Upper bound on Rb in two Higgs doublet model from lepton universality

It has been known that Rb can be enhanced in the two Higgs doublet model if tan β is large. We point out that a similar enhancement in Γ(Z → τ+τ−) is large enough to place a constraint on such a possibility. We obtain a 95% CL upper bound ΔRbRb < 0.73% in this model for the MS mass mb(mZ) = 3.0 GeV. The 1996 world average is ΔRbRb = 0.97%±0.51%. We used the mb(mZ) to determine the bottom Yukawa coupling instead of mb(mb) unlike in previous analyses, and also an improved experimental test of the lepton universality in Z decay, which made our results qualitatively different.