Resolving infrared singularities of QCD through the vertex paradigm
  We furnish details and extensions for the vertex paradigm and related ideas. The vertex paradigm is a method for dealing non-perturbatively with the Schwinger-Dyson equations (SDE) of asymptotically-free (AF) gauge theories such as QCD, even in the face of necessary approximations. It provides a useful truncation for the infinitely-many SDE of the gauge- and renormalization-group invariant Pinch Technique (PT-RGI). We implement the vertex paradigm by successive approximations at the one-dressed-loop level, postulating input tree-level gluon and ghost propagators and a 3-gluon vertex that are well-behaved in the infrared and also satisfy several crucial PT-RGI Ward identities that are QED-like and ghost-free. Good IR behavior is assured by including a (non-running) gauge-invariant dynamical gluon and ghost mass as part of the input. The non-trivial part of the vertex paradigm is that, with our inputs, the one-loop output vertex then satisfies the correct Ward identity from which we can construct the output gluon propagator, by taking account of Nambu-Goldstone-like massless scalars and related technical problems that arise whenever there is dynamical gluon mass generation. The one-loop outputs show a number of desirable features: They are PT-RGI; free of any reference to a coupling (dimensional transmutation); satisfy the ghost-free Ward identities connecting them; give exactly the known one-loop UV behavior; and are free of IR singularities. We give a much simpler illustration of the main principles of the vertex paradigm in a modified $\phi^3_6$ model that is AF. Our successive-approximation scheme is not designed to estimate the gluon dynamical mass, but it shows that there is a lower limit to the mass below which the AF theory breaks down. 
John M. Cornwall
hep-ph hep-th