[Paper Review] Vector like leptons with extended Higgs sector

Sumit K. Garg, C. S. Kim|arXiv (Cornell University)|May 21, 2013

Particle physics theoretical and experimental studies35 references3 citations

TL;DR

This paper investigates a two-Higgs-doublet model (2HDM) extended with a vector-like lepton generation, showing that the additional scalar doublet alleviates electroweak precision constraints (S, T, U parameters), enabling larger Yukawa couplings and mass splittings among vector-like fermions. It further demonstrates that this scenario enhances the Higgs boson decay rate to diphotons, offering a viable explanation for the observed excess in the LHC's diphoton channel.

ABSTRACT

We examined the influence of additional scalar doublet on the parameter space of the Standard Model supplemented with a generation of new vector like leptons. In particular we identified the viable regions of parameter space by inspecting various constraints especially electroweak precision (S, T and U) parameters. We demonstrated that the additional scalar assists in alleviating the tension of electroweak precision constraints and thus permitting larger Yukawa mixing and mass splittings among vector like species. We also compared and contrasted the regions of parameter space pertaining to the latest LHC Higgs to diphoton channel results in this scenario with vector like leptons in single Higgs doublet and pure two Higgs doublet model case.

Motivation & Objective

• To explore the viability of a vector-like lepton generation within an extended two-Higgs-doublet model (2HDM), particularly focusing on electroweak precision constraints.
• To assess how the addition of an inert scalar doublet modifies the parameter space of the SM with vector-like leptons.
• To evaluate the impact of vector-like leptons and the extended scalar sector on the Higgs boson decay rate into two photons (h → γγ).
• To compare the diphoton signal strength predictions in this 2HDM+vector-like lepton model with those in the single Higgs doublet and pure 2HDM scenarios.
• To identify viable parameter regions satisfying electroweak precision, vacuum stability, perturbativity, and unitarity constraints.

Proposed method

• The model includes two Higgs doublets: one that couples to fermions (including vector-like leptons) and one inert doublet under a Z₂ symmetry, ensuring no tree-level flavor-changing neutral currents.
• The electroweak precision parameters S, T, and U are calculated using one-loop contributions from both scalar and vector-like fermion sectors, with analytical expressions derived from the two-point functions of gauge bosons.
• The Higgs to diphoton decay width is computed using one-loop contributions from charged scalar and vector-like fermion loops, employing standard loop functions V₁(x), F₁/₂(x), and C₀(x) with appropriate kinematic invariants.
• Numerical scans are performed over the parameter space, imposing constraints from electroweak precision observables, vacuum stability, perturbativity, and unitarity.
• The diphoton signal strength σ/σ_SM is evaluated and compared across three scenarios: SM with vector-like leptons, SM with single Higgs doublet, and the proposed 2HDM with vector-like leptons.
• The analysis uses the inert doublet model (IDM) framework, where the lightest CP-even scalar is identified as the 125 GeV Higgs boson, and the inert doublet remains uncharged under Yukawa couplings.

Experimental results

Research questions

• RQ1 Can the addition of an inert scalar doublet in a 2HDM alleviate the tension in electroweak precision parameters (S, T, U) arising from vector-like leptons?
• RQ2 What are the viable regions of parameter space for vector-like leptons when both electroweak precision constraints and theoretical consistency (vacuum stability, unitarity, perturbativity) are imposed?
• RQ3 How does the Higgs boson decay rate into two photons (h → γγ) change in the presence of vector-like leptons and an extended scalar sector compared to the SM and single Higgs doublet models?
• RQ4 To what extent can the observed excess in the LHC's diphoton channel (σ/σ_SM ≈ 1.65) be explained by this model configuration?
• RQ5 What are the relative contributions of vector-like fermions and scalar states to the loop-induced Higgs to diphoton decay amplitude in this extended model?

Key findings

• The inclusion of an additional inert scalar doublet enables cancellation between scalar and fermionic contributions to the S, T, and U parameters, significantly relaxing electroweak precision constraints.
• Larger Yukawa couplings and mass splittings between vector-like leptons are permitted in the 2HDM framework compared to the single Higgs doublet case due to improved cancellation in oblique corrections.
• The model predicts a substantial enhancement in the Higgs to diphoton decay rate, with signal strengths reaching up to σ/σ_SM ≈ 1.65 in favorable parameter regions, consistent with the ATLAS excess.
• The CMS diphoton signal strength (σ/σ_SM ≈ 1.11) is also within the allowed range in parts of the parameter space, indicating compatibility with current data.
• Theoretical constraints—vacuum stability, perturbativity, and unitarity—are satisfied in the identified viable regions, confirming the model's consistency beyond phenomenology.
• The diphoton signal strength in the 2HDM with vector-like leptons exceeds that of the SM with vector-like leptons alone and is competitive with the pure 2HDM case, highlighting the role of fermionic loops in enhancing the decay amplitude.