The Top Quark Decay Vertex in Standard Model Extensions
  New physics interactions can affect the strength and structure of the $tbW$ vertex. We investigate the magnitudes and phases of "anomalous" contributions to this vertex in a two-Higgs doublet and the minimal supersymmetric extension of the standard model, and in a top-color assisted technicolor (TC2) model. While the magnitudes of the anomalous couplings remain below 1 percent in the first two models, TC2 interactions can reduce the left-chiral coupling $f_L$ by several percent. 
Werner Bernreuther, Patrick Gonzalez, Martin Wiebusch