[Paper Review] LHC signatures of vector-like quarks

Yasuhiro Okada, Luca Panizzi|arXiv (Cornell University)|Jul 24, 2012

Particle physics theoretical and experimental studies84 references13 citations

TL;DR

This paper provides a comprehensive phenomenological overview of vector-like quarks (VLQs) in the context of LHC searches, analyzing their production and decay signatures across various SU(2)L representations (singlet, doublet, triplet). It presents model-independent constraints from Tevatron and LHC data, showing that VLQs with masses up to 400–600 GeV are excluded depending on decay modes, and details all possible two-body final states and their SM decay chains, offering a reference for future experimental searches and theoretical analyses in BSM physics.

ABSTRACT

This work provides an overview on the current status of phenomenology and searches for heavy vector-like quarks, which are predicted in many models of new physics beyond the Standard Model. Searches at Tevatron and at the LHC, here listed and shortly described, have not found any evidence for new heavy fermionic states (either chiral or vector-like), and have therefore posed strong bounds on their masses: depending on specific assumptions on the interactions and on the observed final state, vector-like quarks with masses up to roughly 400-600 GeV have been excluded by all experiments. In order to be as simple and model-independent as possible, the chosen framework for the phenomenological analysis is an effective model with the addition of a vector-like quark representation (singlet, doublet or triplet under SU(2)) which couples through Yukawa interactions with all SM families. The relevance of different observables for the determination of bounds on mixing parameters is then discussed and a complete overview of possible two-body final states for every vector-like quark is provided, including their subsequent decay into SM particles. A list and short description of phenomenological analyses present in literature is also provided for reference purposes.

Motivation & Objective

• To provide a model-independent overview of vector-like quark (VLQ) phenomenology and LHC search strategies.
• To summarize existing experimental bounds from Tevatron and LHC on VLQ masses and mixing parameters.
• To systematically list all possible two-body decay channels for VLQs of different SU(2)L representations (singlet, doublet, triplet).
• To analyze the impact of VLQs on key LHC processes such as Higgs production and same-sign top quark production.
• To serve as a reference for future experimental searches by compiling and describing key phenomenological analyses in the literature.

Proposed method

• Constructs an effective field theory framework with vector-like quarks transforming as singlets, doublets, or triplets under SU(2)L, coupled to SM quarks and Higgs via Yukawa interactions.
• Applies constraints from flavor-changing neutral currents (FCNCs) and electroweak precision observables to limit mixing parameters.
• Reviews and synthesizes results from Tevatron and LHC searches (e.g., ATLAS, CMS) for resonant production of VLQs in final states with top quarks, W/Z bosons, and jets.
• Performs a systematic classification of all possible two-body decay modes for each VLQ representation, including subsequent decays into SM particles.
• Evaluates contributions of VLQs to Higgs production (gluon fusion, γγ decay) and same-sign top quark production via FCNCs.
• Uses effective Lagrangian techniques and loop calculations to estimate corrections to Higgs couplings and pair production cross sections.

Experimental results

Research questions

• RQ1 What are the current experimental bounds on vector-like quark masses from Tevatron and LHC searches, and how do they depend on decay modes and mixing parameters?
• RQ2 What are the complete set of possible two-body decay channels for vector-like quarks of different SU(2)L representations (singlet, doublet, triplet) and their subsequent SM decay chains?
• RQ3 How do vector-like quarks contribute to Higgs production and decay processes such as gluon fusion and h→γγ at the LHC?
• RQ4 Can vector-like quarks explain the observed forward-backward asymmetry in top quark pair production or bottom quark couplings?
• RQ5 What are the dominant signatures and signal regions for VLQ searches at the LHC, particularly in final states with multiple b-jets or boosted Higgs bosons?

Key findings

• Vector-like quarks with masses up to approximately 400–600 GeV have been excluded by LHC and Tevatron searches, depending on the decay channel and mixing assumptions.
• The $t^{ar{}}$ and $b^{ar{}}$ quarks (top and bottom partners) are the most constrained, with exclusion limits near 600 GeV in specific final states.
• VLQs can enhance the $tH$ decay channel via a dimension-5 operator, leading to observable signals in final states with multiple b-jets at 8 TeV with 15 fb⁻¹ luminosity.
• VLQ contributions to Higgs production via gluon fusion and $h \to \gamma\gamma$ decay are negligible at leading order and remain small even at NNLO due to decoupling.
• Same-sign top quark production via FCNCs mediated by VLQs is predicted to have a cross section of only ~0.01 fb at 14 TeV, below current LHC sensitivity.
• A scenario with a $t^\prime$ singlet decaying to $th$ and $Zt$ or $Wb$ can enhance Higgs signals with boosted $b\bar{b}$ final states, enabling a 5σ discovery at 10 fb⁻¹ luminosity for $t^\prime$ masses between 400–800 GeV.