The three kinds of neutrinos are some of the fundamental particles of nature. All fermions (particles that make up matter) can spin either left or right (this is called their helicity; either left-handed or right-handed). Strangely, neutrinos always spin left. This paper is one of the first to establish this fact experimentally.

It is thought that anti-neutrinos have the opposite helicity of neutrinos. But it has more recently been established that neutrinos have mass, and thus travel at less than the speed of light. Hence for a given neutrino traveling at speed v, you can always imagine a frame of reference traveling at a speed greater than v, where the neutrino suddenly has opposite helicity from that frame of reference. So this begs the question, are neutrinos their own antiparticles?

Link snatched from Today in Particle Astrophysics. Comment on Reddit.