Quarks and Leptons Beyond the Third Generation

TL;DR

This review assesses the case for quarks and leptons beyond the third generation, detailing how extra chiral or vector-like fermions could fit within, or extend, the Standard Model framework. It analyzes quantum-number assignments, mass and mixing constraints from precision electroweak data, vacuum stability, and gauge unification, and explores lifetimes, decay modes, and possible dynamical symmetry-breaking roles. The work also surveys CP-violation implications, including strong-CP solutions involving extra fermions, and surveys current and future experimental searches for long-lived quarks and heavy leptons. Overall, it highlights regions of parameter space where a fourth generation could restore gauge coupling unification, influence Higgs dynamics, or yield distinctive collider signatures, while stressing the need for dedicated searches at current and future experiments.

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.

Figures (26)

• Figure 1: Helium-4 production for $N_\nu=3.0,3.2,3.4$. The vertical band indicates the baryon density consistent with $(D/H)_P = (2.7\pm 0.6) \times 10^{-5}$ and the horizontal line indicates a primeval Helium-4 abundance of $25\%$. The widths of the curves indicate the two-sigma theoretical uncertainty. Figure from Ref. schrammturner.
• Figure 2: Perturbativity and stability bounds on the SM Higgs boson. $\Lambda$ denotes the energy scale where the particles become strongly interacting.
• Figure 3: Perturbativity and stability bounds on the SM Higgs boson as a function of $\Lambda$ for $M_{top}=175$ GeV.
• Figure 4: (a) The trace $T_Q$ at $M_W$ as a function of the trace $T_Q$ and $M_X$ for $N_F=8$ and $T_L=0$. The dotted line denotes the radial quark fixed point. For $T_Q(M_X) > 0.1$, the fixed point is reached in physical time. (b)$T_L(M_W)$ as a function of $T_L(M_X)$ for $N_F=8$ and $T_Q=0$.
• Figure 5: Flow of $\lambda$ and $g_t$ towards fixed points in the standard one-Higgs doublet model. Open circles denote initial points. Crosses denote final fixed points.