Quantum state of a free spin-1/2 particle and the inextricable   dependence of spin and momentum under Lorentz transformations
  We revise the Dirac equation for a free particle and investigate Lorentz transformations on spinors. We study how the spin quantization axis changes under Lorentz transformations, and evince the interplay between spin and momentum in this context. 
Tiago Debarba and Reinaldo O. Vianna
quant-ph