Scientists Just Tore Up a Major Particle Physics Theory

Scientists announced Wednesday that they have found no evidence of the hypothetical “sterile neutrino,” another version of the ghostly neutrino particles ubiquitous in the universe. This discovery comes from the US Department of Energy’s MicroBooNE experiment. Sterile neutrinos are a popular theoretical prediction because they could help explain the cosmos’ mysterious dark matter, if they exist.

The discovery, which was published in Nature Wednesday challenges a popular explanation for anomalies observed in past neutrino experiments that could not be explained by current physics. It could also have major implications for the Standard Model, which is the best particle physics theory we have to explain how the universe works.

“We’re making a very general statement, which is that you can’t just take the Standard Model, add a fourth neutrino and thus explain one of the previous anomalies,” says Justin Evans, professor of particle physics at the University of Manchester in England and one of the spokespersons for MicroBooNE.

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Neutrinos come in three known flavors: electron, muon and tau and can oscillate or switch between them. In the 1990s, physicists observed these elusive particles oscillating in ways that seemed inconsistent with established theory. To explain these strange observations, physicists proposed that what they observed was actually a new fourth type of neutrino: the sterile neutrino. But even though the new results essentially rule out this possibility, they raise exciting new questions.

“Our result is very clear,” says Matthew Toups, senior scientist at Fermilab. “What’s not clear to me is what the experiments that saw anomalies actually saw.”

It is important to note that the results do not completely rule out the prospect of a fourth, unknown type of neutrino.

“We have many reasons to believe that there are more neutrino states,” says André De Gouvêa, a theoretical physicist at Northwestern University. The challenge, he says, is that physicists don’t know whether these other hypothetical neutrinos are detectable with current technology.

Fermilab scientists hope that their results will soon be validated by other neutrino experiments. But already, their attention is turning to answering the larger question posed by their findings: If these were not sterile neutrinos, then what is causing these anomalies?

One possible theory is that there could be a neutrino that decays before reaching the detector, Evans says. Another possibility is that the neutrinos interact with the detector in a way that scientists did not yet know. There are still other hypotheses. Ultimately, this enduring mystery is just one of many enigmas surrounding these elusive particles and their properties.

“There are a lot of very concrete things about neutrinos that we don’t know,” explains De Gouvêa. On the one hand, can neutrinos help explain dark matter? “Maybe dark matter is secretly a sterile neutrino,” he says.

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