In recent years a big effort have been done both with Monte-Carlo simulations and with (semi) analytical methods in order to understand non gauge invariant correlation functions in Quantum Cromodynamics (QCD). It will be explained why such objects have been studied despite of their unphysical nature. After that, the main results for such correlation functions in Landau gauge will be presented. It will be presented the surprising property that such correlation functions show a mass-like behavior for gluons. It will be shown how this feature can be exploited in order to develop a successful perturbation theory for QCD that reproduces correctly those correlation functions even in the low momentum regime.

Uruguay University, Montevideo