Welcome to the "Quanta Science Podcast." Each episode, we bring you stories about developments in science and mathematics. I'm Susan Vallett. Physicists have ruled out a mundane explanation for the strange findings of an old Soviet experiment, leaving open the possibility that the results point to a new fundamental particle. That's next.

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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.

Ben Jones is a neutrino physicist at the University of Texas, Arlington. As it stands right now, there are not really compelling, available explanations. And I would really like to know, personally, what it's caused by. That's why I think it's a particularly exciting anomaly. 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 mis-measurements or inaccuracies that could explain the anomaly. Now they've eliminated another one.

Eric Norman is a nuclear physicist at the University of California, Berkeley. He and his colleagues have announced that one possible solution, an incorrect calculation of the half-life of germanium, can't be the cause. This is not the explanation for the gallium anomaly. It's not that the half-life was wrong. The half-life is correct, and therefore there must be some other explanation. 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.

Vladislav Baranov is a particle physicist at the Institute for Nuclear Research of the Russian Academy of Sciences. He works on the experiment in the Caucasus. We cannot find some future certainty in our experimental procedures and our theoretical calculations. It's a challenge for the future neutrino experiment. Is it a new type of neutrino?

Baranoff says we don't know. 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.

Stephen Elliott is a nuclear physicist at Los Alamos National Laboratory. He worked on the project. The Soviet Union, of course, had a phenomenal group of especially theoretical scientists. And I think the original suggestion for this came out of the Soviet Union. At that point in the late 80s and 90s, the Soviet Union had fantastic raw material and human resources that

but didn't have great access to hard currency and some of the technology that they would like to use for the experiment. Los Alamos was able to provide those types of resources, advanced computing, digitization, that kind of thing. SAGE was constructed at the Baxan Neutrino Observatory, a neutrino physics facility built in the 1960s and 1970s.

A 13,000-foot-tall mountain shielded the facility from cosmic rays and other sources of noise, allowing precise neutrino experiments to take place.

A nearby residential area called Neutrino Village housed the families of the scientists who worked at the facility, as well as visiting international scientists like Elliot. Remember, this was during the days of the Cold War. I was pretty young in those days. I guess I found it as an adventure. I wasn't ever scared. They were very gracious.

SAGE began in 1989 and continued for more than 20 years. That's despite attempts by the Russian government to sell its gallium, a precious metal that's liquid at room temperature.

The project was designed to investigate the solar neutrino problem, a measured deficit of neutrinos streaming from the sun. Specifically, scientists were finding a shortage of electron neutrinos, one of three known types, or flavors. That problem was ultimately resolved in the 2000s with the Nobel Prize winning discovery that neutrinos oscillate between flavors as they travel.

By the time many of the electron neutrinos from the Sun reach Earth, they have become something else. SAGE used a tank of 57 metric tons of gallium. Incoming electron neutrinos would occasionally combine with a neutron inside a gallium atom and convert it into a proton, turning the gallium into germanium.

The scientists counted the germanium atoms in a month-long extraction process. Elliott says they chose gallium for the experiment for a specific reason. It had that low threshold for this reaction. A similar experiment began in Italy in 1991, called GALX. In the mid-1990s, researchers tweaked both experiments to use neutrinos from radioactive elements. They hoped to avoid unknown errors related to the solar neutrino problem.

But both experiments generated roughly 20% less germanium than expected. These were surprise results that couldn't have been caused by the solar neutrino problem. In-Wook Kim, a nuclear physicist at Los Alamos, says the scientists knew precisely the source activity and how many neutrinos are produced.

Soon, the puzzling discrepancy had a name: the gallium anomaly. Here's Barinov. It was really surprising. A follow-up experiment began at Baxan in 2014, called the Baxan Experiment on Sterile Transitions, or BEST.

That experiment uses two gallium chambers instead of one to determine whether the anomaly could be explained by the distance from the source of the neutrinos. Barinov, who's worked on the experiment since 2015, says BEST was constructed to resolve this tension. But both chambers have continued to show a shortfall relative to what models predict. Barinov calls it a really unusual result.

Repeated results from BEST continue to show the anomaly as recently as 2022. One chamber contained only 79% of the expected amount of germanium, the other only 77%.

