A handbook of vector-like quarks: mixing and single production

TL;DR

This work develops a model-independent framework for vector-like quarks that mix with the SM third generation, classifying seven multiplets and deriving electroweak precision constraints by treating one multiplet at a time. It demonstrates that Higgs-sector effects are generally small due to cancellations, but identifies a notable exception: a B–Y doublet can improve LEP/Z-pole fits for Z → bb and predict a detectable Y quark at the LHC, along with distinctive Higgs decay shifts that could be probed at the ILC. The paper also maps out heavy-quark decay patterns, establishes the most promising single-production channels at the LHC, and translates mixing limits into prospective deviations in top couplings, providing a practical guide for collider searches in minimal VLQ scenarios. Overall, it offers a cohesive, phenomenologically rich roadmap for testing VLQs at current and future colliders, while acknowledging that cancellations in more complex models could obscure indirect signals.

Abstract

We obtain constraints on the mixing of vector-like quarks coupling predominantly to the third generation. We consider all (seven) relevant types of vector-like quarks, individually. The constraints are derived from oblique corrections and Z -> b bbar measurements at LEP and SLC. We investigate the implications of these constraints on LHC phenomenology, concerning the decays of the heavy quarks and their single production. We also explore indirect effects of heavy quark mixing in top and bottom couplings. A remarkable effect is the possibility of explaining the anomalous forward-backward asymmetry in Z -> b bbar at LEP, with a hypercharge -5/6 doublet. We also study the impact of the new quarks on single Higgs production at the LHC and Higgs decay.

Figures (8)

• Figure 1: Upper limits on the mixing angles for the different multiplets. The current mass limits from direct searches are also indicated by vertical lines.
• Figure 2: Allowed range for the splitting of the heavy quark masses.
• Figure 3: Allowed branching ratios for the decays of $T$ (left) and $B$ (right) quarks in the different multiplets.
• Figure 4: Maximum single heavy quark production cross sections at the LHC with 8 TeV (left) and 13 TeV (right), for selected quark multiplets. The dotted part of the lines indicate the range of masses already excluded by direct searches. In the left plot, the shaded area corresponds to cross sections below 1 fb, uninteresting for the luminosity $L \simeq 20$ fb$^{-1}$ collected.
• Figure 5: Allowed deviations in top couplings. The dotted part of the lines indicates the range of masses already excluded by direct searches.