Scientists announced on Wednesday that they have found no evidence for the hypothetical “sterile neutrino,” an extra version of the ghostly neutrino particles that are ubiquitous in the universe. The finding comes from the U.S. Department of Energy–run Fermilab’s MicroBooNE experiment. Sterile neutrinos are a popular theoretical prediction because they could help explain the cosmos’ mysterious dark matter, if they exist.
The finding, which was published in Nature on Wednesday, calls into question a popular explanation for anomalies seen in past neutrino experiments that couldn’t be accounted for with existing physics. It could also hold major implications for the Standard Model, which is the best particle physics theory we have to explain how the universe works.
“We are making a very general statement, which is, you cannot just take the Standard Model, add a fourth neutrino and, in that way, explain any of the previous anomalies,” says Justin Evans, a professor of particle physics at the University of Manchester in England and one of MicroBooNE’s spokespeople.
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Neutrinos come in three known flavors—electron, muon and tau—and can oscillate, or flip, between them. In the 1990s physicists observed these elusive particles oscillating in a way that seemed inconsistent with established theory. To explain the weird observations, physicists proposed that what they were observing was in fact a new, fourth kind of neutrino—the sterile neutrino. But although the new results essentially rule that possibility out, they pose new, exciting questions.
“It’s very clear what our result is,” says Matthew Toups, a senior scientist at Fermilab. “What is unclear to me is what the experiments that saw anomalies actually saw.”
Importantly, the results don’t fully take the prospect of an unknown fourth kind of neutrino off the table entirely.
“We have lots of reasons to believe that there are more neutrino states,” says André De Gouvêa, a theoretical physicist at Northwestern University. A challenge, he says, is that physicists don’t know if these other hypothetical neutrinos are detectable using current technology.
The scientists at Fermilab hope their results will soon be validated by other neutrino experiments. But already, their attention is turning to answering the bigger question posed by their findings: If it wasn’t sterile neutrinos, then what caused the anomalies?
One possible theory is that there may be a neutrino that decays before it gets to the detector, Evans says. Another possibility is that neutrinos interact with the detector in ways the scientists didn’t already know about. There are yet other hypotheses. Ultimately, this enduring mystery is just one of the many enigmas surrounding these elusive particles and their properties.
“There are a lot of very concrete things about neutrinos that we don't know,” De Gouvêa says. For one, can neutrinos help explain dark matter? “Maybe the dark matter is secretly a sterile neutrino,” he says.