Quick question regarding superficial degrees of freedom and Ward identities.
For instance in Peskin and Schroeder it is stated that the photon-self energy is superficially quadratically UV divergent but due to the Ward identity it is only logarithmically divergent. I don't see this argument.
The self-energy is given by
How does the Ward identity, or in other words, gauge invariance kill of the divergences?
Best, A friendly helper