LOW-ENERGY QCD AND ULTRAVIOLET RENORMALONS

We discuss the contribution of ultraviolet (UV) renormalons in QCD to two-point functions of quark current operators. This explicitly includ es effects due to the exchange of one renormalon chain as well as two chains. It is shown that, when the external Euclidean momentum of the two-point functions becomes smaller than the scale Lambda(L) associate d with the Landau singularity of the QCD one-loop running coupling con stant, the positions of the UV renormalons in the Borel plane become t rue singularities in the integration range of the Borel transform. Thi s introduces ambiguities in the evaluation of the corresponding two-po int functions. The ambiguities associated with the leading UV renormal on singularity are of the same type as the contribution due to the inc lusion of dimension d = 6 local operators in a low-energy effective La grangian valid at scales smaller than Lambda(L). We then discuss the i nclusion of an infinite number of renormalon chains and argue that the previous ambiguity hints at a plausible approximation scheme for low- energy QCD, resulting in an effective Lagrangian similar to the one of the extended Nambu-Jona-Lasinio (ENJL) model of QCD at large N-c. (C) 1997 Elsevier Science B.V.