The Sensitivity of Higgs Factories to Composite Higgs Models via Precision Measurements

TL;DR

The paper analyzes a representative composite Higgs model in which the Higgs is a pseudo-Nambu-Goldstone boson from an $SU(5)/SO(5)$ breaking pattern, with vectorlike top partners driving the dominant new-physics effects. By explicitly computing the Higgs potential via collective symmetry breaking and performing a comprehensive scan of a three-dimensional parameter space, the authors quantify deviations in Higgs couplings and in the top quark's electroweak interactions. They find percent-level shifts in Higgs couplings and a mass reach for the lightest top partner in the 3–5 TeV range, with distinctive two-region structure in the heavy-quark spectrum and complementary sensitivity from top EW measurements. The study demonstrates that future Higgs factories could discover or constrain natural composite-Higgs scenarios beyond the LHC reach, particularly through precision measurements of Higgs couplings and $t$–$Z$ interactions.

Abstract

We investigate the potential of precision Higgs factory measurements to discover signatures of a representative model of electroweak symmetry breaking in which the Higgs boson arises as a composite Nambu-Goldstone boson. In this model, as in other models of the ``Little Higgs" or Natural Composite Higgs type, the primary perturbations of the Standard Model come from effects of vectorlike top quark partners. We carry out an explicit calculation of the Higgs potential in this model. Applying phenomenological constraints, we are left with a 3-dimensional parameter space. We then present results from a complete scan of this parameter space. The region in which significant departures from the Standard Model predictions extends to models in which the lightest top quark partner has a mass above 3~TeV. Little Higgs models with such heavy top partners also predict significant deviations from the Standard Model in the top quark electroweak couplings, in particular, in the model studied here, in the $t_L$ coupling to the $Z$ boson.

Figures (7)

• Figure 1: The mass ratio of the first and second heavy top partners as a function of $f$. The three curves show the parameter choices, top to bottom: $\lambda_2/\lambda_3 = 1.3, 1.5, 1.7$.
• Figure 2: The mass ratio of the first and third heavy top partners as a function of $f$. The three curves show the parameter choices, top to bottom: $\lambda_2/\lambda_3 = 1.3, 1.5, 1.7$.
• Figure 3: Scatter plot depicting the percent change of the bottom quark Yukawa coupling from the SM value, over the parameter space of the Little Higgs model. The horizontal axis show the mass $M_1$ of the lightest top quark partner. The solid lines show the values in the three scenarios for the ratios of heavy partner masses $\lambda_2/\lambda_3 = 1.3, 1.5, 1.7$ shown in Figs. \ref{['fig:1']} and \ref{['fig:2']}. The color of each point indicates the value of $f$. As in our study Maayergi:2025ybi, the horizontal dotted lines show the lines of 3$\sigma$ significance of three stages of the proposed LCF experimental program. The bottom of these lines is close to the 3$\sigma$ significance value for FCC-ee and CEPC.
• Figure 4: Scatter plot depicting the percent change of the top quark Yukawa coupling from the SM value, over the parameter space of the Little Higgs model. The horizontal axis show the mass $M_1$ of the lightest top quark partner. The annotation of the plot is as in Fig. \ref{['fig:3']}. In this case, only the lowest two horizontal lines are shown.
• Figure 5: Scatter plot depicting the percent change of the Higgs boson coupling to the $W$ boson from the SM value, over the parameter space of the Little Higgs model. The horizontal axis show the mass $M_1$ of the lightest top quark partner. The annotation of the plot is as in Fig. \ref{['fig:3']}.