Only two of the three neutrinos need to be massless, though that would be quite a curious asymmetry, and everyone expects all three to have mass. A massless pa

Only two of the three neutrinos need to be massless, though that would be quite a curious asymmetry, and everyone expects all three to have mass. A massless particle might not have a restframe or experience proper time, but it still propagates through spacetime, and can definitely decay to other massless particles, at least in theory. After all, moving at the speed of light doesn't preclude it from interacting with ordinary matter either. "Luckily", in our universe there are theoretical reasons for photons to be completely stable (e.g. see https://arxiv.org/abs/hep-th/9508018 https://arxiv.org/abs/hep-th/9508018 ), but there's no such general rule.