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Vector-like quarks in a composite Higgs model

Paolo Lodone

TL;DR

<3-5 sentence high-level summary> The paper investigates naturalness in a composite Higgs framework with an $SO(5)/SO(4)$ symmetry breaking pattern, focusing on the role of vector-like quarks in taming the top-loop contribution and in satisfying Electroweak Precision Tests (EWPT). It shows that a non-minimal extension of the top sector, incorporating a full $SO(5)$ quark multiplet, can yield viable spectra with light vector-like quarks in a narrow region of parameter space, while the minimal models fail EWPT constraints. Alternative constructions, including renormalizable Yukawa couplings and an $SU(4)/Sp(4)$ variant, do not reconcile with EWPT. The key result is that light vector-like quarks (down to a few hundred GeV) are possible but inhabit a highly constrained region, with small statistical probability for random parameter choices, suggesting targeted searches at the LHC.

Abstract

Vector-like quarks are a common feature of "composite" Higgs models, where they intervene in cutting off the top-loop contribution to the Higgs boson mass and may, at the same time, affect the Electroweak Precision Tests (EWPT). A model based on SO(5)/SO(4) is here analyzed. In a specific non minimal version, vector-like quarks of mass as low as 300-500 GeV are allowed in a thin region of its parameter space. Other models fail to be consistent with the EWPT.

Vector-like quarks in a composite Higgs model

TL;DR

<3-5 sentence high-level summary> The paper investigates naturalness in a composite Higgs framework with an symmetry breaking pattern, focusing on the role of vector-like quarks in taming the top-loop contribution and in satisfying Electroweak Precision Tests (EWPT). It shows that a non-minimal extension of the top sector, incorporating a full quark multiplet, can yield viable spectra with light vector-like quarks in a narrow region of parameter space, while the minimal models fail EWPT constraints. Alternative constructions, including renormalizable Yukawa couplings and an variant, do not reconcile with EWPT. The key result is that light vector-like quarks (down to a few hundred GeV) are possible but inhabit a highly constrained region, with small statistical probability for random parameter choices, suggesting targeted searches at the LHC.

Abstract

Vector-like quarks are a common feature of "composite" Higgs models, where they intervene in cutting off the top-loop contribution to the Higgs boson mass and may, at the same time, affect the Electroweak Precision Tests (EWPT). A model based on SO(5)/SO(4) is here analyzed. In a specific non minimal version, vector-like quarks of mass as low as 300-500 GeV are allowed in a thin region of its parameter space. Other models fail to be consistent with the EWPT.

Paper Structure

This paper contains 10 sections, 34 equations, 5 figures, 1 table.

Table of Contents

  1. Introduction
  2. Summary of previous works
  3. An extended model in the top sector
  4. Non-minimal model
  5. Minimal values for the masses of the new quarks
  6. Allowed volume in parameter space
  7. Alternative models
  8. A different coupling
  9. A different model: $SU(4)/Sp(4)$
  10. Conclusions

Figures (5)

  • Figure 1: The experimentally allowed region in the $ST$ plane, including contributions "from scalars" and "from cutoff" (see barbieri1, section 2). The dashed arrow shows that an extra positive contribution to $T$ is needed in order to make the model consistent with data. In section \ref{['extendedmodel']} it will be shown that such contribution may come from a suitably extended top sector. This figure is taken from barbieri1.
  • Figure 2: Numerical isoplot of the one loop corrections to $\delta T$ and $A^{bb}/A^{bb}_{SM}$ versus the Lagrangian parameters $(M_X,M_T)$ in the minimal model for $\lambda_T/\lambda_t= 1$. Note that there is no experimentally allowed region.
  • Figure 3: Isoplot of $\delta T$ and $A^{bb}/A^{bb}_{SM}$ in the $(M_X,M_T)$ plane for ($A=1.8$, $B=1.1$, $M_Q=900$ GeV) in the non-minimal model.
  • Figure 4: % Probability of the allowed region (see text).
  • Figure 5: Isolines of $\delta T$ and $A^{bb}/A^{bb}_{SM}$ in the $(M_X,M_T)$ plane for the the $SU(4)/Sp(4)$ model. Plot for $A=B=1, \, M_Q=800$ GeV.