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. 
Chris Beasley and Edward Witten