I know that particles are defined in terms of irreducible representations of the Poincaré group, and that the state of a massive particle is defined by its mass and spin, which are the eigenvalues of the two Casimir operators of the Poincaré group. But what is the relation of the spin of a particle and the representations of the Lorent group? I have already read that some representation of the Lorentz group contain a spin or something like this. I can't find explanation of this in books or in the internet. Can someone give me some reference? ?