In a two-photon process involving a charged scalar field, the full amplitude obeys a Ward identity under replacement of either external photon polarization vector by that photon's four-momentum. At tree level in scalar quantum electrodynamics, both orders of single-photon attachment and the two-photon contact diagram contribute.
Contracting a derivative vertex converts its momentum factor into a difference of inverse scalar propagators. Intermediate terms cancel between the attachment orders and the contact interaction; external inverse propagators vanish for on-shell external states. Omitting the contact term generally leaves a nonzero result, even when both exchange diagrams are present. The cancellation is a practical test of the amplitude's gauge consistency.[ref]

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