Introduction to weak decays and QCD - short distance corrections

There are at present three approaches within the framework of the standard model which are based on two common basic ingredients namely I) perturbative QCD at very short-distances and II) an effective chiral Lagrangian realization at very long-distances. In order of increasing technological complexity these approaches are the 1/Nc expansion approach (where Nc is the number of colors of QCD which is taken arbitraring large with αsNc fixed); the QCD hadronic duality approach; the lattice QCD approach. (orig./HSI)