everyone! I have two points not clear in Peskin & Schroeder's QFT on page 166.
- On Figure 5.6, "Since helicity is conserved, a unit of spin angular momentum is converted to orbital angular momentum", I am really puzzled on this sentence. In my understanding, in this Center-of-Mass frame, the initial total spin is zero, also the finial total spin is zero. For the orbital angular momentum, I see that the "back-scattering" probability is large, so their need to have some orbital angular momentum in principle. But I am really puzzled on how this one unit of spin angular momentum converted to orbital angular momentum. By the wave, the book mentioned the final states is a "p-wave", also what is this "p-wave"?
- I also troubled with the sign in eq.(5.101), I thought their need to have a "minus" sign, the reason as follows: for the equation above (5.101), I thought it should be $$ \bar{\sigma}\cdot (p-k^{\prime})\simeq -\bar{\sigma}^{1}(p-k^{\prime})^{1}=\sigma^{1}\cdot (-\omega\chi) $$ so it has a "minus" sign difference with book, and we have used the approximation $$ p-k^{\prime}\simeq (0,-\omega \chi,0,0) $$
- I also troubled with the sign in eq.(5.101), I thought their need to have a "minus" sign, the reason as follows: for the equation above (5.101), I thought it should be $$ \bar{\sigma}\cdot (p-k^{\prime})\simeq -\bar{\sigma}^{1}(p-k^{\prime})^{1}=\sigma^{1}\cdot (-\omega\chi) $$ so it has a "minus" sign difference with book, and we have used the approximation $$ p-k^{\prime}\simeq (0,-\omega \chi,0,0) $$
If you have any comments on above questions, I am really appreciate it.
