Flavor Democracy Calls for Vector Like Leptons and Quarks

TL;DR

The paper discusses how introducing vector-like leptons and quarks can restore the flavor democracy hypothesis within the Standard Model, addressing fermion mass hierarchies and aligning with experimental constraints.

Contribution

It proposes a more general framework for vector-like fermions that overcomes limitations of current restricted models used in experimental searches.

Findings

Flavor democracy can be revived with vector-like fermions.

Current experimental bounds are limited by restrictive model assumptions.

A comprehensive re-evaluation of decay channels is necessary.

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 ($m_t >> m_b, m_\tau$) 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.