Spin and helicity of massive particles
  It is a well-known fact that helicity is a Lorentz-invariant for massless but not for massive particles. Nevertheless, a satisfactory proof of this fact and a detailed analysis on the relative orientation between spin and the momentum are not readily available. One such analysis was presented by Wigner where a formula for the angle between the spin and the momentum is derived for massive particle states undergoing two successive boosts in orthogonal directions. In this note, using the Wigner rotation, an alternative derivation is provided. 
Cheng-Yang Lee
hep-th