An Analysis of Non-Oblique Corrections to the $Z b \bar b$ Vertex
  We present a model--independent analysis of the $Z b \bar b$ vertex, with the aim of constraining contributions of new physics to the left- and right--handed couplings of the $b$. We find that the left--handed coupling of the $b$ is quite narrowly constrained by present data, but that the right--handed coupling is still largely unconstrained. 
Tatsu Takeuchi, Aaron K. Grant and Jonathan L. Rosner