Deep in the Caucasus Mountains, on the border between Russia and Georgia, an unusual experiment is taking place. In an underground lab shielded by a mountain of rock, highly radioactive material sits inside a vat of liquid gallium, blasting out particles called neutrinos that break the gallium down into atoms of germanium.

The goal is to resolve a little-known mystery of physics: the gallium anomaly. “I think it’s one of the most compelling anomalies in neutrino physics that we have today,” said Ben Jones, a neutrino physicist at the University of Texas, Arlington. Some three decades ago, in a previous version of the current experiment, scientists first detected a dearth of the expected germanium atoms that still can’t be explained.

Since then, physicists have worked to rule out possible mismeasurements or inaccuracies that could explain the anomaly. Now they’ve eliminated another one. Eric Norman, a nuclear physicist at the University of California, Berkeley, and colleagues have announced that one possible solution, an incorrect calculation of the half-life of germanium, can’t be the cause.

“The half-life is correct,” Norman said. “This is not the explanation for the gallium anomaly.”

That leaves few possibilities. One is that some still-unknown experimental defect caused the anomaly. Perhaps a different mismeasurement is throwing things off, or a misunderstanding of nuclear physics. Or maybe, just maybe, the anomaly points to a monumental discovery, the existence of a new type of elementary particle called a sterile neutrino. Sterile neutrinos were initially proposed to explain why the masses of the three known neutrinos are so tiny, but they could also account for at least some of the invisible “dark matter” that fills the cosmos.

“We cannot find some huge uncertainty in our experimental procedures,” said Vladislav Barinov, a particle physicist at the Institute for Nuclear Research of the Russian Academy of Sciences who works on the experiment in the Caucasus. “Is it a new type of neutrino? We don’t know.”

Neutrino Village

At the height of the Cold War, before the fall of the Berlin Wall in 1989 and the subsequent dissolution of the Soviet Union, an unlikely partnership arose in the form of an experiment called SAGE, the Soviet-American Gallium Experiment. “The Soviet Union had a phenomenal group of theoretical scientists,” said Steven Elliott, a nuclear physicist at Los Alamos National Laboratory who worked on the project. But they lacked money and access to certain technologies that would make SAGE possible, he said. “Los Alamos was able to provide those types of resources.”