Measuring Hidden Higgs and Strongly-Interacting Higgs Scenarios

TL;DR

The paper addresses whether LHC Higgs data exhibit deviations from Standard Model couplings due to two well-motivated beyond-SM scenarios: a Higgs portal to a hidden sector and a strongly interacting (composite) Higgs. It develops a model-driven analysis using SFitter to translate observed rates into parameters such as a universal width modifier $\kappa$ (for the hidden sector) or a symmetry-breaking parameter $\xi$ (for composite Higgs) and, in the portal case, an invisible width $\Gamma_{hid}$ with BR_inv. The authors derive 95% CL projections on the key parameters (e.g., $\sin^2\chi$, $\Gamma_{hid}/\Gamma_{tot}^{SM}$, $\xi$) for LHC data at 14 TeV with 30 and 300 fb$^{-1}$ across Higgs masses 110–200 GeV, highlighting how invisible decays and channel combinations enhance sensitivity. They show that, in the portal scenario, BR_inv measurements enable joint determination of $\cos^2\chi$ and $\Gamma_{hid}$, while in the universal strong-interaction case $\xi$ can be constrained to ~10–20% and non-universal patterns can be distinguished by combining channels; degeneracies in non-universal cases can be resolved with sufficient luminosity. The study provides a practical framework for testing and distinguishing Higgs-sector extensions at the LHC, with a clear path to quantifying how closely an observed Higgs matches the SM prediction.

Abstract

Higgs couplings can be affected by physics beyond the Standard Model. We study modifications through interactions with a hidden sector and in specific composite Higgs models accessible at the LHC. Both scenarios give rise to congruent patterns of universal, or partially universal, shifts. In addition, Higgs decays to the hidden sector may lead to invisible decay modes which we also exploit. Experimental bounds on such potential modifications will measure the concordance of an observed Higgs boson with the Standard Model.

Figures (8)

• Figure 1: Correlations between $\Gamma_\text{hid}$ and $\cos^2\chi$ as defined in Eqs.(\ref{['eq:paras']}), based on measuring $\kappa$ and $\mathcal{B}_\text{inv}$. The two parameters are set to $\kappa = 4/9$ and $\mathcal{B}_\text{inv} = 0.5$, respectively, for illustration. The square marks the final solution of $\cos^2 \chi = 2/3$ and $\Gamma_\text{hid}/\Gamma^\text{SM}_\text{tot} = 1/3$ for this parameter set. The estimated 95% CL error bands are explained in the text.
• Figure 2: LHC sensitivity to modified Higgs couplings and no invisible decays $\Gamma_\text{hid} = 0$, based on 30 fb$^{-1}$ of data. Upper: measurement errors as a function of the Higgs mass for $\cos^2 \chi_\text{th}=1.0$ [left], $\cos^2 \chi_\text{th} = 0.8$ [center] and $\cos^2 \chi_\text{th} = 0.6$ [right]. Lower: resulting upper and lower bounds on the mixing parameter $\sin^2\chi$, constrained to the physical range.
• Figure 3: LHC sensitivity to modified Higgs couplings and invisible decays, based on 30 fb$^{-1}$ of data. Upper row: $\Gamma_\text{hid} = 0$; lower row $\Gamma_\text{hid} = \sin^2\chi \, \Gamma^\text{SM}_{\text{tot}}$ for invisible decays. The Higgs mass is fixed to 120 GeV. Left column: extracted $\cos^2 \chi_\text{fit}$ values as a function of $\cos^2 \chi_\text{th}$; Center column: extracted bounds and measurements of $\Gamma_\text{hid}/\Gamma^\text{SM}_\text{tot}$ as a function of $\cos^2 \chi_\text{th}$; Right column: illustration of the correlation between mixing and invisible partial width using $\cos^2 \chi_\text{th} = 1.0$ [upper row] and $0.6$ [lower row].
• Figure 4: LHC sensitivity to modified Higgs couplings based on $30$ [upper] and 300 fb$^{-1}$ [lower row] for un-aligned boson and fermion couplings as a function of the assumed $\xi_\text{th}$ for $m_H=120$ GeV (left), 160 GeV (center) and 200 GeV (right). $\xi$ values close to 1/2, for which the rates are strongly suppressed, are blinded by the gray bars.
• Figure 5: Independent fit of common vector [$g_{VVH}$] and fermion [$g_{ffH}$] Higgs couplings for $\xi_\text{th}=0$, $0.2$ and $0.6$ using 30 fb$^{-1}$. The $\xi$ values correspond to $g_{VVH} = 1$, $0.89$, $0.63$ and $g_{ffH} = 1$, $0.67$, $-0.32$, respectively. The red and orange ellipses show the 68% and 95% CL regions. The green line marks the model prediction, with the black dot indicating $\xi=0$ and the white dots in 0.1 distance, moving to positive values towards the lower left.