Beyond the Minimal Composite Higgs Model (0902.1483v1)

Abstract: The Higgs spectrum of the minimal composite Higgs model, based on the SO(5)/SO(4) coset, consists of a unique Higgs doublet whose phenomenology does not differ greatly from the Standard Model (SM). Nevertheless, extensions beyond this minimal coset structure exhibit a richer Higgs spectrum and therefore very different Higgs physics. We explore one of these extensions, the SO(6)/SO(5) model, whose Higgs spectrum contains a CP-odd singlet scalar, eta, in addition to the Higgs doublet. Due to the pseudo-Nambu-Goldstone nature of these Higgs bosons, their physical properties can be derived from symmetry considerations alone. We find that the mass of eta can be naturally light, opening up the possibility that the SM Higgs decays predominantly to the singlet, and therefore lowering the LEP bound on its mass to 86 GeV. We also show that eta can have interesting consequences in flavour-violating processes, as well as induce spontaneous CP-violation in the Higgs sector. The model can also have anomalies, giving rise to interactions between the SM gauge bosons and eta which, if measured at the LHC, would give quantitative information about the structure of the high energy theory.

• The paper introduces an extension of the Minimal Composite Higgs Model by incorporating a CP-odd singlet within the SO(6)/SO(5) framework.
• It shows that the light scalar can alter Higgs decay channels, potentially lowering LEP limits and providing distinctive signals for LHC searches.
• The study also analyzes anomaly interactions and flavor-changing neutral currents, pointing to new avenues in CP violation research.

Analysis of the Paper "Beyond the Minimal Composite Higgs Model"

The paper "Beyond the Minimal Composite Higgs Model," authored by Ben Gripaios, Alex Pomarol, Francesco Riva, and Javi Serra, explores an intriguing extension to the Minimal Composite Higgs Model (MCHM). The authors focus on a specific coset structure, $SO(6)/SO(5)$, which incorporates an additional $CP$-odd singlet scalar, $\eta$, alongside the Higgs doublet. This framework diverges from the conventional $SO(5)/SO(4)$ model by presenting a richer Higgs spectrum and distinctive phenomenological implications.

Key Concepts and Model Description

The $SO(6)/SO(5)$ model investigates the potential role of pseudo-Nambu-Goldstone bosons (PNGBs) arising from broken symmetries in the strong sector. Here, the scalar singlet $\eta$ may attain a relatively light mass, allowing for scenarios where the Standard Model (SM) Higgs predominantly decays into $\eta$, consequently modifying LEP limits on the Higgs mass to as low as 86 GeV. The authors also examine implications for flavor-changing neutral currents (FCNC) and prospects for spontaneous $CP$ violation within the Higgs sector.

One noteworthy theoretical aspect addressed is the anomaly considerations within this extension. The model posits potential anomalous interactions between $\eta$ and SM gauge bosons, observable at the LHC. Such interactions are dictated by the symmetry structure of the high-energy theory and provide insight into the ultraviolet (UV) completions.

Theoretical and Experimental Implications

The model's implications are both theoretical and experimental:

1. Theoretical Insights:
   • The exploration of non-minimal symmetry structures suggests the possibility of diverse low-energy phenomenology in composite Higgs frameworks. The inclusion of the singlet $\eta$ showcases how alterations in symmetry breaking can lead to modifications in Higgs physics, offering new theoretical playgrounds for emergent phenomena.
2. Experimental Implications:
   • The decay pathways of $\eta$, influenced by its couplings to SM fermions and potential anomaly-induced interactions, open new channels for experimental searches. Notably, the $\eta\rightarrow\gamma\gamma$ decay mode may serve as a key signature, provided significant enough branching ratios.
   • The anomaly interactions provide a unique opportunity to derive quantitative insights into the structure of high-energy physics by measuring decay processes involving $\eta$.

Future Directions

The paper posits essential groundwork for future theoretical explorations and guidance for experimental tests at the LHC and beyond. Future developments could involve:

• Detailed Phenomenological Studies: Rigorous analyses of the parameter space for $\epsilon_i$, the variables controlling $\eta$'s mass and interactions, could bring clearer predictions and guide experimental protocols to search for $\eta$.
• Investigations into Electroweak Baryogenesis and Dark Matter:
  • The model's $CP$-violating elements and the potential for a stable $\eta$ particle suggest applications in electroweak baryogenesis and dark matter candidates, respectively.
• Extended Collider Searches: Identifying collider signatures specific to $SO(6)/SO(5)$ models and validating the presence of any anomaly terms through $\eta$ production and decay will be crucial.

In conclusion, "Beyond the Minimal Composite Higgs Model" articulates a sophisticated picture of Higgs sector physics within composite models, offering extensive theoretical narrative and compelling experimental avenues for potential confirmation. It invites the high-energy physics community to probe deeper into the complex landscape of symmetry-breaking patterns and their observable consequences.