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

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 $V\in \{Z,\gamma\}$ 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.