Higgs Boson Studies at Future Particle Colliders

TL;DR

This work provides a coherent, cross‑project assessment of Higgs physics at future colliders by applying both the κ-framework and SMEFT EFT to projections from HE-LHC, FCC-ee/hh, CEPC, ILC, CLIC, LHeC, and a muon collider. It quantifies expected precision on Higgs couplings to fermions and vector bosons, the Higgs self‑coupling, CP properties, rare decays, and the total width, while rigorously accounting for SM theory uncertainties and correlations. The study demonstrates how lepton colliders offer absolute normalization and sub-percent Higgs coupling accuracy, while high-energy hadron machines complement with strong constraints on heavy‑new‑physics operators and high‑energy growth in SMEFT terms. It also explores beyond‑HL‑LHC options, including muon and multi‑TeV electron‑positron colliders, and discusses the implications for naturalness, dark matter, and electroweak phase transitions. Overall, the report informs strategic planning by contrasting the complementary strengths of proposed facilities and outlining the path toward precision Higgs physics in the coming decades.

Abstract

This document aims to provide an assessment of the potential of future colliding beam facilities to perform Higgs boson studies. The analysis builds on the submissions made by the proponents of future colliders to the European Strategy Update process, and takes as its point of departure the results expected at the completion of the HL-LHC program. This report presents quantitative results on many aspects of Higgs physics for future collider projects of sufficient maturity using uniform methodologies.

Figures (20)

• Figure 1: Time line of various collider projects starting at time $T_0$ as submitted to the European Strategy Update process. Some possible extensions beyond these baseline run plans have been discussed and are presented in more detail in Appendix \ref{['app:addons']}. For the clarification of the meaning of a year of running, see the caption of Table \ref{['tab:colliders']}. Figure \ref{['fig:timelineabs']} in Appendix \ref{['app:inputs']} shows an alternative version of this figure using the earliest possible start date (i.e. the calendar date of $T_0$) given by the proponents.
• Figure 2: Expected relative precision (%) of the $\kappa$ parameters in the kappa-3 scenario described in Section \ref{['tab:kappa_scenarios']}. For details, see Tables \ref{['tab:resultsLHCKappa3']} and \ref{['tab:resultsKappa3']}. For HE-LHC, the S2' scenario is displayed. For LHeC, HL-LHC and HE-LHC a constrained $\kappa_V\leq 1$ is applied.
• Figure 3: Sensitivity at 68% probability to deviations in the different effective Higgs couplings and aTGC from a global fit to the projections available at each future collider project. Results obtained within the SMEFT framework in the benchmark SMEFT$_{\rm ND}$. The HE-LHC results correspond to the $S_2^\prime$ assumptions for the theory systematic uncertainties in Higgs processes Cepeda:2019klc.
• Figure 4: Sensitivity at 68% probability to deviations in the different EW couplings from a global fit to the projections available at each future collider project. Results obtained within the SMEFT framework in the benchmark SMEFT$_{\rm ND}$. Note that $Z$-radiative return measurements at ILC and CLIC are included in the fit. Two different assumptions are considered for the systematic errors. The HE-LHC results correspond to the $S_2^\prime$ assumptions for the theory systematic uncertainties in Higgs processes Cepeda:2019klc. See text for details.
• Figure 5: 68$\%$ probability reach on Higgs couplings and aTGC values for the different lepton colliders from the Global fit SMEFT$_{\rm ND}$, compared with the results obtained assuming infinite precision for the EWPO (scenario SMEFT$_{\rm PEW}$). The difference (partially) illustrates the impact of the EW constraints on the Higgs results. See text for discussion and caveats which apply to this study. The measurements based on $Z$ bosons from radiative return at ILC and CLIC are included in the default fit, and the horizontal red marks indicate the coupling reach when additionally a dedicated $Z$-pole run is taken.