The $R_b$ Excess at LEP: Clue to New Physics at the TEVATRON?

If the $R_b$ excess at LEP is real, then any explanation in terms of renormalizable loop corrections leads to important new decay modes of the $t$ quark and suppresses the $t\to bW$ branching ratio. In the two-Higgs-doublet model, the branching ratio of $Z\to b\bar{b}$ + a light boson which decays itself predominantly into $b\bar{b}$ is at least of order ${10}^{-4}$.