According to the Standard Model, neutrinos should be massless. But they aren’t; they have strangely small masses that don’t seem to fit in with the masses of the rest of the fundamental particles. This fact could possibly be accounted for by a tweak in the theory. Or it could have deep implications for our understanding of the universe.
“It's a picture we've gotten used to, so it doesn't seem very exotic anymore,” says Scholberg, who has been involved in many neutrino experiments over the years. “But it's certainly not part of the original Standard Model.”
“Behind the scenes, there's grumbling noisiness that maybe there's something else out there,” says Kate Scholberg, a neutrino physicist at Duke University. “It could be nothing, or it could be something very exciting.”
This extra neutrino—suggested by results from the Liquid Scintillator Neutrino Detector and the MiniBooNE experiment—wouldn’t match up with the generations of particles in the Standard Model. It would be “sterile,” meaning it likely wouldn’t interact directly with any Standard Model particles. It might even be a form of dark matter.
Other experiments, such as MINOS and IceCube, have published results that are difficult to reconcile with the sterile neutrinos seen by LSND. When Kopp and his colleagues looked for evidence for a fourth neutrino flavor, the numbers just didn’t work. Kopp doesn’t think this rules out sterile neutrinos yet: “It's certainly not impossible that a more complicated scenario with extra neutrinos could fit the data.”
https://www.symmetrymagazine.org/article/already-beyond-the-standard-model
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