Z→bb¯excess and top quark decay

The apparent excess of Z→bb¯ events at CERN 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 top quark and suppresses the t→Wb branching fraction, which goes against what has been observed at the Fermilab 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 ${10}^{\mathrm{-}4}$. © 1995 The American Physical Society.