Systematic study of the single instanton approximation in QCD - art. no. 114020

The single-instanton approximation (SIA) is often used to evaluate analytic ally instanton contributions to the Euclidean correlation function in QCD a t small distances. We discuss how this approximation can be consistently de rived from the theory of instanton ensemble and give precise definitions to a number of different quark effective masses." generalizing the parameter m*, which Aas introduced long ago to account for the collective contributio n of the whole ensemble, We test numerically the range of applicability of the SIA for different quantities. Furthermore. we determine all the effecti ve masses (for random and interacting instanton liquid models) as ell as fr ont phenomenology, and discuss to what extent those are universal.