The leptonic future of the Higgs

TL;DR

This work develops a basis-independent, twelve-parameter SMEFT framework (the Higgs basis) to perform a global fit of Higgs, diboson, and top-associated Higgs observables at future $e^+e^-$ colliders (CEPC, FCC-ee, ILC, CLIC). By combining Higgsstrahlung, WW fusion, $t\bar{t}h$, and $WW'$-type measurements across multiple energies and beam polarizations, the authors quantify how degeneracies in the EFT parameter space can be lifted and quantify the overall constraint strength with the global determinant parameter (GDP). They find that including differential observables and higher-energy runs significantly improves global constraints, with GDPs around a few $\times 10^{-3}$ to $10^{-2}$ when LHC+LEP data are included, and substantial gains from a 350 GeV circular run and from beam polarization at linear colliders. The results highlight strong complementarity between channels, energies, and collider designs, and they provide a framework for comparing future Higgs and electroweak measurements in a consistent EFT setting.

Abstract

Precision study of electroweak symmetry breaking strongly motivates the construction of a lepton collider with center-of-mass energy of at least 240 GeV. Besides Higgsstrahlung ($e^+e^- \to hZ$), such a collider would measure weak boson pair production ($e^+e^- \to WW$) with an astonishing precision. The weak-boson-fusion production process ($e^+e^- \to ν\barν h$) provides an increasingly powerful handle at higher center-of-mass energies. High energies also benefit the associated top-Higgs production ($e^+e^-\to t\bar th$) that is crucial to constrain directly the top Yukawa coupling. The impact and complementarity of differential measurements, at different center-of-mass energies and for several beam polarization configurations, are studied in a global effective-field-theory framework. We define a "global determinant parameter" (GDP) which characterizes the overall strengthening of constraints independently of the choice of operator basis. The reach of the CEPC, CLIC, FCC-ee, and ILC designs is assessed.

Figures (21)

• Figure 1: Leading-order contribution to the Higgsstrahlung process, $e^+e^- \to hZ$.
• Figure 2: Definition of the $\Omega=\{\theta_1,\theta_2,\phi\}$ angles in a $e^+e^- \to hZ$ event (taken from Ref. Craig:2015wwr). Note the two polar angles are respectively defined in the center-of-mass and $Z$ restframes.
• Figure 3: Two contributions to the $e^+e^- \to \nu \bar{\nu} h$ process: weak-boson fusion (left), and $e^+e^- \to hZ, Z\to \nu \bar{\nu}$ (right).
• Figure 4: Leading-order diagrams for the $e^+e^- \to t \bar{t} h$ process. In the SM, the dominant contribution are the ones involving the top Yukawa coupling. Other EFT contributions (including that of four-fermion operators, not depicted) should be well constrained by other measurements.
• Figure 5: Leading-order diagrams contributing to $e^+e^- \to WW$. The $s$-channel diagram on the left with an intermediate $Z$ or photon involves a triple gauge coupling.