  In this thesis we study the possibility that QCD breaks parity at high
temperatures and densities, a scenario that may be tested in heavy ion
collisions. Analytical studies with effective models suggest that QCD may break
parity in dense systems. Besides, P- and CP- odd bubbles may appear in a finite
volume due to local large topological fluctuations in a hot medium. The last
effect may be treated in a quasi-equilibrium description by means of a
non-trivial axial chemical potential. We consider the 'two flavour'
Nambu--Jona-Lasinio model in the presence of a vector and an axial external
chemical potentials and study the phase structure of the model at zero
temperature. These magnitudes could trigger a phase where parity could be
broken in QCD. Analogies and differences with the expected behaviour in QCD are
discussed besides the limitations of the model. We also consider the low energy
realization of QCD in terms of mesons when an axial chemical potential is
present. The eigenstates of strong interactions do not have a definite parity
and interactions that would otherwise be forbidden compete with the familiar
ones. We focus on scalars and pseudoscalars that are described by a generalized
linear sigma model. Finally, we investigate how local parity breaking may
affect vector physics. A modified dispersion relation is derived for the
lightest vector mesons rho and omega. This effect predicts a natural
overproduction of lepton pairs in the vicinity of the rho-omega resonance peak
as well as a polarization asymmetry around this peak. The dilepton excess seems
relevant to explain the anomalous dielectron yield quoted by PHENIX/STAR. We
present a detailed analysis of the angular distribution associated to the
lepton pairs created from these mesons searching for polarization dependencies.
Two angular variables are found to carry the main information related to the
parity-breaking effect.
