[Paper Review] TRSM Benchmark Planes -- Snowmass White Paper

Tania Robens|arXiv (Cornell University)|May 28, 2022

Particle physics theoretical and experimental studies4 citations

TL;DR

This whitepaper presents benchmark planes in the Two-Real-Singlet Model (TRSM), a Z₂⊗Z₂′-symmetric extension of the Standard Model with three CP-even scalars from mixing of a SM-like doublet and two real singlets. It identifies key production and decay channels—especially at future e⁺e⁻ Higgs factories—showing that Z h₁₂₅ and Z h₂/₃ production with subsequent Higgs decays can yield observable cross sections of ∼10–18 fb, enabling discovery of light scalar resonances.

ABSTRACT

In this whitepaper, I briefly review the Benchmark Planes in the Two-Real-Singlet Model (TRSM), a model that enhances the Standard Model (SM) scalar sector by two real singlets that obey a $\mathbb{Z}_2\,\otimes\,\mathbb{Z}_2'$ symmetry. In this model, all fields acquire a vacuum expectation value, such that the model contains in total 3 CP-even neutral scalars that can interact with each other. All interactions with SM-like particles are inherited from the SM-like doublet via mixing. I remind the readers of the previously proposed benchmark planes, and briefly discuss possible production at future Higgs factories.

Motivation & Objective

• To define and analyze benchmark scenarios in the Two-Real-Singlet Model (TRSM) that capture non-standard Higgs production and decay patterns not yet covered by LHC searches.
• To identify viable production channels for light and heavy Higgs scalars at future e⁺e⁻ Higgs factories with √s ≈ 250 GeV.
• To assess the feasibility of detecting exotic Higgs decay chains such as h₃ → h₁h₂ or h₂ → h₁h₁ via Z h radiation.
• To provide quantitative cross-section estimates for Z h production and decay branching ratios relevant for future collider experiments.

Proposed method

• The TRSM scalar potential includes a SM-like Higgs doublet Φ and two real singlets S, X, with a Z₂⊗Z₂′ symmetry ensuring all fields acquire vacuum expectation values and mix into three physical CP-even scalars h₁, h₂, h₃.
• Mass eigenstates are ordered by mass: M₁ ≤ M₂ ≤ M₃, with mixing governed by a rotation matrix; couplings to SM particles are rescaled by κᵢ, where ∑κᵢ² = 1 and |κ₁₂₅| ≥ 0.96 for the SM-like Higgs.
• Production cross sections for hₐ are computed via σ(Mₐ) = κₐ² × σ_SM(Mₐ), valid to all orders in QCD and including full electroweak corrections via Higgs self-interactions.
• Decay branching ratios for hₐ → h_b h_c are calculated using Γₐ→bc / [κₐ² Γ_tot(SM) + ∑ₓᵧ Γₐ→ₓᵧ], with total widths including all SM and BSM decay modes.
• Benchmark planes (BP1–BP6) are defined based on which scalar is identified as the 125 GeV resonance and the dominant decay chain, with kinematic constraints on subsequent decays (e.g., h₂ → h₁h₁ if M₂ > 2M₁).
• LO cross sections for Zh production at e⁺e⁻ colliders (240–250 GeV) are computed using MadGraph5, with Z decay factorized to estimate final-state signatures like Zb̄b, ZW⁺W⁻, or Zb̄bb̄b.

Experimental results

Research questions

• RQ1 Which benchmark planes in the TRSM model produce Higgs resonances with decay chains that are currently under-constrained by LHC searches, particularly involving light scalars?
• RQ2 What are the expected production cross sections for Z h₁₂₅ and Z h₂/₃ at future e⁺e⁻ Higgs factories with √s ≈ 250 GeV, and how do they compare to SM expectations?
• RQ3 Under what kinematic conditions do exotic decay chains such as h₃ → h₁h₂ or h₂ → h₁h₁ become dominant, and what final states do they produce?
• RQ4 Can existing or planned Higgs factory searches cover the parameter space of the TRSM benchmark planes, especially for final states like Zb̄bb̄b or ZW⁺W⁻?

Key findings

• The benchmark planes BP1–BP6 cover a broad range of exotic Higgs decay patterns, including asymmetric decays (e.g., h₃ → h₁h₂) and symmetric decays (e.g., h₃ → h₁h₁ or h₃ → h₂h₂), with kinematic thresholds dictating dominant decay modes.
• Production cross sections for Z h₁₂₅ at 250 GeV e⁺e⁻ colliders are approximately 0.2 pb, consistent with the SM, due to κ₁₂₅ ≥ 0.966 in all benchmark scenarios.
• The maximal cross section for Z h₁₂₅ → Z h₁h₂ in BP1 reaches ∼18 fb, with BRs for h₁₂₅ → h₁h₂ contributing significantly to the final state.
• For Z h₂/₃ production, only BP4 (h₂ → h₁h₁) and BP5 (h₃ → h₁h₁) yield production rates >10 fb, making them optimal for discovery at Higgs factories.
• In BP4 and BP5, when kinematically allowed, the dominant final states are Zb̄bb̄b (due to h₂ → h₁h₁ or h₃ → h₁h₁), with branching ratios in the 40–50% range.
• Below the 2M₁ threshold, h₂ and h₃ predominantly decay to b̄b, making standard Higgs search strategies (e.g., Zb̄b, ZW⁺W⁻) effective, with a cross-over point at M₃ ≈ 135 GeV.