A joint Fermilab/SLAC publication

Daya Bay experiment makes key measurement, paves way for future discoveries


The Daya Bay site in southern China. Image: Lawrence Berkeley Laboratory

An international collaboration of physicists working on a neutrino experiment in southern China announced today they have made a difficult measurement scientists have been chasing for more than a decade.

The results of the Daya Bay neutrino experiment open an important window into understanding the behavior of neutrinos, and now the race is on to determine the implications. Two American experiments, one proposed and one under construction, seem well positioned to take the next steps.

The problem of morphing

Whether or not they travel faster than light, neutrinos are perplexing particles. In the 1960s, scientists studying neutrinos produced in the sun discovered that a portion of them seemed to disappear as they made their way toward Earth.

But the neutrinos weren’t just popping out of existence. It turned out that they were taking on new identities. Neutrinos come in three types, or flavors. As neutrinos move through space, they fluctuate from one flavor to the next.

Physicists were able to measure the probability that two of these three types of flavor fluctuations would occur. But the third, the rarest one, was hardest to pin down.

Today, after just nine weeks of data-taking, physicists from the Daya Bay experiment, including scientists from the Department of Energy’s Berkeley and Brookhaven laboratories, have the answer – to an incredibly high level of precision. They were able to measure how often electron neutrinos oscillated into other flavors using six massive detectors buried in the mountains near the Daya Bay and Ling Ao nuclear reactors. They observed tens of thousands of electron antineutrino interactions to determine this probability.

"This is a new type of neutrino oscillation, and it is surprisingly large," said Yifang Wang of China's Institute of High Energy Physics, co-spokesperson and Chinese project manager of the Daya Bay experiment.

The definite answer delivered by the Dava Bay experiment comes about nine months after the T2K experiment in Japan and the MINOS experiment at Fermilab started to restrict the unknown parameter. Researchers at the Double Chooz experiment in France and the RENO experiment in South Korea also aim to measure this parameter with high precision.

The probability of this type of oscillation occurring is important to experiments that hope to answer two important questions in neutrino physics: How do the three neutrino masses stack up? Could neutrinos be the reason why matter is abundant in our universe and antimatter disappeared?

The problem of mass

The signal Daya Bay measured let scientists determine a quantity called theta one-three. The larger theta one-three is, the more often electron neutrinos and electron antineutrinos oscillate. The experiment determined theta one-three to be about 0.09, which is larger than expected. This is good news for future neutrino oscillation experiments such as the NOvA experiment in Minnesota and the proposed Long Baseline Neutrino Experiment in South Dakota. If theta one-three had been zero or very close to zero, those two experiments would not have been able to achieve their goals.

“Nature has been kind to us,” said physicist Mark Messier of the NOvA experiment. NOvA, currently under construction, will use a 14-kiloton detector constructed of plastic in northern Minnesota to study a beam of neutrinos streaming from Fermilab in Illinois. The experiment aims to answer a crucial question about neutrino masses.

Neutrinos of each mass spend a different percentage of their lives as each flavor of neutrino. One prefers to pass the majority of its time as an electron neutrino. Another spends most of its time in the other two flavors. The third is ambivalent, almost evenly splitting its time between the three options.

Scientists know that two of these types of neutrinos have similar masses to one another. However, they do not know if the third, outcast neutrino is significantly heavier or significantly lighter than the other two. This is called the mass hierarchy problem.

“The only near-term experiment with a crack at solving the hierarchy problem is NOvA,” said Fermilab theorist Boris Kayser.

Measuring theta one-three and determining the mass hierarchy of neutrinos would pave the way to solve another big question, the question of matter-antimatter asymmetry.

The problem of matter

Equal amounts of matter and antimatter should have formed during the big bang. When those bits of matter and antimatter met, they should have annihilated. If this had happened, our universe would be dominated by light and radiation. Instead, it seems to be dominated by matter, and physicists would like to know how that – and we – came to be.

In order for matter and antimatter to exist in different quantities in the universe, they must be governed by different laws. An important part of understanding matter-antimatter asymmetry is finding scientific evidence that this is true.

Calculating whether there is a difference between oscillations of neutrinos and antineutrinos depends on theta one-three. If that measure had been too small, scientists would have known that neutrinos and antineutrinos behaved in similar ways. However, Daya Bay’s precise measure of theta one-three provided evidence that the rulebooks for neutrinos and antineutrinos may not be the same.

Now that scientists know the size of theta one-three, they need make one last measurement of the difference between neutrino and antineutrino behavior. Fermilab’s proposed Long Baseline Neutrino Experiment is in line to make that measurement.

It seems unlikely that an imbalance in the world of the tiny neutrino would make much of a difference to the rest of us. But finding a lopsided relationship between neutrinos and antineutrinos could give us the key to understanding the early formation of our matter-filled universe.

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