Measurement of Higgs couplings and self-coupling at the ILC

TL;DR

The paper outlines a strategy to measure Higgs couplings and the self-coupling at the ILC with full model independence. It describes how to extract absolute $HZZ$ and $HWW$ couplings, the Higgs total width, top-Yukawa, and the trilinear self-coupling by combining recoil mass techniques, exclusive decays, and multi-channel cross-section measurements across staged energies. A global, χ^2-based fit of nine couplings plus $\Gamma_H$ is used to derive precision projections at 250 GeV, 500 GeV, and 1 TeV, demonstrating the ILC's capability to perform a complete, model-independent Higgs program. The results highlight the ILC's potential to validate the electroweak symmetry breaking mechanism and to probe physics beyond the Standard Model.

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 $σ\times\mathrm{Br}$ 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.

Figures (8)

• Figure 1: Mass-coupling relation MassCoupling.
• Figure 2: Major Higgs production processes at the ILC: Higgs-strahlung $e^+e^-\rightarrow {ZH}$ (left), $WW$-fusion $e^+e^-\rightarrow\nu\bar{\nu} {H}$ (middle) and $ZZ$-fusion $e^+e^-\rightarrow e^+e^-{H}$ (right).
• Figure 3: Cross sections of the major Higgs production processes as a function of $\sqrt{s}$ (left) and branching ratios of Higgs decay modes as a function of $m_H$ (right) ILCTDRPhys.
• Figure 4: Recoil mass distribution of $e^+e^-\rightarrow {ZH}$ followed by $Z\to\mu^+\mu^-$ at 250 GeV with a Higgs mass of 120 GeV.
• Figure 5: Reconstructed invariant mass of Higgs in the analysis of $e^+e^-\rightarrow\nu\bar{\nu} {H}$, followed by $H\to b\bar{b}$ (left) and $H\to WW^*$ (right).