Z-]B(B)OVER-BAR EXCESS AND TOP-QUARK DECAY

The apparent excess of Z --> b (b) over bar events at CERN LEP may be an indication of new physics beyond the standard model. However, in ei ther the two-Higgs-doublet model or the minimal supersymmetric standar d model any explanation would lead to an important new decay mode of t he top quark and suppresses the t --> Wb branching fraction, which goe s against what has been observed at the Fermilab Tevatron. In the two- Higgs-doublet model, the branching fraction of Z --> b (b) over bar a light boson which decays predominantly into b (b) over bar would be at least of order 10(-4).