A Systematic Study of the Chirality-Mixing Interactions in QCD
  We present a study of the QCD interactions which do not conserve the chirality of quarks. These non-perturbative forces are responsible for the violation of the U_A(1) charge conservation and for the breaking of chiral symmetry. From a systematic analysis we argue that the leading dynamical sources chirality flips are the interactions mediated by topological vacuum field fluctuations. We study in detail the contribution of instantons and derive a simple model-independent semi-classical prediction. This result can be used to check on the lattice if instantons are the dynamical mechanism responsible both for chiral symmetry breaking and for the anomalous violation of the U_A(1) charge conservation. 
P. Faccioli (E.C.T., Trento)