On the future of Higgs, electroweak and diboson measurements at lepton colliders

TL;DR

The work develops a comprehensive SMEFT-based analysis of Higgs and electroweak processes at future lepton colliders, integrating WW diboson and Higgsstrahlung channels within a 28-parameter framework and evaluating three collider families (CEPC/FCC-ee, ILC, CLIC). It uses statistically optimal observables to maximize sensitivity and compares two fitting approaches to validate results. The key finding is that robust Higgs-coupling determinations at circular colliders require a dedicated $Z$-pole program to constrain EW parameters, while linear colliders need multi-energy runs and beam polarization to mitigate EW-induced degeneracies; radiative-return EW measurements can partly compensate for lacking Z-pole runs. Overall, the paper demonstrates the proven value of a global Higgs–EW EFT program for achieving permille-level Higgs precision without being handicapped by electroweak parametric uncertainties, with significant implications for collider design and program optimization.

Abstract

LEP precision on electroweak measurements was sufficient not to hamper the extraction of Higgs couplings at the LHC. But the foreseen permille-level Higgs measurements at future lepton colliders might suffer from parametric electroweak uncertainties in the absence of a dedicated electroweak program. We perform a joint, complete and consistent effective-field-theory analysis of Higgs and electroweak processes. The full electroweak-sector dependence of the $e^+e^- \to WW$ production process is notably accounted for, using statistically optimal observables. Up-to-date HL-LHC projections are combined with CEPC, FCC-ee, ILC and CLIC ones. For circular colliders, our results demonstrate the importance of a new $Z$-pole program for the robust extraction of Higgs couplings. At linear colliders, we show how exploiting multiple polarizations and centre-of-mass energies is crucial to mitigate contaminations from electroweak parameter uncertainties on the Higgs physics program. We also investigate the potential of alternative electroweak measurements to compensate for the lack of direct $Z$-pole run, considering for instance radiative return to these energies. Conversely, we find that Higgs measurements at linear colliders could improve our knowledge of the $Z$ couplings to electrons.

Figures (13)

• Figure 1: A summary of run scenarios for CEPC, FCC-ee, ILC and CLIC considered in our analysis, with the corresponding integrated luminosities. The impact of beam polarization at the ILC is examined by considering $P(e^-,e^+)=(\mp80\%,\pm30\%)$, $(\pm80\%,0\%)$ and unpolarized configurations.
• Figure 2: Global one-sigma reach of future lepton colliders on Higgs and triple-gauge couplings. The run scenarios and luminosities assumed are listed in \ref{['tab:scenarios']}. LEP and SLD electroweak measurements as well as HL-LHC prospects on Higgs and diboson processes are included in all projections. Modifications of electroweak parameters (shown in \ref{['fig:ew']}) are marginalized over to obtain the prospects displayed as bars, and artificially set to zero to obtain those shown with triangular marks. For the CEPC and FCC-ee, scenarios without the future $Z$-pole ($WW$ threshold) run are shown as light shaded bars (lower edges of the green marks). For ILC, the results with the inclusion of the $A_{LR}$ measurement at 250 GeV are shown with yellow marks. The bottom panel highlights the couplings that are affected significantly EW uncertainties. Numerical results are also reported in \ref{['tab:allcoll']}
• Figure 3: Degradation in Higgs and triple-gauge coupling determinations due to EW uncertainties at future circular colliders. It is obtained by comparison with a perfect EW measurement scenario and quantified as ${\delta g}/{\delta g(\textrm{EW}\rightarrow 0)}-1$ expressed in percent. The dashed and solid lines are respectively obtained with and without new $Z$-pole run. Numerical values are also provided in \ref{['tab:allcoll']}.
• Figure 4: Global one-sigma reach on electroweak couplings for the same scenarios as in \ref{['fig:money']}. Higgs and triple-gauge coupling modifications are marginalized over. Trapezoidal and green marks respectively indicate the prospects obtained with Higgs and $WW$ threshold measurements excluded. The numerical results are reported in \ref{['tab:allcollEW']}.
• Figure 5: A scheme-ball illustration of the correlations between Higgs and EW sector couplings. The $Z$-pole runs are included for both FCC-ee and CEPC. Projections from HL-LHC and measurements from LEP and SLD are included in all scenarios. The outer bars give the one-sigma precision on the individual coupling (see \ref{['tab:allcoll']} and \ref{['tab:allcollEW']}).