Quarks and Leptons Beyond the Third Generation

TL;DR

This paper explores the theoretical and experimental implications of potential additional quarks and leptons beyond the known three generations, discussing their properties, constraints, and significance for particle physics models.

Contribution

It provides a comprehensive overview of the motivations, constraints, and experimental searches for hypothetical fourth-generation fermions, highlighting their impact on theories beyond the Standard Model.

Findings

Mass limits from electroweak data constrain additional fermions.

Possible long-lived states for new quarks and leptons are considered.

Implications for CP violation and dynamical symmetry breaking are discussed.

Abstract

The possibility of additional quarks and leptons beyond the three generations already established is discussed. The make-up of this Report is (I) Introduction: the motivations for believing that the present litany of elementary fermions is not complete; (II) Quantum Numbers: possible assignments for additional fermions; (III) Masses and Mixing Angles: mass limits from precision electroweak data, vacuum stability and perturbative gauge unification; empirical constraints on mixing angles; (IV) Lifetimes and Decay Modes: their dependence on the mass spectrum and mixing angles of the additional quarks and leptons; the possibility of exceptionally long lifetimes; (V) Dynamical Symmetry Breaking: the significance of the top quark and other heavy fermions for alternatives to the elementary Higgs Boson; (VI) CP Violation: extensions to more generations and how strong CP may be solved by additional quarks; (VII) Experimental Searches: present status and future prospects; (VIII) Conclusions.