Light Custodians and Higgs Physics in Composite Models

TL;DR

The paper investigates how light custodial fermions in composite Higgs models modify Higgs couplings, especially the loop-induced $Hgg$ coupling relevant for gluon fusion production. Using a minimal $SO(5)/SO(4)$ PNGB Higgs and a two-site partial compositeness setup with fermions in the ${\bf 5}$ or ${\bf 10}$, the authors show that the $Hgg$ coupling can be either enhanced or suppressed depending on the embedding and spurion structure, with the modification largely controlled by $\partial_v \log(\det M)$ rather than heavy masses. They develop toy models and a 5D-inspired $(10,10,5)$ framework to illustrate how light custodians affect $Hgg$, $Ht\bar t$, and $Hb\bar b$ in an essentially independent way, and they analyze $H\gamma\gamma$ and experimental constraints from LEP, Tevatron, and LHC. A key finding is that bottom-sector custodians can significantly alter $Hgg$ when $b_L$ is highly composite, potentially yielding a gluophilic and $b$-phobic Higgs and thus distinct collider phenomenology. Overall, the work provides a systematic method to quantify Higgs coupling deviations in PNGB composite Higgs models and identifies observable signatures and viable parameter regions for future LHC tests.

Abstract

Composite Higgs models involving partial compositeness of Standard Model fermions typically require the introduction of fermionic partners which are relatively light in realistic scenarios. In this paper, we analyze the role of these light custodian fermions in the phenomenology of the composite Higgs models and show that they significantly modify couplings of the Higgs field. We focus on the coupling to gluons in particular, which is of central importance for Higgs production at the LHC. We show that this coupling can be increased as well as decreased depending on the SM fermion embedding in the composite multiplets. We also discuss modification of the Higgs couplings to bottom and top quarks and show that modifications to all these three couplings Hgg, Htt, and Hbb are generically independent parameters.

Figures (8)

• Figure 1: Higgs coupling to gluons induced by a loop of massive fermions.
• Figure 2: Wavefunction renormalization from mixing with heavy states.
• Figure 3: Modification of the top Yukawa coupling as a function of $\lambda_q$. The solid red line indicates the (constant) trigonometric rescaling for comparison, and the vertical dashed lines indicate the mass of the lightest $t'$ in GeV. Fully composite $t_L$ corresponds to $\lambda_q\rightarrow 3.2$ TeV, $\sin\theta_q\rightarrow 1$.
• Figure 4: Couplings in the model with a single composite 10. The solid red line indicates the simple trigonometric rescaling for comparison. The dotted black line shows the bottom Yukawa relative to its SM value, and the dashed blue line shows the contribution of the bottom quark to the gluon coupling relative to the SM value in the large top mass limit. Vertical dashed lines indicate the mass of the lightest composite $b'$ in GeV.
• Figure 5: Couplings for the ${\bf 10,10,5}$ model, with curves as in Fig. \ref{['single10']}.