Hi, Following my recent https://www.physicsforums.com/showthread.php?t=375638" with density matrix formulations of scattering theory, here's a question I faced on yesterday's particle physics exam. I did some mechanical computation, but I am not fully convinced of what I did (basically computed the density matrix and tried to compute the polarizations). The problem is that I am still not comfortable with the entire machinery. I'd appreciate if someone could discuss this question and explain me the machinery. I would like to work out the solution on my own. Thanks!