I'm trying to understand what the Bjorken sum rule and the polarized structure functions entering it are. I will use equation (2.3) here as a reference for asking the question.
In Peskin, I've only seen the first line of this equation, eq. (18.113). The only QFT context in which I heard 'polarized' and 'unpolarized' is whether we average or not over the initial/final spin states. What does it mean that we need to add another piece to the hadronic tensor which corresponds to the 'polarized scattering'? Is it the part which only matters if we do not average over the spin states?
Is there a simple way to understand the Bjorken sum rule for these $g_{1,2}$ functions? Something in the spirit of what we have for PDFs, which should give the correct total momentum and charge of the hadron?..
