Implications of a heavy top quark for the two Higgs doublet model

Constraints on the two Higgs doublet model are presented, assuming a top quark mass of 174±17 GeV. We concentrate primarily on the ‘‘type II’’ model, where up-type quarks receive their mass from one Higgs doublet, and down-type quarks receive their mass from the second doublet. High energy constraints derived from the W mass, the full width of the Z, and the bb¯ partial width of the Z are combined with low energy constraints from Γ(b→sγ), Γ(b→cτν${\mathrm{¯}}_{\mathrm{\tau }}$) and ${\mathit{B}}^{0\mathrm{-}}$B${\mathrm{¯}}^0$ mixing to determine the experimentally favored configurations of the model. This combination of observables rules out small charged Higgs boson masses and small values of tanβ, and provides some information about the neutral Higgs boson masses and the mixing angle α. In particular, constraints derived from the ρ parameter rule out configurations where the charged Higgs boson is much heavier or much lighter than the neutral Higgs bosons. The agreement of the model with experiment is roughly as good as that found for the minimal standard model. We discuss a scenario where Γ(Z→bb¯) is enhanced relative to the standard model result, which unfortunately is on the verge of being ruled out by the combination of Γ(b→sγ) and ρ parameter constraints. Implications for various extensions of the standard model are briefly discussed.