Radiative corrections to the Z b b-bar vertex and constraints on extended Higgs sectors

TL;DR

This paper analyzes how extended Higgs sectors affect Z boson decays to bottom quarks, showing that certain models worsen the fit to data while others can improve it, and provides formulas for these corrections.

Contribution

It derives general formulas for radiative corrections to Z --> b b-bar in models with arbitrary extended Higgs sectors and applies them to specific models.

Findings

Charged Higgs bosons in certain models worsen the Standard Model fit.

Light neutral Higgs bosons with enhanced b b-bar couplings can improve the fit.

R_b data constrains the masses of charged Higgs bosons in extended models.

Abstract

We explore the radiative corrections to the process Z --> b b-bar in models with extended Higgs sectors. The observables R_b (the Z hadronic branching fraction to b b-bar) and the b-quark asymmetry A_b = (g_L^2 - g_R^2)/(g_L^2 + g_R^2) are sensitive to these corrections. We find that in models containing only Higgs doublets, singlets, or larger multiplets constrained by a custodial SU(2) symmetry, the radiative corrections to R_b involving charged Higgs bosons always worsen the Standard Model fit to electroweak data. Thus, the present R_b data can be used to set lower bounds on the charged Higgs masses in such models. Radiative corrections to R_b involving light neutral Higgs bosons in models with enhanced couplings to b b-bar can improve the Standard Model fit to present data. The Higgs sector alone cannot explain a significant deviation of A_b from its Standard Model prediction. We present general formulas for the corrections to R_b and A_b in an SU(2)xU(1) electroweak model with an arbitrary extended Higgs sector, and derive explicit results for a number of specific models.