Top Quark Compositeness: Feasibility and Implications

TL;DR

The paper investigates the viability of a mostly composite top within a framework where SM fermions mix with a strong BSM sector, emphasizing custodial symmetry and light custodians to suppress dangerous electroweak corrections. It develops a low-energy EFT with higher-dimensional operators encoding top compositeness and computes one-loop contributions to the $\\widehat{T}$ parameter, identifying regions in parameter space where positive $\\\widehat{T}$ helps EWPT fits. It then analyzes collider signatures, notably anomalous top couplings and enhanced four-top processes, showing that LHC and future machines like the ILC can probe or constrain the degree of top compositeness and potentially reveal custodial partners. Overall, the work delineates how top compositeness can be phenomenologically viable and experimentally testable, shaping expectations for next-generation collider searches.

Abstract

In models of electroweak symmetry breaking in which the SM fermions get their masses by mixing with composite states, it is natural to expect the top quark to show properties of compositeness. We study the phenomenological viability of having a mostly composite top. The strongest constraints are shown to mainly come from one-loop contributions to the T-parameter. Nevertheless, the presence of light custodial partners weakens these bounds, allowing in certain cases for a high degree of top compositeness. We find regions in the parameter space in which the T-parameter receives moderate positive contributions, favoring the electroweak fit of this type of models. We also study the implications of having a composite top at the LHC, focusing on the process pp-> t\bar t t\bar t (b\bar b) whose cross-section is enhanced at high-energies.

Figures (11)

• Figure 1: Logarithmic divergent loop diagrams contributing to the SM gauge boson masses, and therefore to $c_{T}$, for the low-energy effective theory of a composite $q_L$, Eqs. (\ref{['effcompoL']}) and (\ref{['effeleR']}). The external lines with a cross correspond to insertions of the Higgs VEV.
• Figure 2: Contribution to $\widehat{T}$ in the $q_L$ composite limit (case (a)) in the $M_{q^*}-c_{L}\xi/\xi_{R}$ plane, where $\xi_{R}=1/4$. The grey area shows the region $-1.7\cdot 10^{-3}<\widehat{T}<+1.9\cdot 10^{-3}$ and the dashed lines show the contribution to $|\widehat{T}|$ equal to 2.8, 4.2 and 5.6 as they respectively move away from the grey area. We have marked with a "$+$" ("$-$") the areas in which the contribution to $\widehat{T}$ is positive (negative). The dotted line corresponds to the holographic composite Higgs model.
• Figure 3: Logarithmic divergent loop diagrams contributing to the SM gauge boson masses, and therefore to $c_{T}$, for the low-energy effective theory of a composite $q_L$ and its custodians (case (b)). The external lines with a cross correspond to insertions of the Higgs VEV.
• Figure 4: Contribution to $\widehat{T}$ in the $q_L$ composite limit (case (b)) in the $M_{q^*}-c_{L}\xi/\xi_{R}$ plane, where $\xi_{R}=1/4$. The grey area shows the region $-1.7\cdot 10^{-3}<\widehat{T}<+1.9\cdot 10^{-3}$ and the dashed lines show the contribution to $|\widehat{T}|$ equal to 2.8, 4.2 and 5.6 as they respectively move away from the grey area. We have marked with a "$+$" ("$-$") the areas in which the contribution to $\widehat{T}$ is positive (negative). We have taken $M_{\rho}=2.3$ TeV (left) and $M_{\rho}=3.6$ TeV (right).
• Figure 5: Logarithmic divergent loop diagrams contributing to the SM gauge boson masses, and therefore to $c_{T}$, for the low-energy effective theory of a composite $t_R$ and its custodial partners (case (b)). The external lines with a cross correspond to insertions of the Higgs VEV and the crosses denotes $M_{q^*}$ insertions.