[Paper Review] Higgs Boson Precision Studies at a Linear Collider

K. Desch|ArXiv.org|Nov 7, 2003

Particle physics theoretical and experimental studies3 citations

TL;DR

This paper presents a comprehensive study of Higgs boson precision measurements at a future electron-positron linear collider (LC), focusing on the Standard Model Higgs and its extensions such as the radion in models with extra dimensions. Using high-luminosity e⁺e⁻ collisions, the LC enables precise determination of Higgs couplings, branching ratios, and trilinear self-coupling, with sensitivity to deviations from the Standard Model due to new physics—particularly through Higgs-radion mixing—outperforming the LHC in model-independent coupling measurements.

ABSTRACT

This report summarizes the progress in the study of Higgs physics at a future linear electron positron collider at center-of-mass energies up to about 1000 GeV and high luminosity. After the publication of the TESLA Technical Design Report, an extended ECFA/DESY study on linear collider physics and detectors was performed. The paper summarizes the status of the studies with main emphasis on recent results obtained in the course of the workshop.

Motivation & Objective

• To assess the capability of a future linear collider (LC) to perform high-precision measurements of Higgs boson properties, including couplings, branching ratios, and self-couplings.
• To investigate the sensitivity of the LC to new physics beyond the Standard Model, particularly in models with extra dimensions and radion-Higgs mixing.
• To compare the LC's precision in measuring Higgs couplings with the LHC's capabilities, identifying synergies and complementary strengths.
• To evaluate the impact of theoretical improvements, such as full one-loop electro-weak corrections, on the accuracy of Higgs cross-section predictions.
• To explore the physics potential of photon-photon collisions at the LC for Higgs studies, particularly in detecting radion resonances.

Proposed method

• Utilizes e⁺e⁻ collisions at center-of-mass energies up to 1000 GeV with high luminosity to produce Higgs bosons via Higgs-strahlung (e⁺e⁻ → ZH) and vector boson fusion (e⁺e⁻ → νν̄H) processes.
• Applies full one-loop electro-weak corrections to theoretical predictions of Higgs production cross-sections, improving precision for both SM and BSM scenarios.
• Analyzes Higgs branching ratios (e.g., H⁰ → bb̄, H⁰ → W⁺W⁻) with high efficiency and low background, enabling precise extraction of coupling strengths.
• Models Higgs-radion mixing in models with large extra dimensions, introducing parameters ξ (mixing), mϕ (radion mass), and Λϕ (coupling scale) to study deviations from SM couplings.
• Evaluates sensitivity to radion signals via the gg → Φ → 4ℓ channel at the LHC and Higgs branching ratio deviations (e.g., BR(H⁰ → bb̄)) at the LC, using statistical significance thresholds (e.g., >2.5σ).
• Conducts comparative studies between LHC and LC, identifying regions where the LC can detect Higgs signals missed by the LHC due to low branching ratios or background suppression.

Experimental results

Research questions

• RQ1 What is the precision achievable in measuring Higgs boson couplings to fermions and gauge bosons at a linear collider with high luminosity?
• RQ2 How sensitive is the linear collider to deviations from the Standard Model in Higgs couplings due to Higgs-radion mixing in models with large extra dimensions?
• RQ3 In which parameter space of radion mass and mixing angle can the LC detect deviations in Higgs branching ratios beyond the 2.5σ threshold?
• RQ4 How do full one-loop electro-weak corrections affect the theoretical prediction of Higgs production cross-sections in e⁺e⁻ collisions?
• RQ5 In which scenarios does the LC provide superior Higgs coupling measurements compared to the LHC, and where is synergy between the two colliders most beneficial?

Key findings

• The LC achieves high-precision measurements of Higgs branching ratios, particularly BR(H⁰ → bb̄) and BR(H⁰ → W⁺W⁻), with low backgrounds and high efficiency, enabling model-independent coupling determinations.
• For a 120 GeV Higgs boson, the LC can detect >2.5σ deviations in BR(H⁰ → bb̄) due to radion mixing in regions where the LHC may be blind, especially for radion masses of 20–200 GeV and mixing angles ξ ≳ 0.1.
• Theoretical cross-sections for Higgs-strahlung and WW-fusion are corrected at the one-loop level, with corrections reaching ±10% depending on Higgs mass, improving prediction accuracy.
• Radion-Higgs mixing leads to significant deviations in Higgs couplings to Z bosons and fermions, especially when mixing parameter ξ is large, with effective couplings modified by up to 30% relative to SM values.
• The LC's sensitivity to trilinear Higgs self-coupling is enhanced by radion admixtures, providing a probe for new physics beyond the SM.
• The LC complements the LHC by enabling precise, model-independent measurements of Higgs couplings—particularly the top-Yukawa coupling and heavy Higgs sector in supersymmetry—where the LHC lacks such sensitivity.