LOW-ENERGY BEHAVIOR OF 2-POINT FUNCTIONS OF QUARK CURRENTS

We discuss vector, axial-vector, scalar and pseudoscalar two-point fun ctions at low and intermediate energies. We first review what is known from chiral perturbation theory, as well as from a heat kernel expans ion within the context of an extended Nambu-Jona-Lasinio (ENJL) model. We derive then these two-point functions to all orders in the momenta and to leading order in 1/N(c) within the framework of this model. We find an improved high-energy behaviour and a general way of parametri zing them that shows relations between some of the two-point functions , which are also valid in the presence of gluonic interactions. The si milarity between the shape of the experimentally known spectral functi ons and the ones we derive, is greatly improved with respect to those predicted by the usual constituent quark like models. We also obtain t he scalar mass M(S) = 2M(Q) independent of the regularization scheme. In the end, we calculate fully an example of a nonleptonic-matrix clem ent in the ENJL-model, the pi+-pi0 electromagnetic mass difference and find good agreement with the measured value.