A 125 GeV composite Higgs boson versus flavour and electroweak precision tests

TL;DR

This work analyzes how a 125 GeV composite Higgs, realized as a pseudo-Goldstone boson and coupled to semi-perturbative top partners, can coexist with stringent flavour and electroweak precision tests within partial compositeness. By exploring three fermion representations (doublet, triplet, bidoublet) and three flavour structures (anarchy, $U(3)^3$, $U(2)^3$), it derives a network of oblique, vertex, and four-fermion constraints, including direct resonance searches. The study finds that anarchic and $U(3)^3$ scenarios face substantial tension, especially from $\epsilon_K$, $R_b$, and dijet-compositeness bounds, whereas $U(2)^3$ can accommodate top partners around the TeV scale in favorable regions. Overall, a limited set of flavour-symmetric constructions appears capable of reconciling a light Higgs with testable top partner signals and associated flavour signatures, informing LHC strategies and precision-flavour probes.

Abstract

A composite Higgs boson of 125 GeV mass, only mildly fine-tuned, requires top partners with a semi-perturbative coupling and a mass not greater than about a TeV. We analyze the strong constraints on such picture arising from flavour and electroweak precision tests in models of partial compositeness. We consider different representations for the composite fermions and compare the case of an anarchic flavour structure to models with a U(3)^3 and U(2)^3 flavour symmetry. Although non trivially, some models emerge that look capable of accommodating a 125 GeV Higgs boson with top partners in an interesting mass range for discovery at the LHC as well as associated flavour signals.