QCD in Infrared Region and Spontaneous Breaking of the Chiral Symmetry

Abstract: The spontaneous breaking of chiral symmetry (SBCS) is one of the most important phenomena of hadron physics. It defines the properties of all the light mesons and baryons. The Chiral Perturbation Theory (ChPT) encodes QCD hadronic correlators at low-energy region in the terms the low-energy constants (LEC) -- the expansion parameters on light quark current masses $m$ and external momenta $p$. The LEC's can be extracted from the phenomenology or from QCD lattice calculations. On the other hand, QCD instanton vacuum/instanton liquid model provides a very natural nonperturbative explanation of the SBCS. It provides a consistent framework for description of the pions and thus may be used for evaluation of the LEC. Our aim is to calculate the vacuum properties and the LEC's within instanton vacuum model and confront with phenomenology and lattice results.