- #1

- 129

- 13

- Summary:
- The ordering of the terms in a Feynman diagram can change its value. Which order is the right one?

Hey there,

In QED, we often have Feynman diagrams involving various numbers of non-commuting components, such as for the one-loop vertex correction (Peskin & Schroeder, page 189):

The diagram this integral corresponds to is:

How do we choose which order to put the propagators and vertex factors in? P & S chose this way, but when I tried to work it out before checking with the textbook, I had ##{ \gamma }^{ \mu }## at the front of the expression (corresponding to the vertex at the 'tree-level' position), followed by the ##{ k }^{ ' }## propagator, and looping around counter-clockwise (I used a ##k-p## photon propagator), which led to me being at least a factor of two out, in particular on the term with quadratic numerator.

In QED, we often have Feynman diagrams involving various numbers of non-commuting components, such as for the one-loop vertex correction (Peskin & Schroeder, page 189):

The diagram this integral corresponds to is:

How do we choose which order to put the propagators and vertex factors in? P & S chose this way, but when I tried to work it out before checking with the textbook, I had ##{ \gamma }^{ \mu }## at the front of the expression (corresponding to the vertex at the 'tree-level' position), followed by the ##{ k }^{ ' }## propagator, and looping around counter-clockwise (I used a ##k-p## photon propagator), which led to me being at least a factor of two out, in particular on the term with quadratic numerator.