The Radial Mode of Composite Higgs Theories at the LHC

Abstract

We examine the potential of the LHC to observe the scalar radial excitation present in extensions of the standard model where the Higgs boson is a pseudo Nambu Golstone boson. These include composite Higgs models as well as the twin Higgs model. These states can be light enough to be seen at the LHC, potentially resulting in additional clues about the nature of the Higgs sector. We present the current status of LHC bounds as well as the future prospects for the the high luminosity LHC (HL-LHC). We identify the most sensitive channels as those where the radial state decays to a pair of Higgs bosons, especially at the high luminosity stage. For the minimal composite Higgs models we study, we make use of the LHC Run 2 data with ${\cal L}=138~{\rm fb}^{-1}$ to extract the $2σ$ mass bounds $m_σ\geq (0.93-1.13)~$TeV, where the values on the interval depend on the parameters of the model. We show that the reach of the HL-LHC for these cases is $m_σ\geq (1.8-2.2)~TeV$, with ${\cal L}=3000~{\rm fb}^{-1}$. For the twin Higgs model radial state, the current bounds are set by Higgs coupling measurements, while for the HL-LHC we obtain the reach $m_σ\geq 1.2~$TeV, corresponding to the lowest symmetry breaking scale allowed by current data.

Figures (6)

• Figure 1: The $\sigma$ total width to mass ratio vs. $m_\sigma$ for the MCHM (left panel) and the THM (right panel).
• Figure 2: Production cross section of $gg \to \sigma\to ZZ$. The solid (dashed) line corresponds to the observed (expected) limit. The bands refer to the $1\sigma$ and $2\sigma$ limits. From data by CMS in Run 2 with $138~fb^{-1}$ integrated luminosity CMS:2024vps. The colored solid and dashed lines correspond to various predictions in the MCHM, for various values of the symmetry breaking scale $f$. See text for details.
• Figure 3: Combination of bounds on the cross section of $gg \to \sigma\to hh$ from CMS:2024vps, with $138~fb^{-1}$ integrated luminosity. The solid (dashed) line corresponds to the observed (expected) limit. The bands refer to the $1\sigma$ and $2\sigma$ limits. The colored solid and dashed lines correspond to various predictions in the MCHM, for various values of the symmetry breaking scale $f$. See text for details.
• Figure 4: Branching fractions of $\sigma$ decays for $f=800~$GeV. The dashed line corresponds to the invisible $\sigma$ width.
• Figure 5: Combination of ATLAS and CMS projections for $pp\to \sigma\to X$ from ATLAS:2025eii, with ${\cal L} = 3000~{\rm fb}^{-1}$ integrated luminosity. Left panel: $X=ZZ$ combination for $Z\to\ell^+\ell^-$. Right panel: $X=hh$, with $hh\to b\bar{b}, \gamma\gamma, 4b's ({\rm merged})$. The solid (dashed) line corresponds to the observed (expected) limit. The bands refer to the $1\sigma$ and $2\sigma$ limits. The colored solid and dashed lines correspond to various predictions in the MCHM, for various values of the symmetry breaking scale $f$. See text for details.