A new result from the Daya Bay experiment has revealed a possible flaw in predictions from nuclear theory.

“Nobody expected that from neutrino physics,” says Anna Hayes, a nuclear theorist at Los Alamos National Laboratory. “They uncovered something that nuclear physics was unaware of for 40 years.”

Neutrinos are produced in a variety of processes, including the explosion of stars and nuclear fusion in the sun. Closer to home, they’re created in nuclear reactors. The Daya Bay experiment studies neutrinos—specifically, electron antineutrinos—streaming from a set of nuclear reactors located about 30 miles northeast of Hong Kong.

In a paper published this week in Physical Review Letters, Daya Bay scientists provided the most precise measurement ever of the neutrino spectrum—that is, the number of neutrinos produced at different energies—at nuclear reactors. The experiment also precisely measured the flux, the total number of neutrinos emitted.

Neither of these measurements agreed with predictions from established models, causing scientists to scramble for answers from both theory and experiment.

Counting neutrinos

To make the record-breaking measurement, Daya Bay scientists amassed the world’s largest sample of reactor antineutrinos—more than 300,000 collected over the course of 217 days. They used six detectors, each filled with 20 tons of gadolinium-doped liquid scintillator. They were able to measure the particles’ energy to better than 1 percent precision. The experiment is supported by several institutions around the world, including the US Department of Energy and the National Science Foundation.

The Daya Bay scientists found that, overall, the reactors they study produced 6 percent fewer antineutrinos than predicted. This is consistent with past measurements by other experiments. The discrepancy has been called the “reactor antineutrino anomaly.”

This isn’t the first time neutrinos have gone missing. During the Davis experiment, which ran in the 1960s in Homestake Mine in South Dakota, physicists found that the majority of the solar neutrinos they were looking for—fully two-thirds of them—simply weren’t there.

With some help from the SNO experiment in Canada, physicists later discovered the problem: Neutrinos come in three types, and the detector at Homestake could see only one of them. A large fraction of the solar neutrinos they expected to see were changing into the other two types as they traveled to the Earth. The Super-Kamiokande experiment in Japan later discovered oscillations in atmospheric neutrinos as well.

Scientists have wondered whether something similar could explain Daya Bay’s missing 6 percent.

Theorists have predicted the existence of a fourth type of neutrino called a sterile neutrino, which might interact with other matter only through gravity. It could be that the missing neutrinos at Daya Bay are actually transforming away into undetectable sterile neutrinos.

Hitting a bump

However, the other half of today’s Daya Bay result could throw cold water on that idea.

In combining their two measurements—the flux and the spectra—Daya Bay scientists found an unexpected bump, an excess of the particles at around 5 million electronvolts. This represents a deviation from theoretical predictions of about 10 percent.

“Experimentally, this is a tour de force, to show that this bump is not an artifact of their detectors,” says theorist Alexander Friedland of SLAC National Accelerator Laboratory. But, he says, “the need to invoke sterile neutrinos is now in question.”

That’s because the large discrepancy suggests a different story: The neutrinos might not be missing after all; the predictions from nuclear theory could just be incomplete.

“These results do not rule out the sterile neutrino possibility,” Friedland says. “But the foundation on which the original sterile neutrino claims were based has been shaken.”

As Daya Bay co-spokesperson Kam-Biu Luk of the University of California at Berkeley and Lawrence Berkeley National Laboratory said in a press release, “this unexpected disagreement between our observation and predictions strongly suggested that the current calculations would need some refinement.”

What comes next

To investigate further, some scientists have proposed building new detectors near smaller reactors with more refined fuel sources—to cut out ambiguity as to which decay processes are producing the neutrinos.

Others have proposed placing detectors closer to the neutrino source—to avoid giving the particles the chance to escape by oscillating into different types. The Short-Baseline Neutrino Program, currently under construction at Fermi National Accelerator Laboratory, will do just that.

Whatever the cause of the mismatches between experiment and theory, these latest measurements will certainly be useful in interpreting results from future experiments, said Daya Bay co-spokesperson Jun Cao, of the Institute of High Energy Physics in China, in the press release.

“These improved measurements will be essential for next-generation reactor neutrino experiments.”