CP-conserving two-Higgs-doublet model: The approach to the decoupling limit

A CP-even neutral Higgs boson with standard-model-like couplings may be the lightest scalar of a two-Higgs-doublet model. We study the decoupling limit of the most general CP-conserving two-Higgs-doublet model, where the mass of the lightest Higgs scalar is significantly smaller than the masses of the other Higgs bosons of the model. In this case, the properties of the lightest Higgs boson are nearly indistinguishable from those of the standard model Higgs boson. The first nontrivial corrections to Higgs boson couplings in the approach to the decoupling limit are also evaluated. The importance of detecting such deviations in precision Higgs boson measurements at future colliders is emphasized. We also clarify the case in which a neutral Higgs boson can possess standard-model-like couplings in a regime where the decoupling limit does not apply. The two-Higgs-doublet sector of the minimal supersymmetric model illustrates many of the above features.