Optimal estimation of Higgs-Gauge Boson couplings at the future $e^+e^-$ colliders

TL;DR

The paper tackles precision constraints on Higgs-gauge couplings within the SMEFT framework at future $e^+e^-$ colliders, focusing on $e^+e^-\to Zh$ at $\sqrt{s}=250$ GeV with the recoil-mass technique. It employs the Optimal Observable Technique to extract dimension-6 Wilson coefficients, distinguishing CP-even and CP-odd operators, and studies both unpolarized and polarized beam scenarios. The main findings show that CP-even operator limits improve substantially over current CMS bounds, with beam polarization providing substantial gains, while CP-odd constraints remain comparatively weaker using inclusive observables; higher luminosities and CP-sensitive observables could further enhance sensitivity. The results underscore the ILC’s capability for per-mille level Higgs EFT tests and guide collider design choices, including polarization strategies and luminosity planning, in comparison with FCC-ee projections. Overall, the work demonstrates how polarized $e^+e^-$ colliders can robustly test SMEFT Higgs-sector deviations and map the structure of possible BSM physics.

Abstract

The proposed $e^+e^-$ collider offers an ideal environment for precise estimation of Higgs boson properties which are of utmost importance to validate the Standard Model of particle physics. We investigate $hVV$ couplings, where $V\in \{Z,γ\}$ with single Higgs production associated with $Z$ boson at the proposed $e^+e^-$ machine with $\sqrt{s}=250$ GeV, within the Standard Model Effective Field Theory (SMEFT) framework. We employ the recoil mass of the dilepton system, to select the signal phase space, i.e, $Zh \to l^+l^-b\Bar{b}$ events. The constraints on the Wilson coefficients (WCs) are obtained using the optimal observable technique (OOT). On comparison with the current experimental limits at $68\%$ CL with $138$ fb$^{-1}$ luminosity, our limits are tighter by a factor ranging from $1.5-10$ for CP even operators, while CP-odd WCs shows comparable limits.

Figures (6)

• Figure 1: Representative Feynman diagrams denoting the production of two leptons and two $b$ quarks at the leading order in the SM.
• Figure 2: Recoil mass distribution normalized to $\mathcal{L}=1000$ fb$^{-1}$ for signal and two background processes viz. $ZZ$ and $ZZ\gamma$. The kinematic effect of ISR are highlighted for $Zh$ and $ZZ$ processes.
• Figure 3: One parameter optimal sensitivity plots from $Zh$ production at the ILC 250 GeV for unpolarized and polarized setups. For the unpolarized case, $\mathcal{L}_{\rm int}=$ 2000 fb$^{-1}$, and for the polarized case each polarization setup i.e. $(+30\%,-80\%)$ and $(-30\%,+80\%)$ corresponds to $\mathcal{L}_{\rm int}=$ 1000 fb$^{-1}$, hence giving a combined luminosity, $\mathcal{L}_{\rm int}=$ 2000 fb$^{-1}$.
• Figure 4: Two parameter 95% CL optimal sensitivity plots from $Zh$ production at the ILC 250 GeV for unpolarized and different polarization setups. For each setup, $\mathcal{L}_{\rm int}=$$1000$ fb$^{-1}$.
• Figure 5: SHAP values for BDT models trained for signal-background segregation.