On p.576 in Peskin & Schroeder, he argues that for a photon propagator with almost on-shell momentum connecting two parts as depicted in the Feynman diagram below, that one can replace the metric in the propagator with the polarisation sum $-g^{\mu\nu}=\sum_i\epsilon^\mu_i\epsilon^{\nu\ast}_i-\epsilon^\mu_+\epsilon^{\nu\ast}_--\epsilon^\mu_-\epsilon^{\nu\ast}_+$. He goes on, then the terms with the unphysical polarisations $\epsilon_\pm$ vanish by the Ward identity.

Feynman diagram

However, as recently discussed in one of my previous questions about the Ward identity (see here), processes like $e^-\rightarrow e^-\gamma$ are unphysical and therefore the Ward identity does not apply to them.

So, how to salvage the argumentation of P&S? Do the terms vanish anyway but for another reason? Or is there another detail I oversaw, that makes the Ward identity still applicable?


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Browse other questions tagged or ask your own question.