration attempted to create a system of combined safety standards from the US and China. But this posed a problem for both sides because the systems differ so much, and neither group of collaborators was totally familiar with the other system. Disagreements arose as to which country’s regulations should be implemented.
“You know, it’s confusing,” said Luk. “Say we use the US requirements or the Chinese requirements: what are the implications? So it took some time to iron those out.”
Wang, now the director of the Institute of High Energy Physics, previously led the BESIII experiment at the Beijing electron positron collider (BESIII was the largest US-Chinese collaboration in China prior to Daya Bay). In the 1990’s Wang worked on the Palo Verde reactor-based neutrino experiment, which was originally planned for a site in California, but was moved to Arizona after five years of delays. Major scientific experiments in the US can face delays due to financing, proper approval from various bodies and organizations, and environmental issues, to name a few. Navigating those issues becomes as much a part of the experiment as the science. Wang says every country has these obstacles, and adds, “I’ve had good experiences in the United States and bad experiences. A worse one [in the US] is worse than in China, but a good one [in the US] is better than in China.”
Ultimately the collaboration was necessary, as neither country could support the experiment on its own. China would cover the cost of civil construction of the tunnels and facilities, and roughly half the cost of building the experimental system; the US would cover the other half of the experimental system. The US commitment came to $35 million dollars. While it is impossible to say exactly how much the project would have cost had it been built entirely in the US, Luk and Edwards say estimates were on the order of $100 million.
The cost benefit was important to the Chinese side as well, according to Wang.
“Everybody seems to now think that China has a lot of money,” said Wang. “But without US participation, getting all the funding in China would have been difficult.”
Wang did his PhD in Florence, and subsequent research at MIT and Stanford University, where he worked with Luk at the KamLAND neutrino experiment in Japan. For the many students of high-energy physics in China who won’t have the opportunity to go abroad, as Wang did, Daya Bay offers an international, competitive particle physics experiment at home. Conversely, the collaboration provides native Chinese collaborators on the US side, like Luk, the opportunity to visit home. Xin Qian, a postdoc at the California Institute of Technology, says he chose to work on the Daya Bay project partly because it would allow him to visit his hometown of Beijing.
The Daya Bay experiment is set to complete construction this year, when the last two of its eight detectors come online. The experiment will continue to run for three to four more years, and ideally will lead to a continuing neutrino physics program in China. But Luk, Wang and Edwards talk in a way that makes it seem as though the experiment has already passed its most difficult days.
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