TheZ-width in the two Higgs doublet model

Abstract

Effects from an extended Higgs sector on the partial widths$\Gamma _{{\rm Z} \to b\bar b}$ and$\Gamma _{{\rm Z} \to \tau ^ + \tau ^ - }$ are analysed with emphasis on enhanced Yukawa couplings to the fermions with the weak isospinT ₃=−1/2. Contributions from charged and neutral Higgs bosons are incorporated. Vertex corrections from a heavy top quark and from charged Higgs bosons are always negative. One can however find regions in the parameter space where neutral Higgs bosons lead to positive vertex corrections. The charged Higgs bosons decouple from the ratio$\Gamma ^{_{{\rm Z} \to \tau ^ + \tau ^ - } } /\Gamma ^{_{{\rm Z} \to \mu ^ + \mu ^ - } }$ if their mass is beyond 80 GeV. This ratio is then sensitive to the neutral sector only.