Composite Higgs under LHC Experimental Scrutiny

TL;DR

The paper investigates whether electroweak symmetry breaking can be realized by a composite Higgs arising from a strongly interacting sector. It develops the SILH effective Lagrangian to parametrize deviations from the SM via $a,b,c$ and higher-dimension operators, and links these deviations to explicit 5D holographic models (MCHM4/5) with an SO(5)/SO(4) structure. It also analyzes constraints from electroweak precision data and direct LHC searches, showing how current data bound the compositeness scale through the parameter $\xi=(v/f)^2$ and how resonances such as vector states and top partners offer direct discovery channels. The work highlights signatures—modified Higgs couplings, vector/fermionic resonances, and altered longitudinal vector boson scattering—that would indicate strong dynamics, and outlines the prospects for future collider runs to probe these scenarios.

Abstract

The LHC has been built to understand the dynamics at the origin of the breaking of the electroweak symmetry. Weakly coupled models with a fundamental Higgs boson have focused most of the attention of the experimental searches. We will discuss here how to reinterpret these searches in the context of strongly coupled models where the Higgs boson emerges as a composite particle. In particular, we use LHC data to constrain the compositeness scale. We also briefly review the prospects to observe other bosonic and fermionic resonances of the strong sector.

Figures (6)

• Figure 1: Composite models built in five dimensional Anti-de-Sitter space-time and their symmetry breaking pattern interpretation. In 5D, the gauge symmetry in the bulk, $G$, is broken by suitable boundary conditions to $H_{UV}$ on the UV brane and to $H_{IR}$ on the IR brane. The low energy theory mimics in 4D a strongly interacting sector invariant under a global symmetry $G$ spontaneously broken to $H_{IR}$ at the IR scale with a subgroup $H_{UV}$ which is weakly gauged. The number of Goldstone bosons is equal to $\textrm{dim}(G/H_{IR})$, $\textrm{dim}(H_{UV}/H)$ being eaten to give a mass to some gauge bosons ($H=H_{UV} \cap H_{IR}$). The remaining $\textrm{dim}(G/H_{IR})-\textrm{dim}(H_{UV}/H)$ massless Goldstones are described on the 5D side by the massless $A_5^{H}$ modes.
• Figure 2: Limits from EW precision data. The grey area is excluded at 99% CL when no additional contributions to $S$ and $T$ beyond the ones of Eq. (\ref{['eq:ST']}) are introduced. The dashed-red (dotted green) lines indicate how this exclusion area is modified in the presence of additional contributions to $S$ ($T$). The logarithmic divergences are cut-off at $\Lambda=4 \pi v/\sqrt{1-a^2}$. The usual parameters $\epsilon_2$ and $\epsilon_b$ used in EW fits are kept fixed to their SM values.
• Figure 3: MCHM5: Rescaled limits from the search of a Higgs boson in various channels set by the ATLAS (top) and CMS (bottom) data presented at CERN in December 2011 Dec11. The various channels are added in quadrature.
• Figure 4: Limits from Higgs searches at LEP, Tevatron and LHC in the plane ($m_h, \xi$) for MCHM4. For the LHC constraints, we have used the data reported at the EPS-HEP 2011 conference EPS, the Lepton-Photon 2011 symposium LP and the ones announced in December 2011 at CERN Dec11. The individual channels are appropriately rescaled according to Eqs. (\ref{['eq:xsec']})--(\ref{['eq:Ggamgam']}) and they are then simply combined in quadrature. The red continuous line delineates the region favoured at 99% CL by EW precision data (with a cutoff scale of 2.5 TeV and after marginalizing over $\epsilon_2$ and $\epsilon_b$), the region below the red dashed line survives for an additional 50% cancellation of the oblique parameters.
• Figure 5: Same as Fig. \ref{['fig:MHCM4searches']} but for MCHM5.