A global view on the Higgs self-coupling at lepton colliders

TL;DR

This work evaluates the precision with which the Higgs trilinear self-coupling can be determined at future lepton colliders using a comprehensive SM EFT framework. It shows that low-energy machines can constrain the coupling indirectly through NLO effects in single-Higgs processes, achieving ~40% precision when combining runs at 240/250 GeV and 350 GeV and performing a global fit that accounts for other EFT deviations. At high energies, Higgs-pair production via Zhh and WW-fusion enables around ~20% precision, with differential analyses of the Higgs-pair invariant mass distribution helping to lift degeneracies. The study also highlights synergy with HL-LHC data and demonstrates that a robust, global approach is essential to accurately extract the Higgs self-coupling across collider scenarios.

Abstract

We perform a global effective-field-theory analysis to assess the precision on the determination of the Higgs trilinear self-coupling at future lepton colliders. Two main scenarios are considered, depending on whether the center-of-mass energy of the colliders is sufficient or not to access Higgs pair production processes. Low-energy machines allow for ~40% precision on the extraction of the Higgs trilinear coupling through the exploitation of next-to-leading-order effects in single Higgs measurements, provided that runs at both 240/250 GeV and 350 GeV are available with luminosities in the few attobarns range. A global fit, including possible deviations in other SM couplings, is essential in this case to obtain a robust determination of the Higgs self-coupling. High-energy machines can easily achieve a ~20% precision through Higgs pair production processes. In this case, the impact of additional coupling modifications is milder, although not completely negligible.

Figures (16)

• Figure 1: One-loop diagrams involving the trilinear Higgs coupling contributing to the main single Higgs production processes: $e^+e^- \to hZ$ (top row) and $e^+e^- \to \nu \bar{\nu} h$ (middle row). The Higgs self-energy diagram (bottom) gives a universal modification to all Higgs production processes via wave function renormalization.
• Figure 2: Left: Value of $C_1$ as a function of the center of mass energy $\sqrt{s}$ for the $e^+e^- \to hZ$ and $e^+e^- \to \nu \bar{\nu} h$ single Higgs production processes. Right: The linear dependence of production and decay rates on the $\delta \kappa_\lambda$, $\delta c_Z$, $c_{ZZ}$ and $c_{Z\square}$ parameters (see \ref{['sec:global1h']} for details on the meaning of these parameters). For $e^+e^- \to \nu \bar{\nu} h$, only the $WW$-fusion contribution is included. The dependence on $\delta \kappa_\lambda$ is amplified by a factor of 500.
• Figure 3: Chi-square as a function of $\delta \kappa_\lambda$ after profiling over all other EFT parameters. Three run scenario are considered for circular colliders, with $5\,{\rm ab}^{-1}$ at $240\,$GeV and $\{0,~200\,{\rm fb}^{-1},~1.5\,{\rm ab}^{-1}\}$ at $350\,$GeV, without beam polarization. The shaded areas cover different assumptions about the precision of TGC measurements. Left: circular lepton collider measurements only. Right: combination with differential single and double Higgs measurements at the HL-LHC.
• Figure 4: Global constraints on $\delta c_Z$ and $\delta \kappa_\lambda$, obtained from single Higgs measurements at circular colliders (left panel) and ILC (right panel), illustrating the improvement brought by $350\,$GeV runs. Dashed lines are for the latter only, while solid lines combined them with the $240/250\,$GeV one.
• Figure 5: One-sigma bound on $\delta \kappa_\lambda$ deriving from single Higgs and diboson production measurements at lepton colliders as a function of the integrated luminosity collected at both $240/250$ and $350\,$GeV. Conservative (solid) and optimistic (dashed) assumptions are used for the precision of diboson measurements.