The biggest major science project in China that has been built through a genuine international collaboration has begun operation. Once fully complete next year, the Daya Bay Reactor Neutrino Experiment – a partnership lead by 19 Chinese and 16 US universities – will begin searching for the final undetermined neutrino "mixing angle", known as θ13.

Neutrinos are difficult to detect because they interact weakly with matter. They come in three "flavours" – electron, muon and tau – that change or "oscillate" from one to another as they travel in space. The oscillation strength between different types of neutrino is characterized by three "mixing angles" – known as θ12, θ23 and θ13 – with Daya Bay designed to determine θ13 by measuring the disappearance of electron antineutrinos.

The US Department of Energy is providing about half of the cost of the $68m facility, with China paying for the other half and all of the civil-engineering costs. The Daya Bay experiment detects electron antineutrinos produced via nuclear beta decay at two neighbouring nuclear reactors – the Daya Bay and Ling Ao power plants, which are around 55 km north-east of Hong Kong.

The new neutrino facility will consist of three experimental halls that contain identical neutrino detectors, each filled with 20 tonnes of gadolinium-doped liquid scintillator. When a neutrino strikes the liquid, a flash of light is produced that is then picked up by a bank of photomultiplier tubes around the liquid.

The first experimental hall, which is around 300 m from the Daya Bay reactor, is now complete, while the second experimental hall – 500 m from the Ling Ao reactor – is expected to be finished in the next few months. Both of these stations, known as "near detectors", are 100 m underground to help shield them against unwanted cosmic rays and each contains two detectors to characterize the beam of electron antineutrinos from the reactors.

A third hall, around 2 km away from both reactors and 300 m below ground, will be ready by June next year. Containing four neutrino detectors, it will measure the electron-antineutrino beam that has passed through the nearer detectors, so that any drop in the strength of the signal will be an indication of neutrino oscillation.

"Among the current generation of reactor neutrino-oscillation experiments for measuring θ13, Daya Bay has the best sensitivity," says Daya Bay co-spokesperson Kam-Biu Luk, of the Lawrence Berkeley National Laboratory in California.

Measuring disappearance

The start-up of the Daya Bay experiment comes hard on the heels of two other neutrino successes. First, in early June, the Tokai-to-Kamioka (T2K) neutrino experiment in Japan for the first time measured muon neutrinos changing into electron neutrinos – a first step to determining θ13. A few weeks later, researchers at the MINOS experiment in the US detected a total of 62 electron neutrinos – 13 more events than the background of electron neutrinos.

At T2K, as well as similar planned experiments such as the NOvA facility being built at Fermilab, the probability of electron neutrino "appearance" depends on two unknown parameters: θ13 and the neutrino phase factor, δ, which is non-zero if neutrino oscillation violates charge–parity (CP) symmetry. Daya Bay, however, is blind to the neutrino-phase factor because the probability of disappearance of electron antineutrinos only depends on θ13, which means that researchers can focus on just its numerical value.

Yifang Wang, co-spokesperson for Daya Bay and a physicist at the Institute of High Energy Physics at the Chinese Academy of Sciences in Beijing, says that the three experiments will be complementary for searching for the phase factor. "If Daya Bay, NOvA and T2K find that θ13 is non-zero, then the CP phase can be jointly measured or strongly constrained," he says.

New collaborations

As Daya Bay is the first major US–China scientific collaboration, Luk expects the facility will provide a good testing-ground for more partnerships between the two countries. "Daya Bay provides a unique opportunity to join forces to tackle a burning question in neutrino physics and, more importantly, to learn how to work together," says Luk.

That view is shared by Wang, who says that Daya Bay will be important for both countries. "We believe we will have a better understanding of each other, and the experience will help us for future collaborations," he says.

Dave Wark of Imperial College London and former international co-spokesperson for T2K says it is good news that Daya Bay has begun running, but warns that it could be some time before the experiment starts to get reliable measurements – given how difficult neutrino-disappearance experiments can be. "If θ13 is large, they have an easier target, but we are still talking about at most few per cent effects in a disappearance experiment so the measurements are tricky," says Wark.