The effect of new physics on R_b = \Gamma(Z->b bbar) /   \Gamma(Z->hadrons)
  Motivated by the $\sim 2\sigma$ discrepancy between the experimental value for $R_b$ and the theoretical prediction of the Standard Model, we examine the effect of new physics on the $Z$-$b$-$\overline{b}$ vertex. Using general results for both new scalars and gauge bosons, we show that two conditions must be satisfied in order for new physics to give observable deviations in the vertex. In particular, the fermion in the loop must transform chirally under $SU(2)\times U(1)$ and it must be massive compared to the exchanged boson. We examine the implications of these results on the 2 Higgs Doublet model and the Left-Right Symmetric model. 
J. T. Liu and D. Ng