What is a neutrino, what is an anti-neutrino?
Neutrinos (or perhaps neutrini?–Pauli was an Italo-American) have anglular momentum–and not much else. The sign of the angular momentum (+ve or −ve) is used to assign a spin, although spin in this sense is abstract, and not very similar to the spin on a cricket ball. If something has spin and is travelling, you can picture it tracing out a helix, as a spot on a ball bowled by Shane Warne would do. One of the particles produced by beta decay is an antineutrino–it has left-handed helicity. If neutrinos are massless and travel at the speed of light, then they can be classified as having positive helicity or negative helicity. The neutrino has the same helicity as a normal screw: you turn clockwise and it goes away from you. The anti-neutrino has the helicity of a left-handed thread. See http://230nsc1.phy-astr.gsu.edu/hbase/particles/neutrino3.html for diagrams. Note that it is only possible to make this distinction if neutrinos travel at the speed of light. If they have mass and trave