Wick Haxton is a theoretical physicist at UC Berkeley. I think everybody was hoping that normally we would go away because we don't need this. But still there's curiosity among us theorists about the possibility of a fourth generation of neutrinos, some sterile neutrino that had not been observed. There still is not any clean understanding of what's going on. So what could be happening here?

Some have floated a possible explanation, that the half-life of germanium-71, the specific isotope produced in the experiment, measured in 1985 to be 11.43 days, was actually longer. The same constant controls germanium-71's decay rate and the rate at which gallium captures neutrinos to produce that germanium.

That means a longer germanium-71 half-life would imply a lower rate of neutrino capture and hence germanium production. That could explain the lack of germanium seen by SAGE, GALX, and BEST. Norman and his colleagues published a reinvestigation of this half-life in Physical Review C in late May.

They arrived at a half-life of 11.468 days, extremely close to the 1985 measurement, ruling out the half-life as the explanation for the gallium anomaly.

While no one ever quite believed the original half-life measurement to be wildly incorrect, researchers still considered it worth checking. Here's neutrino physicist Ben Jones. It was a measurement that needed to be done. It was done very well. It was convincing, I think. Another proposed explanation was that physicists had miscalculated the probability of neutrinos from the source interacting with the gallium.

But in September 2023, Haxton and his colleagues also ruled out this possibility. You can't get rid of the anomaly because the ground state transition alone already gives you a problem. That leaves physicists in an uncomfortable position. Either there is still some error that no one has thought of, or, as Haxton puts it, something unusual is going on with neutrinos.

For instance, the experiments might point to a controversial additional type of neutrino, undetected by most other experiments, that might also help to explain dark matter. The three known "flavors" of neutrinos, which are all millions of times lighter than electrons, interact with other elementary particles via the weak force, which makes them detectable.

But sterile neutrinos would interact only via gravity. If they're much heavier than the known neutrinos, their existence could explain why the known neutrinos are so light, through an inverse relationship hypothesized around 1980 called the seesaw mechanism. But the gallium anomaly would point toward a lighter weight sterile neutrino.

Electron neutrinos would be emitted by the radioactive source sometimes oscillating into a sterile neutrino that wouldn't interact with the gallium. In some models, lightweight sterile neutrinos could comprise a fraction of the universe's dark matter, though not all of it. They would be too light to gravitationally shape the universe in the way dark matter does.

Lindley Winslow, an experimental nuclear and particle physicist at MIT, says they could be a small subset of it.

Other attempts to find sterile neutrinos by studying neutrino oscillation patterns have been largely unsuccessful. Here's Winslow. There is a smaller group of people that really believe, and they might be right, that sterile neutrinos are there. And I think like a couple years ago, it would have been a larger group that thought that, and now the group is sort of shrinking. Kvork Abazajan, an astrophysicist at the University of California, Irvine, calls them the underdogs of the particle physics community.

He says if they do exist... The sterling neutrino at this scale...

would wreak havoc with the early universe. That includes ideas of how atoms formed in the minutes following the Big Bang and the theory of the cosmic microwave background, the remnant heat from the initial expansion of the universe. Abhizajan says you'd expect to see the presence of this extra neutrino. However, Abhizajan says recent work has shown alternative models of the sequence of events in those first minutes can accommodate light sterile neutrinos.

In lieu of other explanations for the gallium anomaly, light sterile neutrinos remain a possibility that we just can't eradicate.

Here's nuclear physicist Stephen Elliott. I've always been a bit skeptical of the sterile neutrino hypothesis, but I can't tell you why it's not right. There's never been a convincing explanation from why the experiment might be wrong. Elliott says Russia's invasion of Ukraine has complicated things, but the collaboration between the U.S. and Russia on BEST is still ongoing for now.

Barinov says the team at Baxan is considering using a new source of neutrinos, such as zinc, to further test the result. They may even construct a third chamber of gallium around the source. For now, the anomaly remains unsolved, with no sign of a resolution on the horizon. Or, as theoretical physicist Wick Haxton says: "It has us all puzzled. I don't think there's any simple explanation for what's been seen."

Arlene Santana helped with this episode. I'm Susan Vallett. For more on this story, read Jonathan O'Callaghan's full article, What Could Explain the Gallium Anomaly, on our website, quantummagazine.org. Quantum Magazine is an editorially independent online publication supported by the Simons Foundation to enhance public understanding of science.