A two-instanton test of the exact solution of N=2 supersymmetric QCD

The exact solution of N = 2 supersymmetric QCD found by Seiberg and Witten includes a prediction for all multiinstanton contributions to the low-energy dynamics. In the case where the matter hypermultiplets are massless, the leading non-perturbative contribution at weak coupling comes from the two-instanton sector. We calculate this contribution from first principles using the exact two-instanton solution of the self-dual Yang-Mills equation found by Atiyah, Drinfeld, Hitchin and Manin. We find exact agreement with the predictions of Seiberg and Witten for all numbers of flavours, We also confirm their predictions for the case of massive hypermultiplets.