Unitary mechanism of infrared freezing in QCD with massive gluons

A simple model free of unphysical infrared singularities has been proposed for the QCD invariant running coupling. This model relies on the assumption that the gluon has a finite mass and, technically, on taking into account the near-threshold behavior of the contributions of Feynman diagrams, so th at the model satisfies the unitarity condition. A one-loop estimate yields a lower bound on the gluon mass, m(gl) > 150 MeV, and indicates that smooth infrared freezing at a level of alpha(s)(Q(2)) less than or similar to 1 o ccurs. Under the additional assumption that the effective mass of the light quarks is approximately equal to the effective mass of the gluon, this con struction admits a shift of the pole to the point Q(2) = 0, which can be co rrelated with the confinement condition.