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A Composite Theory of Higgs and Flavour

Joe Davighi, Gino Isidori

TL;DR

This work presents a composite-Higgs model that embeds a flavour-deconstructed gauge symmetry inside the strong sector, generating a pNGB Higgs $H_3$, a heavy Higgs $H_{12}$, and a link field $\Sigma$ that breaks the flavour structure to the SM. Yukawa hierarchies arise from a combination of partial compositeness for the third generation and direct bilinear couplings for light generations, with a cubic interaction $\text{Tr}(H_3\Sigma H_{12}^\dagger)$ enabling the transfer of couplings to light fermions; a minimal $U(2)$ flavour protection ensures CKM- and chirality-suppressed FCNCs and automatically aligned EDMs. The model predicts TeV-scale dynamics including a heavy $Z'$ and a top partner, with current constraints allowing $M_R\gtrsim3$ TeV and future high-$p_T$ and EW precision measurements offering strong probes, while a WZW term is absent, simplifying topological considerations. Overall, it provides a natural framework addressing both Higgs and flavour hierarchies with reduced tuning, best tested through collider and precision probes and amenable to UV completions with richer gauge structures.

Abstract

We introduce a composite Higgs model in which a flavour deconstructed gauge group is embedded in the strong sector. The pattern of global symmetry breaking yields, as pseudo-Nambu-Goldstone (pNGB) bosons, both a Standard Model (SM)-like Higgs and the link field whose vacuum expectation value breaks the flavour non-universal gauge group down to the SM. Inevitably, a third pNGB appears, transforming as a second Higgs doublet coupled only to the light generations. This heavy Higgs naturally mediates suppressed light Yukawa couplings, providing a solution to the flavour puzzle. At the same time, new physics contributions to flavour violating observables are CKM- and chirally suppressed, while electric dipole moment bounds are evaded through an automatic mass alignment in the light fermion sector. The result is a natural framework for addressing the origin of both Higgs and flavour hierarchies, with reduced Higgs mass tuning and minimised impact in flavour observables, that is best tested by high-$p_T$ and precision electroweak measurements.

A Composite Theory of Higgs and Flavour

TL;DR

This work presents a composite-Higgs model that embeds a flavour-deconstructed gauge symmetry inside the strong sector, generating a pNGB Higgs , a heavy Higgs , and a link field that breaks the flavour structure to the SM. Yukawa hierarchies arise from a combination of partial compositeness for the third generation and direct bilinear couplings for light generations, with a cubic interaction enabling the transfer of couplings to light fermions; a minimal flavour protection ensures CKM- and chirality-suppressed FCNCs and automatically aligned EDMs. The model predicts TeV-scale dynamics including a heavy and a top partner, with current constraints allowing TeV and future high- and EW precision measurements offering strong probes, while a WZW term is absent, simplifying topological considerations. Overall, it provides a natural framework addressing both Higgs and flavour hierarchies with reduced tuning, best tested through collider and precision probes and amenable to UV completions with richer gauge structures.

Abstract

We introduce a composite Higgs model in which a flavour deconstructed gauge group is embedded in the strong sector. The pattern of global symmetry breaking yields, as pseudo-Nambu-Goldstone (pNGB) bosons, both a Standard Model (SM)-like Higgs and the link field whose vacuum expectation value breaks the flavour non-universal gauge group down to the SM. Inevitably, a third pNGB appears, transforming as a second Higgs doublet coupled only to the light generations. This heavy Higgs naturally mediates suppressed light Yukawa couplings, providing a solution to the flavour puzzle. At the same time, new physics contributions to flavour violating observables are CKM- and chirally suppressed, while electric dipole moment bounds are evaded through an automatic mass alignment in the light fermion sector. The result is a natural framework for addressing the origin of both Higgs and flavour hierarchies, with reduced Higgs mass tuning and minimised impact in flavour observables, that is best tested by high- and precision electroweak measurements.
Paper Structure (23 sections, 42 equations, 4 figures)

This paper contains 23 sections, 42 equations, 4 figures.

Table of Contents

  1. Introduction
  2. The Model
  3. Left-universal gauge structure
  4. Symmetry breaking pattern
  5. The one-loop pNGB potential
  6. Yukawa hierarchies and $U(2)$ protection
  7. Third generation Yukawas.
  8. Light Yukawas.
  9. $U(2)$ protection.
  10. Phenomenology
  11. Summary and Outlook
  12. Estimating the pNGB potential
  13. Fermion-pNGB couplings.
  14. Gauge field-pNGB couplings.
  15. Form factor ansatze.
  16. ...and 8 more sections

Figures (4)

  • Figure 1: Effective Yukawa interaction for the light fermions.
  • Figure 2: $H_{12}$-mediated contribution to $\Delta S=2$ amplitudes.
  • Figure 3: Key constraints on the heavy neutral gauge boson predicted by the flavour deconstructed sector of the model. The dashed lines correspond to projections, assuming SM-like measurements, from HL-LHC (for $pp\to \ell\ell$) and FCC-ee (for EWPOs).
  • Figure 4: One-loop diagram generating the scalar trilinear coupling.