File:Right left helicity.jpg

Summary
It's convenient to quantize the spin of spin-1/2 particles along their direction of motion. When the spin and the momentum are parallel, the particle is called "right-handed;" when they are antiparallel, the particle is "left-handed." These are known as helicity states. Red arrows represent the spin; magenta arrows are the momentum. I made this diagram and release it into the public domain. HEL 21:21, 21 February 2007 (UTC)