Power-counting theorem for infrared logarithms in non-Abelian gauge theories

We have developed a general diagrammatic method for precisely determining the power of infrared logarithms for diagrams of arbitrary order in non-Abelian gauge theories. It is applied specifically to the study of the infrared structure of the gauge-group singlet quark form factor for fixed momentum transfer and nonvanishing quark mass. All leading infrared logarithms of the form factor are found to arise from a class of planar quantum-electrodynamic-type diagrams in which, however, the gluon propagators are fully dressed by gluon and ghost loops. Thus the infrared behavior of the massive-quark sector reflects strongly that of the massless-gluon sector, which in turn is controlled by the ultraviolet structure of the gluon self-interaction.