Testing Extended Technicolor with R B * )

Two outstanding questions in particle theory are the cause of electroweak symmetry breaking and the origin of the masses and mixings of the fermions. Because theories that use light, weakly-coupled scalar bosons to answer these questions suffer from the hierarchy and triviality problems, it is interesting to consider the possibility that electroweak symmetry breaking arises from strong dynamics at scales of order 1 TeV. This talk focuses on extended 1) technicolor 2) (ETC) models, in which both the masses of the weak gauge bosons and those of the fermions arise from gauge dynamics. In extended technicolor models, the large mass of the top quark generally arises from ETC dynamics at relatively low energy scales. Since the magnitude of the Cabibbo-Kobayashi-Maskawa matrix element |Vtb| is nearly unity, SU(2)W gauge invariance insures that ETC bosons coupling to the left-handed top quark couple with equal strength to the left-handed bottom quark. In particular, the ETC dynamics which generate the top quark’s mass also couple to the left-handed bottom quark thereby affecting the Zbb̄ vertex. This has been shown 3) to provide a powerful experimental constraint on extended technicolor models – particularly on those models in which the ETC gauge group commutes with SU(2)W . This talk begins by reviewing the connection between the top quark mass and the Zbb̄ vertex in ETC models. Next, the power of the resulting experimental constraint on ETC models with weak-singlet ETC bosons is demonstrated. Several recent attempts 4), 5), 6), 7), 8), 9), 10) to bring ETC models into agreement with experimental data on the Zbb̄ vertex are mentioned, and the most promising one (non-commuting ETC) is discussed.