[Paper Review] h γγCoupling in Higgs Triplet Model

A. Arhrib, Rachid Benbrik+3 more|arXiv (Cornell University)|Feb 29, 2012

Particle physics theoretical and experimental studies3 references3 citations

TL;DR

This paper investigates the diphoton decay width of the Higgs boson (h→γγ) in the Higgs Triplet Model (DTHM), a type-II seesaw extension of the Standard Model. It shows that loop contributions from singly and doubly charged Higgs bosons (H± and H±±) can significantly enhance or suppress the h→γγ rate compared to the SM, offering a viable explanation for the observed diphoton excess near 125 GeV at the LHC, while preserving SM-like rates in other decay channels.

ABSTRACT

We investigate Higgs boson decay into two photons in the type-II seesaw model. The rate of $h o γγ$ gets suppressed/enhanced in this model compared to the Standard Model (SM) due to the presence of the singly and doubly charged Higgs $H^\pm$ and $H^{\pm\pm}$.

Motivation & Objective

• To investigate the h→γγ decay rate in the Higgs Triplet Model (DTHM), a type-II seesaw extension of the Standard Model.
• To determine whether the observed diphoton excess near 125 GeV at the LHC can be explained by new charged Higgs states in DTHM.
• To assess whether the model can simultaneously account for deviations in h→γγ while preserving SM-like rates in other Higgs decay channels.
• To derive constraints on the masses of H± and H±± bosons from diphoton data, using precision measurements at the LHC and ILC.

Proposed method

• The study employs the DTHM scalar potential with a complex scalar triplet Δ, introducing CP-even, CP-odd, singly, and doubly charged Higgs states.
• The Higgs sector is parameterized by seven independent parameters: λ, λ₁–λ₄, μ, and vₜ, with constraints from perturbative unitarity and vacuum stability.
• The partial width Γ(h→γγ) is calculated via loop diagrams involving H± and H±± bosons, using the effective couplings hH±H∓ and hH±±H∓∓.
• The branching ratio Br(h→γγ) is compared to the SM prediction via the ratio Rγγ = Γ(h→γγ)/Γ_SM(h→γγ), which depends on the charged Higgs masses and couplings.
• Numerical scans are performed over the (λ₁, λ₄) parameter space to explore regions of enhancement or suppression in Rγγ.
• Constraints are derived from LHC diphoton exclusion limits and ILC precision measurements, particularly using the γγ option for 2% accuracy on h→bb̄ width.

Experimental results

Research questions

• RQ1 Can the DTHM explain the observed diphoton excess near 125 GeV at the LHC through loop contributions of charged Higgs bosons?
• RQ2 How do the masses and couplings of H± and H±± affect the h→γγ decay width in the DTHM compared to the SM?
• RQ3 Can the DTHM simultaneously suppress or enhance h→γγ while preserving SM-like rates in other Higgs decay channels like h→bb̄, τ⁺τ⁻, WW*, and ZZ*?
• RQ4 What are the viable mass ranges for H±± and H± that are consistent with current LHC diphoton exclusion limits and future ILC precision measurements?
• RQ5 To what extent can the diphoton channel distinguish the DTHM from the Standard Model, given the model's constraints from unitarity and vacuum stability?

Key findings

• The DTHM can enhance the h→γγ decay width by up to a factor of ~2.5 in certain regions of parameter space, particularly when H±± and H± are light and λ₁ is large.
• For m_h ≈ 125 GeV, the model can account for the observed diphoton excess reported by ATLAS and CMS, with Rγγ > 1 in regions where m_H±± < 200 GeV and λ₁ ≈ 8.
• In other regions of parameter space, particularly with large negative λ₄, the h→γγ rate can be suppressed below the SM value, with Rγγ < 1 even for heavy charged Higgs states.
• The model preserves SM-like rates in all other Higgs decay channels (e.g., h→bb̄, τ⁺τ⁻, WW*, ZZ*) due to identical tree-level couplings of the SM-like Higgs h to SM particles.
• The diphoton rate suppression can explain the exclusion of a SM-like Higgs in the 114–115 GeV range by LEP, even if such a state exists, due to reduced h→γγ branching ratio.
• The correlation between m_h and m_H±± in the (m_H±±, Rγγ) plane shows horizontal bands, indicating that Rγγ is primarily sensitive to H±± and H± masses rather than m_h, especially in the SM-like h regime.