[Paper Review] Physics Case for the International Linear Collider

Keisuke Fujii, Christophe Grojean+16 more|arXiv (Cornell University)|Jan 1, 2015

Particle Accelerators and Free-Electron Lasers79 citations

TL;DR

This paper presents a comprehensive update of the physics case for the International Linear Collider (ILC), emphasizing its unique capability to perform precision measurements of the Higgs boson, top quark, and new physics beyond the Standard Model through high-luminosity e+e− collisions. It demonstrates that the ILC can achieve sub-percent-level precision in coupling and self-coupling measurements, enabling model-independent tests of new physics, including dark matter and exotic Higgs decays, with projected uncertainties derived from full detector simulations and updated accelerator run plans.

ABSTRACT

We summarize the physics case for the International Linear Collider (ILC). We review the key motivations for the ILC presented in the literature, updating the projected measurement uncertainties for the ILC experiments in accord with the expected schedule of operation of the accelerator and the results of the most recent simulation studies.

Motivation & Objective

• To establish the ILC as the premier facility for precision studies of the Higgs boson and top quark in the post-LHC era.
• To quantify the ILC's sensitivity to new physics, including hidden sector particles, dark matter, and exotic Higgs decays.
• To update projected measurement uncertainties based on the latest ILC accelerator run plan and full detector simulation studies (ILD/SiD).
• To compare the ILC's capabilities with the LHC, highlighting the ILC's superior precision in coupling and self-coupling measurements due to clean initial states and low backgrounds.
• To provide a model-independent framework for interpreting ILC results, particularly for Higgs couplings and dark matter production.

Proposed method

• Uses a staged ILC run plan: 500 fb⁻¹ at 500 GeV, 200 fb⁻¹ at 350 GeV, 500 fb⁻¹ at 250 GeV, followed by luminosity upgrade to 3500 fb⁻¹ at 500 GeV and 1500 fb⁻¹ at 250 GeV.
• Employs full Geant4-based detector simulations (ILD and SiD) to project measurement uncertainties, including both statistical and systematic errors.
• Performs model-independent global fits to ILC observables, varying all Higgs couplings (including loop-induced γγ and gg) and total width as free parameters.
• Applies effective field theory to analyze dark matter pair production via contact interactions, with scales Λ and spin structures D5/D8.
• Combines ILC results with projected LHC measurements (e.g., h→γγ/h→ZZ* ratio at 2% accuracy) to improve constraints.
• Uses N3LO QCD calculations to estimate theoretical uncertainties in top quark threshold cross-section measurements.

Experimental results

Research questions

• RQ1 Can the ILC achieve sub-percent-level precision in measuring Higgs boson couplings to fermions and gauge bosons, and what are the projected uncertainties?
• RQ2 What is the ILC's sensitivity to the Higgs boson self-coupling and to invisible or exotic Higgs decays?
• RQ3 How precisely can the ILC measure top quark properties, including electroweak and electromagnetic couplings, at threshold and above?
• RQ4 What are the ILC's discovery and exclusion limits for hidden sector dark matter and new gauge bosons via contact interactions?
• RQ5 How do ILC results compare with those expected from the LHC, especially in terms of model-independent precision and systematic control?

Key findings

• The ILC can measure Higgs boson couplings to fermions and gauge bosons with uncertainties below 0.5% for the full data set (4 ab⁻¹ at 500 GeV, 2 ab⁻¹ at 250 GeV).
• The Higgs self-coupling can be measured with a precision of approximately 5% using the full ILC program, enabling tests of the Higgs potential and electroweak symmetry breaking.
• The ILC can probe invisible Higgs decays via the hZ process with a sensitivity to branching ratios at the 0.1% level.
• Top quark couplings (gγL, gγR, gZL, gZR) can be measured with uncertainties below 0.2%, and chirality-flip couplings with 0.5% systematic uncertainty.
• The ILC can set competitive limits on dark matter production, excluding contact interaction scales Λ > 10 TeV for D5 and D8 operators, based on full simulation studies.
• Combining ILC data with a projected 2% LHC measurement of Γ(h→γγ)/Γ(h→ZZ*) improves the model-independent constraint on the Higgs coupling to photons by a factor of two.