HIGHER-DERIVATIVE TERMS IN THE EFFECTIVE ACTION OF N=2 SUSY QCD FROM INSTANTONS

We consider N = 2 SUSY QCD with gauge group SU(2) and N-f flavours of matter with non-zero mass. Using the method of the instanton-induced e ffective vertex we calculate higher derivative corrections to the Seib erg-Witten result in the momentum expansion of the low energy effectiv e Lagrangian in various regions of the modular space. Then we focus on a certain higher derivative operator on the Higgs branch. We show tha t the singular behaviour of this operator comes from values of the mas s of matter at which the charge singularity on the Coulomb branch coll ides with the monopole or dyon singularity. Given the behaviour of thi s operator at weak coupling coming from instantons as well as its beha viour near the points of colliding singularities we find the exact sol ution for this operator. (C) 1998 Elsevier Science B.V.