The Z →bb-bar excess and top decay

The apparent excess of Z → bb events at LEP may be an indication of new physics beyond the standard model. However, in either the two-Higgs-doublet model or the minimal supersymmetric standard model, any explanation would lead to an important new decay mode of the tope quark and suppresses the 6 → Wb branching fraction, which goes against what has been observed at the Tevatron. In the two-Higgs-doublet model, the branching fraction of Z → bb + a light boson which decays predominantly into bb would be at least of order 10-4. (author)