Physics Case for the International Linear Collider

TL;DR

The paper argues that after the Higgs discovery, a linear electron-positron collider like the ILC provides essential, high-precision tests of the Standard Model and powerful probes of new physics. It presents a staged run plan and updated projections showing percent-level Higgs coupling measurements, a top-quark program including a precision threshold scan, and direct searches for hidden sectors (dark matter, Higgsinos, and additional Higgs states) as well as new gauge bosons. Key contributions include model-independent determinations of the Higgs width, precise top mass and couplings, and sensitivity to compressed spectra via ISR and polarized-beam measurements. The work highlights the ILC's role in clarifying the mechanism of electroweak symmetry breaking, testing naturalness scenarios, and guiding future energy-frontier facilities in tandem with the LHC.

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.

Figures (12)

• Figure 1: The Standard Model predicts that the Higgs couplings to fundamental fermions are linearly proportional to the fermion masses, whereas the couplings to bosons are proportional to the square of the boson masses. Left: the CMS fit to the current Higgs data, showing consistency with this prediction, from Ref. CMSHiggs. Right: the expected improvement in the precision in the measurement of the Higgs couplings at the ILC, from Ref. ILCTDR.
• Figure 2: Cross sections for the three major Higgs production processes as a function of center of mass energy, from Ref. ILCTDR.
• Figure 3: Recoil mass distribution for the process: $e^+e^- \to Zh$ followed by $Z \to \mu^+\mu^-$ decay for $m_h=125\,$GeV with $250\,$fb$^{-1}$ at $\sqrt{s}=250\,$GeV, based on Ref. Li.
• Figure 4: Relative precisions for the various Higgs couplings extracted using the model-dependent fit used in the Snowmass 2013 study SnowHiggsReport, applied to expected data from the High-Luminosity LHC and from the ILC. Here, $\kappa_A$ is the ratio of the $A\overline{A} h$ coupling to the Standard Model expectation. The red bands show the expected errors from the initial phase of ILC running. The yellow bands show the errors expected from the full data set. The blue bands for $\kappa_\gamma$ show the effect of a joint analysis of High-Luminosity LHC and ILC data.
• Figure 5: Relative precisions for the various Higgs couplings extracted from a model-independent fit to expected data from the ILC. The notation is as in Fig. \ref{['fig:HiggsILCLHC']}.