Probing CP-violating Higgs-Gauge couplings with Higgsstrahlung at $e^-e^+$ collider

TL;DR

The paper addresses the problem of constraining CP-even and CP-odd Higgs–gauge couplings through SMEFT in Higgsstrahlung at a future $e^-e^+$ collider at $\sqrt{s}=250$ GeV. It combines six dimension-6 operators in a polarized-beam analysis of $e^-e^+\to Zh$, exploiting spin-based observables from $h\to b\bar{b}$, $WW^*$, and $ZZ^*$ decays to access interference effects, including CP-odd contributions. The main contributions are the demonstration that Z-boson polarization and Higgs decay spin correlations substantially enhance sensitivity to CP-odd operators, and the provision of projected 95% CL bounds showing strong complementarity among decay channels and significant improvements over current bounds, with clear saturation behavior set by systematics. The study demonstrates that sub-percent level SMEFT coefficient constraints are achievable at future high-luminosity lepton colliders when leveraging spin observables, beam polarization, and multiple Higgs decay channels, with important implications for precision Higgs physics and BSM sensitivity.

Abstract

We investigate the sensitivity of a future high-luminosity $e^-e^+$ collider operating at $\sqrt{s}=250~\text{GeV}$ to CP-violating and CP-conserving anomalous $hVV$ interactions via the Higgsstrahlung process. The effects of new physics are parameterized in the Standard Model Effective Field Theory~(SMEFT) framework through six dimension-6 operators modifying the $hVV$ vertices. Using polarized beams and exploiting polarization and spin correlation asymmetries reconstructed from Higgs decay products, we perform a comprehensive analysis across the three dominant decay modes, $h\to b\bar{b}$, $WW^\star$, and $ZZ^\star$. The $h\to WW^\star$ channel exhibits the highest sensitivity to $\mathcal{O}_{HW}$ and $\mathcal{O}_{H\widetilde{W}}$, while the $b\bar{b}$ mode constrains the remaining operators with high statistical precision. Sensitivity studies incorporating luminosity scaling and systematic uncertainties show that the projected bounds improve significantly with increasing integrated luminosity, but saturate once experimental systematics exceed the few-percent level. These results highlight the crucial role of spin-based observables and beam polarization in achieving sub-percent precision on SMEFT coefficients at future lepton colliders.

Figures (4)

• Figure 1: 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 2: Sensitivity of asymmetries to the WCs of the dimension-6 operators affecting $hZZ$ and $hZ\gamma$ coupling in $Zh$ production process. The distribution are obtained for $e^-e^+ \to Zh \to l^-l^+b\bar{b}$ process at $\sqrt{s}=250$ GeV, $\Lambda=1$ TeV with initial polarized beams, $(P_{e^-},P_{e^+}) = (\mp0.8,\pm0.3)$ and an integrated luminosity of $\mathcal{L}=1$ ab$^{-1}$ for each polarization setting. No systematic errors are considered.
• Figure 3: In the top row, we show the sensitivity for asymmetries as a function of one WC at a time obtained with $h \to WW^\star$ channel and in the bottom row, the sensitivity for $h\to ZZ^\star$ are shown. Only top eight sensitive asymmetries for each CP-odd WC are shown. The distribution are obtained at $\sqrt{s}=250$ GeV, $\Lambda =1$ TeV with beam polarization $(P_{e^-},P_{e^+}) = (\mp0.8,\pm0.3)$ and an integrated luminosity $\mathcal{L}=1$ ab$^{-1}$. No systematic errors are taken in this analysis.
• Figure 4: Impact of integrated luminosity (left panel) and systematic uncertainties (right panel) on two-dimensional 95% CL contours in the Wilson coefficient plane, using combined Higgs decay channels ($h\to b\bar{b}$, $h \to WW^\star$, $h\to ZZ^\star$) from $Zh$ production at $\sqrt{s} = 250$ GeV $e^-e^+$ collider.