It would be nice if someone commented a couple of propositions by Peskin and Schroeder in their QFT book in p.166. There they say that when the helicity is conserved in the high energy Compton scattering, one unit of spin angular momentum is converted to one unit of orbital angular momentum. From the picture in p.166 it is quite obvious that after the collision one unit of spin angular momentum is lost. However, how do we physically interpret the emergence of orbital angular momentum? Do particles begin to orbit? In the same page it is argued that the final state is a p-wave and that it is somehow related to the conservation of angular momentum. What do they mean by that? And what is a p-wave in this context?