Neutrinos are mysterious particles that have the ability to pass through matter without interacting with it at all. Scientists have theorized about them for decades, but have been unable to turn up any concrete evidence to prove they exist. Until now.

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Back in the mid-1990s, the Liquid Scintillator Neutrino Detector (LSND), an experiment at Los Alamos National Laboratory in New Mexico, found evidence of a mysterious new particle: a “sterile neutrino” that passes through matter without interacting with it. But that result couldn’t be replicated; other experiments simply couldn’t find any trace of the hidden particle. So the result was set aside.

Now, MiniBooNE — a follow-up experiment at Fermi National Accelerator Laboratory (Fermilab), located near Chicago — has picked up the hidden particle’s scent again. A new paper posted to the preprint server arXiv offers such a compelling enough the missing neutrino to make physicists sit up and notice.

If MiniBooNE’s new results hold up, “That would be huge; that’s beyond the standard model; that would require new particles … and an all-new analytical framework,” said Kate Scholberg, a particle physicist at Duke University who was not involved in the experiment.

Public Radio International

The Standard Model of physics has dominated scientists’ understanding of the universe for more than half a century. It amounts to a list of particles that, together, go a long way toward explaining how matter and energy interact in the cosmos. Some of these particles, like quarks and electrons, are pretty easy to imagine:

They’re the building blocks of the atoms that make up everything we’ll ever touch with our hands. Others, like the three known neutrinos, are more abstract: They’re high-energy particles that stream through the universe, barely interacting with other matter. Billions of neutrinos from the sun pass through the tip of your finger every second, but they’re overwhelmingly unlikely to have any impact on the particles of your body.

Electron, muon and tau neutrinos — the three known “flavors” — do interact with matter, though, through both the weak force (one of the four fundamental forces of the universe) and gravity. (Their antimatter twins sometimes interact with matter as well.) That means specialized detectors can find them, streaming down from the sun as well as from certain human sources, such as nuclear reactions. But the LSND experiment, Scholberg told Live Science, provided the first firm evidence that what humans could detect might not be the full picture.

Super-Kamiokande Neutrino Observatory

As waves of neutrinos stream through space, they periodically “oscillate,” jumping back and forth between one flavor and another, she explained. Both LSND and MiniBooNE involve firing beams of neutrinos at a detector hidden behind an insulator to block out all other radiation. (In LSND, the insulator was water; in MiniBooNE, it’s a vat of oil.) And they carefully count how many neutrinos of each type strike the detector.

Both experiments have now reported more neutrino detections than The Standard Model’s description of neutrino oscillation can explain the authors wrote in the paper. That suggests, they wrote, that the neutrinos are oscillating into hidden, heavier, “sterile” neutrinos that the detector can’t directly detect before oscillating back into the detectable realm.

The MiniBooNE result had a standard deviation measured at 4.8 sigma, just shy of the 5.0 threshold physicists look for. (A 5-sigma result has 1-in-3.5-million odds of being the result of random fluctuations in the data.) The researchers wrote that MiniBooNE and LSND combined represent a 6.1-sigma result (meaning more than one-in-500 million odds of being a fluke), though some researchers expressed a degree of skepticism about that claim.

The only problem is that other major neutrino experiments haven’t revealed the same anomaly that LSND and MineBooNE have, so the science is far from settled. Time will tell if the mysterious neutrino truly exists.