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Flavor Democracy Calls for Vector Like Leptons and Quarks

Burak Dagli, Saleh Sultansoy, Ismail Toy

Abstract

There are strong arguments favoring the Flavor Democracy hypothesis (or the Democratic Mass Matrix approach) within the Standard Model framework. However, the large mass of the top quark ($m_t >> m_b, m_τ$) poses an obstacle to the functioning of Flavor Democracy in the three SM family scenario. While a fourth Standard Model generation could have provided a natural resolution, this possibility is now almost entirely excluded by precision data on Higgs boson production and decay rates. The Flavor Democracy hypothesis can be elegantly resurrected through the introduction of Vector-Like Leptons (VLLs) and Vector-Like Quarks (VLQs), which naturally accommodate the observed fermion mass hierarchies while remaining consistent with current experimental constraints. Currently, experimental searches for VLLs conducted by the ATLAS and CMS collaborations rely on a highly constrained Restricted Model. This model imposes a mass degeneracy between charged and neutral VLLs within a doublet and assumes the absence of right-handed neutrinos. Consequently, current results are valid only for the restricted model and do not cover a more realistic, general scenario. Therefore, to accurately reflect the physical reality, it is imperative to conduct a comprehensive re-evaluation that incorporates all viable decay channels.

Flavor Democracy Calls for Vector Like Leptons and Quarks

Abstract

There are strong arguments favoring the Flavor Democracy hypothesis (or the Democratic Mass Matrix approach) within the Standard Model framework. However, the large mass of the top quark () poses an obstacle to the functioning of Flavor Democracy in the three SM family scenario. While a fourth Standard Model generation could have provided a natural resolution, this possibility is now almost entirely excluded by precision data on Higgs boson production and decay rates. The Flavor Democracy hypothesis can be elegantly resurrected through the introduction of Vector-Like Leptons (VLLs) and Vector-Like Quarks (VLQs), which naturally accommodate the observed fermion mass hierarchies while remaining consistent with current experimental constraints. Currently, experimental searches for VLLs conducted by the ATLAS and CMS collaborations rely on a highly constrained Restricted Model. This model imposes a mass degeneracy between charged and neutral VLLs within a doublet and assumes the absence of right-handed neutrinos. Consequently, current results are valid only for the restricted model and do not cover a more realistic, general scenario. Therefore, to accurately reflect the physical reality, it is imperative to conduct a comprehensive re-evaluation that incorporates all viable decay channels.
Paper Structure (16 sections, 31 equations, 7 figures, 1 table)

This paper contains 16 sections, 31 equations, 7 figures, 1 table.

Table of Contents

  1. Introduction
  2. Flavor Democracy and Standard Model
  3. Higgs Mechanism and Mass Eigenvalues
  4. Flavor Democracy Hypothesis
  5. Three SM families
  6. The Fourth SM family
  7. Solution to The Problem: Vector Like Leptons and Quarks
  8. Only One Vector-like Family
  9. Three Vector-like Families
  10. E6 GUT Proposes VLLs and VLQs
  11. Anomalies and VLLs/VLQs
  12. Vector-like Leptons at the LHC
  13. General Remarks on VLL Production and Decays
  14. If $N_e$ is the Lightest VLL
  15. The case uncovered by current analysis
  16. ...and 1 more sections

Figures (7)

  • Figure 1: Cross-sections for pair production of VLLs at the LHC with $\sqrt{s} = 14$ TeV in the case of $m_N=m_E$.
  • Figure 2: Cross-sections for the associated production of VLLs for different ratios of charged and neutral VLL masses.
  • Figure 3: Feynman diagrams for $m_E > m_N + m_W$ case
  • Figure 4: Branching ratios for decays of iso-doublet neutral vector-like lepton.
  • Figure 5: Branching ratios for decays of iso-doublet neutral vector-like lepton (neutral currents only)
  • ...and 2 more figures