Sterile Neutrinos: Mysterious Particle That Shouldn’t Exist May Demand 'New Physics'

In the 1990s, the Liquid Scintillator Neutrino Detector (LSND) in Los Alamos, New Mexico, found a weird anomaly during its experiments on tiny elementary particles called neutrinos. It found far more of a certain type than should have been possible.

Now, after years of controversy and conflicting results, an experiment called MiniBooNE at the Fermi National Accelerator Laboratory near Chicago appears to support the old LSND results. Researchers think it might be evidence of a fabled and highly controversial elementary particle. The MiniBooNE team recently uploaded their results to the academic preprint server arXiv.

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Neutrinos are tiny particles smaller than atoms. They move in strange and mysterious ways, passing through objects and our bodies in the billions, with little impact. The particles oscillate among three "flavors" predicted by the hugely successful standard model of particle physics: electron, muon and tau. From 2002 onward, MiniBooNE zapped neutrinos toward a massive oil tank and monitored how many changed from muon to electron. The experiment detected several hundred more electron neutrinos than predicted.

One explanation for the discrepancy is the existence of an elusive fourth kind of neutrino: the “sterile” neutrino, which interacts only via gravity. The other three neutrino types can interact via the universe’s weak force. Sterile neutrinos, if they exist, are heavier than muon neutrinos, and this greater mass could mean they oscillate between different neutrino types more often. More oscillations means more electron neutrons to be detected.

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If the results are borne out in future research, they would call for a fundamental physics rewrite. "That would be huge, that's beyond the standard model, that would require new particles...and an all-new analytical framework," Kate Scholberg, a particle physicist at Duke University, told Live Science. Scholberg was not involved in the research.

But the existence of sterile neutrinos is still far from certain due to the limits of the MiniBooNE project. In fact, the paper is already kicking up controversy on scientific forums. “I’m very excited about this result, but I am not ready to say ‘Eureka!’” MIT neutrino physicist Janet Conrad, a member of the MiniBooNE collaboration, told Quanta Magazine.

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Even though it may not prove the existence of the fourth neutrino type, the project sheds light on the possible limits of the standard model. “There are other potential cracks in the standard picture,” physicist Scott Dodelson of Carnegie Mellon University told Quanta. “The neutrino paradox could point our way to a new, better model.”

“It’s clear there’s something to be understood, and I certainly hope it’s a fourth neutrino,” Neal Weiner, a theoretical physicist at New York University, told Quanta. “That said, this would be the first discovered particle beyond the standard model, so the threshold for the evidence is obviously very high.”

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