In quantum we have a density matrix for the spin states. The density matrix allows us to specify both polarized states, but also various levels of polarization.
The relativistic version of the spin vector is the angular momentum 4-tensor, with 3 spin components and 3 other components. These other guys (I call them the 'intrinsic boost momentum') have states that are going forwards or backwards at the speed of light. Along with the spin, they are related to the chirality.
But what I'd like to see is a development of a density matrix for this instrinsic boost momentum.
Has anyone come across of nice explanation of this?