In Peskin & Schroeder chapter 19, about trace anomaly in massless QED, the trace of $\Theta^{\mu\nu}$ is given by $$ {\Theta^\mu} _\mu =-\frac{4-d}{4} (F_{\lambda\sigma})^2 + (1-d) \bar{\psi} i \displaystyle{\not}D \psi $$ The one-loop matrix element of $ {\Theta^\mu} _\mu$ is given by the following three diagrams. enter image description here

I do not understand these diagrams. Just as explaination in the P&S, the expectation of $(1-d) \bar{\psi} i \displaystyle{\not}D \psi $ is zero, the one-loop matrix element is from the first term which is can be written as $$-\frac{4-d}{2}A_\mu(-k) (k^2g^{\mu\nu}-k^\mu k^\nu)A_\nu(k)$$ This seems to be the usual photon propagator with somr projection operator inserted. Why not from the usual diagrams like the followings? enter image description here

  • $\begingroup$ Could you clarify what your exact question is? $\endgroup$ – ACuriousMind Nov 11 '14 at 21:37

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