A handbook of vector-like quarks: mixing and single production (1306.0572v3)

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.

• The paper presents a detailed analysis of VLQs mixing predominantly with third-generation SM quarks and their single production mechanisms at the LHC.
• It uses precise evaluations of oblique parameters and LEP/SLC data to impose constraints on VLQ models and mixing angles.
• The study highlights that small mixing angles yield distinct decay channels, offering viable discovery signals in high-mass collider searches.

A Handbook of Vector-Like Quarks: Mixing and Single Production

This paper presents an in-depth analysis of the theoretical and phenomenological implications of vector-like quarks (VLQs). It addresses their mixing with Standard Model (SM) quarks, focusing predominantly on the third generation, and explores their production and decay mechanisms. The work examines various VLQ types and their constraints from precision electroweak data, leveraging both oblique corrections and $Z \to b \bar b$ measurements from LEP and SLC experiments.

Theoretical Context and Constraints

VLQs are hypothesized spin-1/2 particles that deviate from SM fermions by having both left- and right-handed components with identical quantum numbers. Unlike additional chiral fermions, which are ruled out by existing Higgs data and direct LHC searches, VLQs acquire mass through mechanisms independent of the Higgs. This feature allows them to evade stringent experimental mass bounds.

The paper identifies seven relevant VLQ multiplets that can mix with the SM quarks via renormalizable interactions, assuming an extended scalar sector is absent. The analysis is performed under the assumption that VLQs primarily couple to the third generation SM quarks, motivated by the top quark's large Yukawa coupling. The work also covers electroweak contributions, such as those to the S and T oblique parameters and $Z \to b \bar b$ vertex corrections, offering detailed analytical expressions for these effects.

Phenomenological Implications at the LHC

The phenomenological focus lies on the possible discovery signals at the LHC, driven by VLQ mixing with SM quarks. A key aspect is the single production mechanisms of VLQs, which become increasingly significant at high masses where pair production is suppressed by phase space limitations. The research underlines the relevance of VLQs in LHC physics and explores how mixing angles influence both production cross sections and decay branches.

For example, VLQs couple with SM-like partners, such as mixing of new $T$ quarks with top quarks, leading to new decay channels involving SM bosons. However, due to mixing angle constraints from electroweak precision data, these mixing effects yield relatively small deviations in couplings and cross sections at currently accessible energy scales.

Constraining Roles and Potential Observables

The research delineates specific VLQ decay channels, considering kinematically allowed final states involving $W$, $Z$, and Higgs bosons. An important conclusion is that for some multiplets, notable contributions possibly affecting Higgs production and decay are identified, albeit limited by heavy VLQ masses.

For experimental purposes, the allowed branching fractions are crucial for identifying VLQ signals and distinguishing them from SM backgrounds. These predictions are particularly useful for LHC searches, offering guidance on viable mass and mixing parameter regions that align with theoretical models and experimental bounds.

Model-Independent and Specific Scenarios

The analysis remains predominantly model-independent, considering each VLQ multiplet in isolation under simplifying assumptions. However, the paper also acknowledges more intricate scenarios where multiple VLQs or additional new physics could lead to cancellations affecting observable quantities, thereby necessitating dedicated case-specific studies.

Conclusion and Outlook

The paper contributes critically to the understanding of VLQ physics, providing a structured examination of their theoretical placement within the SM extension framework, and their phenomenological implications at collider experiments. The potential ability of VLQs to explain observed discrepancies at $Z$ pole measurements offers an exciting avenue for upcoming LHC runs and future collider studies.

Future studies might focus on refining VLQ constraints using updated electroweak data and further explore their role in advanced Higgs physics scenarios, factoring in synergies with ongoing experimental efforts at the LHC and prospective ILC investigations.