A minimally tuned composite Higgs model from an extra dimension

TL;DR

This paper presents a minimally tuned 5D holographic realization of a composite Higgs model (MCHM$_{14}$) where the Higgs is a pseudo-Goldstone boson from $SO(5)/SO(4)$, and the SM fields arise from chiral zero modes with $q_L\in \mathbf{14}$ and $t_R\in \mathbf{1}$. The authors implement a flat extra dimension with explicit gauge and fermionic sectors, establishing calculable Higgs potential through 1-loop Coleman–Weinberg contributions and two independent invariants that enable electroweak symmetry breaking with minimal tuning. Their numerical analysis shows viable regions with $\xi=(v/f)^2\lesssim 0.04$, a Higgs mass around 125 GeV, and a distinctive spectrum of fermionic resonances including light exotic quarks with charges $5/3$ and $8/3$, notably a light $\Upsilon$ (charge $8/3$) that could be probed at the LHC. The work highlights the tension with the $\hat S$ parameter in 5D and discusses how accidental suppressions can alleviate this tension, yielding testable collider signatures and a concrete, calculable framework for near-term experimental scrutiny.

Abstract

We construct and study the 5D realization of a composite Higgs model with minimal tuning. The Higgs is a (pseudo-)Goldstone boson from the spontaneous breaking of a global SO(5) symmetry to an SO(4) subgroup. The peculiarity of our construction lies in the specific choice of the SO(5) representations of the 5D fermions from which the Standard Model fields arise as chiral zero modes. This choice reduces the tuning of these models to the minimal model-independent value allowed by electroweak precision tests. We analyse the main differences between our 5D construction and other descriptions in terms of purely 4D field theories. 5D models are generally more constrained and show a generic difficulty in accommodating a light Higgs without reintroducing large corrections to the \hat{S} parameter. We propose a specific construction in which this tension can be, even though accidentally, relaxed. We discuss the spectrum of the top partners in the viable regions of parameter space and predict the existence of light exotic quarks, Υ, of charge 8/3 whose striking decay channel Υ\to W^+W^+W^+ b can lead to either exclusion or confirmation of the model in the near future.

Figures (5)

• Figure 1: Distribution of the values of $\sqrt{|a_2|}$ as defined in Eq. \ref{['higgsmass']}. The parameter range is the one defined in Eq. \ref{['scanflat']}. We only keep points with negative $a_2$.
• Figure 2: Left panel: distribution of points coming from the scan with parameter ranges given by Eq. \ref{['scanflat']} as a function of $g_\rho$. Gray: points with $115\,\textrm{GeV}<m_h<400\,\textrm{GeV}$; red: points with $120\,\textrm{GeV}<m_h<130\,\textrm{GeV}$; blue: points with $120\,\textrm{GeV}<m_h<130\,\textrm{GeV}$ which are consistent with the electroweak fit at 99% C.L. assuming a $10^{-3}$ positive contribution to $\hat{T}$. Right panel: distribution of the values of $\sqrt{a_2}$ as defined in Eq. \ref{['a1a2']} and Eq. \ref{['higgsmassprior']}.
• Figure 3: Scatter plot in the $(\xi, g_\rho)$ for those points that can be made consistent with the EWPT at 99% C.L. assuming a $\Delta\hat{T}=10^{-3}$. The black lines are the $2\sigma$ and $3\sigma$ contours from the LEP EW fit. Blue lines indicate the suppression in $\sqrt{a_2}$ which is necessary to achieve $m_h=125$ GeV according to the NDA estimate of the Higgs boson mass of Eq. \ref{['higgsmassprior']}. The color of the points indicates the mass of lightest charge 5/3 top partner in the spectrum (see figure).
• Figure 4: Left panel: Distribution of fermionic resonance masses before EWSB according to their quantum numbers: $\mathbf 9$ in green, $\mathbf{2}_{1/6}$ in blue and ${\bf 1}_{2/3}$ in red. Thicker points are those passing the EWPT constraint. Right panel: mass distributions for the ${\bf 2}_{1/6}$ (blue) and ${\bf 1}_{2/3}$ (red) $SO(4)$ representations without considering EWPT. The red dashed histograms is the expected mass distribution from the NDA scaling $m_\psi=g_\rho f$.
• Figure 5: Mass distributions of the ${\bf 1}_{2/3}$ (top left), ${\bf 9}$ (top right), ${\bf 2}_{1/6}$ (bottom left) and ${\bf 2}_{7/6}$ (bottom right) obtained from the scan with parameter ranges given by Eq. \ref{['scanflat']}. Red: points with $120\,\textrm{GeV}<m_h<130\,\textrm{GeV}$; blue: points with $120\,\textrm{GeV}<m_h<130\,\textrm{GeV}$ which are consistent with the electroweak fit at 99% C.L. assuming a $10^{-3}$ positive contribution to $\hat{T}$.