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Neutrino oscillation, a phenomenon of neutrinos, consists of an apparent regular change of their flavors. Each of the three neutrino flavors, electron neutrino, muon neutrino, and tau neutrino is associated with the type of particle interaction that creates or destroys them, but neutrino detectors at a distance don't detect the same ratio of flavors as should be produced or are detected at a shorter distance, suggesting neutrinos change flavor after they are created (flavor morphing).
The current neutrino theory is that this is an instance of a quantum-mechanical phenomenon termed quantum mixing, in this case being the probabilities of detecting neutrinos of each flavor shifting in time (and as the neutrino travels, the probabilities shifting with travel-distance), preferring each flavor in turn. This particular known-phenomenon requires a particle to have mass, and neutrinos had previously been presumed to be massless, traveling the speed of light (c) in the manner of a photon. What's known of the detected speeds of neutrinos is that they are so close to that of light that any difference can't be measured so such measurements cannot independently determine whether they have mass.
My understanding is that the standard model of particle physics still presumes neutrinos to be massless, a remaining anomaly between the standard model and current neutrino theory. If so, I presume no straightforward update to the standard model that accommodates neutrino mass is yet confirmed independently, and alternate, exotic explanations are not completely eliminated, a motivation for neutrino experimenters to check for Lorentz violations.