Rb and Heavy Quark Mixing

In this talk I summarize a part of the work done in a recent collaboration with C. Burgess, J. Cline, D. London and E. Nardi \cite{b96}. We analyze the observed discrepancy between $R_b(\equiv\gb /\ghad )$ as measured at LEP and its standard model value for signals of new physics. The focus is thereby put on new physics that manifests itself through heavy quark mixing. Heavy quark mixing affects the measured value of $R_b$ in two ways: at tree level (bottom mixing) and at one-loop level (top mixing). One finds that whereas the latter cannot account for the deviation, bottom mixing can in principle do the job.