Dynamical generation of the mass gap in QCD
  We have unambiguously established the dynamical source of the mass scale parameter (the mass gap) responsible for the large scale structure of the true QCD vacuum. At the microscopic, Lagrangian level it is the nonlinear fundamental four-gluon interaction. At the level of the corresponding equation of motion for the full gluon propagator, it is all the skeleton loop contributions into the gluon self-energy, which contain the four-gluon vertices. The key role of the four-gluon interaction is determined by the fact that this interaction survivies when all the gluon momenta involved go to zero, while the three-gluon vertex vanishes in this limit. The mass gap and the corresponding infrared singularities are "hidden" in these terms, and they show up explicitly when the gluon momentum $q$ goes to zero. The general iteration solution (i.e., when the relevant skeleton loop integrals have to be iterated) for the full gluon propagator unavoidably becomes the exact sum of the two terms. The first term is the Laurent expansion in the inverse powers of the gluon momentum squared, starting necessarily from the simplest one $1/(q^2)^2$. Each severe (i.e., more singular than $1/q^2$) power-type IR singularity is accompanied by the corresponding powers of the mass gap. The standard second term is always as much singular as $1/q^2$, otherwise remaining undetermined. The inevitable existence of the first term makes just the principal difference between non-Abelian QCD and Abelian QED. Moreover, the infrared renormalization program of the theory leads to the gluon confinement criterion in the gauge-invariant way. 
V. Gogohia