Emergent axion and Higgs boson from strong dynamics

Abstract

We propose a unified model that simultaneously addresses the hierarchy problem and the strong CP problem by considering the most simple fundamental composite Higgs setup and showing that it can feature a viable emergent axion that could be accessible at collider experiments. To maintain a low decay constant, and therefore address the hierarchy problem, while simultaneously avoiding experimental bounds on the axion couplings, we increase the axion mass via additional small instanton contributions coming from a new hidden gauge sector with a confinement scale larger than $Λ_{\text{QCD}}$. Specifically, the axion will be identified with the CP-odd scalar singlet contained in the $SU(4)/Sp(4)$ coset of the minimal fundamental composite Higgs model. Both the SM color group and the additional hidden sector group are then embedded into a larger non-Abelian \textit{grandcolor} group, such that the topological angles of the two sectors are guaranteed to agree at tree-level. Beyond that, we show that radiative corrections and other CP-violating sources can be controlled. After examining the field content and the gauge structure of the model, we analyze the pNGB spectrum, potential, and couplings, as well as the resulting phenomenology. We focus in particular on the axion potential, to understand under which conditions the CP-odd scalar singlet can solve the strong CP problem while maintaining a naturally low compositeness scale, identifying interesting viable parameter space for an axion in the GeV range.

Figures (3)

• Figure 1: Pictorial representation of the various global symmetry breakings induced by the formation of the fermion condensates, leading to pNGBs.
• Figure 2: FCNC mediated by the singlet. Figure from Esser:2023fdo.
• Figure 3: Main bounds on the axion-top couplings. The black dashed area indicates the parameter space of our model when we assume that the first-generation $\psi$ fermions have the same Yukawas as the up and down quarks. The blue dashed area assumes instead Yukawas similar to the second generation. The thick black dashed line indicates $f_a=2\,$TeV, while the thin one indicates $f_a=10^3\,$TeV, motivated by tuning arguments. The gray shaded region is conservatively neglected due to possible bounds on the induced axion-photon coupling. Adapted from Esser:2024pnc.