Abstract
Like many other models, Composite Higgs Models feature the existence of heavy vector-like quarks. Mixing effects between the Standard Model fields and the heavy states, which can be quite large in case of the top quark, imply deviations from the SM. In this work we investigate the possibility of heavy bottom partners. We show that they can have a significant impact on electroweak precision observables and the current Higgs results if there is a sizeable mixing with the bottom quark. We explicitly check that the constraints from the measurement of the CKM matrix element $V_{tb}$ are fulfilled, and we test the compatibility with the electroweak precision observables. In particular we evaluate the constraint from the $Z$ coupling to left-handed bottom quarks. General formulae have been derived which include the effects of new bottom partners in the loop corrections to this coupling and which can be applied to other models with similar particle content. Furthermore, the constraints from direct searches for heavy states at the LHC and from the Higgs search results have been included in our analysis. The best agreement with all the considered constraints is achieved for medium to large compositeness of the left-handed top and bottom quarks.
Highlights
In the Minimal Composite Higgs Model (MCHM) of ref. [44], which is based on a 5-dimensional theory in Anti-de-Sitter space-time, the bulk symmetry SO(5) × U(1)X × SU(3) is broken down to the SM gauge group on the ultraviolet (UV) boundary and to SO(4) × U(1)X × SU(3) on the infrared (IR)
Composite Higgs Models allow for a smooth deviation from the SM with identical particle content at low energy
A light narrow Higgs boson arises as pseudo-Nambu Goldstone boson from the spontaneous breaking of a strong sector and is separated by a mass gap from the other resonances of the strong sector
Summary
The models given in refs. [15, 44] have been constructed in terms of five-dimensional theories on Anti-de-Sitter space-time and provide a resummation for large values of ξ. The purpose of this work, is to study the effect of bottom partners that arise when the bottom quark mass is generated by mixing with the strong sector. This cannot be achieved by introducing only a single fermion multiplet in the fundamental or spinorial representation of SO(5). If there are no new resonances of the strong sector below the cut-off, apart from the Higgs boson, the model displays the same phenomenology as the one with fermions embedded in the fundamental representation, cf ref. The charge-5/3 fermions only interact with the Higgs boson through small offdiagonal terms and are not relevant for our analysis Their coupling matrix is not given explicitly here. The couplings of the Goldstone bosons with the fermions can be found in appendix A
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