2PI effective action and gauge dependence identities

Abstract.The problem of maintaining gauge invariance when truncating the two-particle irreducible (2PI) effective action has been studied recently by several authors. Here we give a simple and very general derivation of the gauge dependence identities for the off-shell 2PI effective action. We consider the case where the gauge is fixed by an arbitrary function of the quantum gauge field, subject only to the restriction that the Faddeev-Popov matrix is invertible. We also study the background field gauge. We address the role that these identities play in solving gauge invariance problems associated with physical quantities calculated using a truncated on-shell 2PI effective action.