R_b Problem: Loop Contributions and Supersymmetry
  I summarize a part of the work done with Bamert, Burgess, London and Nardi (hep-ph/9602438) in which we find simplified expressions for the possible one-loop contributions to the $R_b$ parameter from new virtual particles, including supersymmetric ones. Our expressions make it easier to identify which models and choices of parameters are best able to solve the $R_b$ problem. 
James M. Cline