Less is More: Non-renormalization Theorems from Lower Dimensional   Superspace
  We discuss a new class of non-renormalization theorems in N=4 and N=2 Super-Yang-Mills theory, obtained by using a superspace which makes a lower dimensional subgroup of the full supersymmetry manifest. Certain Wilson loops (and Wilson lines) belong to the chiral ring of the lower dimensional supersymmetry algebra, and their expectation values can be computed exactly. 
Z. Guralnik, S. Kovacs, B. Kulik