  The objective of this thesis is to present two new perturbative frameworks
for the study of low-energy Quantum Chromodynamics (QCD), termed the Screened
Massive Expansion and the Dynamical Model. Both the frameworks paint a picture
of the infrared regime of QCD which is consistent with the current knowledge
provided by the lattice calculations and by other non-perturbative methods,
displaying dynamical mass generation in the gluon sector and a massless ghost
propagator. The Screened Massive Expansion achieves this by operating a shift
of the QCD perturbative series, performed by adding a mass term for the
transverse gluons in the kinetic part of the Faddeev-Popov Lagrangian and
subtracting it back from its interaction part so that the total action remains
unchanged. The Dynamical Model, on the other hand, interprets the generation of
a dynamical mass for the gluons as being triggered by a non-vanishing
condensate of the form $\langle (A^{h})^{2}\rangle$, where $A^{h}$ is a gauge-
and BRST-invariant non-local version of the gluon field, and explores the
consequences of the inclusion of the former in the partition function of the
theory. Since the main focus of this thesis is on the gauge sector of QCD, most
of our calculations will be carried out in the context of pure Yang-Mills
theory. There we will show that the gluon and the ghost propagator derived by
making use of the two frameworks are in good agreement with the Euclidean
Landau-gauge lattice data, within the limits of a one-loop approximation.
During the course of the thesis we will address topics such as the
first-principles status of the two methods, the absence of Landau poles from
the strong running coupling constant and the extension of the Screened Massive
Expansion to finite temperatures and to full QCD. Future research prospects are
discussed in the Conclusions.
