[Paper Review] Physics Case for the 250 GeV Stage of the International Linear Collider

H. Fujii, Christophe Grojean+22 more|arXiv (Cornell University)|Jan 1, 2017

Particle physics theoretical and experimental studies3 references120 citations

TL;DR

This paper establishes a compelling physics case for the 250 GeV stage of the International Linear Collider (ILC), demonstrating its capability for high-precision, model-independent measurements of Higgs boson couplings to fermions and gauge bosons—achieving sub-1% precision for couplings to W, Z, and bottom quarks. It further shows that this stage enables discovery of new physics beyond the Standard Model through precision Higgs couplings, exotic Higgs decays, and direct pair production of weakly interacting particles, with a luminosity of 2 ab⁻¹ enabling sensitivity beyond the LHC's reach.

ABSTRACT

The International Linear Collider is now proposed with a staged machine design, with the first stage at 250 GeV with a luminosity goal of 2 ab$^{-1}$. In this paper, we review the physics expectations for this machine. These include precision measurements of Higgs boson couplings, searches for exotic Higgs decays, other searches for particles that decay with zero or small visible energy, and measurements of $e^{+}e^-$ annihilation to $W^{+}W^-$ and 2-fermion states with improved sensitivity. A summary table gives projections for the achievable levels of precision based on the latest full simulation studies.

Motivation & Objective

• To establish the 250 GeV stage of the ILC as a standalone, high-impact physics program.
• To demonstrate the ILC's unique capability for model-independent, absolute measurements of Higgs boson couplings.
• To show that precision Higgs coupling studies at 250 GeV can probe new physics at scales beyond the LHC's reach.
• To evaluate the ILC's sensitivity to invisible and exotic Higgs decays, including dark matter portal scenarios.
• To assess the reach for direct production of weakly interacting particles and new resonances.

Proposed method

• Uses effective field theory (EFT) and κ formalism to model deviations in Higgs couplings.
• Performs full simulation studies to project measurement uncertainties for Higgs couplings, W+W⁻, and 2-fermion final states.
• Applies precision measurements of e⁺e⁻ → W⁺W⁻ and e⁺e⁻ → ff to probe new physics scales.
• Evaluates sensitivity to invisible and exotic Higgs decays via missing energy and missing mass reconstruction.
• Compares projected ILC reach to LHC sensitivities and model predictions across multiple new physics scenarios.
• Projects integrated luminosity of 2 ab⁻¹ at 250 GeV to achieve high-precision results.

Experimental results

Research questions

• RQ1 Can the 250 GeV ILC achieve sub-1% precision in measuring Higgs couplings to W, Z, and bottom quarks?
• RQ2 Can the ILC detect deviations in Higgs couplings that signal new physics beyond the Standard Model?
• RQ3 What is the sensitivity of the ILC to invisible and exotic Higgs decays, particularly in dark sector models?
• RQ4 Can the ILC discover new particles via direct pair production when they decay to low-missing-energy final states?
• RQ5 How does the ILC's 250 GeV stage compare in discovery reach to the LHC for new physics in Higgs, W+W⁻, and fermion pair production?

Key findings

• The ILC at 250 GeV can measure Higgs couplings to W, Z, and bottom quarks with precision below 1%, enabling model-independent determination of coupling strengths.
• The ILC achieves higher precision in Higgs coupling measurements than the LHC, particularly for small deviations (≤10%) predicted in many new physics models.
• The ILC can probe new physics scales up to ~10 TeV through precision measurements of e⁺e⁻ → W⁺W⁻ and 2-fermion final states.
• The ILC has significant sensitivity to invisible Higgs decays and exotic decays to dark sector particles, with discovery reach for such decays exceeding that of fixed-target experiments.
• The 250 GeV stage enables discovery of new particles like Higgsinos and dark matter candidates through direct pair production, with sensitivity improved by a factor of 1000 over LEP due to higher luminosity.
• The projected measurement uncertainties in Table 5 confirm that the 250 GeV ILC program exceeds the precision goals set in earlier reports, including the 2013 Snowmass white paper.