[Paper Review] Interplay of vector-like top partner multiplets in a realistic mixing set-up

Giacomo Cacciapaglia, Aldo Deandrea+4 more|arXiv (Cornell University)|Feb 2, 2015

Particle physics theoretical and experimental studies4 citations

TL;DR

This paper investigates the phenomenological implications of two vector-like top-partner multiplets mixing with the top quark sector in a realistic model without down-quark mixing. Using electroweak precision data and LHC constraints, it shows that multi-multiplet mixing significantly alters bounds on masses and mixing angles compared to single-multiplet scenarios, allowing larger mixing and enhanced LHC discovery potential while remaining consistent with precision electroweak and flavor constraints.

ABSTRACT

The ATLAS and CMS collaborations at the LHC have performed analyses on the existing data sets, studying the case of one vector-like fermion or multiplet coupling to the standard model Yukawa sector. In the near future, with more data available, these experimental collaborations will start to investigate more realistic cases. The presence of more than one extra vector-like multiplet is indeed a common situation in many extensions of the standard model. The interplay of these vector-like multiplet between precision electroweak bounds, flavour and collider phenomenology is a important question in view of establishing bounds or for the discovery of physics beyond the standard model. In this work we study the phenomenological consequences of the presence of two vector-like multiplets. We analyse the constraints on such scenarios from tree-level data and oblique corrections for the case of mixing to each of the SM generations. In the present work, we limit to scenarios with two top-like partners and no mixing in the down-sector.

Motivation & Objective

• To study the interplay of two vector-like top-partner multiplets in a realistic mixing setup with no down-quark mixing, minimizing flavor constraints.
• To assess how multi-multiplet mixing affects electroweak precision observables (S, T parameters) and tree-level constraints.
• To evaluate the impact of such mixing on LHC discovery reach for single vector-like top quark production at 14 TeV.
• To identify viable parameter space regions consistent with current experimental bounds from LHC and precision electroweak data.

Proposed method

• Classifies all possible pairs of vector-like multiplets based on their SU(2)L × U(1)Y quantum numbers that can couple to SM quarks via the Higgs doublet.
• Constructs mass matrices for two-top-partner scenarios with no bottom-partner mixing, focusing on top-type (Y=+2/3) and mixed charge states.
• Evaluates oblique corrections (S and T parameters) using loop diagrams involving vector-like fermions, including both left- and right-handed mixing contributions.
• Computes the T-parameter contributions from various loop diagrams involving top-type, bottom-type, and exotic charge states (e.g., X^{8/3}, Y^{-7/3}), with explicit expressions for self-energies Π_T^{LL}, Π_T^{LR}.
• Applies tree-level constraints from fermion mass and mixing angles, and compares with LHC bounds on single vector-like top quark production.
• Performs numerical analysis across multiple benchmark scenarios, including singlet-doublet, doublet-triplet, and doublet-doublet multiplet combinations.

Experimental results

Research questions

• RQ1 How does the presence of two vector-like top-partner multiplets affect the oblique S and T parameters compared to a single multiplet?
• RQ2 What are the combined constraints from electroweak precision data and LHC searches on the masses and mixing angles of two vector-like top partners?
• RQ3 How does the absence of mixing in the down sector affect the allowed parameter space and enhance the LHC discovery potential?
• RQ4 To what extent do multi-multiplet mixing effects modify the bounds derived from simplified models assuming only one vector-like fermion?
• RQ5 What are the viable regions in the parameter space of two vector-like top partners that satisfy both electroweak precision and LHC constraints?

Key findings

• The presence of two vector-like top-partner multiplets leads to significant modifications in the S and T parameters due to interference effects between mixing channels.
• The T-parameter contributions from loops involving exotic charge states (e.g., X^{8/3}, Y^{-7/3}) vanish at p²=0 due to symmetry cancellation, reducing their impact on precision observables.
• Scenarios with two top-partner multiplets allow for larger mixing angles than single-multiplet models, increasing the single production cross-section at the LHC.
• Tree-level constraints and electroweak precision data (S, T) strongly restrict the parameter space, especially when mixing angles are large or masses are degenerate.
• The 3σ exclusion region from tree-level bounds (e.g., on mixing angles) is significantly smaller than the EWP bounds, indicating that EWP data provides stronger constraints in many cases.
• Numerical results show that for doublet-doublet and doublet-triplet configurations, the EWP bounds are more restrictive than LHC bounds at 14 TeV, especially when mixing angles are non-zero.