Hi! I am kinda confused about what gets conserved in QED and what not. So the chirality is always conserved, I got that. So in the massless limit, helicity is too. Now in the massive limit. Are spin and helicity conserved? And if they are, are they at each interaction vertex, or just overall. For example in ##e^+e^-## annihilation, I would expect the 2 particle to have the same spin, such that the z component of the spin angular momentum to be conserved when producing a photon. Is this always true? Then, in ##e^-e^-## scattering, how do you even conserve spin at each vertex. You have a spin half electron emitting a photon and another electron. Does this mean that you need something like ##1/2=1-1/2## so the outgoing electron switches spin? Again, is this always true? And when do you use helicity? In the book I read they say that spin and helicity are not conserved in QED as electric and magnetic field can affect them. But in certain situation they use arguments based on spin and helicity to argue against certain processes. Also it seems to depend on whether you are in a relativistic or non-relativistic regime. Can someone clarify this for me please? Thank you!