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Composite fermions in Electroweak Symmetry Breaking

Riccardo Barbieri, Gino Isidori, Duccio Pappadopulo

TL;DR

The paper develops a general, symmetry-guided framework for electroweak symmetry breaking via strong dynamics that yields composite fermions. It systematically constructs effective Lagrangians for Singlets, Doublets, and Triplets, analyzes their one-derivative interactions and mass-mixing with SM fermions, and evaluates implications for EWPT, flavor, and collider phenomenology. A key result is that Singlets can preserve the CKM structure under a Minimal Flavor Violation pattern, while Doublets and Triplets face significant EWPT and flavor challenges; the collider phenomenology predicts narrow heavy quark partners with sizable decays to $Z$-bosons, offering distinctive LHC signatures. The work highlights the delicate balance between precision constraints and new-physics signals in strongly coupled EWSB scenarios and provides concrete, testable predictions for heavy fermion spectra and their decays.

Abstract

If the electroweak symmetry is broken by some unspecified strong dynamics, composite fermions may exist with definite transformation properties under SU(2)_L x SU(2)_R/SU(2)_{L+R} and may play a role in giving masses by mixing to all the standard quarks and leptons. Assuming this to be the case, we analyze the role of Singlets, Doublets and Triplets in the ElectroWeak Precision Tests and in Flavour Physics. Doublets and Triplets are generically disfavoured. In the Singlet case, we specify the breaking patterns of the flavour group that allow to keep the CKM picture of flavour physics and we discuss the effects of the mixing between composite and elementary fermions. These mixings affect in particular the rather peculiar LHC phenomenology of the composite fermions.

Composite fermions in Electroweak Symmetry Breaking

TL;DR

The paper develops a general, symmetry-guided framework for electroweak symmetry breaking via strong dynamics that yields composite fermions. It systematically constructs effective Lagrangians for Singlets, Doublets, and Triplets, analyzes their one-derivative interactions and mass-mixing with SM fermions, and evaluates implications for EWPT, flavor, and collider phenomenology. A key result is that Singlets can preserve the CKM structure under a Minimal Flavor Violation pattern, while Doublets and Triplets face significant EWPT and flavor challenges; the collider phenomenology predicts narrow heavy quark partners with sizable decays to -bosons, offering distinctive LHC signatures. The work highlights the delicate balance between precision constraints and new-physics signals in strongly coupled EWSB scenarios and provides concrete, testable predictions for heavy fermion spectra and their decays.

Abstract

If the electroweak symmetry is broken by some unspecified strong dynamics, composite fermions may exist with definite transformation properties under SU(2)_L x SU(2)_R/SU(2)_{L+R} and may play a role in giving masses by mixing to all the standard quarks and leptons. Assuming this to be the case, we analyze the role of Singlets, Doublets and Triplets in the ElectroWeak Precision Tests and in Flavour Physics. Doublets and Triplets are generically disfavoured. In the Singlet case, we specify the breaking patterns of the flavour group that allow to keep the CKM picture of flavour physics and we discuss the effects of the mixing between composite and elementary fermions. These mixings affect in particular the rather peculiar LHC phenomenology of the composite fermions.

Paper Structure

This paper contains 20 sections, 70 equations, 2 figures, 1 table.

Table of Contents

  1. Introduction and statement of the framework
  2. Effective Lagrangians
  3. Singlets
  4. Doublets
  5. Triplets
  6. EWPT for Doublets and Triplets
  7. $\Delta S$
  8. $\Delta T$ and $Z\rightarrow \bar{b} b$
  9. Flavour symmetries
  10. Minimal Flavour Violation
  11. Singlets
  12. Doublets and Triplets
  13. Flavour breaking by higher dimensional operators
  14. Composite/elementary mixing effects for Singlets
  15. Tree level
  16. ...and 5 more sections

Figures (2)

  • Figure 1: Constraints in the $t_R$--$M_T$ plane at 95% C.L. from $\Delta F=2$ (red curve) and $Z \bar{b} b$ (blue curve).
  • Figure 2: Production cross sections at LHC: for a pair of composite quarks of mass $M$ (full curve); for a singlet $T\bar{t}$ (dashed curve); for a singlet $T\bar{b}$ (dotted curve).