New instanton effects in supersymmetric QCD

In supersymmetric QCD with SU(N_c) gauge group and N_f flavors, it is known that instantons generate a superpotential if N_f = N_c -1 and deform the moduli space of vacua if N_f = N_c. But the role of instantons has been unclear for N_f > N_c. In this paper, we demonstrate that for N_f > N_c, on the moduli space of vacua, instantons generate a more subtle chiral operator containing (for example) non-derivative interactions of 2(N _f-N_c) + 4 fermions. Upon giving masses to some flavors, one can integrate out some fermions and recover the standard results for N _f = N_c and N_f = N_c -1. For N _f = N_c, our analysis gives, in a sense, a more systematic way to demonstrate that instantons deform the complex structure of the moduli space of vacua.