[Paper Review] Measurement of Higgs couplings and self-coupling at the ILC

Junping Tian, H. Fujii|arXiv (Cornell University)|Nov 26, 2013

Particle physics theoretical and experimental studies7 references4 citations

TL;DR

This paper presents a model-independent strategy for precision measurements of Higgs boson couplings and self-coupling at the International Linear Collider (ILC), using staged running at 250 GeV, 500 GeV, and 1 TeV with high-luminosity data. By combining inclusive recoil mass measurements and global fitting of cross-section and branching ratio data, the ILC achieves sub-percent-level precision on key couplings, including the Higgs self-coupling with a projected uncertainty of 1.9% at 1 TeV under luminosity upgrade.

ABSTRACT

In the Standard Model (SM) the couplings of the Higgs boson to SM particles and itself (self-couplings) are uniquely specified once the masses of the particles in question as well as the Higgs boson mass are given. Precision measurements of these couplings in the future collider experiments are the key to either verifying the mechanism of the electroweak symmetry breaking in the SM or uncovering physics beyond the SM. This article gives an overview of how various Higgs couplings will be measured at the ILC. Emphasis is put on the ILC's capability of performing fully model independent determination of absolute $HZZ$ and $HWW$ couplings, the Higgs total width, and hence various other Higgs couplings, which cover essentially all the crucial ones including the top-Yukawa coupling $Htt$ and the trilinear Higgs self-coupling $λ_{HHH}$. The strategy to get the best precision measurements at the ILC is through staged running, which provides many independent $σ imes\mathrm{Br}$ measurements for different production channels and at different energies. A method of global fitting is discussed to utilize all of the available information and to derive combined precisions.

Motivation & Objective

• To enable model-independent determination of Higgs couplings and self-coupling at the ILC to test the electroweak symmetry breaking mechanism in the Standard Model.
• To overcome limitations of LHC measurements by achieving absolute coupling measurements without theoretical assumptions.
• To provide a complete and precise Higgs program complementary to the LHC, focusing on couplings to Z, W, top quark, photons, and self-coupling.
• To demonstrate the feasibility of measuring the Higgs self-coupling with sub-10% precision at 1 TeV using double Higgs production.
• To establish a global fitting framework that combines multiple independent measurements across energies and production modes for optimal precision.

Proposed method

• Utilizes staged running at 250 GeV, 500 GeV, and 1 TeV to collect data across multiple Higgs production channels: Higgs-strahlung (e⁺e⁻ → ZH), WW-fusion (e⁺e⁻ → νν̄H), and ZZ-fusion (e⁺e⁻ → e⁺e⁻H).
• Employs recoil mass reconstruction in Z → μ⁺μ⁻ decays to measure the absolute cross section of e⁺e⁻ → ZH, enabling model-independent extraction of the gHZZ coupling.
• Applies global χ² fitting to combine independent σ × Br measurements across production and decay modes, with predictions parameterized by couplings and Higgs total width.
• Uses theoretical factors Fi to relate measured cross sections to coupling products, enabling extraction of gHZZ, gHWW, gHtt, and ΓH without assumptions.
• Applies the relation δλ/λ ≈ 0.76 × (δσ/σ) for νν̄HH and 1.66 × (δσ/σ) for ZHH to estimate self-coupling precision from double Higgs production cross sections.
• Incorporates full detector simulation for HH → bbb̄b̄ and HH → bbb̄W* decay modes to evaluate self-coupling sensitivity at 500 GeV and 1 TeV.

Experimental results

Research questions

• RQ1 Can the ILC achieve model-independent measurements of Higgs couplings, including gHZZ and gHWW, without relying on theoretical assumptions?
• RQ2 What is the achievable precision on the Higgs self-coupling λHHH using double Higgs production at the ILC?
• RQ3 How does staged running at multiple center-of-mass energies improve the global determination of Higgs couplings and total width?
• RQ4 To what extent can the global fitting approach reduce uncertainties in Higgs coupling measurements by combining multiple independent observables?
• RQ5 What is the sensitivity of the ILC to deviations from the Standard Model in Higgs couplings, particularly for the top-Yukawa and Higgs self-coupling?

Key findings

• The ILC achieves a projected precision of 0.51% on the gHZZ coupling at 500 GeV and 1 TeV under the luminosity upgrade scenario.
• The gHWW coupling is expected to be measured with a precision of 0.56% at 1 TeV under the luminosity upgrade, significantly improving over LHC constraints.
• The top-Yukawa coupling gHtt is expected to be measured with 1.9% precision at 1 TeV under the luminosity upgrade, enabling sensitive tests of the top quark’s role in electroweak symmetry breaking.
• The Higgs self-coupling λHHH is projected to be measured with a precision of 13% at 1 TeV under the luminosity upgrade, making it accessible for probing new physics beyond the Standard Model.
• The Higgs total width ΓH is expected to be determined with 2.3% precision at 1 TeV under the luminosity upgrade, crucial for consistency checks of coupling measurements.
• The global fitting method successfully combines measurements across energies and channels, reducing uncertainties and enabling a fully model-independent determination of all 10 Higgs parameters (9 couplings and ΓH).