Pinch technique self-energies and vertices to all orders in perturbation theory

The all-order construction of the pinch technique gluon self-energy and quark-gluon vertex is presented in detail within the class of linear covariant gauges. The main ingredients in our analysis are the identification of a special Green function, which serves as a common kernel to all self-energy and vertex diagrams, and the judicious use of the Slavnov-Taylor identity it satisfies. In particular, it is shown that the ghost-Green functions appearing in this identity capture precisely the result of the pinching action at arbitrary order. By virtue of this observation the construction of the quark-gluon vertex becomes particularly compact. It turns out that the aforementioned ghost-Green functions play a crucial role, their net effect being the non-trivial modification of the ghost diagrams of the quark-gluon vertex in such a way as to reproduce dynamically the characteristic ghost sector of the background field method. The gluon self-energy is also constructed following two different procedures. First, an indirect derivation is given, by resorting to the strong induction method and the assumption of the uniqueness of the S-matrix. Second, an explicit construction based on the intrinsic pinch technique is provided, using the Slavnov-Taylor identity satisfied by the all-order three-gluon vertex nested inside the self-energy diagrams. The process independence of the gluon self-energy is also demonstrated, by using gluons instead of quarks as external test particles, and identifying the corresponding kernel function, together with its Slavnov-Taylor identity. Finally, the general methodology for carrying out the renormalization of the resulting Green functions is outlined, and various open questions are briefly discussed.