The symmetries underlying the Standard Model, taken together with the experimental constraints on the form of the neutrino interactions, moreover impose that neutrinos are massless.

I first came in contact with neutrinos when I was writing my last post about the speed of light. There was an experiment that tried to prove that these particles could travel faster than light but it was later discovered that the scientists were actually wrong and the lead scientist, Antonio Ereditato, had to resign. The story was briefly narrated here