------
In the 1960s, when scientists began studying neutrinos streaming out of the sun, they measured only about a third of the output predicted by theory. Oscillation explained why: the missing two thirds were morphing from electron neutrinos into muon and tau neutrinos as they traveled to Earth, but the instruments were set up to see only electron neutrinos. Although the discovery put to bed the so-called solar neutrino problem, it exposed another mystery: according to theory, the only way for neutrinos to switch flavors is for them to have mass—and that is something that the Standard Model did not predict.
------
i also encourage you to read this part
______
But the Higgs mechanism works only on particles that come in both right-handed and left-handed versions, a fundamental difference related to the orientation of their spin relative to their direction of motion. So far neutrinos have been seen only in left-handed form. If they got mass from the Higgs field, then right-handed neutrinos must also exist. But right-handed neutrinos have never been observed, which suggests that if they are real they do not interact at all with any other forces or particles in nature
------
https://www.scientificamerican.com/article/the-neutrino-puzzle/