The last unknown angle in neutrino mixing?

Kilometre-long tunnels in the ground, six nuclear reactors, 200 scientists, giant acrylic vessels containing 20 tonnes of liquid scintillator. It sounds like the plot of a James Bond movie, but actually it is the Daya Bay experiment.

This experiment aims to measure the neutrino mixing angle theta_{13}. With two of the three mixing angles pretty well constrained by data on atmospheric and solar neutrino oscillations this angle is the least well-known of all the parameters in the neutrino-mixing matrix. (Well, except for maybe one...but more on that later.) Daya Bay will either measure it, or---if it is very small---improve the present limit by a factor of 15. In order to do this one wants to optimize the distance so that electron-type neutrinos are at their "oscillations minimum", i.e. the maximum number of them have disappeared. The best distance to do this at is at about 2 km. (You might want to ask why you don't want to go to the "next minimum" in the oscillation pattern, the one corresponding to 2 Pi, rather than Pi.) So what you need is a copious source of electron-type anti-neutrinos, and a big chunk of matter which will convert some of them back into positrons through inverse beta decay.

Now nuclear reactors are just such a source, because for each GW-hour a typical Uranium-based nuclear reactor produces about 10^{20} electron anti-neutrinos. So in the Daya Bay "far" detector they expect to see about 90 neutrino events per day. Which sounds like a lot until you realize that the "far detector" consists of four vessels, each containing 20t of liquid scintillator. That scintillator ensures that when the p anti-neutrino strikes a proton, converting it to a neutron and a positron, both the neutron and the positron can be detected, with a time gap between them that is an (almost) unique signature of the process.

Other experiments have tried this approach before, but one limiting factor has been knowledge of the neutrino flux from the reactors. Daya Bay will solve this problem by taking a ratio of the neutrino rate between "near" detectors and "far" detectors. That technique, combined with the large volume of the detector, and with the promiximity to 17.4 GW of thermal power that is feeding China's growing energy needs, means that Daya Bay can "nail" theta_{13}. If, that is, they can ensure that what is going on at the near and far detectors is essentially identical.

So this experiment becomes all about controlling systematics. Is the scintillator in the near and far detectors exactly the same? Do the PMTs in the near and the far detectors function in the same way? (This is where regular calibration of detectors becomes key.) And it becomes about removing backgrounds: especially in the "far" detector where the count rate is a factor of ten lower. So when Dr Liu spoke on this topic last Thursday he spent some time explaining the cuts they make in their data to remove background events, e.g. cosmic rays activating Lithium-9 that then beta decay to produce a coincidence signal very like that seen when one of the reactor anti-neutrinos interacts with the target. Obviously it is a very delicate business to remove _only_ those events, and none of the signal, and to do so consistently between the near and far detectors. But if those background events are not properly removed they will mess up the measurement of the ratio that is the goal of this experiment.

Which might prompt the question: why all this effort? The millions of dollars (or RMB) on digging tunnels, the millions of dollars more on detector development here in the US, the 200 people working hard to design the best experiment they can. Part of the answer is that theta_{13} is "the gateway to CP violation": the possibility that there is a complex phase in the neutrino-mixing matrix like the one in the quark CKM matrix. And CP violation is a hot topic right now in particle physics. But is that the reason the Chinese government is footing most of the bill for this experiment? And that the US Department of Energy is making a significant commitment to it too? While I like to think that the US and Chinese governments care about CP violation, I don't think they care that much.